Requirements on the design and configuration of small and medium sized solar air conditioning applications. Guidelines

Transcription

1 Requirements on the design and configuration of small and medium sized solar air conditioning applications

2 Requirements on the design and configuration of small and medium sized solar air conditioning applications 2

3 Requirements on the design and configuration of small and medium sized solar air conditioning applications This publication has been produced in the framework of the SOLAIR project which is supported by the Intelligent Energy Europe programme of the European Commission. SOLAIR aims mainly at capacity building, promotion and influencing the process of decision making for the implementation of small and medium sized solar airconditioning (SAC) systems in order to increase the confidence on the technology and to encourage its implementation. April 15, project.eu 3

4 Requirements on the design and configuration of small and medium sized solar air conditioning applications This report was prepared as deliverable D10 in the SOLAIR project on base of material and information provided by all partners in the project. Edited by Edo Wiemken, Fraunhofer ISE. Chapter one Building cooling and air conditioning was prepared by Sašo Medved, University of Ljubljana, Slovenia. Chapter seven Planning tools was prepared by Maria João Carvalho, INETI, Portugal. SOLAIR is co ordinated by target GmbH, Germany Partners in the SOLAIR consortium: AEE Institute for Sustainable Technologies, Austria Fraunhofer Institute for Solar Energy Systems ISE, Germany Instituto Nacional de Engenharia, Technologia e Innovação INETI, Portugal Politecnico di Milano, Italy University of Ljubljana, Slovenia AIGUASOL, Spain TECSOL, France Federation of European Heating and Air conditioning Associations RHEVA, The Netherlands Centre for Renewable Energy Sources CRES, Greece Ente Vasco de la Energia EVE, Spain Provincia di Lecce, Italy Ambiente Italia, Italy 0 SOLAIR is supported by The sole responsibility for the content of this publication lies with the authors. It does not necessarily reflect the opinion of the European Communities. The European Commission is not responsible for any use that may be made of the information contained therein 4

5 Requirements on the design and configuration of small and medium sized solar air conditioning applications Table of content Introduction Building cooling and air conditioning Indoor thermal comfort Cooling demand of buildings Energy conservation principles Fundamentals of solar cooling Impact of climate changes on thermal indoor comfort and energy demand for cooling Technologies applicable for solar thermally driven cooling Chilled water systems Open cycle processes Solar thermal collectors General requirements on solar air conditioning and cooling systems Primary energy saving Requirements on basic system layout Heat rejection system Solar collector system Selection of the appropriate technology All air systems Full air system + chilled water distribution Supply air system + chilled water distribution All water system Small systems: schemes for typical applications Recommendations on monitoring and quality assurance Planning tools Design approaches Rules of Thumb Simple pre design tools SHC Softwaretool (NEGST Project) SACE Solar cooling evaluation light tool SolAC Solar Assisted Air Conditioning Software ODIRSOL Solar Assisted cooling Software Expected new pre design tools Detailed simulation tools System orientated Building orientated Further simulation tool description

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7 Requirements on the design and configuration of small and medium sized solar air conditioning applications Introduction The demand for building cooling and air conditioning is still rapidly increasing. To give an impression: the sales rate in 2008 for small size electrically driven room air conditioners (< 5 kw chilling capacity) was approx. 82 million units worldwide, of which 8.6 million were sold in Europe. It is not surprising that in some areas the peak load in the public electricity grid is evoked during hot summer seasons already by electrically driven air conditioning. In Germany, a country with definitely not the highest demand for cooling and air conditioning, the overall electricity demand for building air conditioning in 2006 was estimated to approx. 5% of the total electricity consumption (14% for the total of air conditioning and refrigeration); in other South European countries this share might be far higher. Building air conditioning is today based mainly on electrically driven mechanic vapour compression technologies. Although for new developed, predominantly large capacity scale developments it is reported about high efficiencies in the compression cycle, for the standard of air conditioning in existing buildings it can be assumed that on an average less than 3 kwh cold are produced with the electricity input of 1 kwh el. Subsequently this implies that approximately 1 kwh primary energy is used for the provision of 1 kwh useful cold. At the same time of peak cooling demand, high amounts of solar radiation are available at many sites and could be used for thermally driven processes, e.g., cooling and air conditioning. The processes are in general well known and not new. Thermally driven cooling was applied within the last decades in niche markets preferably in the large capacity range, using waste heat or heat from combined heat and power production. However, the combination of this technology with solar heat is new and some more complexity arises with this combination. Solar cooling and airconditioning is demonstrated in a few hundred installations so far. Solar thermally assisted cooling and air conditioning can contribute to an environmentally friendly building supply system for the following reasons: considerable savings in primary energy consumption and reduction of CO 2 emissions are possible load relieving of the public electricity grid in terms of both, peak power and energy, thus contributing to grid stabilisation combined use of solar heat for heating, cooling and domestic hot water preparation, thus an all season high utilisation of the solar thermal system no use of working materials with high global warming potential less noise emissions and less vibrations than vapour compression technologies. Thus, support for the market development of this technology is useful; these guidelines, edited in the frame of the SOLAIR 1 project, is one of the supporting activities. 1 SOLAIR Increasing the market implementation of solar air conditioning systems for small and medium applications in residential and commercial buildings (SOLAIR). Supported in the Intelligent Energy Europe Programme of the European Commission. EIE/06/034/S Duration: until 12/ project.eu 7

8 Requirements on the design and configuration of small and medium sized solar air conditioning applications Interaction in the design and layout of a solar thermally driven cooling and air conditioning system, to be considered in the planning phase. The proper design of a solar cooling and air conditioning system and the choice of the components interact to a high degree with the site conditions (climatic conditions) and with the demand for cooling (load conditions). The intention of this guideline is to support the understanding of the interactions and to provide in parallel a picture on the state of the art of solar cooling and air conditioning. As one of the most cost effective measures in the planning of an air conditioning system is the reduction of cooling loads already in the building planning and design phase, chapter one deals with general aspects on building cooling and air conditioning and prepares the reader for the subsequent chapters, focusing on the technical aspects of solar thermally driven technologies. However, some aspects of solar cooling and air conditioning may have found not the adequate attendance in these guidelines, such as e.g. more details on system control or on detailed site oriented installation information. The reason for this lack is the still ongoing process in the development and preparation of such information. The thematic structure of the content underlines the target group of technical orientated planners in the building services and utilities management area, but the guidelines are hopefully useful to anyone, interested on this subject. Finally, a more comprehensive description of solar cooling and air conditioning can be found in the handbook for planners Solar Assisted Air Conditioning in Buildings 2, elaborated in the Task 25 on Solar Cooling within the Solar Heating and Cooling Programme (SHC) of the International Energy Agency (IEA). In the current Task 38 Solar Air Conditioning and Refrigeration, a new edition of this handbook will be launched and available in In the context with the existing handbook, these guidelines may be seen in both ways: as a straightforward introduction into solar cooling and air conditioning on the one hand, and as a market and practically oriented complement to the handbook on the other hand. 2 Hans Martin Henning (Editor): Solar Assisted Air Conditioning of Buildings A Handbook for Planners. Second revised edition ISBN , Springer Wien New York. 8

9 Requirements on the design and configuration of small and medium sized solar air conditioning applications 1 Building cooling and air conditioning The main goal of every building planner is to assure the most pleasant and healthy living environment to people that live in the building. However the challenge here is to attain the optimal indoor comfort with minimal energy consumption and minimal environmental impact. From the engineering point of view the quality of indoor environment is defined by four groups of requirements: thermal comfort, indoor air quality, lighting comfort and noise protection. Concerning energy consumption the most important issue is fulfilling of thermal comfort requirements. Thermal confort Indoor air quality IAQ Lighting confort Noise protection IEQ Figure 1.1 Indoor environment quality could be assured by fulfilling of four groups of requirements 1.1 Indoor thermal comfort Human is a warm blooded being with constant internal temperature (37 ± 0.8 C), which is independent of surrounding temperature and muscle activity. The body produces heat in internal organs with combustion (oxidation) of nutritive substances. This process is called metabolism or basal metabolism. Metabolism is regulated by our body regarding to momentarily activity. Similar as with heat machines, the human body has to give off the excess heat to the environment by means of different heat transfer mechanisms. If such heat transfer from our body to surroundings does not cause any unpleasant sensation the requirements of thermal comfort are fulfilled. 9

10 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 1.2 Human body emits sensible and latent heat into environment using different heat transfer processes. If this process does not cause unpleasant sensation the thermal comfort is provided. The body emits the heat in the form of sensible and latent heat. Sensible heat is emitted with convection and radiation of the body to the surrounding air and surfaces, conduction of heat on the places where we stand and with exhaling the warm air. Latent heat is given off to surroundings with diffusion of vapour trough the skin, evaporation of water on the skin surface and humidifying the exhaled air. heat flux (W) air temperature ( o C) Figure 1.3 Heat transfer mechanisms and heat flux emitted by human body to surroundings depends on air temperature and humidity at low temperatures radiation and convection are the most important mechanisms, meanwhile at air temperatures above 30 C latent heat transfer is dominant, emitted amounts of water vapour as function of air temperature are presented as well Parameters of indoor thermal comfort The importance of individual heat transfer mechanisms is varying with regard to the state of indoor environment which is evaluated with several parameters: air temperature, mean radiant temperature of surrounding surfaces, air velocity and air humidity. Because the amount of heat that the body gives off depends on the difficulty of the work and on the clothes we are wearing, the activity level, which is given in met (metabolism) and clothing which is given in clo (cloth) are two very important additional parameters that affect thermal comfort. 1 Met corresponds with 58 W released by 1 m 2 of human surface area or approximately 100 W in total. During heavy work metabolic rate can reach up to 10 Met and this corresponds to emitted heat flux of 270 W. 10

11 Requirements on the design and configuration of small and medium sized solar air conditioning applications Clo is proportional to thermal resistant of cloths. Characteristic values are 0 clo for a nude body, 1 clo for a business suit and 3 clo for winter clothes. Indoor air temperature Ti is the most evident indicator of proper thermal comfort. In principle, the temperature should be higher on lower activity level and lighter clothing. For building cooling it is important that our body is capable to adapt to the seasonal conditions. Thus the appropriate indoor temperatures are between 20 and 22 o C in the winter and 26 to 27 o C in the summer time when ambient temperature is above 30 C. Mean radiant temperature Tr is mean temperature of the surfaces that surround the living space. It has a strong influence on radiative heat transfer between human body and surroundings. The difference between the indoor air temperature Ti and mean radiant temperature Tr should not be greater than 2K. During the summer, the indoor surfaces or internal window blinds exposed to the solar radiation can warm up to 50 and more C, which can be disturbing. Bright coloured or reflective external window blinds are a good solution for decreasing the mean radiant temperature. The air velocity in the room affects the convective heat losses and evaporation of water, which we are excreting trough the skin and sweat glands. During the heating season our body feel as unpleasant velocities above 0.15 m/s, meanwhile in the summer time we have no comfort problems with higher velocities up to 0.6 or even 0.8 m/s. For example, we can increase the air flow around our bodies with a ceiling fan and it results as feeling the environment around us being cooler. Air humidity affects the latent heat transfer from the bodies to the surrounding air. Therefore in case of higher temperatures the humidity level has to be lower. Air humidity in the buildings is varying because of air conditioning and different sources of water vapour in living spaces (human, plants, cooking, etc.). The air humidity can be given as moisture content of air x, which is defined with the ratio of water vapour mass (in g or kg) added to the mass of one kilogram of dry air (typical values are between 5 to 20 g/kg) or as relative humidity which is defined as ration between actual water vapour pressure and water vapour pressure in saturated air at the same temperature. Values are quoted in percents in range between 0% in dry air and 100% in air saturated with water vapour. At the air temperature Ti between C air humidity should be 70 to 35%, or the moisture content x should not exceed 11.5 g/kg. In practice the air humidity could be reduced by cooling the air beyond its dew point with cooling devices in the rooms or with central air conditioning units. In both cases dehumidification increase the electricity consumption, unless thermally driven cooling engines are used instead of compressor driven cooling systems Integrated indicators of thermal comfort Joint influence of the thermal comfort parameters could be evaluated with the predicted main vote PMV indicator. PMV is an agreed relative assessment scale of thermal comfort in indoor environment. The values of PMV are in the range between 3 (cold), 2 (moderately cold), 1 (pleasantly cold), 0 (neutral), +1 (pleasantly warm), +2 (warm) and +3 (hot environment). The value PMV equal to 0 therefore means neutral environment, positive values mean warmer environment, negative values mean colder environment. The PMV value is established by a mathematical expression or based on measurements of thermal comfort parameters and considering the activity and clothing of the occupancies. The predicted mean vote can be related to percentage of dissatisfied people (PPD), which tells us the percent of dissatisfied people in observed room. 11

12 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 1.4 Instrument for determination of predicted mean vote of indoor environment (PMV); sensor for temperature, velocity and humidity measurements, knobs for Met and Clo input. Figure 1.5 Instrument Correlation between PMV and PPD values. According to the graph at PMV +2 80% of people will be dissatisfied with their thermal environment. Source: [EN ISO 7730, 2005] The demands concerning the indoor thermal environment are defined in many international and national standards and regulations. Thus EN standard defines three levels of comfort expectations: class A (high expectations), class B(normal expectations) and class C (moderate expectations). For class A the PMV must be ± 0.2 (corresponds with PPD < 6%), for class B ± 0.5 (PPD < 10%) and for class C ± 0.7 (PPD < 15%). EN ISO 7730 defines thermal comfort as acceptable if 80% or more inhabitants feel comfortable in such indoor environment. As cooling of buildings is closely related to indoor air temperatures and humidity some other comfort indicators could be used as well. Humid operative temperature is the temperature of the environment with 100% relative humidity in which a human body emits the same total amount of heat as in real environment. The heat stress index is the ratio of the total evaporative heat losses of human body required for thermal comfort and maximum evaporative heat losses possible in the same environment multiplied by factor 100. The decimal value of heat stress index is called skin wettedness. 12

13 Requirements on the design and configuration of small and medium sized solar air conditioning applications 1.2 Cooling demand of buildings Conventional or mechanical cooling Most of the buildings today are cooled with mechanical cooling or air conditioning systems. In both cases a cooling machine is needed. Usually, this is a heat pump which pumps the heat out of the cooler building to the warmer surrounding of the building. In cases of smaller systems (compact cooling units) the air is directly cooled in the evaporator of the cooling unit placed in the room. When dealing with larger buildings central air or water cooling systems are commonly used. In case of air cooling systems the air in the air conditioning device is cooled with chilled water before delivered into the building. In water cooling systems water with temperature between 5 to 7 C is pumped through chilled water pipe distribution systems to the end heat exchangers (e.g. fan coils) installed in each indoor space. Figure 1.6 Fan coil units with coil heat exchanger and fans are end heat exchangers in central water cooling systems. During operation the cooling machine consumes electricity. Because it is working as a heat pump, the amount of heat transferred out of the building is significantly larger than the amount of used electric energy. The ratio between the heat extract out of the building Qc and the electric energy demand W is named coefficient of performance (COP el ). Modern cooling units have COP el between 3 and 5 depending on the cooling power and the type of compressor. In spite of high COP el, these cooling devices still use electricity which is in many countries produced with high emissions of greenhouse gasses. An increased consumption of electricity is characteristic for all modern societies. In Europe the consumption of electricity has increased by a factor 12 within the last 50 years. Today the yearly increase of electricity consumption is twice as high as the increase of fossil fuel consumption. Building cooling systems also have a high factor of simultaneity, which consequently leads to electricity network overload. In Slovenia for example the peak electricity demand has changed from 19 PM in the winter time to 15 PM in the summer time in last three years indicating increased electricity demand for cooling of the buildings. 13

14 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 1.7 Air handling unit of central air conditioning system; air is cooled with chilled water provided by cooling engine. Building heat Q c W Compressor Q od Environment COPel electricity coefficient of performance Q c kwh COP h el = W kwh e Figure 1.8 Cooling machines operates as a heat pump, therefore heat transferred from buildings to the environment is larger than consumption of electricity. The ratio is called coefficient of performance or COP el. Modern cooling units have COP el between 3 and 5. 14

15 Requirements on the design and configuration of small and medium sized solar air conditioning applications Cooling loads and energy demand for cooling of the buildings Cooling loads and energy demand can be calculated using different approaches. In engineering practice VDI 2078 and ASHRAE calculation procedures are often used. Regardless to the method the first step in buildings cooling analyses is the determination of heat gains. Heat gains are divided into sensible and latent heat gains. Sensible heat gains are originated by: solar radiation and heat transfer through windows unsteady heat transfer through opaque building envelopment internal heat gains (human, lighting, appliances,..) heat transfer by air exchange between surrounding and building because of infiltration and ventilation Heat gains through windows and transparent walls can be characterized by several optical parameters: transmittivity of solar radiation t total energy transmittivity g shading factor of shading devices Sf Transmissivity of solar radiation is the ratio between transmitted and incoming solar radiation. Since part of solar radiation is absorbed in glazing, radiation and convection heat flux from the inner glass layer into the interior represent additional heat gains. The sum of heat gains can be expressed by g value as the ratio between sum of solar radiation and heat flux gains and incoming solar radiation on window surface. The g value is the most adequate window characteristic for cooling load determination. Cooling loads through transparent building envelopment could be significantly reduced by selection of effective shading devices. τ= Gi G Gi + qk + q g = G s G' Sf = G Figure 1.9 Transmissivity of glazing is the ratio between transmitted (Gi) and incoming solar radiation (G) (left); total energy transmissivity g of glazing is the ratio between sum of transmitted solar radiation and heat flux transferred from inner glass surface by radiation and convection (Gi + qk+qs) and incoming solar radiation G. (middle); shading factor Sf of shadings is the ratio between transmitted solar radiation G and incoming solar radiation G (right) 15

