SOLAR ENERGY USED FOR HEATING AND COOLING SYSTEMS

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1 Nonconventional Technologies Review Romania, April, Romanian Association of Nonconventional Technologies SOLAR ENERGY USED FOR HEATING AND COOLING SYSTEMS Codruta Bendea 1, Marcel Rosca 2,Gabriel Bendea 3, and Costas Karytsas 4 1 University of Oradea, cbendea@uoradea.ro 2 University of Oradea, mrosca@uoradea.ro 3 University of Oradea, gbendea@uoradea.ro 4 Centre for Renewable Energy Sources and Savings, Greece, kkari@cres.gr ABSTRACT: When hearing about solar energy, most people think about heating their buildings, but only a few are aware of its cooling capacity. Depending of the available amount of radiation, a solar system can stand alone, or have an additional energy source to cover the peak loads. But for times when neither heating, nor cooling are needed, the solar energy may be stored for different periods (from several hours to half a year). After showing the initial status and most recent developments of solar heating and cooling systems, together with possibilities of energy storage for large systems, this paper presents both theoretical and practical approach of a prototype system developed in Greece, by CRES, within HIGH COMBI European project. KEY WORDS: solar energy, space heating and cooling, solar fraction 1. INTRODUCTION For a combined solar system (combi-system) to be able to produce both heat and cold, it needs high performance collectors, capable of delivering fluids with temperatures above 100 C. This high temperature is needed mainly in summer time, when cooling is required. From the operation principle collectors are grouped into two classes: flat plate collectors; concentrating collector systems: compound parabolic collectors (CPC); parabolic through collectors; Fresnel collectors. But most usual collectors belong to the first category - plate collectors - and, in order to increase temperatures, thermal losses must be reduces as much as it can. Thermally driven cooling technologies are relatively well known technologies. The absorption technology for example is to be considered as the oldest cold producing technology. It was developed in 1810 by John Leslie. The first commercially available H2O/LiBr-absorption chiller was on the market in 1945 (Carrier)[1]. Since then, the technology evolved and its status is presented in paragraph 3. The technology of large scale seasonal thermal energy storage (STES) has been investigated in Europe since the middle of the 70 s. First demonstration plants were realized in Sweden in 1978/79 based on results of a national research program. Within the IEA (International Energy Agency), Solar Heating and Cooling program, experiences where worked out and exchanged in Task VII Central Solar Heating Plants with Seasonal Storage (CSHPSS) since Besides Sweden, also Switzerland, Denmark and Germany investigated STES and have built demonstration plants. First research programs focused on basic research including model calculations, laboratory experiments and the construction of small scale pilot plants. The technical and economic feasibility of the storage concepts had to be proven. In Germany, eleven large scale CSHPSS demonstration plants have been built since They are designed for solar fractions of between 35% and 60% of the total annual heat demand for domestic hot water preparation and space heating of the connected residential areas. Several technologies for seasonal heat storage have been further developed and tested within these projects. 2. NEW GENERATION OF SOLAR COLLECTORS To achieve the required temperature range, needed by the cooling machine, with flat plate collectors all heat loss channels - heat conduction, heat convection and heat radiation - have to be minimized. Further measures are required to minimize the reflected component of the incident solar radiation (optical losses). Starting from a standard single glazed collector with high back cover and fringe insulation standards, an adequate step to further minimize heat losses is to introduce a second glass cover (double glazed collectors). Given a selective absorber (typically a=0,95; e=0,05) the main heat loss 10

2 channel between the absorber surface and the glass cover is by convection. Heat losses via convection can effectively be reduced by introducing a second glass cover with spacing of approximately 10 to 15 mm, which minimizes losses via this channel.