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Home , Concentrated solar power, Parabolic trough

HANS MÜLLER-STEINHAGEN, SECTION FRENG AND FRANZ TRIEB INSTITUTE OF TECHNICAL THERMODYNAMICS, GERMAN AEROSPACE CENTRE, STUTTGART, GERMANY Concentrating

A review of the technology

Is solar power the answer to the ever-growing problems of global warming and depleting fossil supplies? In the first of two articles Hans Müller-Steinhagen and Franz Trieb explain the principles and development of and outline its considerable potential for alleviating the constant pressure on our existing resources.

Three main technologies have been While these technologies have reached he limited supply of fossil identified during the past decades for a certain maturity, as has been hydrocarbon resources and the generating in the 10 kW to demonstrated in pilot projects in Israel, negative impact of CO emissions several 1000 MW range: and the USA, significant T 2 on the global environment dictate the improvements in the thermo-hydraulic dish/engine technology, which can increasing usage of renewable performance are still required if such directly generate electricity in sources. Concentrated solar power installations are to achieve the reliability isolated locations (CSP) is the most likely candidate for and effectiveness of conventional providing the majority of this renewable technology, which power plants. This first article focuses energy, because it is amongst the most produces high pressure on present CSP technologies, their cost-effective renewable electricity superheated history and the state of the art. The technologies and because its supply is solar tower technology which second article, in the next issue of not restricted if the energy generated is produces air above 1000°C or Ingenia, looks at the technical, transported from the world's solar belt to synthesis gas for environmental, social and economic ingenia the population centres. operation. issues relating to CSP in the future. 1 SECTION

Technical principles In general, solar thermal technologies are based on the concept of concentrating solar radiation to produce steam or hot air which can then be used for using conventional power cycles. Collecting the , which has relatively low density, is one of the main engineering tasks in solar thermal power plant development. For concentration, most systems use glass because of their very high reflectivity. Other materials are under development to meet the needs of solar thermal power systems. Point focusing and line focusing systems are used, as depicted in Figure 1. These systems can use only direct radiation, and not the diffuse part of sunlight because this cannot be concentrated. Line focusing Figure 1 Technologies for concentrating solar radiation: left side parabolic and systems are easier to handle, but have a linear Fresnel troughs, right side central solar tower receiver and lower concentration factor and hence parabolic dish (Source: DLR) achieve lower temperatures than point focusing systems. about 18% in the medium term. The Because of their thermal nature, Table 1 gives an overview of some of values for other systems are, in general, each of these technologies can be the technical parameters of the different projections based on component and ‘hybridised’, or operated with concentrating solar power concepts. prototype system test data, and the as well as solar energy. Hybridisation Parabolic troughs, linear Fresnel systems assumption of mature development of has the potential to improve and power towers can be coupled to current technology. Overall solar-electric dramatically the value of CSP steam cycles of 10 to 200 MW of efficiencies are lower than the conversion technology by increasing its power electric capacity, with thermal cycle efficiencies of conventional steam or availability and dispatchability, efficiencies of 30–40%. The values for combined cycles, as they include the decreasing its cost (by making more parabolic troughs, by far the most conversion of solar radiative energy to effective use of the power block mature technology, have been heat within the collector and the equipment), and reducing the demonstrated in the field. Today, these conversion of the heat to electricity in the technological risk by allowing systems achieve annual solar-to- power block. The conversion efficiency conventional fuel use if, for example, electricity efficiencies of about 10–15%, of the power block remains essentially the collector has to be repaired. Solar with the aim that they should reach the same as in fuel fired power plants. heat collected during the daytime can

Table 1 Performance data for various concentrating solar power (CSP) technologies

Capacity Concen- Peak solar Annual solar Thermal cycle Land use unit MW tration efficiency efficiency efficiency (solar) m2 MWh–1 y–1 Trough 10–200 70–80 21% (d) 10–15% (d) 30–40% ST 24% (d) 6–8 17–18% (p) 25–70% (p) Frensel 10–200 25–100 20% (p) 9–11% (d) 30–40% ST 25–70% (p) 4–6 Power tower 10–150 300–1000 20% (d) 8–10% (d) 30–40% ST 25–70% (p) 8–12 35% (p) 15–25% (p) 45–55% CC Dish-Stirling 0.01–0.4 1000–3000 29% (d) 16–18% (d) 30–40% Stirl. 25% (p) 8–12 ingenia 18–23% (p) 20–30% GT (d) = demonstrated; (p) = projected; ST ; GT gas turbine; CC combined cycle. net power generation solar operating hours per year Solar efficiency = Capacity factor = incident beam radiation 8760 hours per year 2 SECTION