16 Requirements on the design and configuration of small and medium sized solar air conditioning applications Heat gains through opaque building envelopment depends on absorbed solar radiation (wall orientation and wall surface colour), thermal conductivity of wall materials and heat accumulation of the wall. Heat gains can be calculated by hour to hour analyses of steady heat transfer replacing air temperature differences with reference temperature difference as it is proposed in VDI Since contemporary building envelope elements have low a heat transfer coefficient, heat gains through opaque elements are in most cases small. Internal heat gains are often major reason for overheating. The human body itself emits a heat flux between 100 W and 250 W in condition of heavy activity. Large number of appliances characterized for commercial buildings contribute to large internal heat gains as well. Good daylighting design and use of high efficient compact and LED lamps can significantly reduce the internal cooling loads. Contemporary buildings are sufficiently tight to prevent significant infiltration of ambient air into the building. Nevertheless they must be ventilated to ensure good indoor air quality. Mechanical ventilation must be regulated according to demand to ensure lower cooling load with supply air. Latent heat gains are in general generated in buildings because of different water vapour sources, nevertheless in humid regions supply external air must be dehumidified before supplied to the buildings. For example, a human body emits up to 50 g of water vapour per hour, plants up to 20 g per day. Cooling load indicates heat flux (removed rate of energy) needed for fulfilling requirements of thermal comfort especially regarding to indoor air temperature and humidity. Time dependant heat gains and cooling loads differ by amplitude and time shift because of heat accumulation in building constructions. Cooling loads are calculated for a climate dependant hot summer design day and the daily maximum value is taken as design cooling load of the building. More advanced methods are based on hour by hour analyses using a computer tool, among others TRNSYS is very well known. In such tools, a Test Reference Year as meteorological data source is used for specific locations. The software Meteonorm (CD published by James & James, UK) includes TRY for more than 5000 location world wide. Such tools are most useful for the calculation of energy demand for cooling which taks into account hour by hour cooling load, COP el of cooling machine and overall cooling system efficiency. Detailed descriptions of planning tools are presented in Chapter 7. Important note: The Energy Performance of Buildings Directive (EPBD) requests that the energy demand for cooling must be included into buildings energy performance indicators. As a consequence, in some national regulations the rated power of cooling machines is limited. In Slovenia, for example the permitted power of the cooling machine is 24 W per m 3 of building volume. 16

17 Requirements on the design and configuration of small and medium sized solar air conditioning applications Study cases As an example of computer simulation approaches, annual specific cooling loads and energy (electricity) demand for four business buildings are presented below. All buildings are built at locations with continental climate. Office building 1 Cooling load (W/m 3 ) Cooling load (W/m 2 ) Useful energy demand (heat) (kwh/m 2 ) End energy demand (electricity) (kwh/m 2 ) Office building Office building Shopping centre Tabel 1.1 Specific cooling loads and energy demand of four business buildings Remark: useful energy is related to quantity of heat extracted from indoor air, end energy demand is related to electricity demand of mechanical cooling. 17

18 Requirements on the design and configuration of small and medium sized solar air conditioning applications 1.3 Energy conservation principles The energy demand for cooling of buildings can be reduced by implementation of five principles presented on Figure 1.10: solar radiation controlling, reduction of heat gains thought opaque building envelope, intensive night ventilation, reduction of internal gains and implementation of free cooling techniques. Figure 1.10 Principles of energy conservation for buildings cooling. Source [McQuiston et al., 2005] Shading devices must be external, high reflective for solar radiation and mounted in such a way that enables convective cooling as well as daylighting of the interior. Figure 1.11 shows the temperature profile in an office without shadings and mechanical cooling and in the neighbouring office with external shadings; shading devices are installed in such a way that convective cooling is enabled on both sides of shadings and they are movable to improve shading factor S f all day long and enable optimal daylighting in offices. Temperatura prostora Room v tretjem temperature nadstropju (oc) ( C) Brez Without senčil in haljenja shadings Zunanja lamelna senčila, nehlajen prostor With shadings Hour startingdan 1 v ofletu January Figure 1.11 Only external, high reflective and movable shading devices controls successfully solar radiation heat gains; temperature in an office without shadings and cooling (gray line), and temperatures in office equipped with external shadings as presented on photo (orange line). 18

19 Requirements on the design and configuration of small and medium sized solar air conditioning applications Shading devices could be multi purpose. For example PV modules can be used as external shading device. Following example shows such a case. PV modules are mounted on the part of glass roof of atrium in office building. The result is the reduction of the peak cooling load from 150 kw to 75 kw, meanwhile the heating load remains practically unchanged. In this particular case PV shadings have little influence on daylighting as well. [kw] heating load cooling load -100 Jan. Feb. Mar. Apr. Maj Jun. Jul. Aug. Sept. Okt. Nov. Dec. [kw] heating load cooling load -100 Jan. Feb. Mar. Apr. Maj Jun. Jul. Aug. Sept. Okt. Nov. Dec. Figure 1.12 PV modules as external shading devices on the glass roof of an atrium in office buildings reduce peak cooling load by 50% meanwhile heating demand and daylighting remain practically unchanged (left heating and cooling loads without PV modules, right after PV modules were installed) Heat gains through the opaque envelope could be reduced with light surface colours and quality thermal insulation in combination with a high building construction thermal mass. As a consequence, a significant decrease of temperature amplitude swing at the inner side of the construction and a time lag of several hours can be attained. Modern architecture often requires dark surface colours of walls and roofs. Selective paints can be used in this case to reduce both, surface temperature and resulting cooling loads. Such colours have equal reflectivity of light as ordinary colours, but enlarged near IR reflectivity. This causes a reduction of dark surface temperature during solar noon by 20 C. Even more effective are green roofs and walls. Evapotranspiration by grass and plants reduce cooling loads for 5 to 10 times regarding to dark roofs. 19

20 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 1.13 Additional selective white paint layer (left) pained below green coating (right) reduces peak wall surface temperature up to 15 K Night ventilation can significantly reduce cooling loads but only in case if intensive night ventilation with at least 4 to 5 exchanges of building volume per hour is provided. On the other hand ventilation systems can be supplemented by free cooling techniques like evaporative cooling. Evaporative cooling is most effective in hot and dry areas. It can significantly contribute to cooling power reduction and therefore to the peak electricity demand for mechanical cooling. COP el of such systems are 50 or more. supply air temeprature ( C) T ambient T after evaporative cooling number of hours per year (h) Figure 1.14 Evaporative cooling is most effective at high ambient temperature at solar noon; duration of ambient air and supply air temperatures after evaporative cooling (left); evaporative cooling can significantly contribute to cooling power reduction and peak electricity demand additional for mechanical cooling. Source: [Vidrih, Medved, 2006] 20

21 Requirements on the design and configuration of small and medium sized solar air conditioning applications night day Ta (LHTES inlet temperature) To (measured) To (numerical model) Temperature ( C) Time (h) Figure 1.15 Latent heat storage integrated into ventilation system are cooled down during the night and provide lower supply air temperatures during the next summer day; such system can be combined with other free cooling systems to provide all day free cooling operation. Source: [Arkar, Medved, 2007] Ground heat exchangers can be coupled to mechanical ventilation systems for pre cooling of ventilation air during the daytime in summer days. They are used in smaller buildings, and they have to be planned very carefully, to ensure a high COP el. Mechanical ventilation system can be upgraded with a cold storage as well. Especially effective are the latent storages which are cooled during the night, and at day time they are used to cool the supply air. These systems are more expensive, and are still in a phase of development. Despite the fact that free cooling techniques are effective and can reduce energy demand for cooling greatly they alone cannot guarantee that indoor comfort will be fulfilled all the time. In such cases other energy efficient cooling technology must be implemented the solar cooling. 21

22 Requirements on the design and configuration of small and medium sized solar air conditioning applications 1.4 Fundamentals of solar cooling Principles of desiccant evaporative solar cooling Air is a mixture of different gasses and water vapour. The change of air state can be a consequence of sensible heat transfer during the heating or cooling and the transfer of latent heat because of humidification or dehumidification. For that reason the state of the air should be expressed by the internal energy called enthalpy (h) instead of the air temperature. We can demonstrate the changes of air states in an T x diagram. During the humidification of air, dispersed drops of water in the air, transforms into molecules of water vapour with assistance of internal energy of air. Consequently the air cools down. This kind of natural cooling is very efficient, although it has an side effect of increasing the air s moisture content and it s relative humidity, which can exceed the appropriate levels, defined by thermal comfort. T ( C) φ=0,1 φ=0,2 φ=0, φ= x (g/kg) Figure 1.16 The process of evaporative cooling goes on at constant enthalpy. Air temperature drops, but at the same time moisture content of air (x) and relative humidity (φ) increase. T ( C) φ=0,1 φ=0,2 φ=0, φ= x (g/kg) Figure 1.17 The process of sorption drying (10 > 9) also goes on at constant enthalpy. Air temperature increases as moisture content (x) and relative humidity (φ) decrease. 22

23 Requirements on the design and configuration of small and medium sized solar air conditioning applications In conventional cooling systems air is dehumidified by cooling below the dew point, resulting in condensation of water vapour. The second option for drying the air is using special materials which have the ability of sorption removal of water vapour molecules out of the air. These materials are for example silica gel or lithium chloride. The first one is a solid, the second one is a liquid; however, lithium chloride is also applied in impregnated structures, thus appearing as solid form sorption unit. A side effect of this process is an increase in the air temperature and humidification of the material, which absorbs the water vapour from the air. When heating the sorption material above the temperature of 60 to 70 C the water vapour is released from it and the process can be repeated. In solar driven desiccant evaporative solar cooling systems, this regeneration heat is provided by a solar thermal collector system. In market available applications, the processes are combined with a heat recovery unit to the desiccant evaporative solar cooling cycle, described in detail in Chapter Principle of sorption solar cooling Conventional cooling system use a compressor to compress refrigerant vapour. Sorption cooling processes run in a similar way. However instead of mechanical compressor which uses electricity, only fluid pumps are applied to pump binary mixture of two substances the refrigerant and a substance that absorbs the refrigerant and is called absorbent, in case of an absorption process is applied. In practice a mixture of water (refrigerant) and lithium bromide (absorbent) on the one hand, or ammonia (refrigerant) and water (absorbent) on the other hand is used. Circulation pump electricity consumption is negligible compared to a compressor in a conventional cooling system. Additional energy needed for the operation of sorption cooling systems must be provided in form of heat, which can be produced by high efficient solar thermal system. Alternatively, an adsorption process may be applied, based on the physical process of adsorption of the refrigerant at a solid state sorption material, such as silica gel or types of zeolithes. Since the result ab or adsorption processes is coolant water with temperature of 7 to 10 C all kinds of cooling system can be used. Details of sorption solar cooling can be found in Chapter Impact of climate changes on thermal indoor comfort and energy demand for cooling Predicted climate changes due to anthropogenic emissions will cause an increase in mean atmosphere temperatures and atmospheric IR radiation. For that reason the climate changes will have a strong influence on thermal comfort in the buildings in the summer period and therefore on the energy demand for cooling as well. Based on simulations of a low energy dwelling and an office building without cooling shown on Figure 1.18 and considering a corrected test reference years (TRY) one can find out that the number of overheated hours will strongly increase. In case of mechanical cooling and if the most severe scenario (D, Ta + 3 C, +6 W/m 2 ) is taken into account the energy demand for cooling will increase by 10 times (depending on the location and application). It can be expected, that the cooling demand will increase for 3 to 5 kwh/m 2 of buildings living space. The conditions will be similar as in the year As temperatures will be also higher in the night time, the free cooling systems will be less efficient. 23

24 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 1.18 Low energy and commercial building used in climate change impact simulations. Number of hours [h/a] Number of hours [h/a] TRY A B C D Year 2003 CTRY 0 TRY A B C D Year 2003 CTRY Figure 1.19 Increased overheating hours (Ti > 26 C) in un cooled one family (left) and office (right) building; Scenario A (+1 C), Scenario B (+1 C, +3 W/m 2 ), Scenario C (+3 C), Scenario D (+3 C, +6 W/m 2 ) Source: [Vidrih, Medved, 2006] Cooling demand [kwh/m 2 a] 6,0 5,5 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 Cooling demand [kwh/m 2 a] ,5 2 0,0 TRY A B C D Year 2003 CTRY 0 TRY A B C D Year 2003 CTRY Figure 1.20 Increased specific cooling demand in cooled one family (left) and office (right) building in kwh per m 2 of floor area per year Taking all these facts into account, we can expect that more and more buildings will be cooled in the future, especially every new built building. This gives solar cooling a great possibility to enforce itself in the market. 24

25 Requirements on the design and configuration of small and medium sized solar air conditioning applications References [EN ISO 7730, 2005] Ergonomics of the thermal environment Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal criteria. [EN 15251, 2007] Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics. [McQuiston et al., 2005] F. McQuiston, J. Parker, J. Spitler: Heating, Ventilating, and Air Conditioning, Analysis and Design ; Jonn Wiley&Sons, Inc, 2005 [Vidrih, Medved, 2006] B. Vidrih, S. Medved: The Connection Between the Climate Change Model and a Buildings Thermal Response Model: A Case of Slovenia, Journal of Mechanical Engineering, vol. 52, no. 9/06, Ljubljana, 2006 [Arkar, Medved, 2007] C. Arkar, S. Medved; Free cooling of a building using PCM heat storage integrated into the ventilation system, Solar Energy, vol. 81, no 9, Elsevier Press,

26 Requirements on the design and configuration of small and medium sized solar air conditioning applications 2 Technologies applicable for solar thermally driven cooling The focus in SOLAIR is on solar cooling and air conditioning systems in the small and medium size capacity range. The classification into small and medium aligns with available chiller products; small applications are in this sense systems with a nominal chilling capacity below 20 kw, and medium size systems may range up to approx. 100 kw. Systems in the small capacity range are usually consist of thermally driven chilled water systems, whereas medium sized systems may be open cycle desiccant evaporative (DEC) cooling systems as well. While in the first type of system technology the distribution medium is chilled water in a closed loop to remove the loads from the building, in the latter one supply air is directly handled in humidity and temperature respectively in an open process. Figure 2.1 visualises the two general types of applications. Of course, applications using both types of technology at the same time are possible. In chilled water systems, the central cold water distribution grid may serve decentralised cooling units such as fan coils (mostly with dehumidification), chilled ceilings, walls or floors; but the chilled water may be used for supply air cooling in a central air handling unit as well. The required chilled water temperature depends on this type of usage and is important for the system design and configuration, but the end use devices are not in the focus of SOLAIR and thus are not presented more in detail. ~18 C Chilled ceiling Heat > 60 C Supply air Thermally driven Chiller 16 C - 18 C (< 12 C) 6 C - 9 C Chilled water temperature Fan coil Cooled / Conditioned area Heat > 50 C Return air Supply air Desiccant evaporative cooling (DEC) Conditioned area Figure 2.1 General types of thermally driven cooling and air conditioning technologies. In the figure above, chilled water is produced in a closed loop for different decentral applications or for supply air cooling. In the figure below, supply air is directly cooled and dehumidified in an open cycle process. Source: Fraunhofer ISE. The technologies are outlined more in detail below. Heat is required in both technologies, to allow a coninuous system operation. In the applications surveyed in SOLAIR, the heat is at least to a significant part produced by a solar thermal collector system. 26

27 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 2.2 illustrates that any thermally driven cooling process operates at three different temperature levels: with driving heat Q heat supplied to the process at a temperature level of T H, heat is removed from the cold side thereby producing the useful cold Q cold at temperature T C. Both amounts of heat are to be rejected (Q reject ) at a medium temperature level T M. The driving heat Q heat may be provided by an appropriate designed solar thermal collector system, either alone or in combination with auxiliary heat sources. While in open cycle processes the heat rejection is with the air flow in the system integrated into the process, closed chilled water processes require for an external heat rejection system, e.g., a cooling tower. The type of the heat rejection system is currently turning more into the field of vision, as this component usually is responsible for a considerable fraction of the remaining energy consumption of solar cooling systems. A basic number to quantify the thermal process quality in thermally driven chilled water systems is the coefficient of performance COP, defined as Q cold COP =, Qheat thus indicating the amount of required heat per unit produced cold (more accurately: per unit removed heat). The COP and the chilling capacity depends strongly on the temperature levels of T H, T C and T M. In open cycle desiccant cooling systems, the performance is more difficult to assess, since it depends more strongly on the system operation. It is useful, to define here the performance for the desiccant operation mode only, since in this operation mode heat is required (section 2.2). The performance is then calculated from the enthalpy difference between ambient and supply air, related to the required heat input. Experiences from DEC plants have shown that performance values comparatively to single effect chillers may be achieved. Focussing on chilled water systems, a maximum process performance COP ideal for each temperature level can be derived from thermodynamic laws: COP T T T C H M ideal =. TH TM TC This dependency is discussed more in detail in e.g. [Henning, 2006]. As shown in figure 2.3, the ideal performance of a reversible process is far above the performance, obtained in market available thermally driven chillers. The COP in realised products ranges from 0.5 to 0.8 in singleeffect chillers (absorption or adsorption), and may range to 1.4 in double effect chillers. Q heat T H T M Q reject T C Q cold Figure 2.2 Basic scheme of a thermally driven cooling process. 27

28 Requirements on the design and configuration of small and medium sized solar air conditioning applications ideal double-effect absorption COP adsorption single-effect absorption chilled water temperature: 9 C cooling water temperature: 28 C Hot water inlet [ C[ Figure 2.3 Exemplary curves of the coefficient of performance COP for different sorption chiller technologies and the limit curve for an ideal process. The curves are shown as function of the driving temperature and for a constant chilled and cooling water temperature level. Source: [Henning, Wiemken, 2006] The difference between real and ideal performance of the thermally driven chillers can be expressed with a process quality number ζ PQ : ζ PQ = COP real / COP ideal. Typical vaules of ζ PQ, extracted from market available products, are 0.3. The process quality number allows to assess the advantages of an improved process quality with respect to the required driving temperature. This is shown in figure 2.4. The figure presents the driving temperature as a function of the temperature lift T, which is defined as the difference between heat rejection temperature T M and chilled water temperature T C : T = (T M T C ). As an example, the temperature lift is low in case of high chilled water temperature and wet heat rejection (low cooling water temperatures) and high in case of low required chilled water temperatures and dry cooling. Driving temperatures for two different COP values are included. For each COP curve, the driving temperature depends furthermore on the process quality; therefore, two different quality numbers are assumed. The operation areas of different collector technologies are indicated as well. As an example, a single effect chiller with COP of 0.7, working at T = 35 K, may be driven still with vacuum tube collectors, if the process requires driving temperatures of approx. 100 C (process quality number of 0.4). In case of a lower process quality, the required driving temperature is higher and tracked concentrating collectors are necessary. 28

