Chapter 10 Secondary Applications

Transcription

1 Advanced CSP Teaching Materials Chapter 10 Secondary Applications Authors Franz Trieb 1 Johannes Sattler 2 Reviewers Anette Anthrakidis 2 Christian Faber 2 Thomas Fend 3 Joachim Göttsche 2 Adel Khalil 4 Denis T. G. Wambeogo 2 1 German Aerospace Center (DLR) - Institute of Technical Thermodynamics, System Analysis, Pfaffenwaldring, 38-40, Stuttgart, Germany 2 Solar-Institut Jülich (SIJ), FH Aachen, Aachen University of Applied Sciences, Heinrich-Mußmann- Str. 5, Jülich, Germany 3 German Aerospace Center (DLR) - Solar Research, Linder Höhe Cologne, Germany 4 Cairo University, Faculty of Engineering, Giza, 12613, Egypt

2 Table of Contents Nomenclature... 3 Summary Secondary Applications... 5 Introduction Desalination Seawater Desalination Technologies Pre-Selection of Desalination Technologies Concentrating Solar Power for Large Scale Seawater Desalination Concentrating Solar Power for Small Scale Seawater Desalination Hydrogen Production TC (thermochemical) Cycle Solar Steam Reforming of Natural Gas H 2 Production by Solar Cracking of Hydrocarbons Cost Overview and Comparison Perspectives for Solar Reforming in Sunny Regions Cooling Solar Absorption Chillers Adsorption chillers Solar Drying List of figures List of tables Reference list

3 Nomenclature Symbol Meaning Unit Latin letters P s (T) saturation vapour pressure at the temperature of T Pa q* fraction of refrigerant adsorbed in equilibrium condition kg/kg Q energy J T temperature C or K p pressure Pa W s weight of adsorbent kg W w weight of adsorbate in liquid phase kg W w decrease of water in evaporator kg Greek letters difference operator Subscripts A or abs ads C des e E G th absorber adsorption condenser desorption electric evaporator generator thermal Acronyms A AX C CFC COP CSIRO CSP CT DLR E ED FR G GH GHG GOR HCFC HX, HEX IE MD MED absorber auxiliary energy source condenser chlorofluorocarbons thermal coefficient of performance Commonwealth Scientific and Industrial Research Organisation concentrating solar power cooling tower Deutsches Zentrum für Luft- und Raumfahrt (German Aerospace Center) evaporator electrodialysis freezing generator gas hydrate green house gas gained output ratio hydrochlorofluorocarbons heat exchanger ion exchange membrane distillation multi-effect distillation 3

4 MVC MSF NG PCM PV RO PR SD SMR SOLREF TC TVC V mechanical vapour compression multi-stage flash natural gas phase change material photovoltaic reverse osmosis performance ratio seawater desalination solar distillation solar methane reforming solar reformer thermochemical cycle thermal vapour compression valve Summary This chapter deals with the potentials and possibilities of utilising CSP plants for uses other than solely generating electricity for the grid. Secondary in this case means utilisation of high, medium, or even low temperature heat delivered by CSP-collectors. This may mean either the utilisation of waste heat from turbine processes or direct utilisation of high temperature heat for succeeding processes such as hydrogen generation. Solar cooling is introduced as well as desalination, which has become one of the most important challenges for the future supply of the world s population with fresh water. 4

5 10 Secondary Applications Key questions What are the different possibilities to use CSP waste heat for desalination processes? What are the pros and cons of this utilisation? Solar cooling: what are the economic drawbacks? Introduction Even in optimised CSP-Plants annual solar-to-electric efficiencies do not exceed 15-25%, if they are based on steam turbine technology. This causes the question about the thermal utilisation of the waste heat. Unfortunately the temperature level of this heat is rather low, which limits the performance of possible secondary uses of this heat. However, if the steam turbine processes is modified to reach higher temperature levels, this leads to draw-backs concerning the performance of the primary application. Solar cooling should be an option because the offer of solar radiation often matches with the demand of cooling power during periods of high ambient temperature. However, most of the existing cooling technologies, which are powered by heat (adsorption and absorption cooling machines), are more expensive than simple electrically driven cooling machines. Furthermore, cooling technologies are not easily available in small units. Developing a non-fossil fuel for the traffic sector is one of the most important challenges for the next future. In this chapter a short review is given about the possibilities of solar hydrogen generation via a medium to high temperature process as an alternative to electrolysis. Please note that the entire section on desalination is quoted from the study Aqua-CSP Concentrating Solar Power for Seawater Desalination, which was prepared by the German Aerospace Center, source [1]. 5

