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					5th European Thermal-Sciences Conference, The Netherlands, 2008

APPLICATIONS OF SOLAR HEAT FOR TEMPERATURES RANGING FROM 50 - 2000°C
H. Müller-Steinhagen
Institute for Technical Thermodynamics, German Aerospace Centre (DLR), Stuttgart Institute for Thermodynamics and Thermal Engineering, University of Stuttgart, Germany

Abstract
The thermal utilisation of solar radiation can provide a major contribution to a sustainable energy provision. This ranges from low temperature applications for the building sector through the provision of process heat for industry up to high temperature heat for electricity generation and fuel processing. The stage of development and the market penetration of the various technologies vary widely. In all applications, significant progress has been made over the past decade. In the present review paper, the state of technology is reviewed for different applications of solar heat. Special emphasis is placed on the two main components “collector” and “heat storage”, even though other components are considered as well. Areas for further research and development are identified to reach the required performance level and cost reduction targets within the next 15 years.

1 Introduction
The limited availability of fossil and nuclear energy carriers and the environmental impact associated with the wide-spread application of these fuels are the reasons why the present pattern of energy consumption is not sustainable. Renewable sources of energy will have to be introduced into our heat, fuel and electricity market to a much larger extent and at an accelerated rate. Solar radiation is the most abundant source of renewable energy available to mankind. Its technical potential exceeds the present world energy consumption by a factor of about 3000 /1/. Solar radiation may be used to generate electricity directly via photovoltaic systems, but with a price tag which is still substantially higher than for electricity from conventional fossil and nuclear power plants. Significantly less expensive is the thermal use of solar radiation for heating of water and buildings, for provision of process heat for industrial operations and even as energy input for solar thermal power plants. It is, therefore, not surprising that this form of energy has already been used widely, as indicated in Fig. 1 which shows the globally installed capacity for energy provision from some of the main renewable sources in 2005 /2/. About 50-60% of the European end energy consumption is required for the supply of heat for buildings and industrial processes /3/. Solar thermal energy contributes only about 0,1% to this sector, the rest being predominantly supplied by the burning of fossil fuels. It is obvious, that a significant contribution to climate and resource protection can be made if this sector is expanded further. The major application is the use of solar heat for domestic and industrial water and space heating. However, there are further applications above this relatively low temperature level. The industrial heat demand in Europe for processes such as food, chemicals, petrochemicals, minerals, etc. has been estimated to 300 TWh/year for the temperature range up to 250°C, i.e. to about 8% of the total European end-energy consumption. This huge potential has, so far, hardly been considered at all for solar applications, even though this temperature range can be realised with advanced solar thermal collector technologies.

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Figure 1: Energy from different renewable energy technologies /1/ The production of electricity from fossil fuels contributes significantly to resource depletion and CO2 emissions. Nevertheless, most energy scenarios predict that the use of this highly versatile form of energy will increase considerably in the coming decades. This mismatch may be resolved by solar thermal power plants, where the heat input into conventional steam and/or gas turbine cycles is produced by focussing the solar radiation onto a suitable heat transfer medium. In conjunction with innovative thermal storage technologies, solar thermal power plants in the 100-1000 MW scale may be able to supply dispatchable electricity at competitive cost, in about 10-15 years. These power plants will be installed at sun-rich locations and linked to distant load centres by high voltage, direct current electricity lines. In addition to heat and electricity supply, mobility will be one of the main topics related to a future energy provision. The extensive use of hydrogen as a fuel can only be sustainable if the hydrogen is produced from renewable energy sources. Solar thermal electricity for electrolysis of water may be one option, but the overall process has a low efficiency (similar to photovoltaics). Investigations are underway to split water molecules thermo-chemically at temperatures between 800-1200°C, to produce H2 with greatly reduced energy input. As transition technologies, solar thermal upgrading or reforming of fossil fuels may be an option. Depending on the required temperature level, solar heat may be produced in relatively straightforward absorbers, or in systems which concentrate the solar radiation up to 5000 times before it is absorbed. Design and materials of the absorber are, therefore, essential for efficient conversion of solar radiation into useful forms of energy. In addition to its potential to produce heat at moderate cost, solar thermal technology has the further advantage of heat storage, which can be used to buffer times of low radiation. New heat storage concepts for temperatures from sub-zero (ice storage) to 1000°C, and from short term to seasonal are presently becoming a topic of intense research and development activities. The following paper is subdivided into the four main applications of solar thermal technology, which also correspond to four distinct temperature ranges. Obviously, domestic applications are the most advanced and therefore merit most information. Solar thermal power plant technology is presently booming and will become competitive in the near future. The remaining two technologies may take somewhat longer to a successful market introduction, even though their potential may be very substantial. It must be emphasized that solar technology is not the single one solution for a sustainable energy future. This can only be achieved by an intelligent mix of renewable sources of energy combined with highly efficient conventional power plants and a significantly more rational use of energy.

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2 Solar energy for buildings
2.1 Market development
More than 90% of solar thermal energy presented in Fig. 1 is for the supply of hot water in buildings. Solar thermal hot water systems for household have been sold commercially in the US since the 1870th. Because of the relatively low costs of fossil fuels, their market penetration has only been slow. However, due to environmental concerns, steeply rising fuel costs and improved technology, a dynamic growth phase has started since the beginning of the 21st century. Today, a thermal capacity of 115 GWth has been installed world-wide with about 165 million m2 of collector area. The two main technologies are flat plate and vacuum tube collectors, as shown in Figs. 4 and 5. Unglazed, black polymer absorber tubing is widely used to heat the water of swimming pools. By the end of the year 2007, solar thermal collectors with a capacity of 15350 MWth (21.9 mio m2) had been in operation in the 27 member states of the European Union, out of which 1896 MWth (2.7 mio m2) have been installed in 2007 alone /4/. The detailed capacity of the solar thermal systems in the various member states is shown in Fig. 2. The annual heating energy provided by these systems equals to about 1 billion litres of oil or m3 of natural gas.