29 Requirements on the design and configuration of small and medium sized solar air conditioning applications required driving temp. T H [ C] COP / ξ PQ 1,1 / 0,4 1,1 / 0,3 0,7 / 0,4 0,7 / 0,3 Flat-plate collector Vacuum-tube collector Application examples: Chilled ceilings useful temperature lift T = T M T C [K] Fan-coils; wet cooling Fan-coils; dry cooling 1-axis tracked concentrating collector High temperature lift: ice storage, dry cooling Figure 2.4 Heat source temperature required for different COP/ζ PQ combinations, plotted as a function of the temperature lift. Typical operation ranges of solar collector technologies are included as well as different system application examples (grey marked areas). Source: [Hennng, 2006]. 2.1 Chilled water systems Absorption chillers The dominating technology of thermally driven chillers is based on absorption. The basic physical process consists of at least two chemical components, one of them serving as refrigerant and the other as the sorbent. The main components of an absorption chiller are shown in figure 2.5. The process is well documented, e.g., in [ASHRAE, 1988]; thus, details will be not presented here. The majority of absorption chillers use water as refrigerant and liquid lithium bromide as sorbent. Typical chilling capacities are in the range of several hundred kw. Mainly, they are supplied with waste heat, district heat or heat from co generation. The required heat source temperature is usually above 85 C and typical COP values are between 0.6 and 0.8. Until a few years ago, the smallest machine available was a Japanese product with a chilling capacity of 35 kw. Recently, the situation has improved due to a number of chiller products in the small and medium capacity range, which have entered the market. In general, they are designed to be operated with low driving temperatures and thus applicable for stationary solar thermal collectors. The lowest chiller capacity available is now 4.5 kw. Some examples of small and medium size absorption chillers are given in figure 2.6. In addition to the traditional working fluids H 2 O/LiBr, also H 2 O/LiCl and NH 3 /H 2 O are applied. The application of the latter working fluid with Ammonia as refrigerant ist relatively new for building cooling, as this technology was dominantly used for industrial refrigeration purposes below 0 C in large capacities. An advantage of this chiller type is especially given in applications, where a high temperature lift (T M T C ) is necessary. This is for example the case in areas with water shortage, when dry cooling at high ambient temperatures has to be applied. 29

30 Requirements on the design and configuration of small and medium sized solar air conditioning applications hot water (driving heat) cooling water GENERATOR CONDENSER ABSORBER EVAPORATOR cooling water chilled water Figure 2.5 Scheme of a thermally driven single effect absorption chiller. Compared to a conventional electrically driven compression chiller, the mechanical compression unit is replaced by a thermal compression unit with absorber and generator. The cooling effect is based on the evaporation of the refrigerant (e.g., water) in the evaporator at low pressure. Due to the properties of the phase change, high amounts of energy can be transferred. The vaporised refrigerant is absorbed in the absorber, thereby diluting the refrigerant/sorbent solution. Cooling is necessary, to run the absorption process efficient. The solution is continuousely pumped into the generator, where the regeneration of the solution is achieved by applying driving heat (e.g., hot water). The refrigerant leaving the generator by this process condenses through the application of cooling water in the condenser and circulates by means of an expansion valve again into the evaporator. Figure 2.6a Examples of small absorption chillers using water as refrigerant and Lithium Bromide as sorption fluid. Left: air cooled chiller with a capacity of 4.5 kw of the Spanish manufacturer Rotartica. Middle: 10 kw Chiller with high partload efficiency and overall high COP of the German manufacturer Sonnenklima, shown without housing. Right: Chiller with 15 kw capacity, manufactured by the German company EAW; this machine is also available in capacities of 30 kw, 54 kw, 80 kw and above. Sources: Rotartica, Sonnenklima, EAW. 30

31 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 2.6b Further examples of absorption chillers. Left: Absorption chiller with the working fluid H 2 O/LiBr and a capacity of 35 kw from Yazaki, Japan. This chiller is often found in solar cooling systems, since it was for several years the smallest in Europe available absorption chiller, applicable with solar heat. Currently, a smaller version with 17.5 kw chiller capacity from this manufacturer has entered the European market. Source: Gasklima. Right: This chiller uses water as refrigerant and Lithium Chloride as sorption material. The crystallisation phase of the sorption material is also used, effecting in an internal energy storage. The capacity is approx. 10 kw; the machine is developed by ClimateWell, Sweden, and can operate as heat pump as well. Source: ClimateWell. Figure 2.6c Examples of absorption chillers with the working fluid ammonia water. In principle, these types of chillers are foreseen to provide chilled water at temperatures < 0 C for commercial and industrial cooling, but may be applied for higher chilled water temperature levels under appropriate operating conditions as well. Left: Absorption chiller with 12 kw rated chilling capacity, developed by Pink, Austria; shown without housing. Right: Absorption chiller from Ago, Germany. This chiller is available with 50 kw capacity and with higher capacities. Sources: Pink/SolarNext. Figure 2.7 displays current available hot water driven aborption chillers, sorted by chilling capactiy. The presentation makes no claim to be exhaustive. Double effect machines with two generators require for higher driving temperatures > 140 C, but show higher COP values of > 1.0. The smallest available chiller of this type shows a capacity of approx. 170 kw. With respect to the high driving temperatures, this technology demands in combination with solar thermal heat for concentrating collector systems. This is an option for climates with high fractions of direct irradiation. 31

32 Requirements on the design and configuration of small and medium sized solar air conditioning applications York, Carrier, Trane.. Broad Ago* Thermax EAW Yazaki Robur* Pink* Sonnenklima ClimateWell Rotartica water/libr ammonia/water* water/licl * typical for applications with T cold 0 C Chilling capacity range [kw] Figure 2.7 Typical capacity range of hot water driven absorption chillers. The listed products are market available, either by small series production or fabrication on demand. No claim to be complete. Adsorption chillers Beside processes using a liquid sorbent, also machines using solid sorption materials are available. This material adsorbs the refrigerant, while it releases the refrigerant under heat input. A quasicontinuous operation requires for at least two compartments with sorption material. Figure 2.8 shows the components of an adsorption chilller. Market available systems use water as refrigerant and silica gel as sorbent, but R&D on systems using zeolithes as sorption material is ongoing. To date, only few manufacturers from Japan, China and from Germany produce adsorption chillers; a German company is with a small unit of 5.5 kw capacity on the market since 2007 and has increased the rated capacity in improved versions to 7.5 kw and 15 kw (models of 2008). Typical COP values of adsorption chillers are Advantageouos are the low driving temperatures, beginning from 60 C, the absence of a solution pump and a comparatively noiseless operation. Figure 2.9 shows examples of adsorption chillers, whereas figure 2.10 displays current available adorption chillers, sorted by chilling capactiy. The presentation makes no claim to be exhaustive. An overview on closed cycle water chillers is also presented in [Mugnier et al., 2008]. 32

33 Requirements on the design and configuration of small and medium sized solar air conditioning applications CONDENSER cooling water 2 1 cooling water hot water (driving heat) EVAPORATOR chilled water Figure 2.8 Scheme of an adsorption chiller. They consist basically of two sorbent compartments 1 and 2, and the evaporator and condenser. While the sorbent in the first compartment is desorbing (removal of adsorbed water) using hot water from the external heat source, e.g. the solar collector, the sorbent in the second compartment adsorbs the refrigerant vapour entering from the evaporator; this compartment has to be cooled in order to increase the process efficiency. The refrigerant, condensed in the cooled condenser and transferred into the evaporator, is vaporised under low pressure in the evaporator. Here, the useful cooling is produced. Periodically, the sorbent compartment are switched over in their functions from adsorption to desorption. This is usually done through a switch control of external located valves. Figure 2.9Examples of adsorption chillers. Left: Chiller with 70 kw capactiy of the Japanese manufacturer Nishiyodo, installed for laboratory cooling at the University Hospital in Freiburg, Germany. Adsorpition chillers of similar medium capacity are available from the Japanese manufacturer Mayekawa as well. Middle: Small size adsorption chilllers with 7.5 kw and 15 kw chilling capacity from SorTech company, Germany. Source: SorTech. Right: Small size adsorption chiller in the capacity range 7 to 10 kw of the manufacturer Invensor, Germany. Source: Invensor. 33

34 Requirements on the design and configuration of small and medium sized solar air conditioning applications Nishyodo (JP) SorTech (DE) { SJTU (CN) } Invensor (DE) Mayekawa (JP) water/silicagel water/zeolite { } no detailed information on market status Chilling capacity range [kw] Figure 2.10 Typical capacity range of adsorption chiller brands. The listed products are market available, either by small series production or fabrication on demand. No claim to be complete. Heat rejection Figure 2.2 in section 2 indicates that the amount of heat extracted from the building ( useful cold ) plus the driving heat of the transformation process has both to be charged to the environment at (medium) ambient temperature level. This operation is done by means of a heat rejection system. Figure 2.11 illustrates as an example the difference in the demand of heat rejection between a conventional compression chiller system and an ab or adsorption chiller system. It is evident that heat rejection in thermally driven systems plays a central role in the system development. Compression Sorption QM = Qc + W QM = Qc + QH 1,33 kw 2,4 kw W Compression QH Sorption 0,33 kwe 1,4 kwt 1 kwc Qc = 3 x W 1 kwc Qc = 0,7 x QH Figure 2.11 Example on the demand for heat rejection in a conventional electrically driven compression chiller system (left) and in a (single effect) thermally driven chiller system (right). In the comparison, the chilling capacity is 1 kw in both systems. Typical efficiency numbers have been used. Source: Tecsol. 34

35 Requirements on the design and configuration of small and medium sized solar air conditioning applications In principle, different possibilities and heat rejection technologies may be applicable: 1. wet cooling, either of open type or of closed type, using the evaporative cooling effect 2. dry cooling without evaporation 3. hybrid cooling, allowing for both options: wet and dry cooling 4. geothermal heat rejection by use of ground tubes 5. heat rejection by use of ground water, sea water, river or spring water 6. application of low temperature level cooling water by thus rejecting the medium temperature level heat If applicable in any case, the options 5. and 6. should be preferred, as these applications are connected with the lowest electricity consumption of the different heat rejection possibilities. Unfortunately, application fields of low temperature level heat (~ 30 C) is rarely identified, and sea water cooling is for financial reasons limited to applications direct at costal sites and for large applications. Additionally, the permittance to increase the sea water temperature level by this means is difficult to obtain. Heat rejection using ground tubes is a comparatively new approach and may be of interest, especially when the ground tubes are used for heat pump operation as well during winter, thus contributing to an annual balanced charging and dischcharging of the ground. However, the investment cost for ground tubes are currently still high. An example of such an application in combination with a small adsorption chiller (with heat pump operation) is shown in the SOLAIR Best practice examples [SOLAIR: Best Practice Catalogue, 2008]. The most applied heat rejection technology in combination with thermally driven chillers today is still wet cooling by means of open cooling towers. Figure 2.12 illustrates the principle of such a heat rejection system: the cooling water is sprayed on top of the cooling tower towards the filling material, which increases the effective exchange area between air and cooling water. The main cooling effect is obtained through evaporation of a small percentage of the cooling water (typically < 5%); this loss has to be compensated by fresh water supply. The cooled water then returns to the cooling circuit of the chiller. A fan removes the saturated air in order to keep the process running. The process is very efficient in appropriate climates and in principle, the limitation temperature of the returned cooling water is not far from the wet bulb temperature of the air (3 C to 5 C above the wet bulb temperature). A commercial product is shown in figure Fan Drip-catcher Cooling water distribution Filling material Air inlet Sump Figure 2.12 Typical scheme of an open wet cooling tower. Source: GWA. 35

36 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 2.13 Example of a large wet cooling tower installation. In dry climates, the fan speed of a wet cooling tower can be often decreased in order not to fall below the minimum cooling water temperature of the chiller (e.g., 25 C often defined for absorption chillers), whereas in a very humid climate also the wet bulb temperature often is high. Figure 2.14 displays as an example for more extreme climates monthly averages of the wet bulb temperature at Dubai. During summer, the monthly values are approx. 25 C, indicating that during daytime the obtained return cooling water temperature often may exceed 30 C. Also the ambient temperature levels are very high and during day, up to 40 C ambient temperature is detected, which indicates the limit of dry cooling (limitation temperature: a few C above ambient temperature). In the application with adsorption chiller technology, closed wet cooling towers have to be applied instead of open wet cooling towers. The reason is the connection of the heat rejection circuit with the driving circuit for some seconds during the heat recovery phase, which is activated between the functional interchange of adsorption and desorption partitions of the chiller. The hydraulic pressure conditions do not usually allow for an open cooling water loop. In the closed cooling towers, the tower is equipped with a cooling water heat exchanger, which is sprayed by an external water loop for indirect evaporative cooling. A disadvantage of this technique are lower efficiencies and higher costs. In some countries, regulations exist on the application of wet cooling towers with respect to hygienic aspects. In order to avoid unfavourable growth of bacteria, a water treatment of the cooling water may be necessary. For this reason and for reasons of improving the optical acceptance of heat rejection systems especially in small scale applications, dry cooling is still of interest, although the cooling temperature level as well as the electricity consumption is in general higher (higher power consumption of the fans due to pure sensible cooling). Dry heat rejection in solar thermally driven cooling systems has been applied in a number of demonstration systems for testing this opportunity. Furthermore, a supplier of small capacity adsorption chillers offers a dry cooler with spray function in case of high ambient temperatures, adapted to the chiller. 36

37 Requirements on the design and configuration of small and medium sized solar air conditioning applications Ta [ C] Twb [ C] RH [%] G_Gh [kwh/m2] Ambient air and wet bulb Temperature [ C], rel. humidity [%] Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Global horizontal radiation sum Figure 2.14 Monthly climate data for Dubai site. During summer, very high wet bulb temperautes may be expected during daytime, thus limiting the efficiency of wet cooling towers. At the same time, also the ambient temperature as indicator for dry cooling limits is very high as well. 2.2 Open cycle processes While thermally driven chillers produce chilled water, which can be supplied to any type of airconditioning equipment, open cooling cycles produce directly conditioned air. Any type of thermally driven open cooling cycle is based on a combination of evaporative cooling with air dehumidification by a desiccant, i.e., a hygroscopic material. Again, either liquid or solid materials can be employed for this purpose. The standard cycle which is mostly applied today uses rotating desiccant wheels, equipped either with silica gel or lithium chloride as sorption material. All required components, such as desiccant wheels, heat recovery units, humidifiers, fans and waterair heat exchangers are standard components and have been used in air conditioning and airdrying applications for buildings or factories since many years. However, the appropriate combination of the components to form a desiccant evaporative cooling system (DEC), which is the most common solar driven open cycle system, requires some special experience and attention. The standard cycle using a desiccant wheel is shown in figure The application of this cycle is limited to temperate climates, since the possible dehumidification is not high enough to enable evaporative cooling of the supply air at conditions with far higher values of the humidity of ambient air. For climates like those in the Mediterranean countries therefore other configurations of desiccant processes have to be used. Systems employing liquid sorption materials which have several advantages like higher air dehumidifiation at the same driving temperature and the possibility of high energy storage by means of concentrated hygrocopic solutions are note yet market available but they are close to market introduction; several demonstration projects are carried out in order to test the applicability of this technology for solar assisted air conditioning. A possible general scheme of a liquid desiccant cooling system is shown in figure

38 Requirements on the design and configuration of small and medium sized solar air conditioning applications backup heater humidifier return air 7 6 cooling loads supply air dehumidifier wheel heat recovery wheel Figure 2.15 Scheme of a solar thermally driven solid Desiccant Evaporative Cooling system (DEC), using rotating sorption and heat recovery wheels (source: Fraunhofer ISE). Below: sketch of the DEC unit (source: Munters). The successive processes in the air stream are as follows: sorptive dehumidification of supply air; the process is almost adiabatic and the air is heated by the adsorption heat released in the matrix of the sorption wheel pre cooling of the supply air in counter flow to the return air from the building evaporative cooling of the supply air to the desired supply air humidity by means of a humidifier the heating coil is used only in the heating season for pre heating of air small temperature increase, caused by the fan supply air temperature and humidity are increased by means of internal loads return air from the building is cooled using evaporative cooling close to the saturation line the return air is pre heated in counter flow to the supply air by means of a high efficient air to air heat exchanger, e.g. a heat recovery wheel regeneration heat is provided for instance by means of a solar thermal collector system the water bound in the pores of the desiccant material of the dehumidifer wheel is desorbed by means of the hot air exhaust air is removed to the environment by means of the return air fan. 38