6 10.1 Desalination The general perception of solar desalination today comprises only small scale technologies for decentralised water supply in remote places, which may be quite important for the development of rural areas, but does not address the increasing water deficits of the quickly growing urban centres of demand. Conventional large scale desalination is perceived as expensive, energy consuming and limited to rich countries like those of the Arabian Gulf, especially in view of the quickly escalating cost of fossil fuels like oil, natural gas and coal. The environmental impacts of large scale desalination due to airborne emissions of pollutants from energy consumption and to the discharge of brine and chemical additives to the sea are increasingly considered as critical. For those reasons, most contemporary strategies against a Global Water Crisis consider seawater desalination only as a marginal element of supply. The focus of most recommendations lies on more efficient use of water, better accountability, re-use of waste water, enhanced distribution and advanced irrigation systems. To this adds the recommendation to reduce agriculture and rather import food from other places. On the other hand, most sources that do recommend seawater desalination as part of a solution to the water crisis usually propose nuclear fission and fusion as indispensable option. None of the presently discussed strategies include concentrating solar power (CSP) for seawater desalination within their portfolio of possible alternatives. However, quickly growing population and water demand and quickly depleting groundwater resources in the arid regions of the world require solutions that are affordable, secure and compatible with the environment in one word: sustainable. Such solutions must also be able to cope with the magnitude of the demand and must be based on available or at least demonstrated technology, as strategies bound to uncertain technical breakthroughs if not achieved in time would seriously endanger the whole region. The scope of this chapter is to find adequate combinations of technologies for seawater desalination (SD) and concentrating solar power (CSP) used as energy source. Although the desalination of brackish groundwater is also an option, its resources are rather limited when compared to seawater and the use of groundwater is already today related to strong environmental impacts. Although this option should not be neglected, it is considered here only a minor possible contribution to sustainable water. In the following the focus therefore lies on seawater desalination. Within this chapter in the first place a brief description of the principle and main characteristics of the most important desalination technologies is provided. In the second place, the state of the art of CSP is described Seawater Desalination Technologies There are a large number of different desalination technologies available and applied worldwide. Some of them are fully developed and applied on a large scale, while others are still used in small units for demonstration purposes or for research and development. Table 1 gives a selection of the most commonly applied technologies. 6

7 Table 1: Overview of contemporary desalination methods Separation Energy Use Process Desalination Method Multi-Stage Flash (MSF) Multi-Effect Distillation (MED) Evaporation Thermal Vapour Compression (TVC) Thermal Solar Distillation (SD) 5 Freezing (FR) Water from Salts Crystallisation Gas Hydrate Processes (GH) Filtration/Evaporation Membrane Distillation (MD) Mechanical Mechanical Vapour Compression Evaporation (MVC) Filtration Reverse Osmosis (RO) Salts from water Electrical Chemical Exchange Selective Filtration Ion Exchange (IE) Electrodialysis (ED) Only those desalination technologies were selected for further consideration that have at least reached a semi-commercial state of the art, and that can be realised in sufficiently large units in order to be effectively combined with concentrating solar thermal power stations (CSP). Therefore, only the five technologies highlighted in Table 1 come into consideration. These are thermal desalination methods that evaporate seawater by using heat from combustion or from the cold end of a power cycle, and mechanical methods using filtration through membranes. Vapour compression technologies are mainly used in combination with thermal distillation in order to increase volumes and efficiency of those processes. a) Multi-Stage Flash Desalination (MSF) MSF is a thermal distillation process that involves evaporation and condensation of water. The evaporation and condensation steps are coupled to each other in several stages so that the latent heat of evaporation is recovered for reuse by preheating incoming water (Figure 2). In the so called brine heater, the incoming feedwater is heated to its maximum temperature (top brine temperature) by condensing saturated steam from the cold end of a steam cycle power plant or from another heat source. The hot seawater then flows into the first evaporation stage where the pressure is set lower. The sudden introduction of hot water into the chamber with lower pressure causes it to boil very quickly, almost exploding or flashing into steam. Only a small percentage of the water is converted to vapour, depending on the pressure maintained in this stage, since boiling will continue only until the water cools down to the equilibrium at the boiling point, furnishing the heat of vaporisation. The vapour generated by flashing is condensed on tubes of heat exchangers that run through the upper part of each stage. The tubes are cooled by the incoming feedwater going to the brine heater, thus pre-heating that water and recovering part of the thermal energy used for evaporation in the first stage. This process is repeated in up to 40 stages, whereas mostly around 20 5 Due to unknown reasons, the term solar distillation is exclusively used for small-scale, decentralised solar powered desalting technologies. The creation of this category is rather misleading. Within this chapter large scale options for solar distillation are presented which do not fit into the general perception of this category. Therefore, other terms for large scale solar distillation are used here. 7

8 stages are employed. To maximise water and energy recovery, each stage of an MSF unit operates at a successively lower pressure. The vacuum can be maintained by a steam ejector driven by high pressure steam or by a mechanical vacuum pump. Multi-stage flash (MSF) units are widely used in the Middle East (particularly in Saudi Arabia, the United Arab Emirates, and Kuwait) and they account for 58% of the world s seawater desalination capacity. A key design feature of MSF systems is bulk liquid boiling. This alleviates problems with scale formation on heat transfer tubes. Figure 1: Umm Al Nar East MSF desalination plant, 87,260 m 3 /day (left), Al Khobar Phase II, 267,000 m 3 /day, Saudi Arabia Figure 2: Principle of multi-stage flash desalination (MSF) Large MSF units are often coupled with steam or gas turbine power plants for better utilisation of the fuel energy by combined generation. Steam produced at high temperature and pressure by the fuel is first expanded through a turbine to produce electricity. The low to moderate temperature steam exiting the turbine is then used to drive a thermal desalination process. In this case, the capacity of the low pressure stage of the steam turbine to produce electricity is reduced with increasing temperature of the extracted steam. Multi-Stage Flash plants are usually coupled to the cold end of a steam cycle power plant, extracting steam at C from the turbine to feed the brine heater of the MSF unit. If the temperature is above the condensation temperature of water at ambient pressure, special backpressure turbines are required for such a combined process. Moreover, the reduction of power generation with respect to a conventional condensing steam turbine working at C is 8