Figure 2: Installed solar thermal power in the EU member states /4/ To reach the targets given in the EU White Paper for a Community Strategy and Action Plan, this should increase to 100 million m2 by 2010. The European Solar Thermal Technology Platform (ESTTP) in cooperation with the European Solar Thermal Technology Federation (ESTIF) have produced the cost and growth development scenario given in Fig. 3. While the European market may seem impressive, it pales in comparison with China (ca. 50000 MWth installed by 2006), which boosts about 75% of the world-wide solar thermal capacity. The distribution of solar thermal systems varies significantly. In China, Europe and Japan, mainly flat plate and vacuum tube collectors are installed for heating of water and buildings, while in the US mainly collectors without glass cover are used for heating of swimming pools. Flat plate collectors are the most common in Europe. Evacuated tube collectors can produce higher temperatures and are more efficient, but they are also about 30-50% more expensive. Nevertheless, they comprise 90% of all installations in China. With respect to technology, the European solar thermal systems are the most advanced. In the meanwhile, more than 20% of the solar thermal systems installed in Austria, Germany and Switzerland are used for space heating as well as for water heating. In Germany, every second new installation is such a so-called combi-system.

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Figure 3: Development of solar thermal heat capacity /5 /

2.2 Solar thermal collectors for domestic applications
The solar collector is the key components of a solar thermal system. Its main task is to convert the direct and diffuse solar radiation into heat that can be used for domestic hot water production and / or space heating. The principle set-up of a flat plate collector is shown in Figure 4, indicating the various heat transfer processes which must be addressed in order to optimise the performance.

Figure 4: Principle set-up of a flat plate collector One of the most advanced parts of a high-technology solar collector is the selective coating of the absorber. The task of this coating is to absorb nearly 100% of the short wave radiation originating from the sun in order to convert this energy into heat and to emit only few percent of the long wave radiation from the hot absorber surface. In former times, black paint or black chromium oxide were used as materials for selective coatings. During the past 10 years, new selective coating processes based on PVD (physical vapour deposition) or sputter processes have been developed. The advantage of theses coatings is their improved performance and the fact that they are more environmentally compatible. For further improvement of the performance of flat plate collectors, an anti-reflection coating was developed during the past years. Today the cover glass of top level flat plate collectors and vacuum tube collectors is processed in such a way that the reflection of the sunlight is reduced. In addition to flat plate collectors, vacuum tube collectors are frequently used, because they have reduced thermal losses and can hence reach higher outlet temperatures. Three different concepts for vacuum tube collectors are presently on the market. Figure 5 shows two principles, both based on glass tubes with an internal vacuum. The absorber is located inside the evacuated glass tube. For the

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direct principle shown in the left Figure, the heat transfer fluid of the collector loop flows directly through the absorber via a co-axial tube. In the centre of Figure 5, the heat of the absorber is transferred to the heat transfer fluid of the collector loop via a heat pipe system. Water at low pressure is usually used as heat transport fluid inside the heat pipe.

Figure 5: Vacuum tube collectors (left: direct principle, centre: heat-pipe principle) In addition to the two principles shown in Figure 5, so-called Sydney-collectors are also used. For this type of collector, the vacuum exists in the annular gap between an outer and an inner glass tube. The inner glass tube is equipped with a selective coating and, therefore, also acts as the absorber. A metal sheet transfers the heat from the absorber to a U-tube, through which the fluid of the collector loop is flowing. In order to reduce the required number of Sydney-tubes for a given thermal capacity, CPC reflectors (CPC: compound parabolic concentrator) are located behind the tubes. The thermal performance of solar collectors is described by the collector efficiency curve. This * curve shows the collector efficiency versus the reduced temperature difference Tm which is calculated by dividing the temperature difference between the mean collector temperature (ϑfl,m) and the ambient temperature (ϑamb) by the total solar radiation (Gglob,C) based on the aperture area of the collector. In Figure 6 the collector efficiency curves for different collector types are plotted. Furthermore, typical fields of application are shown in this figure. It can be seen that the choice of the appropriate collector depends on the temperature level at which the collector is operated. For low temperature applications, such as the heating of swimming pools, simple absorbers (uncovered flat plate collectors) are the best choice. For high temperature applications (e. g. solar process heat) vacuum tube collectors are required. For the operation at moderate temperatures in solar domestic hot water systems or solar combi-systems both, flat plate and vacuum tube collectors can be used.

Figure 6: Collector efficiency curves for different collector types

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2.3 Solar domestic hot water systems
To-date most solar thermal systems are used for domestic hot water production in single or small multi-family houses. Depending on product quality, layout and solar radiation conditions, theses systems can supply 60-90% of the annual hot water demand. Typical systems in Northern Europe, e.g. Germany, have a hot water storage volume of approximately 300 litres and a collector area between 3 to 6 m2, depending of the type of collector. In warmer climates, e.g. in Southern European countries or in Turkey, thermosyphon systems are used where the hot water store is installed on the roof together with, or even as an integral part of the solar collector, see Figure 7.

Figure 7: Typical thermosyphon collector Such a system is less expensive, but not suitable for colder climates because of the inevitable heat losses from the external hot water store and the danger of freezing during winter time. Figure 8 shows the schematic set-up of a typical solar domestic hot water system for a single-family house in colder climates. The collectors on the roof heat a transfer medium, which usually is a mixture of water and glycol (anti-freeze). The electronic control unit monitors the temperature difference between the collector and the lower part of the hot water store. When these temperatures are appropriate, a pump is activated to circulate the heat transfer fluid through the collector loop. Through a heat exchanger, the solar heat is transfered into the water in the hot water store. For auxiliary heating, a second heat exchanger, which is connected to the oil- or gas boiler, is located in the upper part of the hot water store.

Figure 8: Schematic set-up of a solar domestic hot water system

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The costs for a solar water heating systems are strongly depending on the technological sophistication required by the ambient climate conditions and on the local cost level for salaries and materials. However, there is a significant potential for cost reduction due to mass production and appropriate design modifications.

