Solar Thermal Power Plants For Central Or Distributed Electricity Generation 20 Years of development and testing at Schlaich Bergermann und Partner W. Schiel, B. Hunt, T. Keck, A. Schweitzer, G. Weinrebe, Schlaich Bergermann und Partner, Germany Abstract Schlaich Bergermann und Partner is developing and optimizing four different technologies for solar thermal electricity generation since the early eighties: - THE SOLAR TOWER (UPDRAFT SOLAR POWER PLANT) - DISH/STIRLING SYSTEMS - DISTRIBUTED COLLECTORS SYSTEMS (PARABOLIC TROUGH) - CENTRAL RECEIVER SYSTEMS (POWER TOWER) The Paper describes in a short form the principle function and the field of application of each system. This is followed by a presentation of the correspondent developments at Schlaich Bergermann und Partner with a comprehensive description of current work and the state of the art of the technologies. Finally the paper goes into details of upcoming realizations of prototypes, pilot plants and commercial plants.
THE SOLAR TOWER (Updraft Solar Power Plant) Principle Solar Towers produce electricity from solar energy. The sun heats air under a big translucent collector roof. A difference in density between the warm air inside the collector and tower and the relatively cold air outside the system drives a powerful airflow. The air flows radial to thehollow tower positioned in the middle of the collector and rises in it to the top. The airflow drives a turbine with generator which is installed in the tower close to the bottom. Solar Radiation Tower Solar Collector Turbines Figure 1: Principle of the Solar Tower: Glass roof collector, Tower, Turbine Besides the simplicity of design and function turbine and generator are the only moving parts the Solar Tower has several advantages compared to other technologies: - 24 hours continuous solar only operation time without the use of additional fossil fuels can be achieved with the heat storage system consisting of water filled hoses placed on the bottom surface of the collector. Heat is stored in the hoses and in the ground during daytime and released in the night. The hoses need to be filled with water only once. Besides this there is no additional water demand.
100 80 natural ground storage water storage 10 cm water storage 20 cm glass roof Power (%) 60 40 into the air into the soil and the water tubes into the air 20 soil Day water tubes Night soil 0 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 Time (h) Figure 2: Left: Principle of heat storage with water hoses under the collector roof of the Solar Tower Right: Representation of the daily net power output depending on the thickness of the water layer - The Solar Tower uses natural air as a working medium. No cooling water is required. This is advantageous in many countries with typically high insolation and shortage of drinking water. - The solar radiation is not concentrated. Thus also the diffuse fraction of solar radiation is exploited to heat the air under the collector roof. Plant operation is therefore possible also when the sky is hazy, partially clouded or even overcast. The Manzanares Prototype The first prototype plant with a tower of 200 m height and a collector surface of 44000m² was erected 1982 in Manzanares, Spain with support of the German Ministry of Research and worked successfully in continuous operation for many years. (Figure 3 ). The goal of this research project was the development and verification of the thermodynamic principles of the Solar Tower to have a qualified base for the design of large scale applications.
Figure 3: Solar Tower prototype in Manzanares, Spain The power output of a Solar Tower is proportional to the level of the global solar radiation, the tower height and the collector surface. A certain power output can be achieved either with a high tower combined with a relatively small collector or with a relatively small tower and a large collector. (Figure 4). From a constant product of height times surface results a approximately constant power output. For the final design and dimensioning only the specific cost figures for the components at the specific site lead to clear and optimized results. annual energy production [GWh/year] 1 600 1 400 1 200 1 000 800 600 400 200 0 tower height H k [m] 1 500 1 000 800 600 445 0 1000 2 000 3 000 4 000 5 000 6 000 collector diameter D coll [m] Figure 4: Yearly net energy production of Solar Tower Plants versus collector diameter and height of Tower A single Solar Tower can be designed with an appropriate high tower and big collector to have an output of 100 to 200 MW. Several Solar Tower plants like this can already replace nuclear power plants (Figure 5).