9 considerable (Figure 3). On the other hand, an advantage of combined generation is that the condenser required for a conventional plant is substituted by the desalination unit. In this case, the feedwater must include enough water for desalination and cooling. Figure 3: Principle of substituting the condenser of a steam cycle power plant by a thermal desalination unit (left) and typical reduction of steam turbine power capacity at increasing condensing temperature (right). The squares show the typical operating range of MED and MSF plants The MSF process requires a considerable amount of steam for the evaporation process and also significant amounts of electricity to pump the large liquid streams (Table 2). To this adds the power reduction induced within the steam cycle. Two different performance indicators are used, that yield however similar values: the performance ratio (PR) is the ratio of product water and input heat, while the gained output ratio (GOR) is defined as the mass of water product per mass of heating steam. A typical gain output ratio for MSF units is 8. MSF is specially suited for desalination if the quality of the feedwater is unfavourable (high salinity, temperature and contamination), as the system is very robust. A MSF plant has a typical heat requirement of kj/kg product. The specific electricity consumption is in the order of 3-5 kwh/m³. To this adds a loss of electricity from the steam turbine due to the higher cold end temperature equivalent to 6-8 kwh/m 3. b) Multi-Effect Desalination (MED) Multi-effect desalination (MED) is also a thermal distillation process (Figure 4 and Figure 5). The feedwater is sprayed or otherwise distributed onto the surface of the evaporator surface (usually tubes) of different chambers (effects) in a thin film to promote evaporation after it has been preheated in the upper section of each chamber. The evaporator tubes in the first effect are heated by steam extracted from a power cycle or from a boiler. The steam produced in the first effect is condensed inside the evaporator tubes of the next effect, where again vapour is produced. The surfaces of all the other effects are heated by the steam produced in each preceding effect. Each effect must have a lower pressure than the preceding one. This process is repeated within up to 16 effects. The steam produced in the last effect is condensed in a separate heat exchanger called the final condenser, which is cooled by the incoming sea water, which is then used as preheated feedwater for the desalination process. 9

10 MED has gained attention due to the better thermal performance compared to MSF. In principle, MED plants can be configured for high temperature or low temperature operation. At present, they operate at top brine temperatures below 70 C to limit scale formation and corrosion. The top brine temperature can be as low as 55 C which helps to reduce corrosion and scaling, and allows the use of low-grade waste heat. If coupled to a steam cycle, the power losses are much lower than those obtained when coupling a MSF plant (Figure 3), and even standard condensing turbines may be used instead of back-pressure turbines. The MED process can have several different configurations according to the type of heat transfer surface (vertical tube falling film, vertical tube climbing film, horizontal tube falling film, plate heat exchanger) and the direction of the brine flow relative to the vapour flow (forward, backward, or parallel feed). MED systems can be combined with heat input between stages from a variety of sources, e.g. by mechanical (MVC) or thermal vapour compression (TVC). MED-TVC systems may have thermal performance ratios (similar to the gained output ratio, distillate produced to first stage energy input) up to 17, while the combination of MED with a lithium bromide -water absorption heat pump yielded a thermal performance ratio of 21. Figure 4: Multi-effect desalination unit with thermal vapour compression (left) and complete plant (right) When coupled to the cold end of a steam cycle power plant, MED plants (without TVC) typically have a heat consumption of kj/kg in the form of process steam at less than 0.35 bar that is withdrawn from the steam turbine, and a specific electricity consumption of kwh/m 3, mainly for pumping and control, which are fairly independent from raw water salinity, contamination or temperature. MED-TVC plants are driven with motive steam above 2 bar, mostly between 10 and 20 bar. 10

11 Figure 5: Principle of multi-effect desalination (MED) c) Reverse Osmosis (RO) Reverse osmosis (RO) is a membrane separation process that recovers water from a saline solution pressurised to a point greater than the osmotic pressure of the solution (Figure 6). In essence, membrane filters hold back the salt ions from the pressurised solution, allowing only the water to pass. RO membranes are sensitive to ph, oxidisers, a wide range of organics, algae, bacteria, depositions of particulates and fouling. Therefore, pre-treatment of the feedwater is an important process step and can have a significant impact on the cost and energy consumption of RO, especially since all the feedwater, even the amount that will eventually be discharged, must be pretreated before being passed to the membrane. Recently, micro-, ultra- and nano-filtration has been proposed as an alternative to the chemical pre-treatment of raw water in order to avoid contamination of the seawater by the additives in the surrounding of the plants. RO post-treatment includes removing dissolved gases (CO 2 ), and stabilizing the ph via the addition of Ca or Na salts, and the removal of dangerous substances from the brine. Pressurising the saline water accounts for most of the energy consumed by RO. Since the osmotic pressure, and hence the pressure required to perform the separation is directly related to the salt concentration, RO is often the method of choice for brackish water, where only low to intermediate pressures are required. The operating pressure for brackish water systems ranges from bar and for seawater systems from 50 to 80 bar (the osmotic pressure of seawater with a salinity of 35 g/kg is about 25 bar). 11

12 Figure 6: Principle of desalination by reverse osmosis (RO) Figure 7: Specific electricity consumption of reverse osmosis plants with and without energy recovery system as function of raw water salinity 12

13 Figure 8: Left: Pressure cylinders containing the separation membranes of a reverse osmosis plant in Barcelona, Spain, with 30,000 m 3 /day Desalting capacity; Right: RO-stacks and high pressure pumps of a 30,000 m 3 /day desalination plant in Gran Canaria, Canary Islands d) Thermal Vapour Compression (TVC) Vapour compression is added to a multi-effect distiller in order to improve its efficiency. Vapour compression processes rely on the reuse of vapour produced in the distiller as heating steam after recompression. The vapour produced in one stage is partially recompressed in a compressor and used to heat the first cell. The vapour is compressed either with a mechanical compressor (mechanical vapour compression, MVC) or with a steam ejector (thermal vapour compression, TVC). For thermal vapour compression, motive steam at higher pressure is withdrawn from another process, e.g. a steam power cycle or industrial process steam [1]. Figure 9: Principle of thermal vapour compression (TVC) 13