2.4 Solar combi-systems
Solar combi-systems are systems that contribute to both, domestic hot water preparation and space heating. Present solar combi-systems typically consist of a solar collector field with an area from 10 m2 to 20 m2 and a hot water storage tank with a volume in the range of 0.7 – 1.5 m3. If such systems are installed in a „typical“ middle European single-family house, they can save 20 - 30 % of the primary energy required for domestic hot water preparation and space heating. There are various designs of solar combi-systems available. Figure 9 shows, as an example, a very common design with a solar combi-store as the key component. Compared to a typical solar hot water system, the main difference is that a combi-store uses the circulation water of the space heating system as storage medium. The combi-store is designed to store the heat delivered by the solar radiation and by a conventional fossil fuel fired boiler. The potable hot water is obtained from the combi-store via an internal or external heat exchanger.

Figure 9: Typical solar combi-system New developments in the field of solar combi-systems are systems where the burner is directly integrated into the combi-store. Due to a higher level of pre-fabrication, installation costs are reduced and failures during the installation process can be avoided. Over the last years, a trend towards larger solar combi-systems could be observed. These systems are designed in such a way that up to half of the energy required for domestic hot water preparation and space heating is provided by solar radiation. Typical collector areas for these systems are between 20 - 40 m2, and the volume of the hot water store is in the range of 2 - 4 m3 /6/. In many cases these systems are combined with biomass burners and, therefore, provide completely CO2 neutral heat to the house. To reach higher solar shares in the total heating energy, even larger collector areas and, more importantly, thermal storage capacities must be available, as shown in Figure 10 for a typical single family house in Germany /6/. It is obvious that large storage volumes are required which, in existing buildings, may not be realised even with large, locally assembled heat stores made from glass fibre enforced polymers /7/.

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Figure 10: Influence of collector area and hot water storage volume on solar share The commercially available thermal storage technology is, therefore, not efficient enough to facilitate a sufficient reduction of the CO2 footprint of European houses. Improved heat storage concepts with high capacity, efficiency and transfer rate are required at moderate cost. ESTTP aims at an increase in storage density by a factor of 8 in comparison to water by the year 2030 /5/. For this purpose, heat stores which use the latent heat of fusion or evaporation (with so-called Phase Change Materials) /8,9/, or the heat of sorption /10/ are presently investigated and demonstrated in pilot installations. Figure 11 shows the specific thermal storage capacity of various materials which can be used for the technically more suitable phase change between solid and liquid phase.

Figure 11: Specific thermal storage capacity of various phase change materials and of water, as a function of temperature or temperature difference (based on 20 °C) If processes based on sensible and latent heat storage are used, some losses through the containment walls to the ambient can not be avoided. Contrariwise, sorptive and thermo-chemical processes allow thermal storage for an almost unlimited period of time, since heat supply or removal occurs only if the two physical or chemical reaction partners are brought into contact. Of particular interest is in this context the adsorption of water vapour in a porous medium. Figure 12 shows the general process of charging and discharging.

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Figure 12: Operation of a sorption heat store Commercially available sorbents, such as zeolites or silica-gel are able to adsorb large quantities of water while releasing up to 4200 kJ/kgH2O of heat. Even though the physical mechanisms of adsorption are well understood, no technically and economically promising heat storage system has been realised, so far. Several different concepts have been suggested and tested in the laboratory; usually water vapour is ad- and desorbed in a closed system. An alternative system has been suggested by Kerskes, where the humidity of the air in the heated room is directly used for discharging of the heat store /11/. At present, both latent and sorptive heat storage technology is still under development in research laboratories and industry. Several companies offe latent heat stores for selected applications (e.g. cold storage in automotive applications, ice storage for air conditioning or micro-encapsulated paraffins to increase the thermal mass of buildings). No sorptive heat stores are commercially available to-date. To obtain even higher storage densities and almost zero heat losses, reversible chemical reactions may be used. However, system development for such technologies is still at its infancy, with some laboratory scale investigations being underway.

2. 5 Large solar heating systems
Besides the application of solar heat in single- and small multi-family houses, the use of solar thermal energy for domestic hot water preparation and space heating in large public/office buildings or residential buildings/communities are promising applications. According to /2/, there are presently 87 large solar thermal systems in Europe with a total capacity of 120 MWth. The most common large domestic solar systems, such as the one shown in Fig. 13, are for water heating and have collector areas in the range of 100 to 300 m2.

Figure 13: Large solar thermal collector system in Friedrichshafen/Germany

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The advantage of such systems is the reduced cost in comparison with the installation of numerous individual, smaller solar systems. The disadvantage is the more complicated system integration and control. Because of the high costs of land in central Europe and due to individual preferences, these large collectors are usually installed on the roof of buildings. Three options are available: • on-site installation with the system assembled directly on the roof • large collector modules industrially prefabricated with an area of 8–12 m² • solar roofs that make up complete roof modules. There are also systems with collector area from 2.000 - 10.000 m2 and seasonal heat stores with a water-equivalent storage volume of 3.000 to 10.000 m3, which provide up to 50% of the heating and hot water demand of housing developments and large building complexes. In this case, the large heat storage facilities are charged with solar heat during sunshine hours, and discharged in late autumn and winter. Figure 14 shows the schematic set-up of a solar assisted district heating system with a seasonal heat store.

Figure 14: Solar assisted district heating system with a seasonal heat storage Depending on ground and climate conditions, four concepts have been realised and investigated, namely hot water heat storage, gravel/water heat storage, duct heat storage and aquifer heat storage /12/. Also shown in Fig. 14 is the hot water heat storage in Friedrichshafen (Germany) with a volume of 12,000 m3. To reach thermal energy costs in the range of 4-7 cts/kWh, improved storage concepts/materials, large collector systems from new materials and effective control strategies need to be developed and demonstrated. An important finding in most pilot installations to-date was that heat losses significantly exceeded the previously estimated values.

2.6 Solar cooling systems
During the summer months, cooling of public and private buildings, office buildings, hospitals and hotels becomes more and more important. At the same time, the tendency to increase the solar share of typical solar combi-systems for domestic water and space heating by increasing the collector area results in longer stagnation periods during the summer. Hence, a large share of the solar thermal energy available during the summer time is not used. By efficient use of the available solar energy over the whole year, the solar gains and the economic efficiency of solar thermal systems can be significantly enhanced. The increased efficiency of flat plate and vacuum tube collectors makes it possible to add a thermally driven, ad- or absorption cooling system to an existing solar combisystem, resulting in a so called “solar plus combi-system” /13/. This benefits greatly from the fact that supply (of sunshine) and demand (of cooling) coincide widely.