Figure 2 Schematic diagram of a steam cycle power plant with a parabolic trough collector and a thermal generate 1 MWh of solar electricity per To generate electricity, the fluid year with CSP, a land area of only flowing through the absorber tube – (Source: DLR) 4–12 m2 is required. This means, that usually synthetic oil or water/steam – 1km2 of arid land can continuously transfers the heat to a conventional be stored in concrete, , and indefinitely generate as much steam turbine power cycle (Figure 2). ceramics or phase-change media. At electricity as any conventional 50 MW With the sunlight concentrated by night, it can be extracted from storage coal- or gas-fired . about 70–100 times, the operating to run the power block. Fossil and temperatures achieved are in the range of 350 to 550°C. renewable such as oil, gas, coal Line focusing systems and biomass can be used for co-firing With 354 MW of parabolic trough 2 the plant, thus providing power As schematically shown in Figure 1, power plants (about 2 million m of capacity whenever required. line focusing systems use a trough-like area) connected to the grid in Moreover, solar energy can be used mirror and a specially coated steel southern California, parabolic troughs for co-generation of electricity and absorber tube to convert sunlight into represent the most mature CSP heat. In this case, the high value solar useful heat. The troughs are usually technology. In the solar electricity energy input is used with the best designed to track the along one generating systems (SEGS) plants possible efficiencies of up to 85%. axis, predominantly north–south. The developed since the 1980s in Possible applications include the first parabolic trough systems were California, a synthetic thermal oil is combined production of electricity, installed in 1912 near Cairo (), to used for operating temperatures up to industrial process heat, district cooling generate steam for a 73 kW pump 400°C. In a steam generator, this heat- 3 and sea water . which delivered 2000 m /h of water for transfer oil is used to produce slightly It is generally assumed that solar irrigation (see Figure 3). At the time, superheated steam at 5–10 MPa concentrating systems are economic this plant was competitive with coal- pressure, which then feeds a steam only for locations with direct incidence fired installations in regions, where the turbine connected to a generator to radiation above 1800 kWh m–2 year–1. cost of coal exceeded 10 German produce electricity. No new plants have 11 Typical examples are Barstow, USA, Marks per tonne (Stinnesbeck, 1914 ). been built since 1991, because with 2500–2700 kWh m–2 year–1 and Almeria, Spain, with Figure 3 First parabolic 1850–2000 kWh m–2 year–1. Today, all trough plant in installations would have capacity Egypt factors of 25%, equivalent to about (Source: 2000 full load operating hours per year, Stinnesbeck, 1914)11 with the aim of using solar operation for base load with ingenia storage and larger collector fields. To 3 SECTION

declining fossil-fuel prices in the resulted in unattractive economic predictions for future plants. However, the performance of these power plants has been continuously improved. For example, the Kramer Junction site (see Figure 4) has achieved a 30% reduction in operation and maintenance costs during the last Figure 4 Parabolic trough concentrating solar collector field of the 150 MW (5 × 30 MW) steam cycle solar electricity generating systems at Kramer five years. In addition, trough Junction, California (Source: KJC) component manufacturing companies have made significant advances in improving absorber tubes, process know-how and system integration. It is estimated that new plants, using current technology with these proven enhancements, will produce electrical power today for about 10 to 12 US cents/kWh in solar only operation mode. Performance data for the nine SEGS plants are given in Table 2. Despite the promising technology, the initiator of these plants, LUZ International Ltd, did not succeed. There were several reasons for LUZ’s failure:

Direct steam generating energy prices did not increase as parabolic trough of the DISS projected in the mid 1980s project at Plataforma Solar the value of the environmental de Almeria, Spain benefits was not recompensed a changing undefined tax status did not allow to the necessary profit o be realised.