14 e) Mechanical Vapour Compression (MVC) Mechanical vapour compression processes are particularly useful for small to medium plants. MVC units typically range in size up to about 3000 m 3 /day while TVC units may range in size to m 3 /day. MVC systems have between one and three stages, most of them only have a single stage, while TVC systems have several stages. This difference arises from the fact that the pressure and temperature increase by the mechanical compressor and its capacity are limited [1]. Figure 10: Single stage mechanical vapour compression desalination process (MVC) Exercises Name at least 3 seawater desalination technologies Describe briefly the multi-stage flash desalination (MSF) process Describe briefly the multi-effect desalination (MED) process Describe briefly the desalination process by reverse osmosis (RO) 14

15 Pre-Selection of Desalination Technologies Table 2 shows some of the characteristics of the four leading desalination technologies. The purpose of this comparison was to select the most appropriate thermal and mechanical desalination method for the combination with CSP, and to find a plausible combination that could be representative for large scale dissemination. Comparing MSF and MED, it becomes clear that MED is more efficient in terms of primary energy and electricity consumption and has a lower cost. Moreover, the operating temperature of MED is lower, thus requiring steam at lower pressure if connected in co-generation to a steam cycle power plant. Thus, the combination of CSP with MED will be more effective than a combination of CSP and MSF desalination. Thermal vapour compression is often used to increase the efficiency of an MED process, but it requires steam at higher pressure if connected to a steam power cycle. Comparing the mechanical driven desalination options, reverse osmosis has a lower electricity consumption and cost per unit product water than the mechanical vapour compression method. Table 2: Characteristics of the two main thermal desalination technologies and the two main mechanical desalination technology options. The figures refer to seawater as the raw water source. The low performance characteristics of MSF and MVC marked in red have lead to the selection of MED and RO as reference technologies for this study. The range shown for MED/TVC covers simple MED as well as combined MED/TVC plants. (*Power consumption does not include power losses induced by cogeneration due to increasing outlet temperature at the turbine; **Plant cost increases with product water quality and energy efficiency) Energy used thermal mechanical Process MSF MED/TVC MVC RO State of the Art commercial commercial commercial commercial World Wide Capacity 2004 (Mm³/d) Heat Consumption (kj/kg) Electricity Consumption (kwh/m³)* Plant Cost ($/m³/d)** Time to Commissioning (months) Production Unit Capacity (m³/d) < < < 3000 < Conversion Freshwater / Seawater 10-25% 23-33% 23-41% 20-50% Max. Top Brine Temperature ( C) (max) Reliability very high very high high moderate (for seawater) Maintenance (cleaning per year) several times Pre-treatment of water simple simple very simple demanding Operation requirements simple simple simple demanding Product water quality (ppm) < 10 < 10 < The much lower primary energy consumption of RO and the slightly lower cost compared to MED suggests that RO might be the preferred desalination technology anyway. However, if MED is coupled to a power plant, it replaces the cost of the condensation unit of the steam cycle and partially uses waste heat from power generation for the desalination process. In this case, not all the primary energy used must be accounted for the desalination process, but only the portion that is equivalent to a reduction of the amount of electricity generated in the plant when compared to conventional cooling at lower temperature, and of course the direct power consumption of the MED process. 15

16 Processes combining thermal and mechanical desalination may lead to more efficient future desalination systems. However for simplicity, only separated processes have been used for our comparison. Exercises The following desalination technologies are either thermal or mechanical processes: Multi-Stage Flash Desalination (MSF) Mechanical Vapour Compression (MVC) Multi-Effect Desalination (MED) Thermal Vapour Compression (TVC) Reverse Osmosis (RO) Task: For each of the technologies, write in the underlined space next to it whether the process is thermal or mechanical. 16

17 Concentrating Solar Power for Large Scale Seawater Desalination Concentrating solar power plants can generate electricity which can be used for membrane desalination via reverse osmosis. Being thermal power stations, CSP plants can also be used for combined heat and power. Thus, also thermal desalination methods like multi-effect or multi-stageflash can be coupled to and powered by CSP, either directly or in co-generation with electricity. The different configurations for desalination powered by CSP are shown in Figure 11. Heat Only Power Only Combined Heat & Power Solar Field Storage Solar Field Storage Solar Field Storage solar heat fuel grid solar heat fuel Power Plant solar heat heat fuel Power Plant MED RO MED Water Power Water Power Water Power Figure 11: Different configurations for desalination powered by CSP. Left: Solar field directly producing heat for thermal multi-effect desalination. Center: Power generation for reverse osmosis (RO). Right: Combined generation of electricity and heat for multi-effect desalination (MED). Table 3 shows selected characteristics of CSP/MED and CSP/RO plants. 17

18 Table 3: Selected characteristics of CSP/MED and CSP/RO plants System CSP/MED CSP/RO Site Selection limited to coast CSP may be anywhere, RO must be at the coast, while the public grid can be used for interconnection Flexibility interdependent operation independent operation possible if plants interconnected through the public grid Optimal Irradiance defined by coastal site CSP can be placed at site with higher irradiance, but certain amount of power is then lost by transmission to RO plant, and dry cooling leads to lower efficiency Storage Options molten salt, concrete, low temperature hot water storage possible, PCM Water Quality Other Uses independent of raw water quality, very high quality of product water industrial co-generation of process heat, district cooling, integrated systems for power, cooling, desalination for tourism and rural development molten salt, concrete, phase change materials (PCM) may be favourable for brackish raw water and if low product water quality is allowed power only Exercises Sketch at least 2 different configurations for desalination powered by CSP. 18