The construction materials required for a Solar Tower Plant like concrete, steel and glass are available everywhere. Solar Tower Plants can thus already today be erected even in countries with less advanced industrial development. Local Industries will be sufficient for almost all requirements during the erection. Investment in high technological production facilities is not necessary. The specific countries require therefore no foreign exchange and can use own human and material resources. Thus even in poor countries the realization of a big plant is feasible. Figure 5: Solar Updraft Tower Plants in the desert (artist s impression). Current Projects 200 MW Solar Tower Plant in Australia: In collaboration with a large construction company SBP develops a 200 MW Solar Tower project in Australia close to the city Mildura approximately 500 km northwest of Melbourne. The design comprises of a Tower with a height of 1000 m and a diameter of 120 m. The Collector will have a diameter of 7000 m. Taking in account the local meteorological boundary conditions, an annual net energy production of app. 650 GWh can be expected. The prototype in Manzanares had a single turbine with a vertical turning axis. The power plant in Mildura will have 32 individual turbines with horizontal axis positioned around the circumference of the tower foot.
The goal of the first project phase was to get a detailed cost estimate for the single plant components. Local and international companies are involved to get a reliable cost basis for the calculation of the levelised energy costs. It is expected that the preliminary design is completed in mid 2004. The start of construction is foreseen in 2005. The start of operation will then be in 2007. DISH/STIRLING SYSTEMS Dish/Stirling Systems are characterized among other solar thermal power plants on one hand by their high efficiency and on the other hand by a high modularity in their application. Principle A Dish/Stirling System consists of the main components parabolic concentrator, tracking system, solar heat exchanger (receiver) and a Stirling motor with generator (Figure 6). Stirling Solar Radiation Concentrator Receiver Tracking System Figure 6: Schematic representation of a Dish/Stirling system. The concentrator tracks the sun biaxially in a way that the optical axis of the concentrator always points to the sun. The solar radiation is focused by the parabolic concentrator onto the solar receiver which is situated close to the focal point of the parabola. The receiver feeds the high temperature thermal energy into the cycle of the Stirling motor, which converts it highly
efficient into mechanical energy. The directly coupled generator finally converts it to electric energy. System Properties and Applications Dish/Stirling Systems have been developed so far with power outputs ranging from several kwel to 50 kwel and concentrator diameters up to 17 m. Recent developments are concentrated on power output from 10 to 25 kwel. In this range both stand alone and grid connected systems were designed and tested. Due to the low thermal inertia of the energy conversion, a Dish/Stirling System reacts very quick on changes in solar thermal input. Thus steady state operation is achieved already shortly after the start up of the system. If power output is required independent from the existing meteorological conditions, in the evening or by night like in many applications, the Dish/Stirling system can, besides the use of batteries, be configured as hybrid system. Hybrid system means that additional fossil energy sources (e.g.: bio gas) can be used to add thermal energy to the solar thermal energy to stabilize the power output or for prolongation of operation time. Hybrid Systems have already been successfully tested in longtime operation. The biaxial tracking system is driven by electric motors. The tracking position is found with sensors and with a tracking algorithm that calculates the actual sun position. For the control of these drives and for the whole system including Stirling engine, micro controllers or PCs are used. The operation is therefore fully automatic and remote control via internet is optional. The field of possible applications covers on one hand the support of smaller or bigger grid systems, and on the other hand stand alone systems can power for example water pumps or desalination plants. If Dish/Stirling systems are installed gathered in clusters, applications up to 10 MW can be realized. Above this limit other solar thermal systems are economically more efficient. Installed Systems Beginning in 1980 several projects started in the USA and the first modern Dish/Stirling systems with 25 kwel were erected and tested. In 1984 SBP erected 2 units with a concentrator diameter of 17m and a power output of 50 kwel (Figure 7). A summary of early Dish/Stirling system development until 1993 can be found in [4].