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Solar thermal air-conditioning and cooling systems are still a relatively new technology, and at present less than 100 systems are in operation in Europe. Hence, standardised system concepts and proven design guidelines are not available up to now. Most installations are in excess of 50 kW, even though smaller systems for individual houses or offices are under development /14/.

2.7 Future R&D requirements
Even though solar collectors and systems for domestic applications are a mature and reliable product, there are still substantial R&D requirements to reduce the heat costs to well below those that may be obtained with fossil fuels. In a recent strategy paper co-authored by German manufacturers and research institutes on behalf of the German Federal Ministry for the Environment /15/, the following topics have been particularly emphasized: innovative storage and collector concepts efficient and cost effective materials adaptive control simulation software for integration into buildings and town planning

Due to the high metal prices, polymers are increasingly investigated as potential replacements for collector and heat store components. This provides the additional benefit of reduced weight. The selective absorber surfaces produced with sputter or plasma techniques already achieve high collector efficiencies. Additional improvements are due to self-cleaning, anti-reflective nanocoatings of the glass cover and so-called CPC reflectors. However, high collector efficiencies lead to high surface temperatures of up to 250°C during stagnation (i.e. without circulation of heat transfer medium). This must be taken into consideration for future developments and materials selection. At present, aging effects due to stagnation temperatures have been found to be insignificant /16/.

3 Industrial process heat
The provision of solar process heat between 100°C and 250°C with suitable collector and heat storage technologies could provide a significant contribution to the reduction of fossil fuel consumption and the associated emission of greenhouse gases. However, the application of solar systems for process heat to-date has been limited to a moderate number of pilot installations for water heating (e.g. Fig. 15a), for application in the food and beverage industry and for combinations of power/heat/air conditioning of hospitals and hotels (e.g. Fig. 15b).

Figure 15: a) Car wash facility in Austria /23/, b) SOLITEM collector field for a hotel in Turkey

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For the lower end of the temperature range, high performance flat plate or vacuum tube collectors may be used. Higher temperatures require concentrating solar systems, i.e. smaller versions of the parabolic trough or heliostat systems, which will be described in the section on solar thermal power plants. More recently, several companies have developed Fresnel concentrators (see Fig. 16) with flat mirrors which may be used instead of the parabolic trough concentrators – with lower efficiency but also at a potentially lower price.

Figure 16: Fresnel collectors for temperatures up to 300°C To improve the application of solar process heat, significant further R&D activities are required: new heat transfer fluids and construction materials suitable for higher temperatures adaption of proven technologies for operation at elevated temperatures further development of concentrating collector design direct steam generation in collectors standardised systems and testing proceedures automatic operation of solar supported processes coordinated process control of solar thermal and process systems development of thermal storage concepts for elevated temperatures to buffer the intermittent supply of heat

For storage of industrial waste heat and for solar heat significantly above 100°C, the conventional hot water heat stores can not be used anymore. Suitable concepts for this temperature range include solid sensible heat stores, liquid salts, phase change materials, steam storage and reversible chemical reactions. Selection and application of these storage types ultimately depends on the specific industrial process and must be analysed and designed in each case. A vast amount of energy in the temperature range of 100-300°C is needed to generate process steam at low or intermediate pressure for applications in food processing, production of cardboard and paper, in the textile industry, manufacturing of construction materials, rubber and other commodities. For such applications, sensible heat storage is unpractical and inefficient, since the majority of energy transfer during evaporation occurs at almost constant temperature. Improved phase change materials or steam storage systems could lead to more economic thermal energy storage solutions under these circumstances. The selection of the PCMs depends strongly on the operation conditions of the respective application. At present, the main emphasis is directed to alkali metal nitrates and nitrites and their mixtures. For example, the eutectic mixture of the binary system KNO3-NaNO3 has been identified as an excellent system for processes using saturated steam at around 25 bar. While this approach has already frequently been suggested, only limited actual experience is nevertheless available in this temperature range. Most problems result from the low thermal conductivity of salts, particularly in the solid phase, but cycling stability, handling and costs are playing an important role, as well. To overcome the low thermal conductivity of the salt systems, increased surfaces area and increased thermal conductivity using expanded graphite (EG) have been

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investigated. Using EG/PCM-composites the effective thermal conductivity can be increased from below 0.5 W/(mK) to 3-20 W/(mK). Several concepts for the realisation of such composite heat stores have been investigated, including infiltration into extruded graphite blocks (Fig. 17a) and graphite fins in a salt bath (Fig. 17b)) /17/. Pilot measurements in the 100 kW range are presently performed on the Plataforma Solar in Almeria/Spain.

Figure 17: a) extruded graphite/salt composite b) PCM salt heat store with graphite fins

4 Solar thermal power plants
Solar thermal power plants concentrate the direct component of solar radiation to provide the heat input for conventional steam or gas turbine power plant cycles. They are recognised as suitable technology for bulk electricity generation in the 10-1000 MW range /18/. The energy pay-back time of solar thermal power plants is around 0.5 years; their life-cycle CO2 emissions are similar to those of wind, hydro and nuclear power, i.e. substantially below those of fossil fuel fired power plants Nine plants with a total capacity of 354 MW peak electricity have been built in the Californian Mojave desert during the 1980s, which since then have been feeding more that 15 TWh of electricity into the Californian grid at a cost of less than 12 €-ct/kWh. For example, Figure 18 shows the 5 x 30 MW parabolic trough plant at Kramer Junction.

Figure 18: 5 x 30 MW parabolic trough plant in Kramer Junction Due to the relative drop in energy prices, no additional commercial solar thermal power plants have been built since 1991, even though research and development activities have continued in organisations such as DLR, CIEMAT, SANDIA and Weizmann Institute. The recent dramatic increase in fossil fuel prices as well as the now widely accepted negative impact of combustion emissions on the environment have initiated renewed interest in this technology. Triggered by market introduction support programmes, new solar thermal power plants with a total capacity exceeding 10 GW are presently under construction or detailed engineering design in several countries, see Fig. 19. If the present progress in R&D and industrial realisation continues over the next 10-15 years it should be possible, to reach the cost target of 5-7 Euro cents per kWh /19/.