19 Concentrating Solar Power for Small Scale Seawater Desalination The configurations shown in Figure 11 can also be applied to small-scale seawater desalination systems in a capacity range below 1 MW or 1000 m³/day, respectively. There are cases for directly applying heat from parabolic troughs or linear Fresnel collector fields to thermal MED desalination (Figure 11, left), or to realise small scale co-generation systems in the 10 kw range using parabolic-dish-stirling engines (Figure 12). An important issue for small systems is the usual up-scaling of specific system costs when downscaling the size of the collector fields. Conventional parabolic troughs or central receivers will hardly be competitive when they are scaled down to units smaller than 1 MW. In this market segment, CSP will have to compete with PV- and wind-powered RO-systems and with nonconcentrating solar thermal collector systems. However, low-temperature parabolic trough and linear Fresnel systems are likely to be competitive in this market segment, as they offer low cost and a unique possibility of energy storage by hot water at temperatures below 100 C. Considerable amounts of energy (35 kwh/m³) can be stored in hot water in the temperature range between the maximum storage temperature of e.g. 95 C and the operating temperature of an MED plant of e.g. 65 C. It may be feasible to directly heat and store incoming seawater for later processing in hours without sunshine. Thus, fluctuating solar energy input would not affect continuous operation of the desalination plant. Small part of the solar collector field or a different source could be used to provide the relatively small amounts of electricity required by MED. Figure 12: Left: Low-temperature parabolic trough for direct steam generation from SOLITEM, Center: Linear Fresnel from NOVATEC-Biosol, Right: Dish-Stirling engine from Schlaich, Bergermann & Partner There is a considerable market for small-scale solar systems for seawater and brackish water desalination in remote urban and in agricultural areas. In order to apply these technologies to rural development, their technical and economic feasibility must be assessed for specific sites and applications, and pilot plants must be built to demonstrate reliability of system operation. For an overview of present activities see the sources listed in the original report by following the website link given in the bibliography. Exercises Describe a problem that parabolic trough and solar tower power plants have if they are to be used for small scale seawater desalination. 19

20 10.2 Hydrogen Production Hydrogen and its secondary products methane, methanol and sun diesel 6 have potential of becoming the fuel of the future in the mobility sector. Many leading car manufacturers have identified the potentials of hydrogen as a fuel and have built cars on commercial basis. The cars work with fuels cell stacks, combustion engines or in hybrid mode. Nevertheless, there are still some problems associated with hydrogen: Hydrogen is not a naturally occurring direct fuel in the way as natural gas or oil is, i.e. it has to be produced by means of extracting it from fossil fuels or by splitting it from other gas or fluids e.g. water. In other words, additional energy is required to obtain hydrogen. Due to the mentioned characteristics, hydrogen can therefore be categorised as a synthetic fuel. When combusting/consuming hydrogen (e.g. in a car), the process is emission-free and the byproduct is clean water. Unless produced from water by utilisation of renewable energy, hydrogen is not a clean fuel as such: Using CO 2 -emmissive energy to produce hydrogen means that hydrogen as a fuel is indirectly CO 2 -emmissive [2]. Moreover, hydrogen needs to be stored either in fluid form at cryogenic temperature (near zero Kelvin) as the normal boiling point is at C (20.35 K), in a compressed gaseous state at psi ( bar) or in solid-state materials (e.g. metal hydrides) in order for it to be utilised for the mobility sector [3]. One difficulty with utilising hydrogen for the mobility sector is the low volumetric energy density of hydrogen at room temperature. To store hydrogen more useful forms e.g. liquid, however, again requires large amounts of energy. Hydrogen has not yet had its break through as to date there is little demand. Hence, the production facilities and the distribution network are not yet as developed as those for petroleum products e.g. fuel oil or gasoline. As an example, setting up the infrastructure in and for the USA will cost an estimated $1 trillion to provide the nations requirement for hydrogen fuel for cars [4]. Alongside renewable energy technologies such as wind energy converters installed in large scale, solar thermal power plants have a promising potential on the hydrogen market. Solar and wind technologies are becoming increasingly accepted by the public, cheaper and therefore more competitive and commercial. Hence, they are valuable long-term options for environmentally friendly electricity generation now and in the future. Although still claimed expensive these technologies will become cheaper as the technology improves and more large-scale installations are being built. Cost estimations have shown that the electricity demand of the US market for producing hydrogen fuel for all its road vehicles will be the same as the current amount of generated electricity in the USA. With the scale-up of solar thermal power plants and wind parks, the future costs of these technologies will be significantly reduced compared to the current costs. Moreover, this will be the case as demand rises [4]. Some quick facts about hydrogen as a fuel: Nowadays, hydrogen is still a bulk chemical rather than an energy vector. Based on figures from the year 2003, the global annual production is about billion Nm 3 /year (In more detail, 6 Methanol and sun diesel could be produced by CSP plants in a process that first produces hydrogen (e.g. in a reformer, in a thermochemical cycle or by means of solar cracking) which is then followed by a conversion to the synthetic fuels methanol and diesel. As the diesel fuel is produced synthetically from solar energy, it was given the name sun diesel. 20