In all these early systems high efficient and thus expensive Stirling motors were used. The further development at SBP was focused on the more simple and thus slightly less efficient V160 engine of SOLO Kleinmotoren GmbH which made the whole system economically more efficient. This Stirling engine with two single acting cylinders in V configuration has passed 20 years of continuous development and is with 300.000 accumulated operating hours the most mature engine on the market. Figure 7: 25 kwel system (McDonnel Douglas), 50 kwel systems (SBP) in Saudi Arabia (right) In the following two generations 8 units with concentrator diameters from 7.5 8.5 m and 9.5 kwel were installed. These concentrators were designed in the stretched membrane technology. In this technology thin metal membranes and a cylindrical ring form a drumlike housing. With a hydro pneumatically forming procedure the front membrane is plastically deformed into a parabolical shape. Thin glass mirrors are glued to the front membrane to form a high efficient concentrator. The structure is extremely stiff when it is supported during operation by a slight underpressure in the concentrator. Six of these units were erected between 1992 and 2000 at the Plataforma Solar de Almeria in southern Spain and tested in continuous operation (Figure 8) [5]. With more than 40.000 cumulated operating hours the systems have the broadest operational experience worldwide. In 1998 SBP started within a Spanish/ German consortium the development of the next generation 10 kwel Dish/Stirling systems under the designation Eurodish. Characteristic for the new generation is the concentrator shell construction with 8.5 m diameter out of
fiberglass reinforced resin (Figure 9). The energy conversion unit is the advanced SOLO Stirling 161. With the working fluid helium or hydrogen, working pressure from 20 to 150 bar and a working temperature of 650 C an overall efficiency of approximately 20% can be achieved [6]. Figure 8: 9kWel systems in Spain (SBP) Figure 9 : 10 kwel Eurodish in Spain (SBP)
Recent developments and projects All systems so far were built in single piece production and have therefore a high investment cost level. Additionally Stirling motors still require regular maintenance. The biggest goals for further development are an increase of reliability and further cost reduction. In the USA three consortia are trying to bring Dish/Stirling systems into the market [7]. Stirling Energy Systems (SES) together with Boeing and the Swedish company Kockums developes a 25 kwel system. Science Application International Corporation (SAIC) and Stirling Thermal Motors (STM) are working on a 22 kwel unit, while WG associates together with Sandia National Laboratories installed a 10 kwel Dish/Stirling system using the SOLO V161 engine (Figure 10). Different concentrator types (metal membrane SAIC, metal/glass SES, sheet metal/sandwich- WGA) are used. SBP is working currently with several German partners supported by the German ministry of the environment (BMU) on measures for cost reduction and on first steps into the market. As part of the project demonstration plants will be erected in Seville, Spain, Odeillo, France and Würzburg, Germany [8]. Figure 10: US systems: 22 kwel (SAIC) and 10 kwel (WGA) The next step will be a plant with 0,5 to 1,0 MWel respectively 50 to 100 Dish/Stirling systems. With this step the first series production can be achieved and at the same time the operational experience can be extended. It is expected that levelized energy costs of Dish/Stirling systems can reach and soon beat photovoltaics and first niche markets can be penetrated.
Comparable projects are also planned by competing groups in USA. A 1,0 MWel plant is planned to be installed in Nevada. With series production, levelized energy cost between 0.15 and 0.2 /kwh depending on site conditions are expected. American studies predict for big series even 8 USCent/kWh for this technology. DISTRIBUTED COLLECTORS SYSTEMS (PARABOLIC TROUGH) The main part of the commercially produced solar thermal electricity comes from Parabolic Trough Power Plants. Among all solar thermal power plants this is the most advanced and mature technology. Principle The reflective surface of a parabolic mirror surface concentrates solar radiation onto a vacuum insulated receiver tube which is located in the focal line (Figure 11). The heat transfer fluid (HTF) in the tube is a thermo oil which is thus heated up to 400 C. Mirrors and Receiver tube are tracking the sun along one axis. By serial connection of single collectors, loops with the length of several 100 meters can be generated. The hot oil of all loops is collected and fed into the steam generator in the central power block. A conventional steam turbine (steam temperature 370, 100 bar) with generator converts thermal to mechanical and electrical energy, respectively. Figure 11: Principle of a parabolic trough collector.
Installed Systems In the Mojave desert of California nine Parabolic Trough Power Plants produce electricity from the sun since almost 20 years. The SEGS (Solar Energy Generating System) Power Plants have a nominal capacity of 354 MWel (Figure 12). In case of bad weather an additional fossil heating (natural gas) guarantees a constant output of the plant. The first plant (SEGS I) with 14 MWel was put in operation in 1984, the last plant (SEGS IX) with 80 MWel started operation in 1990. In this development many details were optimized and efficiency was increased. In parallel operation and maintenance cost were decreased significantly. Bild 12: Arial view of the power plants SEGS IV- VII in USA Recent Developments and Projects After almost ten years of standstill in the Parabolic Trough technology the development was intensified again in the 90es. Several prototypes and demonstration units were erected to prepare the realization of further commercial plants. A European consortium developed a weight and cost optimized collector. SBP was responsible for design and engineering within the consortium. In the years 2000 and 2001 two prototypes (EuroTrough I and II, Figure 13) were erected and tested on the Plataforma Solar de Almeria in Spain with financial support of the European Commission.