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Figure 19: Project development for solar thermal power plants Solar thermal power plants require direct solar radiation in excess of about 1800 kWh/m2/year and are hence only suitable for locations in the sunbelt of this planet. Nevertheless, the generated electricity may be transferred to other locations via high voltage, direct current power networks with only minor losses. Additional costs for transmission from North Africa to Northern Europe are below 2 €-ct/kWh /20/. Less than 1% of the Sahara desert area would be required to supply the 2006 world electricity consumption with solar thermal power plants.

4.1 New power plants under construction
In February 2006, Spanish company Acciona started the construction of 65 MWel parabolic trough plant Nevada Solar One with an investment cost of about 250 million US$. This power plant operates in solar-only mode without heat storage or fossil fuel back-up. Due to the excellent solar radiation conditions in Nevada and the limitation to operation during solar hours, electricity generation costs of 14 US-ct per kWh can be achieved. Nevada Solar One went on-line in July 2007. Near Seville/Spain, Abengoa Solar has been building a 10 MWel solar tower power plant with the support of the European Union and in cooperation with German engineering company Fichtner and DLR. This plant, shown in Figure 20, has gone into operation in April 2007. A second solar tower plant with a capacity of 20 MWel is approaching completion at the same site.

Figure 20: Solar thermal power tower plants PS10 (right) and PS20 (left) near Seville/Spain

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In the PS10 plant, 700 mirrors, each with an area of 140 m2, focus on a receiver at the top of an 80 m tower. Saturated steam with a moderate temperature of 260 °C is generated; heat storage will allow continuing electricity production for up to 1 hour under cloudy conditions. Average annual efficiencies (defined as the ratio of generated electricity per year divided by the direct radiation on the mirrors) are above 16%. With a total investment of around 36 Mio. €, this power plant is not yet economic, even under the conditions of the Spanish energy feed-in law which provides about 25 €cts/kWh for electricity from solar thermal power plants. However, solar tower technology has the long-term potential to produce electricity at lower costs than trough plants, due to the higher efficiencies which may be achieved at receiver temperatures up to 800 °C /18/. On July 20th of 2006, Spanish companies ACS/Cobra and German Solar Millennium Group started construction near Almería/Spain of the 50 MWel parabolic trough plant ANDASOL 1, which will be followed by identical plants ANDASOL 2 & 3 in the next couple of years. The layout of these plants is depicted in Figure 21.

Figure 21: ANDASOL 1 plant layout. Its collector area of over 510,000 m2 will make Andasol 1 the world’s largest solar power plant. It will generate approximately 179 GWh of electricity per year to supply some 200,000 people with solar electricity after a construction time of barely two years. The parabolic trough collector field consists of 596 solar collector assemblies each 150 m long, with an aperture of 5.70 m and tracked by a single hydraulic drive. Curved float glass mirrors from Flabeg GmbH are mounted on a metal space frame structure to approximate the ideal parabola. The absorber tube developed and manufactured by Schott AG is surrounded by an evacuated glass tube and coated with a special selective optical surface to minimize thermal losses. Thermal oil is heated to temperatures up to 400°C, and generates steam at 375°C and 100 bar for driving a conventional Siemens steam turbine. The collector field is designed to provide, under good solar conditions, more energy than the turbine can accept. This surplus energy is used to charge a heat storage, which can provide the required energy input to the turbine system during periods of insufficient solar radiation. The storage consist of two large tanks of 14 m height and 36 m diameter, each containing 25,000 tons of storage medium (nitrate heat exchanger salt mixture) which is equivalent to 6 full-load hours of operation capacity. Yearly electricity production is thus extended to close to 4000 hours. Heat is transferred from or to the thermal oil in an oil-to-salt heat exchanger. The salt is pumped through this heat exchanger from the cold tank to the hot tank during charging and vice versa during discharging

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periods. Electricity generation costs with the ANDASOL plants are anticipated at around 21 €-ct per kWh, but this will be dispatchable, high value electricity due to the buffering and extending effects of the heat stores. Another parabolic trough solar power plant is presently constructed by Abengoa Solar in Algeria to supply 15 MW to an existing combined cycle power plant. Similar plants have been commissioned in Morokko and Egypt. Iran has contracted Fichtner Solar and DLR to do the detailed engineering of a 10 MW parabolic trough plant as an extension to an existing conventional power plant in Yazd. There are also construction activities in Australia. In a first stage of development, collectors with Fresnel arrangement of flat mirrors developed by the Solar Heat and Power Company will supplement 1 MW of thermal energy for feed water preheating in a 650 MW coal-fired power plant. Provided that commissioning in this year will be successful, an extension to the equivalent of 38 MW of electrical power has already been ordered. Due to the large demand, the Spanish government extended its subsidy programme from 200 MWel to 500 MWel solar thermal electricity, which are expected to go on-line before 2010. Spanish contractor Abengoa has already announced further plants with a total power of at least 100 MWel. The electricity supplier Iberdrola wants to develop seven power plants at various locations with a total capacity of 350 MWel. The German Solar Millennium group is also planning seven 50 MWel parabolic trough power plants in Spain. Additional Spanish projects accounting for more that 1 GW are in an early development phase. In the presence of German Chancellor Angela Merkel, Solar Millennium Group also signed a memorandum of understanding with the Chinese government for construction of 1000 MWel of solar thermal power plants by 2020. In February 2008, Abengoa Solar has announced financial closure of a contract to construct a parabolic trough plant with 280 MWel in the US. The US company SES has recently announced the signature of a contract with a Californian energy provider for the installation of about 38,000 dish-stirling systems (see Fig. 22) with a total capacity of 850 MWel by 2010.

Figure 22: Dish-Stirling systems for generating electricity in the 10-30 kW range, each

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New feed-in tariffs supporting solar thermal electricity have recently been announced for Portugal, Italy and Greece and will generate further market opportunities. It must be assumed that the successful completion of the first projects will significantly accelerate future expansion of this technology, since technical and economic risks become much more quantifiable. Furthermore, investment companies world-wide have entered into the solar thermal power plant market so that the actual number of projects which are presently under development or consideration is almost impossible to follow.