39 Requirements on the design and configuration of small and medium sized solar air conditioning applications Regenerator regeneration air Q H concentrated solution driving heat LiCl/water solution storage supply air Absorber Q M rejected heat diluted solution Figure 2.16 General scheme of a liquid desiccant cooling system (top). The supply air is dehumidified in a special configured spray zone of the absorber, where a concentrated salt solution is diluted by the humidity of the supply air. The process efficiency is increased through heat rejection of the sorption heat, eg., by means of indirect evaporative cooling of the return air and heat recovery. A subsequent evaporative cooling of the supply air may be applied, if necessary (heat recovery and evaporative cooling is not shown in the figure). In a regenerator, heat e.g. from a solar collector is applied, to concentrate the solution again. The concentrated and diluted solution may be stored in high energy storages, thus allowing a decoupling in time between cooling and regeneration to a certain extent. Bottom: a liquid desiccant cooling demonstration system is installed at the Solar Info Center in Freibug, Germany, for airconditioning of 310 m² office area. The air volume flow rate is 1500 m³/h. The system was developed and installed by the German company Menerga. The ventilation system is at the left side of the figure, the solution storages are located right hand side in the foreground. The storage in the background is part of the solar thermal driving heat source, consisting of 17 m² flat plate collectors. Sources: Fraunhofer ISE. In general, desiccant evaporative cooling is an interesting option if centralized ventilation systems are used. At sites with high latent and sensible cooling loads, the air conditioning process can be splitted into dehumidification by means of a thermally driven open cycle desiccant process, and an additional chilled water system to maintain the sensible loads by means of e.g. chilled ceilings with high chilled water temperatures, in order to increase the efficiency of the chilled water production. More details on open cycle processes are given in [Henning, 2004/2008] and in [Beccali, 2008]. 39

40 Requirements on the design and configuration of small and medium sized solar air conditioning applications 2.3 Solar thermal collectors A broad variety of solar thermal collectors is available and many of them are applicable in solar cooling and air conditioning systems. However, the appropriate type of the collector depends on the selected cooling technology and on the site conditions, i.e., on the radiation availability. General types of stationary collectors are shown in figure 2.17, and construction principles of improved flat plate collectors and evacuated tube collectors are given in figure 2.17a c. The use of cost effictive solar air collectors in flat plate construction is limited to desiccant cooling systems, since this technology requires the lowest driving temperatures (starting from approx. 50 C) and allows under special conditions the operation without thermal storage. To operate thermally driven chillers with solar heat, at least flat plate collectors of high quality (selective coating, improved insulation, high stagnation safety) are to be applied. Typical efficiency curves for collectors are displayed in figure 2.18 (steady state efficiency for two different radiation conditions; no dynamic behaviour is reflected in this figure). For two different sites in Spain, Barcelona and Huelva, the annual gross energy yields for typical stationary collectors as well as for concentrating parabolic trough collectors are compared in figure 2.19, to visualise the high dependency of the collector types on the site conditions. Figure 2.20 presents two examples of stationary collector installations, used for solar cooling and air conditioning. glas cover insulation collector frame absorber with air channels solar air collector glass cover flat plate collector insulation absorber with fluid channels collector frame glass cover CPC collector reflector insulation absorber with fluid channel collector frame evacuated tube collector evacuated tube evacuated glass tube optionally: reflector Absorber with fluid channel (forward/return) Figure 2.17 Examples of stationary collectors, applicable for solar cooling. Source: SOLAIR didactic material base / Fraunhofer ISE. 40

41 Requirements on the design and configuration of small and medium sized solar air conditioning applications Glass 1 st layer of air Convection barrier (teflon foil) 2 nd layer of air Absorber Insulation 120 mm Figure 2.17a Example of a flat plate collector with minimised heat losses through improved insulation thickness and an additional convection barrier (teflon foil). Source: S.O.L.I.D. Other flat plate collectors have been improved through anti reflective coatings or using double glass cover for more supression of heat losses. Those improved flat plate collectors are more appropriate in solar cooling systems than standard flat plate collectors. Figure 2.17b Examples of evacuated tube collectors with direct fluid flow through the collector. The figure on the left reflects the traditional construction principle, whereas the right hand figure shows todays often favourized type, as in this solution tightness problems of the evacuated volume have been mainly solved. Source: Fraunhofer ISE Collecting pipe Heat exchanger (condenser) Insulation Glass pipe Heat pipe Figure 2.17c Example of an evacuated tube collector with heat pipe principle. Advantage: the pipe is already freezingprotected and stagnation safe, but not the collecting pipe system. Disadvantage: highest cost of vacuum tube collectors. Source: 41

42 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 2.18 Typical efficiency curves of stationary collectors, calculated from parameters related to the aperture area of the collectors. The curves are drawn for ambient temperature of 25 C and 800 W/m² radiation level (top) as well as for 400 W/m² (bottom). The figure includes very roughly the application range of the most interesting cooling technologies. As the graph represents only steady state operation conditions and only exemplary sets of efficiency curves, is not sufficient to decide for a specific type of collector in a system to be planned. Although the efficiency curves of stationary collectors may be theoretically drawn also for higher temperatures, they have been cut in the figure at temperatures > 20 C, since there is only little experience with these collectors at higher temperatures (and thus pressure levels). For the higher temperature range, the efficiency range of a 1 axis tracked concentrating collector is included as an example. Source: Fraunhofer ISE. FK ST Flat plate collecotr, standard product FK AR Flat plate collector, 1 cover glass, anti reflective coated FK HT Flat plate collector, 1 cover glass, convection barrier foil, improved insulation VRK CPC Evacuated tube collector, direct mass flow, Sydney type with external CPC reflector 42

43 Requirements on the design and configuration of small and medium sized solar air conditioning applications energy yield [kwh/m²] Barcelona FPC CPC EFPC ETC ETC/CPC CPC PTC energy yield [kwh/m²] Huelva FPC EFPC CPC ETC ETC/CPC CPC PTC temperature [ C] temperature [ C] Figure 2.19 Typical gross energy collector yields as a function of the collector output temperature, calculated for the sites Barcelona and Huelva, Spain. FPC: standard flat plate collector, CPC: flat plate collector with concentrating parabolic compound mirrors (low concentration ratio), ETC: evacuated tube collector, ETC/CPC: evacuated tube collector with concenetrating parabolic compound mirrors, PTC: parabolic trough collector. Source: SOLAIR didactic material base /Aiguasol. Figure 2.20 Examples on solar collectors. Left: Flat plate CPC collector, installed at the National Energy Research Centre INETI in Lisbon, Portugal. The collector is the driving heat source for a DEC system, located in one of the office buildings of INETI. Source: INETI. Right: Evacuated tube collector at the wine storage building in Banyuls, France. This collector provides heat for the solar autonomous operation of an absorption chiller with 52 kw chilling capacity. Source: Tecsol. It is important, to have a common understanding on the referenca area, when efficiency curves or costs per m² collector or other area related issues are discussed. In general, there are three different area definitions as shown in figure 2.21: the gross area, the aperture area (indicating the projected light catching area of the collector), and the absorber area. Especially in evacuated tube collectors, these area values within one collector type may deviate by more than 25%. The absorber area has to be defined precisely, when e.g. pipe shaped absorbers are applied (absorber area may increase the gross area). 43

44 Requirements on the design and configuration of small and medium sized solar air conditioning applications Bruttofläche Gross area Aperturfläche Aperture area Absorberfläche area a a Aperture area: n x a Aperture area CPC-mirror Figure 2.21 Definitions of collector areas (of course, to be multiplied by the length). Figure 2.22 Examples of 1 axis tracked concentrating solar sthermal collectors. Left: Fresnel collector for hot water preparation in the medium temperature range up to 200 C. The mirrors are tracked to focus the direct radiation towards the absorber, located above the mirror area. Advantage: low sensitivity to high wind speeds. Source: PSE, Germany. Right: Parabolic trough collector, developed by Button Energy, Austria. The collector is designed for steam production and is part of research project with a solar thermally driven steam jet ecector chiller at AEE INTEC, Austria. Concentrating collectors are also in the focus of interest for solar cooling. In principle, they can be applied for providing driving heat (steam of hot water) at temperatures above 150 C for 2 effect apsorption chillers. With the higher COP thermal > 1.0, to be expected in this application, less driving heat capactiy has to be installed and consequently, the heat rejection system can be designed smaller. However, double effect chillers are currently not market available in the small capacity range; providing driving heat for chillers in applications, where it is necessary to overcome a high temperature lift from chilled water temperature to heat rejection temperature (e.g., low 44

45 Requirements on the design and configuration of small and medium sized solar air conditioning applications chilled water temperature demand, combined with dry cooling). A typical configuration is thus a concentrating collector combined with an ammonia/water absorption chiller; new concepts for solar cooling, such as the use of a steam jet ejection chiller (research status). The use of tracked concentrating collectors is generally more appropriate at sites with a high direct radiation fraction on the global radiation sum. However, a more detailed analysis is necessary in order to determine the yields of such a system. Figure 2.22 shows two examples on concentrating collectors. For collectors operating in the medium temperature range, results from market surveys are available at [Task 33/IV, 2008]. References [Henning, 2006] Hans Martin Henning: Solar cooling and air conditioning thermodynamic analysis and overview about technical solutions. Proceedings of the EuroSun 2006, held in Glasgow, UK, June, [Henning, Wiemken, 2007] Hans Martin Henning, Edo Wiemken: Solar Cooling. Proceedings of the ISES Solar World Congress, Bejing, China, [ASHRAE, 1988] ASHRAE handbook (1988) Absorption Cooling, Heating and Refrigeration Equipment; Equipment Volume, Chapter 13. [Henning, 2004/2008] Hans Martin Henning (Ed.): Solar Assisted Air Conditioning in Buildings A Handbook for Planners. Springer Wien/NewYork. 2 nd revised edition 2008; ISBN [Mugnier et al., 2008] D. Mugnier, M. Hamdadi, A. Le Denn: Water Chillers Closed Systems for Chilled Water Production (Small and Large Capacities). Proceedings of the International Seminar Solar Air Conditioning Experiences and Applications, held in Munich, Germany, June 11 th, [Beccali, 2008] Marco Beccali: Open Cycles Solid and Liquid based Desiccant Systems. Proceedings of the International Seminar Solar Air Conditioning Experiences and Applications, held in Munich, Germany, June 11 th, [SOLAIR: Review technical solutions, 2008]. Task 2.1: Review of available technical solutions and successful running systems. Cross Country Analysis. Public accessible report in SOLAIR. project.eu [SOLAIR: Best practice Catalogue, 2008] Task 2.2: Best Practice Catalogue. Public accessible report in SOLAIR. project.eu [MEDISCO, 2006] Mediterranean food and agro industry applications of solar cooling technologies. Contract (EU INCO). Coordination: Politcnico di Milano, Italy. Duration: [Zahler, 2008] Chr. Zahler, A. Häberle, F. Luginsland, M. Berger, S. Scherer: High Teperature System with Fresnel Collector. Proceedings of the International Seminar Solar Air Conditioning Experiences and Applications, held in Munich, Germany, June 11 th, [Task 33/IV, 2008] Werner Weiss, Matthias Rommel (Editors): Process Heat Collectors State of the Art within Task 33/IV. Brochure compiled in IEA SHC Task 33 and SolarPACES Task IV: Solar Heat for Industrial Processes. Published by AEE INTEC, Gleisdorf, Austria, shc.org/task33 45

46 Requirements on the design and configuration of small and medium sized solar air conditioning applications 3 General requirements on solar air conditioning and cooling systems 3.1 Primary energy saving Any type of air conditioning and cooling is connected to the use of primary energy sources, to provide electricity or heat in order to operate the end user equipment. Today, the primary energy sources used in most of the countries are predominantly composed of fossil fuels and thus their use is linked with greenhouse gas emmissions. Consequently, the basic requirement on a solar airconditioning and cooling system is the saving of primary energy and reduction of greenhouse effect supporting emmissions 3. This pre condition effects the configuration and design of solar air conditioning and cooling systems; this will be discussed in the following example. We consider a solar thermally assisted cooling system with a fossil fueled gas boiler as back up heat source and compare the primary energy input with a conventional electrically driven vapour compression chiller system. The general system scheme is shown in Figure 3.1. The backup heater is foreseen in order to cover the heat demand of the chiller in periods of low solar heat availability but still present cooling demand. Boundary conditions in this comparison are space heating is not considered in the reference system, energy input for dry heat rejection is included in the Coefficient of Performance of the vapour compression chiller COP VCC in the solar assisted system, the thermally driven chiller is characterised by the thermal Coefficient of Performance COP TDC. Energy effort for heat rejection is considered separately specific primary energy demand per kwh useful cold are calculated on base of estimated average consumption data and conversion numbers, defined below. Figure 3.1 Reference system and solar assisted cooling system, considered in the primary energy demand estimation of this section. Source: Fraunhofer ISE. 3 In very few countries, the primary energy sources used for electricity generation and heat supply may be already based to a high fraction on renewable energy sources. In this case, solar cooling systems do not mainly contribute to fossil fuel savings. Then, the systems may be more evaluated on base of efficiency and economic or other criteria, e.g., whether a local system using renewable energy sources is more efficient or more economic in comparison to the central energy supply systems. Since these are very rare situations, such cases will be not considered here. 46

47 Requirements on the design and configuration of small and medium sized solar air conditioning applications The governing equations, to estimate the primary energy demand are thus given in Figure 3.2: Figure 3.2 Equations to calculate the primary energy demand for the soalr cooling system and reference system as shown in figure 3.1. Based on average specific consumption data. Source: Fraunhofer ISE. with PE C,sol = primary energy demand of the solar cooling system [kwh PE ] PE C,ref = primary energy demand of the reference system [kwh PE ] PE rel = difference in primary energy saving, related to PE C,ref [ ; %] COP TDC = produced cold per unit heat input [kwh cold /kwh driving_heat ] COP VCC = produced cold per unit electricity input [kwh cold /kwh electricity ] Q cold = useful amount of cold produced [kwh] SF C = solar fraction of driving heat to thermally driven chiller [ ] η boiler = efficiency of fossil fueled boiler [ ] η PE,fossil fuel = primary energy efficiency of fossil fuel [ ] η PE,grid = primary energy efficiency of electricity grid [ ] f el,solar = specific electricity demand of solar system (pumps) [kwh electricity /kwh solar heat ] f el,tdc = specific electricity demand of thermally driven chiller [kwh electricity /kwh cold ] f el,hr = specific electricity demand for heat rejection [kwh electricity /kwh rejected heat ] The general dependency between the primary energy demand and the solar fraction is displayed in Figure 3.3. The primary energy demand is shown per unit kwh produced cold. The demand for the reference system depends on COP VCC only, thus results in horizontal lines. The primary energy demand for the solar assisted system decreases with increasing solar fraction, but varies with the thermal COP TDC. Exceeding a certain solar fraction, the primary energy demand of the solar assisted system falls below the primary energy demand of the reference system, and the solar thermal driven solution saves primary energy. Consequently, the system has to be designed in an appropriate way to guarantee the average solar fraction in the cooling period. 47

48 Requirements on the design and configuration of small and medium sized solar air conditioning applications 2 specific primary energy per unit of cold no primary energy saving thermal system, low COP conventional system 0.5 thermal system, high COP saves primary energy solar fraction cooling Figure 3.3 General dependency of specific primary energy demand of a solar thermally driven coolings system and a conventional reference system. The solar assisted system uses a fossil fueled gas boiler as a heat back up. Source: Aiguasol. For a specific set of parameters, the relative primary energy saving is shown in percent of the primary energy demand of the reference system in Figure 3.4 as an example. The parameters used here are outlined in the legend of the figure. It has to be kept in mind that in the calculations the auxiliary heat is provided by a fossil fuel driven boiler. The savings are calculated for two different COP VCC values of the reference system: 3.5 (top figure) and 2.5 (bottom figure), whereas the latter corresponds more to the experience in realised building air conditioning applications. The figure reveals that in general very high solar fractions are required in combination with 1 effect thermally driven chilling technology, to achieve primary energy savings. With a reference chilling system with COP VCC = 3.5, and a solar assisted system with an average COP TDC = 0.5, relative primary energy savings of > 20% may be achieved with less than 10% heat input from the boiler only. The situation improves in combination with 2 effect chilling technology, but despite the fact that this technology is not applicable in all climates with solar heat, the solar fractions required are still high. For example, considering a reference system with COP VCC = 3.5, a solar fraction of at least 70% has to be obtained with a 2 effect chiller (average heat ration assumed as COP TDC = 1.0) in order to achieve relative primary energy savings of above 20%. 48

49 Requirements on the design and configuration of small and medium sized solar air conditioning applications 80% 60% 2-effect rel. primary energy saving delta_pe 40% 20% 0% -20% -40% -60% 1-effect COP VCC = 3.5 COP_TDC = 1.0 COP_TDC = 0.7 COP_TDC = 0.5 Reihe4-80% Solar Fraction of thermally driven chiller driving heat SF C 80% 2-effect 60% rel. primary energy saving delta_pe 40% 20% 0% -20% -40% -60% 1-effect COP VCC = 2.5 COP_TDC = 1.0 COP_TDC = 0.7 COP_TDC = 0.5 Reihe4-80% Solar Fraction of thermally driven chiller driving heat SF C Figure 3.4 Relative primary energy savings of a solar assisted cooling system (see figure 3.1) as a function of the solar fraction. Top: in comparison to a reference system with an average COP VCC = 3.5; bottom: in comparison to a reference system with an average COP VCC = 2.5. The values of COP TDC between 0.5 and 0.7 indicates roughly the operation range of 1 effect absorption and adsorption chiller system, whereas values above approx. 1.0 correspond to 2 effect absorption technology. Source: Fraunhofer ISE. Parameters applied in the calculation: COP TDC = [kwh cold /kwh driving_heat ] COP VCC = 3.5 (top); 2.5 (bottom) [kwh cold /kwh electricity ] η boiler = 0.9 [ ] η PE,fossil fuel = 0.95 [ ] η PE,grid = 0.38 [ ] f el,solar = 0.02 [kwh electricity /kwh solar heat ] f el,tdc = 0.01 [kwh electricity /kwh cold ] f el,hr = 0.03 [kwh electricity /kwh rejected heat ] 49