[2] A next improvement step is to reduce heat conduction between the glass covers by reducing the pressure in the clearance. Heat conduction in a gas is indirectly proportional to the mass of the atoms constituting the gas. So gases made up of heavy elements like neon, argon, krypton (inert gases) exhibit reduced heat conductivity, which makes the filling of the clearance between the covers with any of these gases an appropriate measure to further decrease heat conduction. Radiation losses are primarily determined by the temperature difference between the covers respectively between the sky temperature and the top-cover. The previously introduced measures have a positive impact on reducing radiative losses, as the surface temperature of the glass covers is reduced. After treatment of the different heat loss channels, losses associated with incident solar radiation are considered. It is desirable to get the highest possible proportion of the incoming radiation on to the absorber plate. Possible optical loss mechanisms are: reflection on the glass covers (typically 4-5 %); absorption in the glass cover: The fundamental reason for reflection on the air/glass interface is different propagation velocities of an electromagnetic wave in air and in the material glass. Anti-reflective coatings on the glass cover can mitigate reflection losses. An antireflection coating can be made by choosing a film of a certain thickness and refractive index so that the reflections from its upper and lower surfaces are out of phase and will interfere destructively. Through destructive interference the intensity of reflected radiation is reduced and transmission intensity in the same amount increased (given constant absorption). Absorption of radiation in the glass cover comes from incorporated chemical elements (contaminations) into the glass. The energy of the absorbed radiation is used to trigger excitations in the electronic structure of the contaminating chemical compounds. A typical absorption causing contaminant for solar cover glasses is iron oxide in its form Fe 2 O 3. Concentrating systems focus solar radiation on to a small area called the receiver and thereby achieving high radiation intensities. All above discussed heat loss mechanisms are area proportional. In the case of concentrating systems the small receiver surface area accounts for reduced heat losses as compared to flat plate collectors. The absorbing surface of the receiver can be designed to be selective identical to selective absorbers used in flat plate collectors. Typically the receiver is bonded with a glass tube ideally evacuated to reduce heat conduction between the receiver surface and the glass-tube wall. By this measure the surface temperature of the glass tubing stays comparably low, yielding low radiation losses between the receiver shielding and ambient. The combination of high radiation intensities and diminished heat losses result in operating temperature levels well suited for the mid temperature range. Most market available flat plate collectors utilize basically all approaches discussed above to lower optical and heat losses. Double glazing, antireflective coating and filling of the space between the two covers with an inert gas push the performance limits of some flat plate collectors to an operation temperature of approximately 150 C. 3. THERMALLY DRIVEN COOLING TECHNOLOGIES Nowadays, the thermally driven cooling technologies commonly used can be divided into two groups, open cooling processes and closed cooling processes and further into processes with solid or liquid sorbent. Figure 1 gives an overview. Figure 1. Overview on thermally driven cooling technologies [Source: Fraunhofer ISE] 3.1 Closed cycle cooling machines In closed cycle cooling processes chilled water is produced. This can be used afterwards in conventional cold distribution systems to cool down spaces (by fan-coils, cooled roofs, etc.) or processes. As shown in the table above, two main processes exist: the absorption and the adsorption. 11

3 3.1.1 Absorbtion chillers Absorption chillers use liquid refrigerant/sorbents solutions. Figure 2 shows the scheme of a closed cycle single stage absorption process, where a solution of refrigerant and solvent is heated under high pressure in the generator. Figure 2. Scheme of a closed cycle absorption system [3] The refrigerant evaporates, the concentrated solution flows through a restrictor to the absorber (low pressure level). The refrigerant vapour condenses in the condenser by rejecting heat through a heat rejection unit. After that, it is expanded to a lower pressure level via another restrictor. At this low pressure level, the refrigerant evaporates again while absorbing heat at a low temperature level. This is the actual cooling process. Hereafter the refrigerant vapour is drawn to the absorber, where it is absorbed by the concentrated solution. The heat of this reaction also has to be dissipated by the heat rejection unit. The emerged diluted solution is pumped to the higher pressure into the generator, where the regeneration process starts again. Before entering the generator, the diluted solution is preheated by the hot concentrated solution coming from the generator through a solution heat exchanger. As the solution pump only needs to pump incompressible fluids and does not need to compress a gaseous refrigerant, it is much smaller than the compressor in a conventional