21 approximately 30% is provided from NG by reforming, 50% from oil by reforming, 20% from coal by gasification and 2-3% by electrolysis). The largest consumers are from the chemical industry and steel production (50%) and petrochemical industry (33%) [5]. The virtual value is 100 billion /year. The growth rate is about 10%/year. Only 4% are traded commercially. The production of ammonia alone generates about 250 Mt CO 2 /year [6]. For the current space technology, hydrogen is still the only fuel suitable for the propulsion of rockets into space. The currently available methods of producing hydrogen are shown in Figure 13. Figure 13: Hydrogen production processes, edited from [7] Only those methods that find application for solar technologies are discussed in more detail. Solar hydrogen production is still a field that is explored and experimented with. The following examples are given for projects that are or were conducted at the Institute of Technical Thermodynamics of the German Aerospace Center (DLR) TC (thermochemical) Cycle Research on the thermochemical cycle was conducted in the project Hydrosol. The objectives of the project were [6]: to develop novel active redox materials for water splitting and regeneration reactions at moderate temperatures ( C) the design, construction and test operation of a prototype reactor for continuous hydrogen production based on a thermochemical cycle applying mixed iron oxide feasibility of operability of solar thermal two-step hydrogen production evaluation of techno-economic potential of the technology 21

22 Figure 14: Thermochemical cycle, edited from [6] The cycle is a two-step redox thermochemical cycle using mixed iron oxides. The equations are [8]: 1. Endothermal step at C: MO ox MO red + ½ O 2 (1) 2. Splitting (also called water dissociation) at C: MO red + H 2 O MO ox + H 2 (2) System: e.g. MO = (Zn,Y)Fe 2 O 4 Y = Ni or Mn (3) Costs: ct/kwh 22

23 Figure 15: Receiver with quartz window and a ceramic honey comb absorber structure [8] The thermochemical water splitting method has to deal with a major problem as the products hydrogen and oxygen are an explosive gas mixture. Moreover, the process could also take place at temperatures of around 2500 C (it is, however, not clear as to how this temperature could be achieved). This would, of course, require high-temperature materials (special ceramics), which would, however, be difficult to handle and are expensive [9]. Exercises Hydrogen production: Write down the two-step redox thermochemical cycle using mixed iron oxides of a thermochemical cycle (TC). 23

24 Solar Steam Reforming of Natural Gas Research on solar steam reforming of natural gas has been conducted in the EU projects SCR, SOLASYS and the recent study SOLREF 7, all in which the German Aerospace Center participated. Figure 16: Solar steam reforming of natural gas [6] The following illustration shows the steam reforming process. The reformer shown can be taken as a black box, as there are several reformer design options. 7 SOLREF: abbreviation for Solar Steam Reforming of Methane Rich Gas for Synthesis Gas Production 24

25 Figure 17: Process of steam reforming and chemical equation [7] Steam reforming can be realised in different ways, as shown in Figure 18. Figure 18: Possibilities of steam reforming [8] Description of the different reforming processes [8]: a) Separated/allothermal: The reformer is externally heated ( C). The operation of a heat storage system is possible. Research projects were conducted e.g. project Asterix at the German Aerospace Center in the late eighties and beginning of the nineties. b) Indirect and e.g. tubular: Here the reformer wall is directly irradiated (up to 850 C). The efficiency of the reformer is stated to be approx. 70%. Ongoing research is conducted at CSIRO in Australia and in Japan. Research in Germany and at WIS in Israel was conducted in the eighties and nineties. c) Integrated direct and volumetric: The catalytically active absorber is directly heated by concentrated solar energy. Efficiencies above 90% are possible in future designs (efficiency = increase of sensible and chemical power of the gas mixture divided by the incoming solar power). Projects are or were coordinated by the German Aerospace Center: (SCR, SOLASYS, SOLREF); further research in Israel and Japan. 25

26 Reformer designs Tubular concept reformer Through ongoing research at CSIRO, Australia, a kw th reformer was designed with a tubular concept, as shown in Figure 19. The catalyst is packed in-between the inner and outer tubes; the inner tube is purely for countercurrent heating of the feedwater stream [8]. Figure 19: kw th reformer designed by CSIRO, Australia [8] Reformer DIAPRRef Ongoing research at WIS, Israel: Integrated concept for a 10 kw th reformer (DIAPRRef) as shown in Figure 20. Figure 20: 10 kw th reformer (DIAPRRef) at WIS, Israel [8] 26

27 SOLREF Solar Reformer, State-of-the-art (SOLASYS) This reformer, a direct irradiated volumetric reactor receiver as shown in Figure 21, was realised in the EU-project SOLASYS (duration ) [8]. Figure 21: Direct irradiated volumetric reactor receiver [8] Results [8] In the gas absorbed power: 100 to 220 kw th (more power was not available) Reforming temperature: 700 to 765 C Operation pressure: 4 to 9 bar a Conversion of methane: max. 78% (close to theoretical equilibrium) Main objectives of project SOLREF [8] Develop an advanced 400 kw th solar reformer Investigate various catalyst systems Simulate mass and heat transport and reaction in porous absorber Perform thermodynamic and thermochemical analyses to support the system design phase Operate the reformer with gas mixtures which represent the variety of possible feedstock on the solar tower at WIS, Israel, producing partly solar hydrogen Evaluate new operation strategies Pre-design of a 1 MW th prototype plant in Southern Italy Conceptual layout of a commercial 50 MW th reforming plant Assess on potential markets including cost estimation and environmental, socio-economic, and institutional impacts Exercises Hydrogen production: Name at least 2 ways of realising steam reforming. 27