50 Requirements on the design and configuration of small and medium sized solar air conditioning applications 3.2 Requirements on basic system layout From the considerations outlined in the previous section, the following conclusions may be drawn for the layout and design of solar cooling systems in order to comply with the primary energy saving targets: the use of a fossil fuel based heat backup source in thermally driven cooling applications is critical and has to be avoided completely or may be applied in exceptional cases only (extreme room air states). Only in systems using 2 effect chiller technology, a certain amount of heat provided by fossil fuel is acceptable, depending from the quality of the considered reference system if the room air states have to comply with set values of temperature and humidity, thus a backup system is necessary, a vapour compression chiller as a backup is the preferred solution. In this case, any energy unit of cold produced by the solar thermally driven part lowers the primary energy consumption of the total system (fuel saver operation of the solar system) alternatively, the backup heat source is operated with fuels from renewable sources or with waste heat and thus does not contribute to greenhouse gas emmissions if the heat input from fossil fuel sources to the thermally driven chilling equipment cannot be avoided for different reasons, the design of the solar cooling system should be supported by simulation caclulations to identify the appropriate component sizes, system configuration and ontrol strategies in order to obtain the primary energy saving target. 50

51 Requirements on the design and configuration of small and medium sized solar air conditioning applications Solar thermal system Other heat source(s) Heat Vapour compression chiller(s) Thermally driven chiller(s) Chilled water Cooling Heating Solar thermal system Other heat source(s) renewables, waste heat Thermally driven chiller(s) Heat Other heat source(s) Chilled water Cooling Heating Figure 3.5 Recommendations on the basic layout of solar cooling systems. Top: the thermally driven chiller is driven by solar heat alone. In case such an autonmous solar cooling operation does not comply with the air conditioning requirements (set values of room air states may be not achieved at all times), a vapour compression chillers is used as a cold back up. Bottom: the thermally driven chiller is the only source for cold production. Heat is provided either by the solar system or by other heat sources, operated with renewable fuels or by waste heat. Fossil fuel driven heat sources, if existing, are used for space heating and/or domestic hot water preparation only. Source: Fraunhofer ISE. The Figures 3.5 and 3.6 summarises the preferred general system layout schemes. Figure 3.5 shows the schemes for thermally driven chilled water systems, whereas Figure 3.6 suggests the scheme for open cycle desiccant cooling systems. However, the analysis for this type of systems is more complex and is not covered by the equations given in section 3.1. The reference system in this case is a vapour compression chiller for dehumidification and cooling the supply air. In many conventional air conditioning systems, a subsequent air heating after dehumidification through supply air cooling below the dew point is necessary. This operation is not required in a desiccant cooling system. Thus, more detailed analysis is necessary to identify the primary energy impact of fossil fueled back up heat sources. Nevertheless, as a rule of thumb, waste heat or renewable heat sources are recommended for desiccant cooling systems as well as shown in Figure

52 Requirements on the design and configuration of small and medium sized solar air conditioning applications Solar thermal system Other heat source(s) Summer air-conditioning: renewables, waste heat; Supply air heating winter: no conditions Heat Vapour compression chiller(s) Desiccant cooling system Chilled water Additional cooling Conditioned air Figure 3.6 Recommendations on the use of heat sources in open cycle desiccant cooling systems. Source: Fraunhofer ISE. 3.3 Heat rejection system Even if the heat supply for solar thermally driven chilled water system is gained totally from a collector or other non fossil heat sources, auxiliary energy demand is required e.g. for pumps, cooling tower fans and system control. From experiences in solar cooling systems it is known that the electricity demand for cooling water pumps and cooling tower fans is dominating and usually far above the electricity demand for collector pumps, driving heat pumps and control. As an example, Figure 3.7 shows the distribution of annual auxiliary electricity demand for a solar adsorption cooling system. 68% of the electricity demand is used for heat rejection in this example. Pump heating circle 2 1% Pump solar circle 5% Pump heating circle 1 4% Ventilator cooling tower 30% Pump chilling circle, primary 22% Pump cooling tower 38% Figure 3.7 Example on the share of annual electricity demand in a solar thermally driven cooling system (171 m² vacuum tube collectors, 70 kw adsorption chiller, closed wet cooling tower. Application: University Hospital Freiburg; Monitoring data of 2003). Source: Fraunhofer ISE. 52

53 Requirements on the design and configuration of small and medium sized solar air conditioning applications PE rel Figure 3.8 Example on the influence of the electricity demand for heat rejection on the relative primary energy savings. Source: Fraunhofer ISE. Figure 3.8 reveals the influence on the specific electricity demand by varying f el,hr for heat rejection. The calculation was executed using the equations given in section 3.1 with a fixed parameter set (compare to figure 3.4; COP VCC =3.5, solar fraction approx. 0.9). It is evident that the heat rejection curcuit has to be hydraulically designed with care in order to minimize the power demand. Otherwise, the aimed target of primary energy saving will be not achieved. Figure 3.8 shows as an example the effect on the power consumption of fans for different motor techniques applied. The lowest power consumption is here achieved with electronically controlled, brushless DC motors (EC technology) Power consumption [W] Air volume flow rate [m³/h] Figure 3.9 Example on power consumption of different motor techniques in fans. Source: EBM Pabst. Possibilities to decrease the power demand for heat rejection are, beside a carefully designed hydraulic system part, the control of the cooling tower fans and the use of the medium temperature level heat. Some of the chiller manufacturers and system distributors are already reacting to this problem with the development of appropriate cooling tower control strategies, e.g., presented in [Clauß et al., 2007], [Kühn et al., 2008]. An improvement in the market situation is furthermore that first system providers starts with the composition of kits, consisting of 53

54 Requirements on the design and configuration of small and medium sized solar air conditioning applications matched chiller and heat rejection components and peripheric hydraulic components. An example is shown in figure Figure 3.10 Example of solar cooling kits from system providers, mainly consisting of the thermally driven chiller, a matched heat rejection unit and some hydraulic components. Source: SolarNext AG. Another option is the application of ground tubes for heat rejection. With this technology, the electricity demand is decreased effectively, since no energy for fan power operation is required. A realised system using this technology is described in [Núñez et al., 2008]. A novel approach, currently being tested in the frame of a pilot project, is the integration of a latent heat storage into the heat rejection loop. The latent heat storage decreases the power demand for heat rejection during daytime; during night, the storage is recovered at low ambient temperatures efficiently. The concept is outlined in [Keil et al., 2007]. 3.4 Solar collector system General remarks on different types of collectors and on their applicability in thermally driven solar cooling and air conditioning plants were presented in section 2. Many experience in the configuration and hydraulic design of small and large solar thermal plants is available and spreaded widely to the solar thermal companies. Information on this subject and on planning and design of solar thermal systems may be found for example in [Schenke et al., 2007], [Weiss (Ed.), 2004], [VDI 6002, 2004], [DGS, 2008], [Peuser et al., 2002]. However, in most of the published information on solar thermal system configuration and design, the special operation conditions for solar cooling systems are not considered. In case the solar thermal collector is the dominating heat source for the operation of a thermally driven chiller, the heat flux of the collector has to be approximately matched to the required heat flux of the chiller. To give an example, a thumb rule will be used here to estimate an appropriate specific size of the collector for a solar cooling system: 54

55 Requirements on the design and configuration of small and medium sized solar air conditioning applications A spec = G coll η 1 coll,design COP design Example G coll =0.80kW/m 2 η coll,design =0.5 COP design =0.75 ==> A spec = 3.3 m 2 per kw cooling power Source: SOLAIR didactic material base with A spec = specific collector are per installed kw thermally driven chilling capacity [m²/kw cold ] G coll = irradiation at collector surface [kw/m²] η coll,design = collector efficiency at design condition (driving temperature) [ ] COP design = thermal COP of chiller at design conditions [ ] It has to be kept in mind that this is a very rough estimation, not considering any real system component data, site conditions and part load operation. Nevertheless, the estimated specific collector size is within the range of A spec = 3 to 4 m²/kw, found in many realised solar cooling systems with the collector as main heat source. Furthermore, we consider typical hot water volume flow rates in thermally driven chillers. An example is shown in the data sheet of the Suninverse absorption chiller in Figure 3.9. The nominal volume flow rate in the driving circuit is 1.2 m³/h, and with an assumed chilling capacity of 10 kw and a COP of 0.75, the hot water returns from the chiller with a temperature difference of 9 K to the input temperature level. This corresponds well to an average temperature difference in the driving circuit of approx. 10 K or even smaller in many systems. 55

56 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 3.9 Example on typical volume flow rates: Specifications of the Suninverse absorption chiller. Source: Sonnenklima. The volume flow rate, related to the estimated specific collector area in the example above, results in specific volume flow rates per unit collector area of approx. 40 liter/m². From the typical values of this example, it may be concluded: in solar cooling systems, the volume flow in the collector is usually in the medium and high flow level in order to match the required heat flux in the driving circuit of the chiller. Low flow systems are not appropriate; a buffer storage disconnects the mass flows between collector and absorption chiller and is usefull to bridge short periods of low collector heat production without stopping the chilling process, but due to the high flow rates in the chiller, the collectors are more short term storages. The correct matching between the mass flows is therefore still necessary; the temperature difference between supply and return flow to the chiller is usually low (between 5 and 10 K). This fact, combined with the relative high volume flow rate indicates that the stratification effect in the storage is low. In fact, the buffer storage is acting more as a 56

57 Requirements on the design and configuration of small and medium sized solar air conditioning applications mixing bottle, exchanging energy from the solar collector to the chiller, rather than a stratified tank. Consequently, the average temperature difference between collector fluid input and output is in general small. This should be considered in the hydraulic design of the collector. Figure 3.10 provides an overview of typical mass flow rates as a function of the chilling capacity and temperature difference between input/output in all three hydraulic circuits of a thermally driven chiller. For the design of the collector system, the volume flow rates in the driving circuit of the chiller are of interest. 30 Volume flow rate [m³/h] dt=3k dt=5k dt=7k Chilling capacity [kw] 30 Volume flow rate [m³/h] dt=5k dt=10k dt=15k Driving heat power [kw] 50 Volume flow rate [m³/h] dt=5k dt=7k dt=9k Heat rejection capacity [kw] Figure 3.10 General overview on volume flow rates in the chilled water, driving heat and heat rejection circuit. Shown as a function of the respective thermal capacities, whereas the dots represents corresponding data sets at a COP of 0.7. Three different temperature differences dt in each circuit are considered. Calculated for pure water in the loops. Source: Fraunhofer ISE. 57

58 Requirements on the design and configuration of small and medium sized solar air conditioning applications A critical issue in the use of high efficient collectors ist the stagnation safety. In case of non use of the solar thermal energy at high radiation levels (e.g., non operation of the cooling system, storage at maximum temperature, pump malfunction), steam generation and its propagation into the hydraulic system is possible and must be considered in the planning of the collector. All components concerned in the system thus have to be planned stagnation temperature safe. Furthermore, a dissociation of the collector fluid into its components (e.g., water, glycol) is possible as well, when non freezing fluids have been applied. This again requires for a careful selection of an appropriate collector. Experimental and theoretical results on this subject are summarised in [Rommel et al., 2007], [Hausner, Fink, 2002]. Measures to encounter stagnation problems are for example emergency heat dissipation system. It has to be ensured that such a system is operable at blackouts of the public grid as well; drain back system. In this case, the collector fluid is completely removed from the collector, whenever the circulation pump stops. A special design of the collector system is required. This technology permits to avoid stagnation and freezing risks. Safetey components such as expansion vessels, airvent and safety valves are skipped from the solar loop while a special design of the collector system is required (intermediary small storage tank and draining layout of the pipes). In this case, the collector fluid (water or water glycol) is completely removed from the collector, whenever the circulation pump stops. A recent solar heating and cooling system of 7.5 kw cold (adsorption chiller from Sortech) plus 25 m² double glass flat plate collectors in Perpignan in 2008 has shown that a drainback strategy did not modify the collector efficiency and the electric consumption of the primary loop pump for heating and cooling purposes; pure water as collector fluid. This concept does not avoid danger of stagnation, but the consequences are better to manageable, since only water vapour is exhausting. Pure water systems require at central European locations with danger of freezing special effort in piping insulation and collector circuit control and is applicable with vacuum tube collectors only. The collector circuit is closed. If the syestem pressure of the collector system is not conflicting with the pressure conditions in the remaining hydraulic heating system, a heat exchanger is not required in the solar loop, thus reducing installation cost and increasing thereby slightly the efficiency of the collector system. In any case, the additional electricity consumption for these measures has to be assessed. It should be finally noted that a well designed solar thermal system has the capability to produce on an average > 50 kwh heat per kwh electricity, consumed by the circuit pumps. References [Clauß et al., 2007] V. Clauß, A. Kühn, F. Ziegler: A new control strategy for solar driven absorption chillers. Proceedings of the 2 nd International Conference Solar Air Conditioning, Tarragona, Spain, 2007 [Kühn et al., 2008] A. Kühn, J.L. Corrales, F. Ziegler: Comparison of control strategies of solar absorption chillers. Proceedings of the EuroSun2008, Lisbon, Portugal, 2008 [Núñez et al, 2008] T. Núñez, B. Nienborg, Y. Tiedtke: Heating and cooling with a small scale solar driven adsorption chiller combined with a borehole system. Proceedings of the EuroSun2008, Lisbon, Portugal,

59 Requirements on the design and configuration of small and medium sized solar air conditioning applications [Keil et al., 2007] C. Keil et al.: Design and operation of a solar heating and cooling system with absorption chilller and latent heat storage. Proceedings of the 2 nd International Conference Solar Air Conditioning, Tarragona, Spain, 2007 [Schenke et al., 2007] A. Schenke, H. Drück, R. Croy, H.P. Wirth: Analyse und Evauluierung großer Kombianlagen zur Trinkwassererwärmung und Heizungsunterstützung. Abschlussbericht zum BMU Verbundprojekt: Systemuntersuchung großer solarthermischer Kombianlagen. FKZ B. November 2007 [Weiss (Ed.), 2004] W. Weiss (Editor): Solar heating systems for houses A design handbook for solar combisystems. Pukblished within the IEA Solar Heating and Cooling Programme, Task 26 (Solar Combisystems). ISBN , 2004 [VDI 6002, 2004] VDI Guideline 6002: Solar heating for domestic water General principles, system technology and use in residential building; September 2004 [DGS, 2008] Leitfaden solarthermische Anlagen (Planning & installing solar thermal systems). Published by Deutsche Gesellschaft für Sonnenenergie e.v. 8 ht edition, berlin.de [Peuser et al., 2002] F. Peuser et al.: Solar thermal systems successful planning and construction. Solarpraxis Berlin, ISBN , 2002 [Rommel et al., 2007] M. Rommel et al.: Entwicklung von Techniken zur Beherrschung des Stillstandsbetriebs. Schussbericht zum Teilprojekt StagSim im Verbundprojekt Systemuntersuchungen großer solarthermischer Kombianlagen xl.info [Hausner, Fink, 2002] R. Hausner, Chr. Fink: Stagnation behaviour of solar thermal systems. A Report of IEA SHC Task 26, Solar Combisystems. November shc.org/publications/task26/index.html 59

60 Requirements on the design and configuration of small and medium sized solar air conditioning applications 4 Selection of the appropriate technology In section 2 different technologies of solar thermally driven cooling and air conditioning technologies were presented in brief. The selection of the appropriate technology is the first step in the planning process and for those planners and decision makers in the building sector, who are not familiar with solar cooling applications, a support in this phase is helpful. For this reason, a decision scheme was created first within the Task 25 Solar Assited Air Conditioning of Buildings [IEA SHC Task 25] and a respective guideline document is available at the web page of Task 25 [Henning, 2004]. The general approach for the decision of the technology is outlined in simple decision schemes in the following, starting with an overview on the complete decision scheme in figure 4.1 and discussing the different decision possibilities in the subsequent descriptions. A basic assumption in the schemes is that both, temperature and humidity of the conditioned areas are to be controlled. A pre condition is the calculation of the cooling loads based on at least the design case and of the required hygienic air change rate. Depending on the cooling load as well as according to the desire of the users or owners, either an all air system, an all water system or combined air/water systems are possible to remove heat and humidity out of the building. The subsequent basic technical decision is then, whether or not the hygienic air change is sufficient to cover also cooling loads (sensible plus latent). This is typically the case in areas with a requirement for high ventilation rates, such es e.g. seminar and lecture rooms. However, a supply/return air system makes only sense in a well designed tight building, since otherwise the leakages through the building shell are too high. In cases of supply/return air systems, thermally driven desiccant cooling systems as well as thermally driven chiller technologies are applicable; in all other cases, only thermally driven chillers can be used in order to apply solar thermal energy as driving energy source. Items of the design, which are not considered in this decision scheme, are necessity and choice of a backup system for the cold production or possibilities to allow a solar autonomous operation of the solar air conditioning system; flexibility in comfort conditions, e.g., to allow certain deviations from the desired air states; economical issues; availability of water for humidification of supply air or for cooling towers; installations covering heating demand in winter; comfort habits for room installations: fan coils may show lowest investment cost, but have to be connected to a drainage system in order to allow dehumidification; chilled ceilings and gravity cooling systems require for high investment cost, but provide high comfort. 60

61 Requirements on the design and configuration of small and medium sized solar air conditioning applications BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air air change DISTRIBUTION MEDIUM All All water system (chilled water) TECHNOLOGY Installation of of centralized air air handling unit feasible and desired? yes no Supply air air system + chilled water system temperate and extreme Climate Thermally driven chiller, chilled water network 6 C - 9 C Climate temperate and extreme Conv. AHU, thermally driven chiller, chilled water network 6 C - 9 C Building construction appropriate for supply // return air air system (building no tight enough)? yes Climate temperate extreme Climate temperate extreme Hygienic air air change able to to cover cooling load? yes no Full air air system (supply and exhaust air) + chilled water system DEC system, standard configuration, chilled water network 12 C - 15 C DEC system, special configuration, chilled water network 12 C - 15 C Conv. AHU, thermally driven chiller, chilled water network 6 C - 9 C DEC system, standard configuration DEC system, special configuration Conv. AHU, thermally driven chiller 6 C - 9 C All All air air system: Full air air system (supply and exhaust air) Figure 4.1 Basic decision scheme to identify a technology path for solar thermally assisted air conditioning. The lowest required chilled water temperature level, indicated in the branch Technology, is determined by the question whether air dehumidification is realised with conventional technique (i.e., cooling the air below the dew point), or whether air dehumidification is achieved in a desiccant process. In the latter case, the temperature of chilled water if needed at al may be higher since it has to cover only sensible loads. Source: Fraunhofer ISE. A general system scheme of a system which contains both, open desiccant cycle and closed chilled water chiller, is shown in figure 4.2. Solar thermal heat is provided to both applications. The scheme includes different backup system options as well: on the heat side by other heat sources (e.g., gas burner, connection to a district heating network or co generation plant, etc.) and on the chilled water side a backup compression chiller. A realised system with solar thermal assisance usually consists of a sub system of this figure according to the solutions, following the different paths in the decision scheme. These sub systems are outlined in the following. 61