compression cycle and therefore consumes only a fraction of energy Adsorbtion chillers Adsorption chillers use solid sorption materials as adsorbents. The water-silica gel working pair is commonly used adsorbent-refrigerant materials in presently available adsorption chillers. Figure 3 shows a schematic diagram of an adsorption chiller. The system consists of two sorbent compartments (1 and 2), an evaporator and a condenser. The sorbent in compartment 1 is regenerated (desorption) using hot water from the external heat source, e.g. the solar collector. The desorbed water vapour is condensed in the condenser. At the same time, the sorbent in compartment 2 (adsorption) adsorbs the water vapour entering from the evaporator. The water in the evaporator takes up heat from the chilled water circuit and evaporates. During this process cold is produced. After a pre-set time or adsorption in compartment 2 has reached a certain value, the hydraulics are switched, so that compartment 2 is connected to the hot water circuit (i.e. desorption) and 1 is connected to the cooling water (adsorption). In between these two phases a heat recovery phase is implemented: water is circulated between the heat exchangers of compartment 1 and 2 cooling down the hot and desorbed compartment 1 and heating up the adsorbed and cold compartment 2. This process is an essential step to minimize negative effects by the thermal inertia of the system and increase the COP of the system. Figure 3. Scheme of a closed cycle adsorption system [3] 3.2 Open cycle cooling machines Open cycle cooling processes are those, in which the air that has to be cooled (or dehumified) is in direct contact with the working medium of the cooling process itself. Because the permanent contact between the refrigerant and air, the first must always be water. These processes consist of a combination of sorptive air dehumidification and evaporative cooling (desiccant and evaporative cooling DEC). Open cycle cooling processes can be driven with moderate temperatures (45 C to 95 C). This makes them very attractive to be combined with solar thermal collectors. Additionally, no cooling tower is needed, because the re-cooling is integrated in the process. 12

4 4. SEASONAL STORAGE Seasonal heat storage offers a great potential for substituting fossil fuels using waste heat from cogeneration heat and power plants (CHP) or solar energy for domestic hot water preparation and space heating. Large scale seasonal storages in (solar assisted) district heating systems have lower specific investment costs and reduced relative thermal losses in comparison to decentralized heating systems.[5] Solar assisted district heating systems with seasonal thermal energy storage aim at a solar fraction of 50% or higher of the total heat demand for space heating and domestic hot water preparation. The seasonal time shift between solar irradiance and heat demand is matched by means of long term heat storage. Central solar heating plants or solar assisted district heating systems may be realized with various components and system configurations. The design of a seasonal thermal energy storage depends on the system design (i.e. with or without heat pump, integration of the auxiliary heating device) and the operation parameters (number of charging cycles, maximum and minimum operation temperature). A typical configuration of a solar assisted district heating system is illustrated in Figure 4. Although construction above ground may be preferable from the technical point of view, for some reasons (optical, thermal losses, statical) seasonal thermal energy storages are mostly buried or - at least - partially buried. Integration in the landscape is of major importance especially as in most cases a seasonal thermal energy storage will be located within or next to residential areas. There are several disadvantages resulting from the construction below surface level. First of all additional costs arise for the excavation. Secondly, the static is more complex due to the soil pressure. Furthermore, a construction with rear ventilation of the thermal insulation is hardly possible. Hence, construction in moist soil requires measures that prevent the insulation from getting wet. The wall of a buried thermal energy storage is an assembly of several layers. The complexity of the design of such a composite wall arises due to the fact that on the one hand the envelope has to guarantee protection of the thermal insulation from moisture penetration from the inside and the outside but on the other hand desiccation in case the thermal insulation is already wet has to be enabled. Figure 4. Solar assisted district heating system with seasonal thermal energy storage [4] 13