Figure 13: Prototype system Eurotrough II (410 m² collector surface) Encouraged by the results the partners SBP, Solar Millennium AG and FlagSol started to project a 50 MWel commercial plant in Spain. To reduce the up scaling risk from the prototype (410m²) to the power plant (550.000m²), a demonstration loop with 4360 m² surface was planned as an intermediate step. The task for SBP in this step was: - optimization of static and design (Figure 14 and 15) - reduction of manufacturing and erection cost - Analysis of efficiency - Planning of large scale fabrication and erection This demonstration loop (Figure 16) is in continuous operation as an integrated component of the SEGS V (Figure 12) power plant at Kramer Junction, USA since 2003.
Figure 14: Static model of single trough collector element (68m² collector surface) Figure 15: Deformation analysis under wind load for a single trough collector element (68m² collector surface)
Figure 16: Demonstration Power loop (SBP et al.) in California USA The economic and technical analysis of the demonstration unit certifies the collector to be a real improvement compared to the existing designs. The material usage could be reduced because of an increased stiffness of the structure. Optimized alignment procedures have led to a higher collector efficiency. The improvement of the on site erection was emphasized. A new designed receiver tube developed by Schott, Germany, with a highly selective coating will be implemented in 2004 to further increase the efficiency. Besides the classic Parabolic Trough systems there are alternative developments under way. On Fresnel Collectors the parabolic surface is divided into individually tracked stripes of mirrors installed close to the ground. Thus the Receiver tube can remain at a fixed position but requires a secondary concentrator to achieve the necessary efficiency. A demonstration unit in a technical scale is the next step for this technology. Recent Projects The Solar Millennium AG, SBP and FlagSol together with ACS Cobra, Spain, are projecting two power plants close to Guadix, Spain (Figure 17). The first of these plants, ANDASOL I, should start erection in 2005. A thermal storage will allow 9 h of operation at nominal power output (night operation). The parabolic trough plant is supposed to produce app. 50 MWel and will dispose of a thermal salt storage system. Each plant will feed 157 GWh per year of pure solar electricity into the Spanish grid. Basis for cost-effective operation are the feed-in-tariffs for solar thermal power generation ( Prima ), issued in September 2002 and March 2004.
AndaSol Figure 17: 50 MWel plant ANDASOL I in southern Spain Key data of the AndaSol I Power Plant - Nominal capacity 49,9 Mwel - Yearly electricity production 157 GWh - Total number of single collector elements (68m²) 7500 - Total collector aperture Area 510000 m² - Turbine type Condensing Turbine - Turbine inlet conditions 100 bar / 370 C CENTRAL RECEIVER SYSTEMS (POWER TOWER) Characteristic for Power Tower plants is a field reflecting mirrors (heliostats) that focus solar radiation onto a central receiver that is positioned on a tower in the middle of the heliostats. The heliostats are parabolic or spherical shaped mirrors that track the sun biaxially. At the receiver, the solar energy is converted into thermal energy and a heat transfer fluid (air, molten salt, water/steam) feeds it into a conventional steam turbine cycle that finally drives a generator. To get constant steam parameters also during cloud passages either a heat storage system or additional heating (e.g.: with natural gas) with fossil fuels is foreseen. Installed were prototype testing plants up to 10 MWel in France, Israel, Italy, the former USSR, Spain and the USA. Commercial Power plants are not in operation up to now.