4.2 Research and development activities
Mass production and design/construction experience alone will not be sufficient to reach the required cost reduction. Accompanying R&D activities required to reach the cost target of 5-7 Euro cents per kWh within the next 10-15 years are outlined in /19/. Some of the more innovative approaches are described in the following. 4.2.1 Direct solar steam generation

The present parabolic trough plant design uses a synthetic oil to transfer energy to the steam generator of the actual power plant cycle. Direct solar steam generation in the absorber tubes of parabolic trough collectors is a promising option for improving the economy of solar thermal power plants, since all oil-related components become redundant and steam temperature (and hence efficiency) can be increased. Steam temperatures up to 400 °C at 100 bar pressure have been reached within the framework of a European project for over 6000 operating hours at the Plataforma Solar de Almería. The test loop with 700 m length and an aperture of 5.70 m has been custom designed and constructed for the purpose of demonstrating safe operation and controllability under constant and transient operating conditions. The most reliable process control is achieved if the steam-water mixture is separated after about 500 m, with the water being recycled and the steam being passed on into the superheating section. For this, Siemens AG and DLR have developed a steam separator with a pressure drop below 1 bar and 95% separator efficiency, which has already achieved the cost target of less than 10,000 € per unit. An additional area of development are absorber tubes with selective surfaces which can operate at temperatures above 500°C for an extended period of time. Dynamic modelling of the operation of solar thermal power plants with direct steam generation is essential for optimised operation, due to transients occurring during daily start-up/shut-down as well as changing radiation conditions. Using the simulation tool Dymola/Modellica, DLR has been able to produce validated control algorithms for complete collector fields, which are refined in present projects /21/. Due to the positive experience with the test loop in Almería, a European consortium has started project development for a 5 MWel demonstration plant with direct steam generation. 4.2.2 Heat storage

One of the advantages of solar thermal power plants is that heat can be readily stored to provide buffer capacity for uninterrupted operation and to extend the production time. Present technology involves sensible heat storage in molten salts or oil. In cooperation with civil engineering company ZÜBLIN AG, DLR has developed a concrete sensible heat storage technology for temperatures up to 400 °C, which may reduce storage costs by up to 50%. A pilot unit is shown in Figure 23.

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Figure 23: Concrete heat storage pilot facility For direct steam generation, latent heat storage is considerably more attractive than sensible heat storage, since a large amount of heat can be stored at almost constant temperature. For the desired temperature range between 300°C and 400°C the heat of solidification of salt mixtures (so-called phase change materials, PCMs) may be used, see also Section 3 on industrial process heat. Another challenge will be the development of suitable heat storage for solar tower plants, where operating temperatures up to 1000°C will be reached. 4.2.3 Volumetric solar receiver Present solar tower plants use bundles of steel tubes on top of the central tower to absorb the concentrated solar heat coming from the heliostat field. However, the temperatures which can be achieved in such a system are limited by the thermal stability of the steel and the poor heat transfer inside the tubes. In order to reach higher temperatures, and hence achieve higher efficiencies, the concept of the volumetric receiver was developed. A wire mesh is directly exposed to the incident radiation and cooled by air flowing through that mesh. Such a receiver can easily achieve 800 °C. For even higher temperatures, the wire mesh screen is replaced by porous SiC or Al2O3 structures, as shown in Fig. 24 /23/. This set-up has been tested on the Plataforma Solar de Almería in Spain at 200 kWth scale, reaching temperatures in excess of 1000°C at receiver efficiencies close to 90%.

Figure 24: Ceramic absorber module (left) and 200 kWth HITREC test receiver (right)

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A 1.5 MWel demonstration plant for this technology with 20,000 m2 heliostat area and a 50 m tower is presently built near Jülich/Germany. The investment costs of about 13 million € are shared by several German federal/state ministries (MWME, STMWIVT, BMU). After a 5 year period of investigations by a consortium of company KAM, University of Applied Science Jülich and DLR, this facility is expected to feed electricity into the German grid for another 15 years. 4.2.3 Solar gas turbine

High efficiencies may be reached with solar-heated gas turbines, which may be increased further in combined cycle processes (Figure 25). These systems have the additional advantages that they can also be operated with natural gas during start-up and with a high fossil-to-electric efficiency when solar radiation is insufficient. Hence, no shadow capacities of fossil fuel plants are required and high capacity factors are provided all year round. In addition, the specific cooling water consumption is reduced significantly compared to steam cycle systems.

Figure 25: Solar combined cycle plant Gas turbines required high solar receiver outlet temperatures and heat transfer efficiencies. In the volumetric receiver shown in Figure 26, heat transfer from concentrated solar radiation to air occurs within a highly porous SiC absorber /23/. To contain the pressurised air, the absorber is covered by a dome-shaped quartz glass window. Secondary concentrators are installed directly before the receiver to reduce losses.

Figure 26: Volumetric receiver for solar gas turbines For the simulation of solar-hybrid gas turbine systems, software tools have been developed in order to optimise system configuration, reduce system cost and maximise annual solar electricity production /24/. Numerous optimisation studies have been performed for different plant locations, heliostat arrangements and system configurations, using solar radiation data with high local and time-wise resolution. Electricity production costs below 6 €-ct/kWh will be possible in the future at good locations and with large centralised solar thermal power plants.

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To demonstrate this concept, an international consortium of CIEMAT, DLR, ORMAT and Abengoa have realised a complete pilot system consisting of one tubular and two volumetric receivers together with a modified gas turbine at a power level of 250 kW. Recently, joint funding form the European Union and industry has been obtained to construct a 4.6 MWel solar gas turbine plant in Spain.