28 H 2 Production by Solar Cracking of Hydrocarbons The decarbonisation, i.e. separation of hydrogen and carbon can be realised by solar cracking of hydrocarbons (e.g. methane). The chemical equation for methane is CH4 C + 2H 2 (4) The solar cracking process takes places inside a special receiver at a temperature >1300 C and atmospheric pressure. The receiver needs to be a closed system and is therefore equipped with a high-temperature resistant quartz window. CH 4 is continuously added in the cracking process and is cracked at a conversion rate of 70%. The cost is indicated at 8 Euro-ct/kWh /Dahl et al/ (0 14 Euro-ct/kWh depending on the use of carbon). Since March 2006, a EU project, SOLHYCARB, is in progress [6]. Figure 22: Upper illustration: Adapted receiver for cracking of hydrocarbons (source: Hirsch et al), Lower illustration: Diagram showing cracking process [6] Exercises Hydrogen production: Explain the term decarbonisation. Hydrogen production: At what temperature does solar cracking of hydrocarbons take place? 28

29 Cost Overview and Comparison The following table gives an overview and comparison of the different possibilities of producing hydrogen (including wind and biomass). Table 4: Assessment of relevant H 2 pathways until 2020 [8] H 2 production cost Positive impact on security of energy supply Positive impact on GHG emission reduction Natural Gas SMR*** Natural gas Solar- SMR*** Grid Electricity electrolysis Wind electrolysis Biomass TC-Cycle 7-8* /GJ 12-14* /GJ 31 /GJ /GJ /GJ ** /GJ 18-21** /GJ /GJ modest modest high high high high high neutral modest high negative high high high modest neutral * assuming a natural gas (NG) price of 4 /GJ NG ; NG Solar-SMR: expected cost for large scale, solar-only ** assuming a natural gas price of 12 /GJ NG including sequestration cost *** SMR: Steam Methane Reforming 29

30 Perspectives for Solar Reforming in Sunny Regions In the medium term, a single site demonstration facility is desired, which shall already significantly reduce the GHG CO 2. Figure 23: Perspectives for solar reforming in sunny regions [8] 30

31 10.3 Cooling For countries with a hot climate, the cooling of buildings (e.g. offices, apartments), public transportation systems etc. is very important, just as it is important for countries with cold climate to assure adequate heating. Moreover, needed in either case, it is important to provide refrigeration for perishable food. Like heating, cooling also requires large amounts of energy which must be provided by means of using conventional or renewable energy sources and is therefore also expensive. Improving the housing standards, for example by insulation, so that less cooling (and heating) is required, will, in effect, be cheaper than providing additional cooling (or heating). However, as most buildings are not meeting these high energy standards, it is essential to analyse the possibilities for solar cooling. The electricity demand for a conventional refrigeration unit could, for example, be covered with a photovoltaic system. Solar thermal cooling can be done with absorption and adsorption chillers, open adsorption (DECtechnology) and liquid sorption [10]. All these solar technologies require only low or medium temperature heat. Usually the heat is provided by a flat plate solar collector system, but it could also be provided by CSP plants (e.g. parabolic trough, Fresnel or solar tower technology). The primary function of a CSP plant will be electricity production and cooling could be implemented as a secondary application. As no solar cooling system using heat provided by a CSP plant exists to date, only the ordinary (solar) cooling technology is explained. The theory behind absorption and adsorption chillers shall now be discussed in more detail Solar Absorption Chillers Absorption cooling technologies are well established on the market. They are usually employed in cases, where heat is easily and cheaply available (waste heat). A simple solar-operated absorption air conditioner consists of a solar collector with storage and an absorption cycle air conditioner [11, p. 582]. Unlike with electric chillers (i.e. food preserving refrigerators) absorption chillers use heat and not electro-mechanical energy for compressing the refrigerant vapours to a high-pressure. A thermal compressor consists of an absorber (A), a generator (G), a pump and a throttling device, which are the components needed to replace the mechanical vapour compressor [12], as shown in Figure 24. The entire absorption air conditioner consists of the components: condenser (C), evaporator (E), heat exchanger (HX) to recover sensible heat, cooling tower (CT), an auxiliary energy source (AX), and the components of the thermal compressor [11, p. 582]. 31

32 Figure 24: Schematic diagram of a solar-operated absorption air conditioner, edited from [11, p. 582] The processes taking place in the cycle are shown in Figure 25 in more detail. Here, the heat is provided by a flat plate solar collector. Figure 25: Schematic diagram of an absorption cooling system, edited from [13] 32