Receiver Concentrated Solar Radiation Direct Solar Radiation Direct Solar Radiation Heliostats Figure 18: Principle of a Power Tower Tower Heliostats Concentrated Radiadition Air - Receiver Burner 700 C Steam Generator Turbine Luftkreislauf Pump Steam Cycle G 200 C Blower Condenser Figure 19: Schematic of Power Tower Plant with air as a heat transfer fluid Components Heliostat Field. The Heliostat field consists of several hundreds up to thousand individual heliostats. The concentrator surface of heliostats range between 40 and 150m². A maximum of 200 m² was installed. The heliostat field causes more than 50% of the investment costs of the plant. Therefore big efforts were undertaken to develop economical heliostats with high
optical efficiency, high reliability. Two different design principles can be identified: Glass/metal facetted heliostats and Glass/metal membrane heliostats. Figure 20: Facetted Glass-Metal Heliostat Facetted Glass-Metal Heliostats. Their concentrator usually consists of a framework structure installed on a torque tube, and a multitude of rectangular single mirrors, the so called facets, with a size of 2 to 3 m 2 each. The tracking unit comprises a vertical pedestal bolted to the foundations with an azimuth/elevation gearbox on top, to which the torque tube is connected (Fig. 20). Membrane Heliostats. To avoid the high construction and installation expenditures associated with single facetted designs, and to achieve a high optical quality at the same time, heliostats on the basis of stretched metal membranes were conceived. Their concentrator consists analogous to the concentrators of some Dish/Stirling systems of one or more drums, which again are made from a metallic pressure ring and stretched membranes on the front and backside (Fig. 20). Thin glass mirrors are glued to the front side membranes to achieve the desired reflectivity. Using a small fan or pump, a slight underpressure is maintained in the concentrator plenum. Thus the membrane deforms elastically, and the flat mirror becomes a concentrator. Using justifiable effort, the optical quality achieved with large stretched membrane heliostats is significantly higher than that obtained with glass-metal heliostats of comparable size. An additional advantage is the fact that by changing the
underpressure inside the concentrator, the focal length of the heliostat can easily be adjusted. Thus it is unnecessary to manufacture, install and adjust special facets for heliostats depending on their respective distances from the receiver, as it is the case with glass-metal heliostats. Instead, one common concentrator is used for all heliostats positions. SBP have designed and built stretched metal membrane heliostats with 44 m 2 and 150 m 2 [Fig. 21]. Both types are characterized by an excellent optical quality [15,16]. Figure 21: Metal Membrane Heliostats with 150m² (left) and 44m² reflective area (right) Receiver. Receivers of Power Towers convert the concentrated solar radiation - reflected onto them by the heliostat field - into heat. In current designs, the radiative heat flux ranges from 600 to 1000 kw/m². Power Tower receivers can be classified based on the respective heat transport fluid used: Air, molten salt, water/steam. Today the favourite media are air and molten salt. Other components. Other components like steam circuit, balance of plant, control and instrumentation are not described here, as they are very similar to those used in fossil-fuelled power plants. Current Projects Solgate. The Solgate system demonstration project is already operational. Solgate couples solar energy into a gas turbine process instead of a steam turbine process [14], enabling the realization of the next generation of Power Towers as highly efficient combined cycle power plants that make optimum use of the high exergy content of concentrated solar radiation. The
Solgate pilot plant with closed volumetric receiver, equipped with a secondary concentrator and ceramic absorber, achieves an electrical power output of 250 kw in hybrid operation (i.e. combined operation using solar radiation and natural gas). With the Spanish feed-in tariff for solar electricity of roughly 0.16 /kwh (as of February 2004) as an incentive, two Power Tower projects are being developed: PS10. Based on the very positive experience with air as heat carrier medium in the TSA project [17], a Spanish led European consortium plans to construct and operate a 10 MW Power Tower named PS10 in southwest Spain. Its open volumetric receiver will be installed on a 90m high tower. The receiver has the form of a half cylinder (height and diameter 10.5m respectively). An integrated thermal storage with a capacity of 33 MWh is planned. The steam process is designed for 460 C and 45 bar [12]. Solar Tres. This project is based on the know-how and experience gathered during design, construction and operation of the 10 MW Solar Two system near Barstow, California. Hence the name Solar Tres (Spanish for Solar Three ). The 10 MW Power Tower design comprises a molten salt tube receiver and an integrated thermal salt storage, enabling continuous operation of the plant [13]. Prospects Intensive design and testing of central and decentralized solar thermal power systems using different concepts is currently underway. All technologies described before have already left the stage of prototype. They are poised on the verge of commercialization or have already reached that stage. Still, for all this technologies it is currently true that the associated power generation costs are higher than those for conventionally generated electricity. Thus, to further decrease power generation costs, more research and development efforts are needed, but also the operation of power plants under real-world conditions. The multitude of activities in the area of project development let expect that this goal will be reached within the next few years. Taking the unavoidable rise of costs for the provision of fossil fuels in the mid and long term into consideration, it becomes very probable that solar thermal power generation will gain ground on the energy markets in the near future, and that it will play an important role in the long run.
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