5 Solar hydrogen
On the longer term, solar fuels will also become a viable option. The effective conversion of the huge energy potential of solar radiation into chemical fuels, such as hydrogen, is a subject of primary technological interest. Thermo-chemical cycles were originally developed in the 1970s and 1980s to be coupled to nuclear power plants, for a thermodynamically highly efficient provision of hydrogen – mainly as fuel for transportation applications. Since the maximum achievable temperature in nuclear reactors is limited, direct thermal dissociation of water has never been an option, because temperatures above 2500°C are required to yield sufficient amounts of hydrogen. Two other problems are difficult to solve in this respect: a) the materials necessary for the reactor construction and b) the separation of hydrogen and oxygen without recombination at the relevant temperature level. In thermo-chemical cycles, water splitting is carried out in two or more consecutive steps. With increasing number of steps, the temperature of the cycle can be reduced, but the overall efficiency of the process decreases. The most prominent cycles are based on redox reactions involving either sulphuric acid or metal/metal-oxide pairs. From the beginning, the use of concentrated solar radiation was evaluated as a primary energy source for thermo-chemical cycles as well, because it is the only alternative to reach the necessary temperatures without burning carbon containing fuels. The design of a first solar reactor was published by US company General Atomics in 1982. The main challenges of thermo-chemical cycles are the necessary temperatures of more than 850°C, and the handling of corrosive, abrasive, or toxic chemicals. The current state-of-the-art of solar water splitting chemistry is focused on materials that can act as effective water splitters at relatively low temperatures in a two-step water splitting process. According to this idea, in the first step the redox material (usually the higher-valence-state of a metal oxide) is reduced to deliver some of its lattice oxygen; in the second step the reduced material is oxidized by taking oxygen from water vapour and producing hydrogen, according to the general scheme shown below.
MO oxidized → MO reduced + O2 MO reduced + H2O → MO oxidized + H2

A number of possible reaction systems, such as the ZnO cycle, are under intense investigation at present, and will be described elsewhere. In the following, the European HYDROSOL project will be discussed which has been successfully undertaken by the Aerosol and Particle Laboratory of the Greek research organisation CERTH/CPERI, the German Aerospace Center DLR, the Danish SME StobbeTech, and Johnson Matthey in the UK. In the follow-up project HYDROSOL II, the Spanish research centre CIEMAT has joined the consortium. The basic idea was to combine a ceramic support capable of achieving high temperatures when heated by concentrated solar radiation, with a redox pair system suitable for water dissociation and regeneration at these temperatures in such a way that the complete process can be realised in a single solar energy converter. The purpose of this project was thus twofold: firstly the development

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of novel redox pair materials for the water dissociation and regeneration reactions at moderate temperatures (800-1200°C) and of the associated coating technology on supports, and secondly the integration of the developed material technologies into a solar reactor suitable for incorporation into solar energy concentration systems, to open the road towards a complete hydrogen fuel production unit based on solar energy. An innovative solar reactor was designed for the production of hydrogen from the splitting of water molecules. The reactor contains no moving parts and is constructed from special refractory ceramic thin-walled, multi-channelled (honeycomb) monoliths (see Figure 27) optimized to absorb solar radiation and develop sufficiently high temperatures. The monolith channels are then coated with the active water splitting materials. The overall reactor looks similar to the familiar catalytic converter of modern automobiles.

Figure 27: a) Ceramic honeycomb absorber

b) twin reactor arrangement.

When steam passes through the reactor, the coating material splits water molecules by incorporating its oxygen. Pure hydrogen is left in the effluent gas stream with no need for expensive and complex gas separation in post-processing operations. In a subsequent step, the oxygen “trapping” material is regenerated (i.e. releases the absorbed oxygen) by increasing the amount of solar heat absorbed by the reactor and increasing its temperature as a result. Hence a cyclic operation is established. To produce a continuous hydrogen stream, two reaction chambers are operated in parallel: one for water splitting and one for reduction. At the end of the reactions, heating conditions and gas flows are switched-over. Experiments in the 10 kW scale in the solar furnace at DLR in Cologne, see Figures 28a/b, proved that it is possible to split water at relatively low temperatures around 800 °C and recycle the catalyst bed at about 1200 °C /25/. The cyclic operation of solar thermo-chemical splitting of water was successfully demonstrated in a twin reactor (Fig. 27b)), producing hydrogen exclusively at the expense of solar energy. 54 consecutive cycles of constant H2 production were performed with a single redox material sample in a four-day continuous trial production of hydrogen.

Figure 28: a) DLR solar furnace,

b) HYDROSOL reactor

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The evaluation of the economic potential of the process and detailed cost analyses indicate that, with further technical improvements, it will be possible to reduce the production costs of hydrogen from today’s 20 Eurocent/kWh to less than 10 Eurocent/kWh (HHV) in the long-term. Based on the very promising results, this work is presently continued in the HYDROSOL II project, with the main aim of scaling-up the process to a 100 kW plant which will be operated on a socalled solar tower on the Plataforma Solar de Almería (PSA) in southern Spain. A major challenge of the project will be the realisation of the operation strategy which requires an alternating movement of the large heliostats during operation to fit the requirements of the quasi-continuous chemical process and to guarantee at the same time a most efficient use of the available radiation. During a transition period from fossil fuels to hydrogen, solar upgrading or reforming of natural gas and crude oil may be a viable option, as it helps to reduce the consumption of fossil fuels and accelerates the installation of concentrating solar plants. In these processes, a significant percentage of the required high temperature process heat will be supplied by solar radiation.

5 Conclusions
There are no doubts that solar energy will make an essential contribution to a future, sustainable energy supply. The main use of solar radiation will be to produce thermal energy in a wide range of temperatures. The basic technology for these applications has been developed in recent years and is increasingly used; thermal energy costs, however, are still higher than for competing conventional technologies. The environment will always benefit from solar thermal technologies. Present energy pay-back times range from 0.5 years for solar thermal power plants, via 1-2 years for domestic hot water systems to 2-4 years for solar combi-systems for hot water and space heating. Solar thermal technologies, in general, do not depend on raw materials with limited availability. To overcome the initial cost barrier, which is typical for most innovations, political support in form of legislation and market introduction incentives will still be required. The extent and duration of these subsidies varies for the different solar thermal technologies and depends greatly on the cost development of the fuels which need to be replaced. While small domestic solar thermal systems are more or less competitive at present oil/gas prices, large systems with seasonal heat storage will need at least another decade to reach this target. In some applications, industrial solar hot water systems may already be a viable option, whereas systems for process heat at higher temperatures still require significant moral and financial support. For several reasons, solar thermal power plants are presently experiencing an unprecedented renaissance and may reach full commercial competitiveness in 10-15 years. Solar production of hydrogen will become a pre-requisite for a successful establishment of a hydrogen economy. However, this technology is still a long way from commercialisation, even though promising technologies have been developed in recent years. All solar thermal technologies have significant potential for cost reduction. There is enough evidence that mass production alone will not be sufficient to achieve the required targets and market penetration. Substantial research and development efforts will be required over the next decades which depend, of course, on the necessary degree of innovation in the various applications. In common for all applications is the development or the qualification of new materials, numerical codes for optimised design of components, simulation of complete systems for advanced control strategies, automated manufacturing techniques and quality control and assurance procedures. Furthermore, the problem of intermittent, diurnal und seasonal availability of solar heat will have to be addressed. This requires the development of new storage technologies with greatly improved performance.

