Trough solar systems use parabolic curved, trough shaped reflectors focus the sun's energy onto a receiver pipe running at the focus of the reflector. Because of their parabolic shape, troughs can focus the sun at 30-60 times its normal intensity on the receiver pipe. The concentrated energy heats a heat transfer fluid (HTF), usually oil, flowing through the pipe. This fluid is then used to generate steam which powers a turbine that drives an electric generator. The collectors are aligned on and east-west axis and the trough is rotated to follow the sun to maximize the suns energy input to the receiver tube. Click flow diagram above to see full size flow diagram of the new plants beig built in Spain. Current cost of electricity from these plants is $0.10 to $0.12 per kWh. The current goal of ongoing development by EERE is to reduce the cost to $0.035 to $0.043 per kWh by 2020.
Trough collector solar power, also called solar electric generating systems (SEGS), represent the most mature concentrating solar power (CSP) technology, with 354 MW of installed capacity in nine plants, built between 1984 and 1991, all located in the Mohave desert of California. There are several generations of plants, the newer generations incorporating features that would reduce the cost of electricity if they were new plants. All plants are still operating and producing power for the electrical grid at 99% availability.
The technology has been stagnant since 1993, but four new projects are now being developed.
- In the US, a 1 MW plant is being built by Arizona Public Service's at its Saguaro Power Plant and a 64 MW plant is being built in Eldorado Valley near Boulder City, NV. Solargenix is the prime contractor on both projects in conjunction with EERE/NREL. SHOTT North America is supplying the receivers. The Boulder City plant is currently scheduled to go on line in Jone 2007. Funding for the American projects has been difficult, but on 10/4/04 the Western Governors Association and EERE announced funding and support of a 1000 MW goal of concentrating solar power for the region by 2010. Included in this program is support for the two afore mentioned projects. Nevada Power and Sierra Pacific Power Company have signed long term contracts to buy the power produced by the 64 MW plant.
- Two identical 50 MW plants are being built by Solar Millinium AG in Granada, Spain. The first plant has received a €5 million (~$6.25 million) grant from the EU’s Fifth Framework Program and financial support from the German Ministry for Environment. To encourage this type of technology the Spanish government has set up regional funding programs. This is coupled with a bonus of 12 Eurocents per kWh generated from solar power has been instrumental in bringing such projects to Spain.
- A 150 MW facility that is to be expanded to 500 MW In the Negev desert of Israel is being developed by Solel (the successor company to Luz). Solel will establish a consortium to finance and build the power station, which it estimates will cost $1 billion. The state has promised to buy the electricity under the terms of a contract to be signed by the parties.
The first solar trough plants were constructed in California by Luz International. The first started up in 1984, the last in 1991. Altogether, nine such plants were built, SEGS I–VII at Kramer junction and VIII and IX at Harper Lake and Barstow respectively. In February 2005, all but two (I and II) of the Kramer Junction SEGS plants were acquired by FPL Energy and Carlyle/Riverstone. A natural gas system “hybridizes” the plants and contributes up to 25% of their output, a feature that allows operation later at night or on cloudy days to meet the requirements of the grid. FPL now runs these systems, making it the largest solar power generator in the United States. All of the power generated from the SEGS projects is sold to Southern California Edison under long-term contracts negotiated by Luz back in the 1980s.
Development work is now proceeding on several components of the plants:
- SolarGenix, the contractor for the two new American plants has developed a new structural support system for the collector. The design uses an aluminum frame patterned after the Luz design, but superior in terms of structural properties, weight, manufacturing simplicity, corrosion resistance, manufactured cost and installation ease. One key point of in design of the structure is that it has to have extreme tolerances and structural rigidity in order to focus the mirrors precisely on the receiver pipe. High resistance to large wind loadings on the large structure is a very important parameter. Solargenix used computer modeling techniques that were unavailable at the time that the original Luz design was made. The collector assembly has been tested at the NREL test facility. A new improved generation of reciever tubes supplied by SHOTT will be used in these plants.
- FLAGSOL Gmbh, who supplied all the collector assemblies at the California plants is supplying the trough assembly for Solar Millenium, has gone through a similar development process to develop a new steel structure. The collector assembly has been tested at the Kramer Junction SEGS plants in California.
- Industrial Solar Technology Company (IST) has developed a concentrator system that incorporates the reflective surface as part of the collector structure. This results in a very lightweight, low-cost concentrator module that is also very strong.
- Solel's solar collector assembly includes an upgraded version of the Luz parabolic collector. It is designed for mass production and cost reduction by economies of scale.
The reflectors are composed of individual concentrator modules that are a consist of a steel support structure with a mirror mounted on it. Development efforts are aimed at reducing the thickness of the mirror, improving the reliability of the glass to metal seal, surface coatings on the mirrors to improve their performance and development of a composite concentrator modules with lightweight, front-surface mirrors instead of heavy (4 mm) glass mirrors that were used on the original SEGS plants.