However, three operating companies took over the plants and are delivering 800–900 million kWh of electricity to the Californian grid every year, reaching today a total accumulated solar electricity production of almost 9 billion kWh (12 billion kWh including operation), which is roughly half of the solar electricity generated world wide to date. The plants had a total turnover of over US$1.5 billion. While the plants in California use a synthetic oil as a fluid within Enhanced parabolic trough Linear Fresnel collector at the the collectors, and a separate heat structure of the EUROTOUGH Solarmundo test facility in exchanger for steam generation, efforts project facility at Plataforma Liege, Belguim Solar de Almeria, Spain to achieve direct steam generation within the absorber tubes are underway in the Figure 5 Highlights of line concentrating systems DISS and INDITEP projects sponsored ingenia development in Europe by the European Commission, with the (Source: DLR, Flagsol, Solarmundo) aim of reducing costs and enhancing

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efficiency by 15–20% each. Direct solar steam generation has recently been demonstrated by CIEMAT and DLR on the Plataforma Solar in Almeria, Spain, in a 500 m long test loop with an aperture of 5.78 m (Figure 5, top), providing superheated steam at 400°C and 10 MPa. Two-phase, steam–water flow in a large number of long, parallel and horizontal absorber tubes is a major technical challenge. Constant turbine inlet conditions must be maintained and flow instabilities must be avoided, even in times of spatially and temporally changing insolation. Control strategies have been developed based on extensive experimentation and modelling of two-phase flow phenomena (Eck, 20014; Steinmann, 200210) A European industrial consortium has developed the EURO-TROUGH Figure 6 The EURO-DISH parabolic dish concentrator with a Stirling motor- collector, which aims to achieve better generator in the focal point at the CIEMAT solarthermal test centre performance and cost by enhancing the Plataforma Solar de Almeria, Spain (Source: SBP) mechanical structure, and the optical and thermal properties of the parabolic which may be particularly useful in Point focusing systems troughs (Figure 5, middle). A prototype desert climates. Acting like a large, Dish/Stirling systems segmented blind, it could shade crops, was successfully tested in summer Parabolic dish concentrators are pasture and water sheds to protect 2003 under real operating conditions at relatively small units that have a motor them from excessive evaporation and the Californian solar thermal power generator mounted at the focal point of provide shelter from the cold desert plants within the PARASOL project the reflector. The motor-generator unit sky at night. However, the funded by the German Federal Ministry can be based on a or a performance of the linear Fresnel for the Environment. small gas turbine. Several dish/engine system has so far only been tested in Another European consortium has prototypes have successfully operated a 50 m installation in Belgium; further developed a collector with segmented over the last 10 years, ranging from modelling and experimental will flat mirrors following the principle of 10 kW (Schlaich, Bergermann and be required to determine under what Fresnel (Figure 5). The linear Fresnel Partner design), 25 kW (SAIC) to the conditions it may be more cost- system also shows a good potential 400 m2, 100 kW ‘big dish’ of the effective than the parabolic trough for low cost steam generation, and Australian National University. Like all system with direct steam generation. provides a semi-shaded space below, concentrating systems, they can additionally be powered by fossil fuel or Name SEGS I-II SEGS II-VII SEGS VIII-IX biomass, providing firm capacity at any time. Because of their size, they are Site Dagget Kramer Junction particularly well suited for decentralised Capacity 14 + 30 MW 5 × 30 MW 2 × 80 MW power supply and remote, stand-alone Commissioning year 1985–1986 1987–1989 1990–1991 power systems. Within the European Annual solar-electric project EURO-DISH, a cost-effective efficiency 9.5–10.5% 11.0–12.5% 13.8% 10 kW Dish-Stirling engine for Maximum working decentralised electric power generation temperature 307–350°C 370°C–390 °C 390°C has been developed by a European Investment 3800–4500 $/kWel 3200–3800 $/kWel 2890 $/kWel consortium with partners from industry Electricity cost 0.27–0.18 $/kWh 0.18–0.12 $/kWh 0.14–0.11 $/kWh and research (Figure 6). Annual output 30 GWh/y + 80 GWh/y 5 × 92 GWh/y 2 × 250 GWh/y ingenia Table 2 Data for the nine commercial solar electricity generating systems in California, USA 5 SECTION