62 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 4.2 General scheme of a complete system including desiccant technique and thermally driven water chiller. To provide cooling in the conditioned areas, several solutions are possible: a fan coil system which may be used in summer and winter, a radiative cooling system such as chilled ceilings, or a ventilation system providing fresh cooled and dehumidified air. Source: Fraunhofer ISE. 4.1 All air systems It is considered that the installation of a centralised supply/return air system is feasible and the required air change rate is sufficient to cover all sensible and latent coolings loads. In this case, an all air system is possible; no other cooling equipment is required. A pre condition is a tight and very well designed building with measures to reduce the cooling demand, such as use of energy saving equipment, efficient shading, minimising artificial lighting through dayligthing concepts, night ventilation (e.g., in combination with phase change materials), etc. Another example is a seminar room with a high occupation rate; in such room the required fresh air amount may be high enough to purge the sensible loads completely. The installation site is considered in a moderate, continental climate with temperate outdoor humidity and temperature conditions. Thus, a standard cycle of a desiccant evaporative cooling (DEC) system is applicable. The respective decision path and a scheme of the standard DEC application is shown in figure 4.3. The solar thermal collector system provides heat for the regeneration of the dehumidification unit as well as for supply air heating support in winter. Additionally, not shown in the figure, room heating with radiative heating systems may be supported in winter as well. A backup heat may be necessary for room heating in winter and to provide additionally regeneration heat to the dehumidification unit, in case the collector power is low but dehumidification is still necessary. 62

63 Requirements on the design and configuration of small and medium sized solar air conditioning applications BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air air change DISTRIBUTION MEDIUM TECHNOLOGY collector storage backup Installation of of centralized air air handling unit feasible and desired? yes regeneration heat humidifier Building construction appropriate for supply // return air air system (building tight enough)? yes exhaust air ambient air dehumidifier wheel heat recovery wheel heating return air cooling loads supply air Hygienic air air change able to to cover cooling load? yes Climate temperate All All air air system: Full air air system (supply and exhaust air) DEC system, standard configuration Figure 4.3 Decision path of a standard DEC system configuration for temperate climates and the corresponding general system scheme. The system configuration shown reflects the standard scheme of a solid DEC system with rotating sorption wheel, but a liquid desiccant cooling system may be considered as well. Source: Fraunhofer ISE. In principle, two different system operation strategies are possible: solar autonomous air conditioning mode during summer. In this case, only solar thermal produced heat is used for regeneration of the sorption unit. This operation mode is applicable, when the cooling loads are mainly caused through external solar gains and the load pattern thus are quite well in coincidence with the solar radiation. However, a perfect coincidence will be never achieved. Consequently, the probability of deviations between the actual room air states and the desired air states has to be accepted. Storages may overcome gaps in solar thermal power availability to a certain extent. The storage can be implemented either as a hot water storage as shown in figure 4.3, or, in case a liquid desiccant cooling system is implemented, as chemical storages for concentrated and diluted solution; solar assisted air conditioning. This operation mode is required, when the building load corresponds not well to the solar thermal power availability pattern, or in case the the room air states have to comply with the set values. In this case, a backup system (e.g., gas heater, connection to district heating network, etc.) provides heat to guarantee a continuous operation even at low solar radiation periods. The use of the backup system may be minimised through storages, either a hot water storage or chemical storages as mentioned above in liquid DEC systems. For this technology, glazed flat plate collectors of good quality may in general provide the required driving temperatures in the range of 55 C to 70 C. In applications without necessity of a hot water storage (e.g., liquid DEC with internal solution storages, or with a high coincidence of 63

64 Requirements on the design and configuration of small and medium sized solar air conditioning applications load pattern or usage with daily radiation availability), solar air collectors may be applied as well. An example for such a system operation is given in [Hindenburg et al., 2005] A conventional air handling unit with supply/return air system and with supply air heating and cooling/dehumidification is another option for temperate climates as well as in more extreme climates with high ambient air humidity and high ambient temperatures. In this configuration, solar thermal heat is used to operate a thermally driven chiller, which is connected to the water/air heat exchanger in the supply air channel. This technology solution is shown in figure 4.4. In this example, the air handling unit is equipped with with an evaporative cooler in the return air, which in combination with the heat recovery unit allows pre cooling of the fresh air. BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air air change DISTRIBUTION MEDIUM TECHNOLOGY collector storage backup heat rejection chiller Installation of of centralized air air handling unit feasible and desired? yes return air Building construction appropriate for supply // return air air system (building tight enough)? yes cooling heating supply air cooling loads Hygienic air air change able to to cover cooling load? yes Climate temperate extreme All All air air system: Full air air system (supply and exhaust air) Conv. AHU, thermally driven chiller 6 C - 9 C Figure 4.4 In both, temperate and extreme climates, a conventional air handling unit in combination with a thermally driven chiller can be applied. Solar thermal heat is used to operate the chiller, which either can be an absorption chiller or an adsorption chiller. Source: Fraunhofer ISE. In conventional air handling units, heating of the supply air is often required simultaneousely after dehumidification with low chilled water temperatures in the chilled water/air heat exchanger in order to prevent too low supply air temperatures (e.g., below 18 C). In principle, water from the heat rejection circuit may be used for this purpose with the advantage that on the one hand no additional heat is used for this reheating, and reducing the energy demand for heat rejection on the other hand. This possibility is indicated in the figure with the light grey lines between heat rejection circuit and supply air heating circuit. Depending on the chiller technology, the type of collector will be selected. In an absorption chiller system, either high quality flat plate collectors or evacuated tube collectors may be applied, whereas in combination with an adsorption chiller high quality flat plate collectors may be sufficient. For the definitive decision on the type of collector, the radiation availability at the site and the available area has to be considered. 64

65 Requirements on the design and configuration of small and medium sized solar air conditioning applications In extreme climates, the standard DEC cycle is often not sufficient, to meet the required supply air states with respect to temperature and humidity. Several special configurations of the DEC cycle can be considered, of which one is a desiccant evaporative cycle in combination with a chilled water system. A sketch of such a possible configuration is shown in figure 4.5. Dehumidification is mainly achieved by a desiccant wheel and the desiccant cycle is maintained using solar thermal heat for regeneration of the sorption unit. The chilled water system is used for additional predehumidification and pre cooling (heat exchanger in front of the dehumidification wheel) and for a subsequent supply air cooling (2 nd heat exchanger in the supply air channel). The chilled water can be either produced by e.g. a conventional electrically driven compression chiller or by thermally driven closed chilled water systems. An advantage of this technology solution is that the chilled water has to be provided at comparatively high temperatures (> 12 C), due to the high dew point temperature in extreme climates (for pre dehumidification) and due to the dehumidification, which is mainly achieved with the thermally driven desiccant process. This results in a favourable and efficient operation of the chiller. The system configuration as described above and shown in figure 4.5 was realised in a demonstration plant at Palermo, Italy. However, the regeneration heat in this case is not provided by solar thermal collector, but by use of a co generation unit. The produced electricity is simultaneousely driving the compression chiller. But in principle, such systems can be operated with solar heat as well. The plant in Palermo and other possible special configurations of DEC systems are discussed more in detail in [Henning et al., 2005]. BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air air change DISTRIBUTION MEDIUM TECHNOLOGY solar heat exhaust return Installation of of centralized air air handling unit feasible and desired? humidifier yes ambient supply Building construction appropriate for supply // return air air system (building tight enough)? yes dehumidifier wheel chilled water heat recovery wheel chilled water Hygienic air air change able to to cover cooling load? yes Climate extreme All All air air system: Full air air system (supply and exhaust air) DEC system, special configuration Figure 4.5 In climates with high ambient humidity and high ambient temperatures, a special configuration may be necessary, if a desiccant cycle is included. The main dehumidification is done in this example in a solar thermally driven sorption process. However, the desired air states of supply air may be not achieved in extreme climates. For this reason, additional chilled water at high chilled water temperatures can be applied for pre cooling and pre dehumidification and for a final adjustment of the supply air to the set temperature. Source: Fraunhofer ISE. 65

66 Requirements on the design and configuration of small and medium sized solar air conditioning applications 4.2 Full air system + chilled water distribution Figure 4.6 shows the decision path for situation where the building meets all requirements to install a full air system (supply/return air), but the air change rate is not sufficient to remove all the sensible loads. This may be for example the case in office buildings with normal occupation, but high internal loads through equipment, large glazed facades, etc. It is further assumed that the building is located in a temperate climate, thus, a desiccant evaporative cooling system is able to remove all latent heat from the conditioned area. In this example, a standard DEC configuration can be applied in combination with a chilled water network. The chilled water then serves either chilled ceilings or fan coils (without need for dehumidification). This separation between latent load handling (by the DEC system) and sensible load handling allows an efficient operation of the chiller, since the chilled water can be provided at high temperatures (typically > 12 C). Solar thermal heat is used to provide heat for the regeneration of the sorption unit, e.g., the dehumidification wheel. In principle, a liquid desiccant system is possible as well instead of the drawn solid desiccant system. In both cases, the required regeneration temperature from the solar collector is < 75 C for most materials, used in the sorption unit. Consequently, in most systems a glazed flat plate solar collector of good quality is sufficient for the system operation. BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air air change DISTRIBUTION MEDIUM TECHNOLOGY collector storage backup conventional chiller Installation of of centralized air air handling unit feasible and desired? yes Building construction appropriate for supply // return air system (building tight enough)? yes exhaust air ambient air dehumidifier wheel Climate temperate regeneration heat heat recovery wheel humidifier heating return air cooling loads supply air Hygienic air air change able to to cover cooling load? no Full air air system (supply and exhaust air) + chilled water system DEC system, standard configuration, chilled water network 12 C - 15 C Figure 4.6 In this decision, a full air system is possible, but not sufficient to cover all loads in the building. At temperate climates, the latent laods may be covered e.g. with a standard DEC configuration and remaining sensible loads are to be removed with an additional chilled water network in the building. In this example, solar thermal heat is the driving source for the DEC system operation. Source: Fraunhofer ISE. 66

67 Requirements on the design and configuration of small and medium sized solar air conditioning applications BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air air change DISTRIBUTION MEDIUM TECHNOLOGY collector storage backup heat rejection chiller Installation of of centralized air air handling unit feasible and desired? yes return air cooling loads supply air Building construction appropriate for supply // return air air system (building tight enough)? yes Climate cooling heating temperate extreme Hygienic air air change able to to cover cooling load? no Full air air system (supply and exhaust air) + chilled water system Conv. AHU, thermally driven chiller, chilled water network 6 C - 9 C Figure 4.7 In both, temperate and extreme climates, a conventional air handling unit in combination with a thermally driven chiller can be applied. Solar thermal heat is used to operate the chiller, which either can be an absorption chiller or an adsorption chiller. In addition to the configuration shown in figure 4.4, chilled water is used for the operation of decentralised cooling units (fan coils, chilled ceilings, etc.), since in this example the air flow rate is not sufficient to cover all sensible loads. Chilled water has to be provided at low temperatures due to the necessity of supply air dehumidification. Source: Fraunhofer ISE. For the same decision path, figure 4.7 presents a technological solution, which may be applied in temperate climates as well as in extreme climates. A conventional supply/ return air system is used, whereas the chilled is prepared by a thermally driven system, using solar heat. In comparison to figure 4.4., the chilled water system is extended to a chilled water network to maintain decentral cooling installations for sensible heat removal. Another possibility for extreme climates, e.g. at Mediterranean sites, is again a special configuration of a desiccant cycle as discussed above and shown in figure 4.5, but also with an extended chilled water network for additional sensible load removal. This configuration is presented in figure

68 Requirements on the design and configuration of small and medium sized solar air conditioning applications BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air air change DISTRIBUTION MEDIUM TECHNOLOGY exhaust solar heat humidifier return cooling loads ambient supply Installation of of centralized air air handling unit feasible and desired? yes dehumidifier wheel chilled water heat recovery wheel chilled water Building construction appropriate for supply // return air air system (building tight enough)? yes Climate Conventional chiller extreme Hygienic air air change able to to cover cooling load? no Full air air system (supply and exhaust air) + chilled water system DEC system, special configuration, chilled water network 12 C - 15 C Figure 4.8 The application of a desiccant cycle in a special configuration is possible in extreme climates as well. The example presented here is similar to the configuration in figure 4.5, but additionally, a chilled water network serves decentralised cooling installations in the building in order to extract the remaining sensible cooling loads. Again, the required chilled water temperature in this application is comparatively high (typically > 12 C), thus allowing for an efficient chilled water production. Solar thermal heat is used to provide regeneration heat for the sorption wheel. Source: Fraunhofer ISE. 4.3 Supply air system + chilled water distribution It is considered that a central air handling unit is desired. However, in a building which is not sufficiently tight, the installation of a supply/return air system is problematic since either outside air is sucked inot the buioding (internal pressure lower than external), or is lost through the building shell (internal pressure higher than external). In such a case an air handling unit to provide fesh air only would be installed. Fresh air is cooling and dehumidified and sensible loads not covered by the fesh air are purged by other means. An example might be a chilled ceiling system. The decision path for such a configuration and a possible realisation sketch is shown in figure 4.9. A thermally driven chiller, operated with solar heat, supplies chilled water to the air handling unit and to decentralised cooling installations via a chilled water network. Dehumidification is realised in the supply air handling unit. Thus, the chilled water temperature has to be sufficient low. Of course, the chilled water delivered to e.g. chilled ceilings is to be mixed to higher temperatures by controlled valves. In general it is also possible to use the chilled water return flow from the air handling unit as an inlet to a chilled ceiling, but the hydraulic scheme is more complex and therefore not shown in the figure. The technology shown in figure 4.9 is in general applicable in temperate as well as in extreme climates. With this technology decision, either absorption chillers or adsorption chillers may be applied. The selection of the chiller type is subject to a more detailed planning process, considering the cooling 68

69 Requirements on the design and configuration of small and medium sized solar air conditioning applications load pattern, exact required level of chilled water temperature, cost, etc. At least a high quality flat plate solar collector is required as driving source. Depending on the load structure and on the requirements on the room air states, either a solar thermal autonomous cooling operation during summer is possible, or a backup system to support the cooling process when necessary is needed. BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air change DISTRIBUTION MEDIUM TECHNOLOGY collector storage backup heat rejection chiller Installation of of centralized air air handling unit feasible and desired? yes supply air Building construction appropriate for supply // return air air system (building tight enough)? no Supply air air system + chilled water system cooling heating Climate temperate extreme Conv. AHU, thermally driven chiller, chilled water network 6 C - 9 C Figure 4.9 In this example, only a supply air system installation is possible for building quality reasons or due to limited space for the air handling system. A desiccant evaporative cycle is then not applicable. Supply air cooling and dehumidification and, if required, additional cooling via e.g. chilled ceilings is realised by means of a thermally driven chiller. Source: Fraunhofer ISE. 4.4 All water system In case the installation of a centralized air handing unit is not ferasible or desired, the only technical solution to use solar thermal erngy for building air conditioning is the installation of a thermally driven chiller to supply cold water to a chilled water network. An example might be an office building or large residential building, which disposes not of the space necessary for the installation of a duct system. Independently from the climate conditions, a low temperature of the chilled water (approx. between 6 C and 9 C) is required in order to allow for air dehumidification in a fan coil system. This technical solution is shown in figure The driving source for the chiller is a solar collector system. At least high quality flat plate collectors are required to provide heat to the chiller, which either can be an adsorption or an absorption chiller. Whenever possible, solar gains should be used in winter for space heating, either in the identical fan coil units or in separate indoor units. The use of solar heat in winter is highly recommended for all examples shown before, wherever heating demand occurs, even if the presented figures focus more on the summer operation of the air conditioning systems. 69

70 Requirements on the design and configuration of small and medium sized solar air conditioning applications BUILDING START Cooling load calculation (building parameters, e.g., materials, geometry, orientation; internal loads, meterological conditions) cooling load, required hygienic air air change DISTRIBUTION MEDIUM TECHNOLOGY collector storage backup heat rejection chiller Installation of of centralized air handling unit feasible and desired? no All All water system (chilled water) cooling temperate Climate extreme heating Thermally driven chiller, chilled water network 6 C - 9 C Figure 4.10 Decision path for an all water system and a possible technical solution. No air handling unit is desired or feasible in this example. A thermally driven chiller to operate a chilled water network is then the solution, to use solar thermal heat as driving source. Source: Fraunhofer ISE. References [IEA SHC Task 25] Solar Assisted Air Conditioning of Buildings.Task25 in the Solar Heating and Cooling Programme of the International Energy Agency (IEA). Completed in shc task25.org/ [Henning, 2004] Hans Martin Henning: Decision scheme for the selection of the apporporate technology using solar thermal airconditioning. Guideline document in IEA Task 25, October [Hindenburgv et al., 2005] Carsten Hindenburg, Lena Schnabel, Thorsten Geucke: Solar desiccant cooling system with solar air collectors four years of operation with 100% solar fraction in summer. Proceedings of the International Conference Solar Air Conditioning. October 2005, Bad Staffelstein, Germany. [Henning et al., 2005] Hans Martin Henning, Tullio Pagano, Stefano Mola, Edo Wiemken: Micro tri generation for indoor air conditioning in the Mediterranean climate. Applied Thermal Engineering 27 (2007),