The heat collection element (HCE) is composed of a stainless steel pipe with a glass tube surrounding it, with the space between evacuated to provide low thermal losses from the pipe. The pipe is coated with a material that improves the absorption of solar energy. Several improvement have been made or developments are underway to improve performance:
- Solel has supplied all the HCE's to date. In its newest model, the UVAC 2003, they have increased the annual average performance by more than 20% over the assemblies used in the Luz plants. Absorptivity, emissivity (radiative loss) and transmittivity (of the glass) all have been improved.
- IST's innovations include a receiver that incorporates a highly efficient blackened nickel selective surface and an anti-reflective coating on the glass envelope. The receiver pipe surface has a high absorptance for incoming light in the visible range, and a low emittance in the infrared wavelength.
- Shott Rohrgas has announced it is planning on producing HCE's and presumably will supply them for the Spanish plant.
- The seal between the glass and the pipe has not been as reliable as desired and development of better seal materials/seal configuration is underway.
- The HCE was connected to the stationary hot oil piping with a flexible tube in the Luz plants. Replacing the flexible tube with a ball joint will reduce the pressure drop in the hot oil piping piping by 50% and significantly reduce the parasitic power losses in the plant.
The fluid going through the receiver pipe is routed through a thermal storage system which permits the plant to keep operating for several hours after sunset while the electrical demand is still relatively high. The thermal storage system (to be used in Spain) is a two tank system in which the HTF flows through the solar field and then through a heat exchanger where it gives up a portion of its heat to heat a nitrate salt solution that is stored in a hot salt tank. The slightly cooled HTF continues on to the power generation system. At night the hot salt solution flows through the same heat exchanger heating up the HTF for generating power. The cooler oil flows from the heat exchanger to the cold storage tank where it stays until daytime when it is reheated and returned to the hot storage tank.
Several companies are involved in developing heat transfer fluid/thermal storage systems. The leading candidate appears to be a system designed by Sandia National Laboratories. Only a few details are available. The following is courtesy of Solarplaces:
- Sandia has tested a thermocline storage system that uses a single tank that is only marginally larger than one of the tanks in the two-tank system. A low-cost filler material, which is used to pack the single storage tank, acts as the primary thermal storage medium.
- Nexant has developed a near-term thermal storage option that uses biphenyl-diphenyl-oxide HTF in the solar field and then passes it through a heat ex-changer to heat molten salt in a thermal storage system.
- Kearney and Associates is investigating using a lower temperature molten salt as the HTF in the solar field as an innovative approach for reducing the cost of thermal storage for troughs.
- Work at the University of Alabama and NREL is looking into using a new class of fluids known as organic salts (or ionic liquids) as the HTF and thermal storage media in a parabolic trough plant. Organic salts have the primary advantage of being liquid at room temperatures
The Plataforma Solar de Almería (PSA), the European Test Center for Solar Energy Applications, in southeastern Spain is Europe's test facility for concentrating solar power development. They are developing direct steam generation technology to eliminate the thermal oil system that is currently used in solar trough plants, which would greatly simplifies the plants. The two-phase heat transfer that occurs in the receiver tube required evaluation as there are unique condition that occur in the tube. Results of testing to date indicate that the heat transfer was very sensitive to the small changes in temperature that occur in solar troughs and that their measurement accuracy was not good enough to correlate the data properly. PSA is also developing new trough collector assemblies and coatings to improve the absorption of solar energy on the receiver tubes and to decrease the reflectivity of glass.
Status of new plants:
APS' 1 MW solar trough plant at the Saguaro power plant will heat mineral oil to between 250° and 500°F, lower temperature than used in other plants. Flowing through a heat exchanger, the heated oil will vaporize pentane, a hydrocarbon, which in turn will drive the turbine. The vapor then will condense back into liquid form and the cycle will repeat. This approach represents a departure from existing solar thermal plants, which use steam driven turbines. The capital cost for a steam turbine would be too great for such a small plant, instead, APS will use a 1-MW turbine similar to a geothermal plant’s.
In 2004, German company Solar Millennium AG started work on two parabolic-trough solar power plants in Grenada, southern Spain. Two 50 MW solar trough power plants, AndaSol-1 and 2, are being built jointly by ACS Cobra and Solar Millennium in the region of Andalucia, with a 510,120 m² solar collector field and 6 hours of molten salt storage. Construction will start in autumn 2005 and will be completed in 2007. ACS Cobra and Solar Millennium have started development of various 50 MW follow-up plants in Southern Spain.
The $1 billion Solel plant will be built in phases, in the first phase, a 150 MW plant will be build at a cost of approximately $250 million. The plants will incorporate a new trough Assembly incorporating their UVAC. No starting date for construction has been announced.
Revised 11/12/05 to update data on Solergenix Bouder City plant.
ACS Cobra, Madrid, Spain
Flagsol Gmgh, Koln, Germany
Solargenix Energy LLC, Raleigh, NC
Solar Millennium AG, Erlangen Germany
Solel Solar Systems LTD, Beit-Shermesh, Israel
"Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts", Executive Summary (47 pages), Full Report (344 pages), Sargent and Lundy LLC, October 2003
"Parabolic Trough Systems", EERE
Plataforma Solar de Almería, European Test Center for Solar Energy Applications, Tabernas Desert, Spain
"2 x 50 MW AndaSol Power Plant Projects in Spain", SolarPACES