5 5. GREEK PROTOTYPE FROM HIGH COMBI PROJECT The proposed system is considered advanced, mainly because it has the following features: Use a combination of boreholes and water storage. Have small dimensions, if compared with the usual size of existing seasonal storages: (total volume of about 400 m³ while usual seasonal storage tanks have volumes bigger than 1000 m³). Use the rejected heat during summer cooling operation to warm the earth around the water tank. Use low (or zero) cost insulation materials. One of the main aims of the storage design is to minimize the heat losses by the use of boreholes around the main storage tank. During cooling operation, the cooling machine receives thermal energy (Q h ) at high temperature (i.e., C) from the collectors field and provides the useful cooling (Q c ) services to the building. As in every heat driven system, the sum of the heat provided for operating the chiller at high temperature and of the heat extracted from the building (at low temperature), namely (Q h +Q c ), has to be rejected at a mid temperature level. Often the external ambient air is used as heat sink for the heat rejection (e.g., through a cooling tower). In this HighCombi system, Q h +Q c is delivered to the boreholes (that are positioned around the water storage tank). Consequently, the earth surrounding the storage will be heated, thus reducing the losses of the water storage tank. When, during summer, there is solar energy available but no need for cooling or domestic hot water, the produced heat will be delivered into the water tank storage. The storage will be heated up to about 90 C in summer. The combination of a good insulation (yellow area in the figure), an additional low (or zero) cost insulation (light blue area) and the surrounding earth heated by the boreholes will hopefully maintain the storage high temperatures, so that it will cover a substantial amount of the heating load during winter. Obviously, a part of the heating load will be covered directly by the solar gains during winter. A schematic representation of the system can be seen in Figure 5. The characteristics of the underground thermal storage are described in Figure 6. Figure 6. Position of several elements of the Greek prototype The structure of the tank wall is shown in Figure 7 and it comprises multiple layers for both anticorrosion and heat losses protection. Figure 7. Multilayered (composite) vertical side wall of a seasonal thermal energy storage Figure 5. Scheme of the Greek HIGH COMBI plant in summer operation mode An example of temperature values, both for the water inside the storage tank and for the water inside the underground heat exchanger, taken in mid- October, is shown in Figure 8. Also, for the same day, temperatures measured in the key points of the system are presented in Figure 9, as a print screen of the monitoring program. 14

6 Figure 8. Temperatures of the seasonal storage system the one hand new promising solar thermal applications, such as solar cooling and solar process heat, have to be developed and demonstrated also with solar fractions between 10%- 30%. On the other hand the development of new storage concepts and integration of other renewable energy sources than solar are prioritized ( Despite more than 20 years of international research activities there is still no economical and technical state of the art of seasonal heat storage. The prototype system presented in this paper was commissioned in the winter of After that, a period of control devises adjustment followed. The solar combi-system (heating and cooling) having also an underground seasonal energy storage system has been fully functional since A complete monitoring system provides information about temperatures, flow rates and energy consumptions for all subsystems. 7. ACKNOWLEDGEMENTS Many thanks have to be said to the people from the solar department of CRES. 8. REFERENCES (HEADING 1) Figure 9. Scheme of the Greek HIGH COMBI plant in summer operation mode 6. CONCLUSIONS In Germany, research and development activities on solar assisted district heating systems with seasonal thermal energy storage were funded within the programme "Solarthermie-2000", which was initiated in 1993 for a period of 10 years. It has been extended by the Solarthermie2000plus a program that was running until Whereas the main objective of the "Solarthermie-2000" was to improve and demonstrate the technical and economic feasibility of systems with large scale seasonal thermal energy storage, new priorities are in the focus of the Solarthermie2000plus program. On 1. Maake, W., Eckert, H.J.: Taschenbuch der Kältetechnik, 16 Auflage, Müller, Karlsruhe, Germany, (1978). 2. Weiss, W., Rommel, M.: Medium Temperature Collectors, IEA_SHC Task 33, Subtask C, umtemperaturecollectorstask33.pdf 3. Henning, H.M.: Solar Cooling, Presentation at the ISES Solar World Congress, Beijing (2007). 4. Bodmann M., Mangold D., Nußbicker J., Raab S., Schenke A., Schmidt T.: Solar unterstützte Nahwärme und Langzeit-Wärmespeicher, Forschungsbericht zum BMWA / BMU- Vorhaben (Februar 2003 bis Mai 2005), Stuttgart Germany, (2006). 5. High Combi project: 15