71 Requirements on the design and configuration of small and medium sized solar air conditioning applications 5 Small systems: schemes for typical applications Most of the solar air conditioning systems which were realised until the year 2005 are systems in the medium and large scale range. This development was forced due to the available product size in thermally driven chillers, which was a minimum of 35 kw chilling capacity at that time. All of the plants were individually designed; no standard system schemes for both, hydraulics and control schemes, were available. Consequently, the number of different hydraulic layouts is nearly as high as the number of pilot and demonstration system installations. It is evident that any targeted progress in real market penetration of solar cooling and airconditioning demands for a higher degree of standardisation in the system layout, especially in the hydraulic schemes. Furthermore, for systems in the small capacity range, e.g., for residential application, the goal is to minimize the planning effort nearly to zero. This finally demands to preassemble the system kernal, i.e., to define fixed hydraulic schemes and the respective components (chiller, piping, pumps, valves, expansion equipment, heat rejection, control, etc.) and to give recommendations on the appropriate type and size of the collector and e.g. on the use of the backup system, if those components are not included into the package system. The discussion on the appropriate system schemes is currently still ongoing in different levels of which the IEA SHC Task 38 is one. However, first companies are in between on the market offering complete solar cooling systems for sale. In this section, some basic ideas on system schemes will be presented and discussed briefly. However, complete drawings with details on the hydraulic components such as valve, piping and pump descriptions can not be outlined here. heat rejection collector storage boiler chiller cooling heating Figure 5.1 Scheme of a possible technical solution of a small solar thermally driven chilled water system. Source: Fraunhofer ISE. A basic sketch of a small solar cooling system is shown in figure 5.1, containing already the most important components. The characteristics of this configuration are No bypass possibility of the hot side storage. Advantage of this concept: solar heat is always transferred first into the storage; thus, the storage keeps always a buffer function. The flow rates between collector and driving circuit of the chiller are discoupled, the scheme is simple and requires a simple charging/discharging control of the storage only. A disadvantage is that in case of low storage temperature the thermal inertia of the system is high, i.e., before fluid at high temperature can be applied to the chiller, the storage has to be charged first. This results in a delay of the chiller operation start; The boiler is installed separately, thus injects no heat into the solar hot water storage. Advantage: higher utilisation of the solar collector and the boiler can be operated for space heating or domestic hot water preparation (not shown in the figure) without unfavourable 71

72 Requirements on the design and configuration of small and medium sized solar air conditioning applications thermal support of the chiller (see section 3). Additionally, the boiler may be operated in exceptional cases at high room temperatures far from the comfort range for example, when not sufficient solar power is available. Disadvantage: larger boiler capacity required than in a buffer integrated boiler; higher instability of hot water temperatures. Furthermore, the success of common solar combi systems (solar assisted heating and hot water preparation) in some European countries is connected with a compact and pre assembled system layout, in which the solar hot water storage with an integrated top level boiler plays a central role; Mixing valve in the hot water driving circuit of the chiller: the necessity of this device depends on the control strategy of the chiller. In principle, it aims as a capacity control of the chiller by limiting the driving temperature. In some chillers, this possibility is already integrated. Alternatively, a control of the mass flow in the driving circuit may be considered; Mixing valve in the heat rejection circuit: this safety equipment avoids too low cooling water temperatures, entering the chiller (special requirements for absorption chillers). The necessity of this device depends on the control requirements of the chiller. However, a continuous or stepwise control of the cooling tower fan should be used in any case before applying the mixing valve, since the fan control is an effective measure to decrease parasitic electricity consumption; Direct supply of chilled water to the consumers: no chilled water storage is foreseen in figure 5.1. The chiller is operated as far as the thermal solar power allows and as far as heat from the building has to be removed. Depending on the control and operation of the decentral cooling installations, part load operation of the chiller may occur often, but also a stop of the chiller operation at small cooling demand periods may be expected. A chilled water storage can improve this situation: the chilled water production is to a certain degree discoupled from the demand and the chiller can more often operate at full load. Solar heat can be still utilised and chilled water can be stored even if there is no cooling demand in the building, since due to the efficiency of 1 effect sorption processes, it is in general more useful to store chilled water than hot water. Disadvantage: higher investment cost for the chilled water storage and additional hydraulics (pump and control). In the project Solar Combi+ [SolarCombi+, 2008], supported by the European Commission within the Intelligent Energy Europe program of the EACI, a virtual case study on small scale solar cooling systems was carried out; through extensive simulation calculations a base of annual performances of selected system configurations in different application fields was provided. In the beginning of the study, typical system schemes had to be defined. It turned out soon that yet no common understanding of the best system solution exists among the partners from different European countries. The knowledge on appropriate solar cooling installations is distributed to numerous working groups, the number of different technical approaches is high and the total number of realised plants is not sufficient to condense the experience into one optimal system scheme. Furthermore, national regulations may also affect the general layout; to give an example, a regulation in Spain concerning the design of solar thermal systems does not allow the direct integration of a backup heater into the solar hot water storage. 72

73 Requirements on the design and configuration of small and medium sized solar air conditioning applications Loads Heating Boiler DHW Collector Chiller Cooling heat rejection Source: Fraunhofer ISE Loads Heating DHW Collector Boiler Chiller Cooling heat rejection Source: Fraunhofer ISE Figure 5.2a (top) and 5.2b (bottom) Two system schemes for small solar cooling systems proposed within a case study in the project SolarCombi+. The schemes are discussed more in detail in the text. Source: Fraunhofer ISE. However, it was agreed to focus on mainly two different system schemes as master configurations, which are presented in figure 5.2a and 5.2b. In both cases it is assumed that heating demand and cooling demand has not to be covered at the same time, which is a realistic assumption for small and medium buildings. In the scheme 5.2a, the auxiliary boiler is installed seperately from the collector system. The following operation modes are possible: 1. Space heating and domestic hot water preparation with the boiler; no sufficient solar thermal energy is available. The cold return from the DHW heat exchanger is passed through the solar buffer storage in order to be heated or pre heated by solar energy. The chiller is not in operation; 2. Space heating support by the solar thermal system: either thermal energy from the hot water storage is used and, if necessary, subequently heated with the boiler to the desired set point temperature, or heat direct from the collector system is used. This depends on the control of the collector circuit pumps. Likewise in I., the chiller is not in operation; 73

74 Requirements on the design and configuration of small and medium sized solar air conditioning applications 3. In the summer operation mode, the chiller is in operation. To allow a quick start of the chiller in the morning, the storage is bypassed through simultaneous operation of the chiller driving circuit pump and of the primary and secondary collector circuit pump. This requires for an adequate layout of the pumps in all circuits. In case the collector temperature is not sufficient high, the fluid is subsequently heated in the boiler. However, attention should be payed to the recommendations on the use of auxiliary heat sources in such a case as dicussed in section 3; 4. When the collector system is not operated during chiller operation (stop of collector circuit pumps), solar heat is removed from the storage and, if necessary, subsequently heated in the boiler (again: recommendations on the used of auxiliary heat sources). The bypass possibility of the hot water storage leads to a comparatively high complexity in the hydraulic scheme and of the control strategy. An appropriate control strategy has to be developed to allow nevertheless an efficient use of the storage, since a thermal storage is normally connected with high investment cost. However, this solution is favourised by some system providers and working groups. Hot water is always passing the heat exchanger of the boiler, whether this component is active or not and thus resulting in a higher pressure drop in the heat supply circuit. In the configuration of scheme 5.2b, the hot water storage is as the central heat distribution system always in use; the heat back up system is integrated into or attached to the storage. This solution is today applied in numerous solar combi systems (solar thermal heating and DHW preparation support), as these systems are pre assambled to a certain degree and simplifies the installation of the total solar thermal system. The development of this type of solar combi systems has surely accelerated the success of solar heating support installations. Beside advantages in the installation, the hydraulic system is less complex than the scheme 5.2a and the collector control is indepent from the operation of the remaining system parts. Fossil fueled boiler operation 70 C - 90 C 50 C - 65 C driving heat domestic hot water Figure 5.3 Basic idea on a heat back up control, when the boiler is integrated into the hot water storage, but a back up support of driving heat for the chiller is not intended. The operation level of the boiler is below the operation range for the driving circuit of the chiller. During winter, when the chiller is not active, the temperature operation range for the boiler may be switched to higher levels, if necessary. Source: Fraunhofer ISE. However, the integration of the heat back up into the storage in the context with solar cooling requires a high attention on the control of the boiler with respect to the primary energy balance, commented in section 3. In any case, the boiler should not heat up during the cooling season the storage temperature for DHW use, when subsequently this hot water is used directly as driving heat source for the chiller. This requires that the operation range between DHW temperature and chiller operation has to be clearly separated for the boiler control. Figure 5.3 gives an idea on such a control strategy: the boiler is operated for domestic hot water preparation in a narrow temperature range only, e.g., up to 65 C. The chiller however starts at storage temperatures of > 70 C, the additional heat for this level has to be prepared by the solar thermal system. During chiller operation, the driving fluid temperature returning to the storage is still high (e.g., 60 C) and thus above the starting level of the boiler for DHW preparation. In case adsorption chilling technology is used, a special issue of interest in the system configuration arises from the heat recovery period during the chiller operation. It is part of the adsorption process that in between the hydraulic switching from adsorption to desorption (see section 2), a short period of heat recovery between the two compartments of the chiller is included (typically with a duration of approx. 15 s). This results in short but distinct fluctuations in 74

75 Requirements on the design and configuration of small and medium sized solar air conditioning applications the hot water temperature, returning from the chiller driving circuit. Figure 5.4 shows as an example the temperature levels as monitored in an adsorption chiller system. The fluctuations especially in the return of the driving circuit may require for additional measures in the system control, when this temperature level is foreseen to be used in the control. An appropriate measure is the smoothing of the temperature value in a storage, either the main hot water storage or in a special return flow buffer hot water circuit T_hot_supply T_hot_return temperature [ C] cooling water circuit T_cold_supply T_cold_return chilled water circuit, primary: T_chilled_supply T_chilled_return minute of the day Figure 5.4 Typical for the operation of adsorption chillers are temperature fluctuations in the hydraulic circuits. Especially the hot water return temperature fluctuations have to be considered in the system control and configuration. Source: Fraunhofer ISE. The number of existing system configurations is currently nearly as high as the number of total installed systems, this is an expression of the lack of standards and pre defined systems in solar cooling applications today. However, providers of solar cooling systems are well aware on the benefits of standardised configurations and on their effects on cost reduction and system operation reliability especially in the small scale capacity range. The tendency for more standardised approaches and system schemes can be thus viewed on the web pages of e.g. SolarNext SOLution ution.com or ClimateWell System configurations of three different solar cooling applications are also described in detail for plants with the High Combi project [High Combi, 2008]. In this project, three demonstration systems will be realised with different innovative topics. Figure 5.5 shows the configuration of the smallest system, equipped with the Rotartica Solar 7 chiller (4.5 kw chilling capacity). In the Best Practice Catalogue of SOLAIR, further examples of configuration schemes can be viewed for some of the installations in the SOLAIR data base [SOLAIR, 2008]. 75

76 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 5.5 System scheme of the the Rotartica Solar 7 solar cooling system, installed within the project High Combi. Source: Project deliverable D6 State of the art of similar applications in High Combi [High Combi, 2008] References [SolarCombi+, 2008] Identification of most promising markets and promotion of standardised system configurations for the market entry of small scale combined solar heating & cooling applications (SOLAR COMBI+). Supported in the Intelligent Energy Europe Programme of the European Commission. EIE/07/158/S Duration: until 02/ [High Combi, 2008] High solar fraction heating and cooling systems with combination of innovative components and methods (High Combi). Task 2, Deliverable 6: State of the art of similar applications, July Supported by the European Commission. TREN/07/FP6EN/S / Duration: until [SOLAIR, 2008] Increasing the market implementation of solar air conditioning systems for small and medium applications in residential and commercial buildings. Task 2.2 / Best Practice Catalogue, June Supported in the Intelligent Energy Europe Programme of the European Commission. EIE/06/034/S Duration: until 12/ project.eu 76

77 Requirements on the design and configuration of small and medium sized solar air conditioning applications 6 Recommendations on monitoring and quality assurance With some hundred installations, solar cooling is still in an initial phase of market penetration. In this phase it is important to obtain information on the achieved benefits of the installation and on the overall efficiency of the system. Thus, monitoring plays an important role in both, attesting the contribution of the systems to primary energy savings, thereby demonstrating the potential of solar cooling, and supporting a reliable and optimised operation of the system. The latter issue is directly related to an increased economic value of the installation. Since solar cooling installations are more complex than e.g. solar thermal systems for heating support, more effort has to be taken into account in the monitoring. In the planning of the monitoring system, the selection and number of monitoring positions depends on the level of interest in the system performance assessment. Roughly, the levels of system operation assessment may be classified into basic functional control: status of system operation for simple trouble shooting. This level of system surveyllance provides at least status information on e.g. a reliable operation of the solar system (pump control) and other key komponents in order to detect significant system failures and to avoid unplanned operation of a back up system. This is a minimum of required information to be provided. No information on the system performance is given. Especially the capability of the chiller cannot be surveyed (except the internal chiller control reveals such information); basic global performance assessemt: integrated values of e.g. delivered solar heat, produced cold, auxiliary heat, total electricity consumption. With these informations, a simple overall energy balance can be calculated and the benefits in primary energy saving and CO 2 avoidance may be estimated, when compared to data of a reference system. Cost figures can be derived; detailed system analysis: monitoring data with high time resolution are accessible for all relevant hydraulic circuits, the radiation is monitored, electricity meters are installed, position signals of valves are recorded. With this effort, efficiency of the components may be assessed, heat flux through the systems can be traced. It allows of course the information gain of the previous level, but at any time scale desired. Furthermore, this monitoring level allows to identify weak points in the system control and thus the data should be used for system optimisation. The latter detailed monitoring level is currently applied in several pilot and demonstration plants of solar cooling. The costs are high for both, monitoring equipment and installation as well as for the time consuming data evaluation. As in general the effort for a detailed system analysis is independent from the size of the cooling system (nearly identical number of hydraulic circuits and componts in all systems), it is evident that this monitoring level will not become a standard for small size systems on the market. However, this monitoring level is important in the ongoing development of solar cooling systems. Figure 6.1 gives an example on minimum requirements on monitoring in order to allow a comparison with a reference system in terms of energy use and energy input. For a more detailed analysis of the solar cooling system, the energy flux in between the components has to be recorded as well (figure 6.1c). Figure 6.2 gives another example on the required monitoring signals, necessary for a detailed system analysis. No monitoring is applied at the ventilation systems, since the solar thermally driven chilled water production is in the focus of this example. Each heat flux monitoring point Q consists of two temperature signals and a mass flow signal, since for a detailed monitoring not only the heat flux measured by a heat flux meter is of interest, but also the absolute temperature 77

78 Requirements on the design and configuration of small and medium sized solar air conditioning applications and mass flow values. Not indicated in the figure are additionsally sensors which may ease the error detection in case of system trouble significantly, such as position signals of three way valves, pump operation signals, etc. Primary energy conversion E fossil fuel boiler system boundary: reference system heat supply air-conditioning heating, hot water E other chiller, el. compr. heat rejection Q cooling electricity waste heat Figure 6.1a To obtain a very rough picture on the energy input of a conventional heating and cooling system (reference system), the useful energy output (indicated by the red and blue bars) has to be assessed as well as the electricity consumption of the whole supply system (light blue triangle). The fossil fuel demand may be estimated from average boiler efficiencies. Primary energy other media conversion E V E fossil fuel water solar radiation solarcollector boiler other Q system boundary: solar assisted system heat storage chiller, thermal heat supply air-conditioning heat rejection heating, hot water Q cooling electricity waste heat Figure 6.1b For a performance comparison of a solar cooling system with a reference system shown in figure 6.1a, at least the indicated energy values have to be recorded. The results (e.g., annual energy data) may be compared with the estimated data of the reference case in order to calculate the benefits in fossil fuel saving. However, no detailed information on the components is available, such as collector efficiency and chiller performance. 78

79 Requirements on the design and configuration of small and medium sized solar air conditioning applications solar radiation Primary energy conversion E fossil fuel G,T solarcollector boiler Q Q system boundary: solar assisted system heat storage heat supply air-conditioning heating, hot water other media V water other Q chiller, thermal E E E Q heat rejection Q cooling electricity E waste heat Figure 6.1c A detailed analysis of the system requires for more monitoring effort than shown in figure 6.1.b. Additionally, not only the heat flux but also the temperature levels and differences, mass flow rates and status signals from the system control are of interest (not shown in the figure). At this level, unsufficient system behaviour may be detected and system control strategies can be optimised as well. Source: Fraunhofer ISE Figure 6.2 Example on a detailed monitoring in a solar cooling application with two small absorption chillers, providing chilled water for supply air cooling and chilled ceilings. The coloured bars indicate precisely calibrated twin temperature sensors and a volume flow meter for heat flux measurements; the triangles denote electricity meters. In practice, groups of pumps are monitored by one electricity meter. Further signals in the monitoring not shown in the figure are position signals of valves. All data are recorded as mean values of 60 second time intervals. Source: Technikerschule Butzbach/Fraunhofer ISE 79