33 Principle of Absorption Cooling / Refrigerating The simple absorption cycle is realised with a mixture of two fluids, where one fluid acts as the absorbate or refrigerant and the other as the absorbent. The two most common fluid mixtures on the market are water (H 2 O) lithium bromide (LiBr) and ammonia (NH 3 ) water (H 2 O). Note that the fluid written to the left of the dash is the refrigerant (i.e. cooling medium) and to the right of the dash the absorbent [10], [12]. Water-lithium bromide (H 2 O-LiBr) and ammonia-water (NH 3 - H 2 O) mixtures find application in closed cycles, for example, to produce chilled air [10]. When considering a CSP plant for providing heat for a chiller, then it will most likely be an ammonia chiller. This is due to the simple reasons that the evaporation temperature of ammonia can be as low as -60 C and industrial process would then be possible. Using a chiller with water as the cooling medium has the disadvantage that the evaporation temperature needs to be kept to temperatures no less than 4-5 C to prevent freezing. The heat provided for the desorption process must have a temperature of 90 C to 140 C, depending on the chiller technology [10]. This temperature range can be handled by a flat plate solar collector or CSP plant. Keeping in mind that the boiling point of a liquid substance increases with the pressure, the absorption cycle is explained with references made to Figure 25. In the beginning of the absorption cycle these fluids are separated from one another and recombined again at the end of the cycle. In detail, the absorption cycle works as follows [12]: 1. The low-pressure refrigerant vapour, which is evaporated in the evaporator, passes through tubes to the absorber, where it is condensed and absorbed by the absorbent at low pressure. Thereby it releases a large amount of heat (the heat must be removed by a cooling system). 2. In a next step the liquid diluted refrigerant-absorbent solution is pumped to a generator which operates at a high pressure. The pumping requires far less amounts of electricity than the mechanically compression work done by electric chillers. Heat is added to the highpressure generator e.g. from the solar panel, which has the effect that the refrigerant desorbs 8 from the absorbent and vaporises. The remaining concentrated liquid solution left in the generator is passed through a pressure-reducing valve into the absorber for re-use, where it is recombined with the low-pressure refrigerant vapour, which is fed from the evaporator to the absorber. On the way to the absorber the higher temperature liquid solution passes through a heat exchanger, where it passes its heat to the counter flowing liquid refrigerant-absorbent solution, which is pumped to the generator. 3. The vaporised refrigerant flows from the generator to the condenser, which has the function to reject the heat and condense the vaporised refrigerant to high-pressure liquid. 4. The cycle is completed by throttling the liquid with an expansion valve, which has the function to lower the liquids pressure to that in the evaporator. With the liquid at low pressure, its boiling point is reduced. The liquid is evaporated by absorbing heat and provides useful cooling. 8 Desorbing is defined as the process of removing a sorbed substance by means of reversing adsorption or absorption. 33

34 Such a typical (ideal) absorption cycle is shown in a pressure-temperature-concentration diagram in Figure 27. The diagram shows an example cycle for a LiBr-H 2 O solution. The given values are for demonstration of the process only. As water is used as cooling medium here, the refrigeration application is restricted to temperatures of no less than 4-5 C. With reference to the schematic of Figure 25, it is evident that the condenser and generator are connected by a tube. Due to the solar heating, the pressure in the generator is slightly higher than in the condenser. However, the pressure in both the condenser and generator is dominated by or dependent on the condenser fluid coolant temperature. Similarly, the pressure in the evaporator and absorber is dominated by or dependent on the temperature of the cooling fluid that cools the absorber. In the pressure-temperature-concentration diagram the cycle is shown in bold lines and contains the components absorber, heat exchanger and generator, as shown in Figure 26. Figure 26: Cut-out of schematic of Figure 24 showing the thermal compressor and heat exchanger, edited from [11, p. 582] The letter A corresponds to the absorber, G to the generator and HX to the heat exchanger in both diagrams of Figure 26 and Figure

35 Figure 27: Pressure-temperature-concentration diagram for the cooling substance LiBr-H 2 O showing an idealised cooling cycle, edited from [11, p. 583] In the generator the concentration of LiBr is increased as the water is evaporated. In this example the concentration increases from 55 to 60% while the equilibrium temperature of the solution is raised by 10 C from 72 to 82 C at the dominating condenser pressure. In the absorber, the solution concentration is decreased from 60 to 55% as the LiBr-salt-solution is diluted with the water vapour and the solution temperature is reduced from 48 to 38 C at the dominating evaporator pressure [11, p. 583]. As described before, the higher temperature liquid solution passes through a heat exchanger while it flows to the absorber. There the solution passes its heat to the liquid refrigerant-absorbent (H 2 O-LiBr) solution, which is pumped to the generator. When setting up the steady-state energy balance for the absorption cooler it becomes apparent that the energy input to the evaporator and generator must equal the energy that is removed from the absorption cooler by the coolant which flows through the absorber and condenser, and also by losses to the ambience. The equation therefore becomes: Q + Q = Q + Q + Q (5) G E A In order to determine the performance of the absorption chiller and to have an index for the calculation of collector costs, the so-called thermal coefficient of performance, COP, was defined as the ratio of the energy input in the evaporator Q E to the generator Q G [11, p. 584]: QE COP = (6) Q G The COP increases with increasing temperature of the heat available. Therefore it makes sense to use CSP as a heat source. C losses 35

36 Exercises Cooling: Explain the absorption cycle of an absorption chiller. Cooling: How many fluids are required for operating an absorption chiller? Cooling: Name 1 refrigerant (also called absorbate) and 1 absorbent that can be used in an absorption chiller. Cooling: The absorption cycle of an absorption chiller takes place in 4 chambers. Name at least 3 chambers. What is the COP of an absorption chiller useful for? Give also the formula Adsorption chillers For adsorption refrigeration cycles, adsorption is the adhesion or accumulation of a vaporised refrigerant (e.g. water or methanol) on the surface of a solid sorbent material (i.e. the adsorbent). An adsorption chiller uses a solid sorbent material such as silica gel or zeolith to adsorb the refrigerant (in this case water) on its surface. Another example for an adsorbent-refrigerant pair is active carbon-methanol. A conventional adsorption chiller consists of two reactors, an evaporator and a condenser, as shown in Figure 28 below. Each reactor is made up of a solid sorbent material bed and a heat exchanger. For better understanding of the working principle of a conventional adsorption refrigeration cycle it should be kept in mind that in the cycle, reactor 1 and 2 switch their function. This means that if reactor 1 is in adsorption mode, then reactor 2 is in desorption mode and in the next turn reactor 1 is in desorption mode and reactor 2 is in adsorption mode, and so on. Figure 28: Adsorption chiller, edited from [14] 36