Figure 7 Solar II central receiver plant in Barstow, California (Source: SNL)

Central receiver systems project GAST in the early 1980s receiver easily achieved 800°C and Central receiver (or power tower) showed that tube receivers where not was used to operate a 1 MW steam systems use a field of distributed appropriate for that purpose, because cycle. A ceramic thermal heat storage mirrors – – that individually of an inadequate heat transfer and was used for night time operation. This track the sun and focus the sunlight local overheating of the tubes. Thus, concept has been validated at 2.5 MW on the top of a tower. By the concept of the volumetric receiver (thermal) level in tests conducted at concentrating the sunlight 600–1000 was developed in the 1990s within the the Plataforma Solar in Almería. In this times, they achieve temperatures from PHOEBUS project, using a wire mesh installation, the solar energy is 800°C to well over 1000°C. The solar directly exposed to the incident harvested by 350 heliostats of 40 m2 energy is absorbed by a working fluid radiation and cooled by air flowing area each. For even higher and then used to generate steam to through that mesh (Figure 8). This temperatures, the wire mesh screens power a conventional turbine. In over 15 years of experiments worldwide, power tower plants have proven to be technically feasible in projects using different heat transfer media (steam, air and molten salts) in the thermal cycle and with different designs. At Barstow, California (see Figure 7), a 10 MW pilot plant operating with steam from 1982 to 1988, and subsequently with molten salt as the heat transfer and energy storage medium, has now several thousand hours of operating experience delivering power to the electricity grid on a regular basis. Early approaches with central receivers used bundles of steel tubes on top of the tower to absorb the concentrated solar heat coming from the heliostat field. The Californian Figure 8 Volumetric receiver (Source: DLR) 10 MW test plant Solar II used molten salt as heat transfer fluid and as the ingenia thermal storage medium for night time operation. In Europe, air was preferred as the heat transfer medium, but the 20 MW air cooled central receiver 6 SECTION

Figure 9 REFOS pressurised receiver concept (Source: DLR)

are replaced by porous SiC or Al2O3 vessel with a parabolic quartz window Conclusions structures. for solar radiation incidence. This Concentrating solar power technology The high temperatures available in design is shown in Figure 9. for electricity generation is ready for the solar towers can be used not only to Since December 2002, this market. Various types of single- and drive steam cycles, but also for gas absorber has been successfully used dual-purpose plants have been turbines and combined cycle systems. to operate a 250 kW gas turbine at analysed and tested in the field. In Since such systems promise up to over 800°C. Combined cycle power addition, experience has been gained 35% peak and 25% annual solar- plants using this method will require from the first commercial installations, electric efficiency when coupled to a 30% less collector area than plants in use world-wide since the beginning combined cycle power plant, a solar using equivalent steam cycles of the 1980s. Solar thermal power receiver was developed within the (Figure 10). Ceramic volumetric plants will, within the next decade, European SOLGATE project for absorbers with an operating provide a significant contribution to an heating pressurised air by placing the temperature of over 1200°C are under efficient, economical and environmentally volumetric absorber into a pressure development for this purpose. benign both in large-scale

Figure 10Schematic of a combined cycle system powered by a volumetric central receiver using pressurised air as heat transfer fluid (Source: DLR) ingenia 7 SECTION