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In addition to environmental and long-term financial advantages, solar thermal technologies will provide a number of further benefits, which are more difficult to quantify. This includes a reduction of the dependency from imported fossil fuels as well as a growth in local employment.

Literature
1. Müller-Steinhagen, H. and J. Nitsch: The Contribution of Renewable Energies to a Sustainable Energy Economy. Trans IChemE, Part B, Process Safety and Environmental Protection, Vol. 83, pp. 285-297 (2005) 2. Weiss, W., Bergmann, I. and Faninger, G.: Solar Heat Worldwide. AEE INTEC (2007) 3. Energy Scenario of the European Commission (200) 4. Stryi-Hipp, G.: How can Solar Thermal contribute to the 20% goal - The view of the Solar Thermal Technology Platform ESTTP. EUSEW, 29 January 2008, Brussels 5. European Solar Thermal Technology Platform ESTTP: Solar Thermal Vision 2030 (2006) 6. Drück, H., Heidemann, W. and Müller-Steinhagen, H.: Potenziale innovativer Speichertechnologien für solare Kombianlagen. Tagungsband 14. OTTI Symposium Thermische Solarenergie, pp.104-109 (2004) [13] 7. Bachmann, S., Drück, H. und Müller-Steinhagen, H.: Vorstellung und Untersuchung eines neuen Konzepts für große Warmwasserspeicher. Tagungsband 14. OTTI Symposium Thermische Solarenergie, ISBN 978-3-934681-55-2 (2007). 8. Ebert, H.-P.: Forschungsnetzwerk LWsNet: Grundlagenaspekte in der aktuellen PCMForschung. Tagungsband zum Statusseminar Thermische Energiespeicherung (2006) 9. Buschle, J., Steinmann, W.-D., Tamme, R. (2006) Latent Heat Storage for Process Heat Applications, , ECOSTOCK 2006, Stockton, New Jersey, 31.5–2.6.2006. 10. Hauer, A., Lävemann, E.: Möglichkeiten offener Sorptionsspeicher zum Heizen, Klimatisieren und Entfeuchten. Tagungsband zum Statusseminar Thermische Energiespeicherung (2006) 11. Kerskes, H., Sommer, K., Müller-Steinhagen, H.: An Effective Application of an Open Adsorption Process for Solar Thermal Heat Storage. Eurosolar, Glasgow (2006) 12. Mangold, D. and Müller-Steinhagen, H.: Central Solar Heating Plants with Seasonal Storage in Germany. Proc. World Chemical Engineering Conference, Melbourne, Australia (2001) 13. Decision Scheme for the Selection of the Appropriate Technology Using Solar Thermal AirConditioning - Guideline Document, IEA Solar Heating and Cooling Programme, Task 25. 14. Brendel, T., Spindler, K. and Müller-Steinhagen, H.: Development of an Absorption Solar Cooling System for Experimental and Demonstration Purposes. Environmental Technology Conference, Almeria/Spain (2006) 15. Strategiepapier der Solarthermiebranche: Zukünftiger Forschungsbedarf. Prepared for Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (2004). 16. E. Streicher, S. Fischer, H. Drück, H. Müller-Steinhagen: Impact of Ageing on Thermal Efficiency of Solar Thermal Collectors. Proceedings, Sept. 2007, ISES Int. Solar World Congress. Bejing, China 17. Tamme, R., Steinmann, W-D. and Laing, D.: High temperature Thermal Energy Storage Technologies for Parabolic Trough. Journal of Solar Energy Engineering, 126, 2 (2004), pp. 794-800.

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18. Müller-Steinhagen, H; Trieb, F.: Concentrating Solar Power Plants. Royal Academy of Engineering, INGENIA Issues 18 & 20 (2004). 19. Pitz-Paal, R.; et al.: Development Steps for Concentrating Solar Power Technologies with Maximum Impact on Cost Reduction. Proceedings of ISEC2005 ASME International Solar Energy Conference, August 6-12, Orlando, USA 20. Trieb, F.: www.dlr.de/tt/trans-csp 21. Eck, M.; Steinmann, W.-D.: Modelling and Design of Direct Solar Steam Generating Collector Fields. Journal of Solar Energy Engineering, Vol. 127, August 2005, 371-380 22. Fend, T.; Pitz-Paal, R.; Reutter, O.; Bauer, J.; Hoffschmidt, B.: Two novel high-porosity materials as volumetric receivers for concentrated solar radiation. Solar Energy Materials & Solar Cells 84 (2004) 291-304 23. Buck, R., Bräuning, T., Denk, T., Pfänder, M. Schwarzbözl, P. Pitz-Paal, R. et al.: Solar-Hybrid Gas Turbine-based Power Tower Systems (REFOS). Journal of Solar Energy Engineering (ASME), 124, 1, 2002, pp. 2 – 9 24. Schmitz, M. Schwarzbözl, P., Buck, R., Pitz-Paal, R.: Assessment of the potential improvement due to multiple apertures in central receiver systems with secondary concentrators. Solar Energy , Vol. 80, 1, 2006, pp. 111-120. 25. Roeb, M., Sattler, C., Klüser, R., Monnerie, N., de Oliveira L., Konstandopoulos, A.G., Agrafiotis, C., Zaspalis, V.T., Nalbandian, L., Steele, A.M., Stobbe, P.: Solar Hydrogen Production by a Two-Step Cycle based on Mixed Iron Oxides. Journal of Solar Energy Engineering, 128, S. 125-133 (2006).


				
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