80 Requirements on the design and configuration of small and medium sized solar air conditioning applications In the IEA Task38 Solar air condition and refrigeration [IEA SHC Task 38] of the Solar Heating and Cooling Programme (SHC), more detailed guidelines on monitoring and on defined monitoring levels are developed and will be available at the end of the Task. Within this activity, three monitoring levels are proposed, characterised by a progressive increase in the number of sensors and data amount: Level One: Basic Information on Primary Energy Ratio and Costs. At this level, minimum requirements are defined in order to allow a comparison between different solar cooling systems with respect to their primary energy efficiency and their economic performance. A limited number of heat flux meters and one overall electricity consumption meter is required. Conversion factors for the primary energy evaluation and the evaluation parameter Primary Energy Ratio are defined as well. The approach is applicable for chilled water systems and for open cycle systems; Level Two: Simple analysis of the solar energy resource management. At this level, extended monitoring is especially applied to the solar system (pyranometer for radiation measurement and additional heat flux meters). The exploitation of the collector system and the thermal losses in the storage can be analysed more in detail; Level Three: Advanced monitoring procedure. The monitoring system is expanded in order to apply the FSC method (Fractional Solar Consumption), developed in IEA Task 26 for solar combi systems. The method was extended in IEA Task 32 for solar heating systems with large storages [Letz, 2002], [Weiss (Ed), 2003]. Additional monitoring points may be necessary to identify the energy flux in the system in detail. A conventional heating and cooling system is defined without solar heat supply for reference calculations. The method is applicable for closed cycle systems as well as for open cycle systems. Other important measures for the quality assurance are not directly related to the plant monitoring, but related to regulary maintenance and system checks. The chiller manufactures provide suggestions on maintenance, adapted to the type of chiller, such as vacuum checks, analysis of the solution etc. The maintenance, to be done in different time scales ranging from semi annual until e.g. 6 years checks, comprises several checks such as mechanical tests, vacuum checks, analysis of the solution, etc. An example on the checks for a specific type of chiller is presented in the SOLAIR training material data base in part D3: operation and maintenance [SOLAIR Training, 2008]. Further maintenance has to be applied to the heat rejection system and to the collector system. An example for a preventive maintenance plan of a collector system is shown in figure 6.3. At least, quality assurance measures have already to be considered in the early step of system planning and when the calls for tender are prepared. To give some examples: the minimum of useful solar collector field productivity (kwh/m² per year should be determined for the heating mode and for the cooling mode (e.g., derived from an annual COP number of the cold production process) from the planned collector array. The real achieved useful yield should be monitored, thus an adequate monitoring equipment has to be considered in the planning phase; It is recommended that the call for tender includes the obligation to guarantee a minimum annual yield of the collector system for the foreseen application and temperature range. This measure pushes the provider of the collector system to present realistic yield data of the collector system, this gives a more reliable base for economic assessments. In case the yield was not achieved under normal working conditions, a certain penalty has to be paid by the collector provider. Details on the definition of normal working conditions and on the reference annual radiation have to be specified in the contract. At least a collector yield monitoring is necessary in this case; Additionally, the collector or system provider should specify the risks of collector stagnation and guarantee a stagnation safe behaviour of the system in such an event through adequate 80

81 Requirements on the design and configuration of small and medium sized solar air conditioning applications design of the hydraulic components. A stagnation situation may always occur, for example in case of electricity or pump breakdown. An option is also to induce a stagnation situation after installation of the system and to prove the stagnation safety with this method; The contract with the supplier of the chiller shall include the verification of the chilling capacity at given operation conditions after the system installation and start of the operation. For this reason, a monitoring of at least the heat flux and temperature levels in the hydraulic circuits of the chiller is necessary; The overall electricity consumption of the system should be outlined in the tender and, as a more severe criteria, the electric efficiency of the total system could be specified. In the first operation year(s), the real electricity consumption is then to be inspected by an appropriate monitoring equimpent. Similar criteria may be exposed e.g. for the water consumption in open cycle systems or in case wet cooling towers are intended to be installed. 81

82 Requirements on the design and configuration of small and medium sized solar air conditioning applications COLLECTORS FIELD Item Frequency (months) Description Collectors 6 VI differences with the original one VI differences between collectors Glasses 6 VI condensations and dirtiness Joints 6 VI cracking, deformations Absorber 6 VI corrosion, deformations Frame of the collector 6 VI deformation, movement, ventilation spaces Connexions 6 VI escapes Structure 6 VI degradation, corrosion, checking of the screws HEAT EXCHANGER Equip Frequency (months) Description Heat exchanger 12 WC efficiency 12 Clean Submersible heat 12 WC efficiency exchanger (loop) 12 Clean TANKS Equip Frequency (months) Description Tanks 12 Dust in the inferior part Sacrifice anode 12 Check wear Electrical anode 12 Check good working Insulation 12 Check there is no humidity HYDRAULIC LOOPS Equip Frequency (months) Description Refrigerant 12 Check density and PH Tightness 24 Do a pressure proof Outdoor insulation 6 VI degradation protection, unions and no humidity Indoor insulation 12 VI unions and no humidity Automatic purge 12 WC and clean Manual purge 6 Take the air out Pumps 12 WC and tightness Closed expansion vessel 6 Check the pressure Automatic filling system 6 WC actuation Cut valve 12 WC actuations (open and close) Security valve 12 WC actuation Distribution loop 6 Check the pressure ELECTRICAL AND CONTROL PARTS Equip Frequency (months) Description Electrical part 12 Check the electrical board is closed to avoid the entrance of dust Differential control 12 WC actuation Sensors 6 WC actuation Thermostat 12 WC actuation Energy-meter 6 Write the produced energy Figure 6.3 Example on a preventive maintenance plan of an entire solar collector systems. VI = visual inspection; WC = working control. Source: Aiguasol. 82

83 Requirements on the design and configuration of small and medium sized solar air conditioning applications References [Letz, 2002] T. Letz: Validation and background information of the FSC procedure. Technical report of subtask A, IEA SHC Task shc.org/outputs/task26/a_letz_fsc_method.pdf [Weiss (Ed), 2003] W. Weiss (Ed): Solar Heating Systems for Houses A Design Handbook for Solar Combisystems. IEA SHC Task 26, James&James Ltd London, pp , [IEA SHC Task 38] Task 38 Solar air conditioning and refrigeration, executed in the Solar Heating and Cooling Programme of the International Energy Agency IEA. shc.org/task38 [SOLAIR Training, 2008] Increasing the market implementation of solar air conditioning systems for small and medium applications in residential and commercial buildings (SOLAIR). Supported in the Intelligent Energy Europe Programme of the European Commission. EIE/06/034/S Duration: until 12/2009. Training modules available at the SOLAIR web page; Tools and Products: Training modules and materials; D_3_Operation_Maintenance.pdf. Prepared project.eu 83

84 Requirements on the design and configuration of small and medium sized solar air conditioning applications 7 Planning tools 7.1 Design approaches Design with regard to solar assisted air conditioning mainly means Selection of the proper thermally driven cooling equipment for the selected air conditioning system Selection of the proper type of solar collectors for the selected air conditioning system and thermally driven cooling equipment Sizing of the solar collector field and other components of the solar system with regard to energy and cost performance The two first items were discussed in section 2. For sizing of the system different design approaches can be followed as exemplified in Figure 7.1. In the following sections these different design approaches are described and its advantages and drawbacks referred, as well as, its application limits. Accuracy, reliability of results, details of design information Rules of thumb Collector cost per heating capacity Cost of solar heat for given climate Load - gain - analysis for given climate and load Anual cost based on loadgain-analysis Computer design tool with predefined systems Open simulation platform Required system information, effort for parametrization Figure 7.1 Different design approaches. Accuracy is closely related with the complexity. Reference [Henning, 2004/2008]. 84

85 Requirements on the design and configuration of small and medium sized solar air conditioning applications 7.2 Rules of Thumb In section 3 of this document, an initial rule of thumb is given. The expression: 1 Aspec = Gη COP coll,design design with A spec = specific collector are per installed kw thermally driven chilling capacity [m²/kw cold ] G = irradiance at collector surface [kw/m²] η coll,design = collector efficiency at design condition (driving temperature) [ ] COP design = thermal COP of chiller at design conditions [ ] gives a rough idea of the collector area to be installed in a solar air conditioning system. This simple rule: + Allows a very quick assessment (guess) about the required collector area, if the efficiency of the collector and the COP of the thermally driven cooling equipment is known Neglects completely the influence of the variation of radiation on the collector during day and year Any information on the specific site and load is neglected Neglects completely part load conditions 4 of cooling load in thermally driven cooling equipment In reference [Henning, 2004/2008] other simple rules of thumb are introduced. For determination of Collector First Costs a simple calculation can be performed considering the collector efficiency curve obtained according to [EN :2006]: η =η a 0 1 t m t G a a 2 ( t t ) m G a 2 with η 0 a 1, a 2 t m t a = optical efficiency = collector heat loss coefficients = collector temperature (average between input and output temperature) = ambient temperature The power delivered by the solar collector operating at a temperature t m is: q q = Aη G A = ηg The collector area for kw power produced, A spec, is: A spec 1kw = η G 4 Part load conditions correspond to the working conditions of a cooling machine that are not the optimal ones do not correspond to those of highest COP. 85

86 Requirements on the design and configuration of small and medium sized solar air conditioning applications If a collector has 50% efficiency at an average temperature of 80ºC and considering that the irradiance incident on the collector is 800 W/m 2, the collector specific area, i.e, the area necessary to produce 1kW power is 2,5 m 2. Considering the collector specific cost, based on information of current solar thermal systems installed, i.e., the collector cost per collector area, the cost of collectors per power unit produced can be determined by: Cost = A heat,power spec Cost spec If the collector specific cost is 500 /m 2 and collector specific area is 2,5 m 2 /kw the collector costs per power unit is 1250 /kw. Also in this case the calculation made: + Allows a rough comparison of different solar collectors, if the collector parameters and the operation temperature of the thermally driven cooling equipment are known Neglects completely the influence of the variation of radiation on the collector during day and year Any information on the specific site and load is neglected Neglects completely part load of cooling load and thermally driven cooling equipment All calculations made until this moment considered only power produced by the collector. Solar Thermal collectors do not produce a constant power, but a variable power which depends of the variation of the irradiance incident on the collectors due to variation of weather conditions. To determine the energy produced by a solar thermal collector, information on climatic data is needed. With hourly values of irradiance incident on the collector and the knowledge of collector efficiency curve and collector incidence angle modifier it is possible to calculate to maximum energy produced by the collector at a fixed working temperature. References [Horta et al, 2008, 2008a] give the necessary methodology for this calculation. Form this methodology an annual gross energy produced by the solar thermal collectors is obtained, Q gross, which can be expressed in kwh. The annual cost of the heat produced by the solar thermal system Solar heat cost can than be calculated considering: Cost = Cost annual f spec annuity where f annuity is the annuity factor that takes into account the interest rate of the investment and the lifetime of the collector system. Cost Cost heat = Q annual gross where Q gross is the annual collector heat production at a given site and a given operation temperature. In this case the calculation made: + Allows a good comparison of different solar collectors using their parameters and the radiation data of a specific site + The maximum possible heat production of a specific solar collector for a given site (annual meteorological data file) and a given constant operation temperature is determined Any information about the load profile is neglected Method neglects completely part load of cooling load and thermally driven cooling equipment 86

87 Requirements on the design and configuration of small and medium sized solar air conditioning applications For calculation of Q gross some software tools usually for the design of solar thermal systems for hot water preparation can be used. Some of these software tools use monthly averages of climatic data, others use hourly data. 7.3 Simple pre design tools More or less simple pre design tools are available for free download, which have been developed in the frame work of different European Projects. Table 7.1 lists the ones that were identified up to now. Software SACE: Solar cooling evaluation light tool SHC SoftwareTool (NEGST project) Reference / Source Reference [Hans Martin Henning, 2003] project.eu/218.0.html Reference [Sabatelli, V. et al., 2007] technologie.de/html/publicdeliverables3.html EasySolarCooling See reference [Wiemken, E. et al (2004)] Not available SolAC Solar Assisted Air Conditioning Software Table 7.1 List of pre design software tools Reference [Franke, U. et al (2005)] shc task25.org/english/hps6/index.html A simple description of each of these tools is given here focusing mainly on its capabilities and limits of application SHC Softwaretool (NEGST Project) An example of a simple software tools using monthly data was developed in the framework of the NEGST project [NEGST ( )]. Is available for free download at Figure 7.2 is a picture of the main window of this tool. This software tools allows for the determination of solar collector area required to achieve a given overall primary energy saving with respect to the most common conventional cooling system. The program considers both cooling and heating loads on monthly bases. The user needs to input the energy load per square meter of room area to be conditioned and the area of the room. The solar thermal system has to satisfy these loads considering that the heating load is satisfied directly and that the cooling load is satisfied by a cooling machine with a specific COP. As an example, the Solar Load is determined and listed in Table 7.2, where a COP of 0.7 for the cooling machine was considered. 87

88 Requirements on the design and configuration of small and medium sized solar air conditioning applications Month Heating Load [kwh] Cooling Load [kwh] Solar Load [kwh] 1 1, , ,273 1, ,477 2, Table 7.2 Calculation of monthly values of solar load, considering known values of cooling and Heating load of a building. The monthly energy delivered by the solar thermal system is determined based on the calculation method phi fchart [Duffie, J. and W. Beckman, 2006]. Q gross is calculated for different collector areas. The primary energy savings is calculated in each case and the result is represented on the graph on the left (see Figure 8.2). The results corresponding to the chosen fraction of primary energy saved are graphically represented in the right graph and can also be visualised in the form of table (View results). Figure 7.2 Main window of the SHC pre design tool 88

89 Requirements on the design and configuration of small and medium sized solar air conditioning applications There is no detailed information on the calculation procedure adopted for primary energy. It is not guaranteed that equations given in Figure 3.2 of section 3 are fully adopted. Also it is not possible to know which values of conversion of primary energy for electricity and fossil fuels are adopted. This pre design software tool: Takes into account average climatic conditions as well as average load (heating and cooling) Allows for the determination of a collector area as a function of primary energy savings Method neglects completely part load of cooling load and thermally driven cooling equipment SACE Solar cooling evaluation light tool This software was developed in the framework of the European Project SACE: Solar air conditioning in Europe. Project summary and deliverables can be found at the SOLAIR website project.eu/218.0.html. The software can be downloaded for free. The objective of this software is to allow a quick pre feasibility study of solar assisted air conditioning systems. The annual solar fraction for heating and cooling is calculated based on an hour by hour comparison of needed heat for a thermal driven cooling machine and available solar heat. It performs parametric studies which are a function of specific collector area, i.e. ratio between collector area and room area, as well as, storage capacity. The storage capacity is defined in terms of time allowed to satisfy the annual peak load. The information obtained is solar fraction and solar thermal collector efficiency. The software has available a set of load files for seven locations and three different building types (Hotel, m 2 ; Office, 930 m 2 ; Lecture room, 216 m 2 ). Load files for other locations and buildings can be generated with external commercial software tools, but in such a case the actual room area should be correctly introduced in the SACE tool software window. Figure 7.3 SACE tool main window For calculations it needs a load file (heating and cooling load) and a weather file with hourly values. The solar system is mainly characterized by collector efficiency parameters. The building is represented by its area and the HVAC equipemnet by two operating temperatures (heating and cooling), efficiency of heating system and COP of thermal chiller. 89

90 Requirements on the design and configuration of small and medium sized solar air conditioning applications This pre design software tool: + Takes into account the hourly data for climatic conditions as well as hourly load files (heating and cooling) + Allows for the determination of solar fraction as a function of parameters like specific collector area and storage buffer volume Method neglects completely part load of cooling load and thermally driven cooling equipment SolAC Solar Assisted Air Conditioning Software This software was developed by ILK Dresden in the frame work of Implementing Agreement Solar Heating & Cooling Task 25 Solar assisted air conditioning systems. The software is available for free download at: shc task25.org/english/hps6/index.html Documentation of the software is also available after downloading (reference [Franke, U. and Seifert, C. (2005)]) The input data for the programme is: weather data including solar radiation (hourly data) load files including heating and cooling loads (hourly data) An example of the system different components can be seen in Figure 7.4. Four different units are considered in this software: Solar system Cooling device Air handling unit Cooling and heating components in the room These units can have different configurations chosen by the user (see Figure 7.5). The results of the simulation are available as the hourly power requirement of a unit system (the system is considered to be formed by the above referred four components). The models adopted for each component are described in detail in reference [Franke, U. and Seifert, C. (2005)], but the source code is not available. It is not possible to add other components. 90

91 Requirements on the design and configuration of small and medium sized solar air conditioning applications Figure 7.4 Graphical representation used by the software to represent the system components considered (from left to right) Solar system; Cooling device; Air handling unit; cooling and heating components in the room, as well as input data (source [Franke, U. and Seifert, C. (2005)]) Figure 7.5a Solar system options Figure 7.5b Cooling device options Figure 7.5c Air handling unit options Figure 7.5d Options for cooling and heating components in the room 91

92 Requirements on the design and configuration of small and medium sized solar air conditioning applications This pre design software tool: + Takes into account the hourly data for climatic conditions as well as hourly load files (heating and cooling) + Performs hourly calculations of energy demand of the main components and determines yearly values + Considers part load behaviour of thermally driven cooling equipment + Also includes and economical analysis Only considers pre defined systems No full primary energy analysis possible, since the electricity demand of some components is not considered ODIRSOL Solar Assisted cooling Software The ODIRSOL software has been developed in partnership between CSTB and TECSOL. It aims at being a decision tool for designers and planners. The tool is based on dynamic simulations with TRNSYS, in order to provide a technical and economical assessment of a detailed solar cooling project using single effect absorption chillers. Simulations are covering simple configurations with hot and/or cold back up and/or hot and/or cold storage. An online help is available for most of the steps, as well as examples of projects of several sizes. The hourly results of a yearly load simulation of the building to be cooled have to be provided by the user. All the data used in the program are presented in a data base included in the software, in the directory ODIRSOL\ Interface\ Data. The data are French data on period. Content and method The first step consists in selecting the hydraulic configuration; 4 choices are available, with or without storage. Then the user has to feed up hourly meteo and cooling load data under the required format from a TRNSYS PREBID study, or coming from selected thermal hourly simulation software. The user also provides other geographical simple data. From those data, the software will automatically pre size all components of the installation. The pre sizing method is based on simple ratios, rules of thumb and on a database of commercially available products. In the main screen, the user has the possibility to modify each component, to fill his own feature or to choose other components in the database. 92