grid-connected dispatchable markets Überhitzung in 12 Sugarmen, C., Ring, A., Buck, R., and remote or modular distributed Parabolrinnenkollektoren’, VDI Uhlig, R., Beuter, M., Marcos, M.J., markets. Parabolic troughs, central Fortschrittsberichte, Vol. 6, No. 464. Fernandez, V. (2002) ‘Solar-hybrid receivers and parabolic dishes will be 5 Geyer, M., Lüpfert, E., Osuna, R., gas turbine power system’, installed for solar/fossil hybrid and Esteban, A., Schiel, W., Schweitzer, Proceedings of 11th SolarPACES solar-only power plant operation. In A., Zara, E., Nava, P., Langenkamp, International Symposium on parallel, decentralised process heat for J., Mandelberg, E. (2002) Concentrated Solar Power and industrial applications will be provided ‘EuroTrough – parabolic trough Technologies, by low-cost concentrated collectors. collector developed for cost September 4–6, Zurich, Switzerland Following a subsidised introduction efficient solar power generation’, 13 SUN-LAB Snapshot (2000) Solar phase in green markets, electricity Proceedings of 11th SolarPACES Two Demonstrates Clean Power for costs will decrease from 14 to 18 Euro International Symposium on the Future, US Department of cents per kilowatt hour presently in Concentrated Solar Power and Energy Southern Europe towards 5 to 6 Euro Chemical Energy Technologies. cents per kilowatt hour in the near Sept. 4–6, Zurich Useful Internet sites future at good sites in the countries of 6 Keck, T., Schiel, W., Reinalter, W., http://www.kjcsolar.com the Earth’s sunbelt. After that, there will Heller, P. (2002) ‘EuroDish – an http://www.eurotrough.com be no further additional cost in the innovative dish/stirling system’, http://www.solarmundo.be emission reduction by CSP. This, and Proceedings of 11th SolarPACES http://www.dlr.de/TT/solartherm/ the vast potential for bulk electricity International Symposium on solargasturbine generation, moves the goal of long- Concentrated Solar Power and http://www.klst.com/projekte/eurodish term stabilisation of the global climate Chemical Energy Technologies. http://www.solarpaces.org into a realistic range. Moreover, the Sept. 4–6, Zurich. http://www.energylan.sandia.gov/sunlab/ problem of sustainable water resources 7 León, J., Zarza, E., Valenzuela, L., and development in arid regions is Hennecke, K., Weyers, D., Eickhoff, addressed in an excellent way, making M. (2002) ‘Direct steam generation use of highly efficient, solar powered – three years of operation of DISS co-generation systems. However, Project’, Proceedings of 11th during the introduction phase, strong SolarPACES International political and financial support from the Symposium on Concentrated Solar Franz Trieb has worked in the field responsible authorities is still required, Power and Chemical Energy of renewable since 1983. and many barriers must be overcome. Technologies. Sept. 4–6, Zurich. After the implementation of These topics will be addressed in the 8 Price, H., Lüpfert, E., Kearney, D., hydrogen storage for an second article. Zarza, E., Cohen, G., Gee, R., autonomous Mahoney, R. (2002) ‘Advances in system at the University of References and additional reading parabolic trough solar power Oldenburg, 1 Bockamp, S., Griestop, T., Fruth, technology’, ASME Journal of Solar Germany, he M., Ewert, M., Lerchenmüller, H., , 124, 109–125. followed a two-year Mertins, M., Morin, G., Häberle, A., 9 Romero, M., Marcos, M.J., Osuna, postgraduate Dersch, J. (2003) Solar Thermal R., Fernández, V. (2000) ‘Design course ‘Renewable Power Generation (Fresnel), and implementation of a 10 MW Energy’ at the PowerGen solar tower power plant based on National University 2 Buck, R., Bräuning, T., Denk, T., volumetric air technology in Seville of Tacna, Peru. Pfänder, M., Schwarzböezl, P., (Spain)’, Proceedings of the Solar Since 1994, he has Tellez, F. (2002) ‘Solar-hybrid gas 2000, Solar Powers Life–Share the been project turbine-based power tower Energy, June 17–22, Madison, manager at the Institute of systems (REFOS)’, J. Solar Energy Wisconsin. Technical Thermodynamics of the Engineering, 124, 2–9 10 Steinmann, W-D. (2002) ‘Dynamik German Aerospace Center (DLR), 3 Becker, M. et al. (2002) The Future solarer Dampferzeuger’, VDI working on solar energy resource for Renewable Energy 2, EUREC Fortschrittsberichte, Vol. 6, No. 467. ingenia assessment by satellite remote Agency, James & James (Science 11 Stinnesbeck, L. (1914/1915) sensing, market strategies for Publishers) London ‘Sonnenkraftmaschinen’, Keller´s concentrating solar power and 4 Eck, M. (2001) ‘Die Dynamik der Monatsblätter, Bergstadtverlag, Vol. renewable energy scenarios. solaren Direktverdampfung und 3, No. 1 8 SECTION

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