Linear Concentrators Research and Development
As part of its research program in concentrating solar power (CSP), the U.S Department of Energy sponsors research and development (R&D) for linear concentrator systems. The goals for linear concentrator R&D—specifically, for parabolic troughs—are to
- Improve the performance and lower the cost of parabolic trough collector systems
- Support the development of next-generation trough fields
- Support the expansion of the U.S. trough industry.
This page summarizes key trough R&D activities by the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories to attain the above goals in three areas:
Trough Solar Field R&D
The trough solar field consists of a solar collector assembly that incorporates curved mirrors and a heat-collection element or receiver. R&D is reducing the delivered energy costs of these trough systems. DOE's activities are helping to significantly improve the performance of these systems, with specific R&D to improve the optical efficiency of parabolic troughs and reduce receiver heat losses. Representative projects focus on optical measurement techniques, optical modeling, advanced absorber materials, receiver heat-loss R&D, and hydrogen mitigation.
An explanation of the configuration of the VSHOT system for measuring the optical accuracy of trough collectors.
Credit: Sandia National Laboratories
Optical Measurement Techniques
Parabolic trough collectors concentrate sunlight onto a fluid-carrying receiver tube to achieve the high temperatures needed to ensure high steam power-cycle efficiency. Optical accuracy of the entire collector assembly is crucial for achieving proper concentration. NREL has developed the Video Scanning Hartmann Optical Test (VSHOT) method, which uses laser beams to determine the accuracy of the reflector surface. This technique, in high demand by industry, is being extended to measure larger reflector apertures and longer sections of reflector. VSHOT can also be used to characterize dish and power tower optics.
The Topographic Optical Photographic (TOP) alignment system, developed by Sandia, allows rapid and accurate in-field optical alignment of troughs by general maintenance workers. Sandia has used the TOP system to measure and adjust the alignment of troughs at two Solar Electric Generating Station (SEGS) plants in California.
VSHOT evaluates one reflector section at a time either in the laboratory or in the field. But to evaluate the optical accuracy of a large field of collectors, NREL is developing a "Distant Observer" technique, which uses a camera positioned over the collector field. Analyzing the images of the receiver tubes mirrored in the reflectors allows one to evaluate the overall field accuracy and identify trouble spots. These trouble spots can then be corrected accurately and efficiently using the TOP technique. The Distant Observer technique could also use infrared cameras to identify underperforming receiver tubes in the field.
Before (left) and after (right) images of trough mirrors using the TOP technique. Note that the reflection of the receiver in the right image has been straightened as a result of the technique.
Credit: Sandia National Laboratories
Optical Model Development
NREL's SolTRACE computer model was specifically designed to allow detailed optical analysis of solar concentrators. Originally an in-house tool, it is being improved and expanded because many members of the rapidly growing CSP industry continue to use SolTRACE due to their familiarity with it and its ease of use. Commercial software codes are used to handle more-advanced modeling requirements beyond the scope of SolTRACE. Elements of such models developed at the laboratories are shared with industry to reduce the learning curve associated with these powerful programs.
Advanced Absorber Materials
A key to achieving lower costs from parabolic trough power plants is to increase the operating temperature, which increases power-cycle efficiency and reduces required storage size. But higher operating temperatures require an absorber coating on receivers with a lower thermal emittance. NREL has developed a prototype absorber coating that can achieve a significantly improved emittance of 0.07, coupled with a solar absorptance of 95% and is stable above 500°C. NREL has partnered with industry to bring this promising technology to market.
This field infrared camera and GPS unit, mounted within a van, are measuring receiver temperatures of a parabolic trough system.
Credit: Sandia National Laboratories
Receiver Heat-Loss R&D
NREL is expanding the capability of its receiver heat-loss measurement rig to allow faster tests and testing of longer receiver tubes. This apparatus can test the newest industry receiver tubes and tubes incorporating NREL's latest high-performance absorber coating. Researchers have tested new Schott and Solel receivers, as well as tubes from the field with and without hydrogen contamination.
For field measurements, an infrared camera with an associated Global Positioning System (GPS) unit can measure the glass temperature of receivers at a rapid rate of 6,000 receivers in one day. These glass temperatures can be analyzed to estimate the field heat losses.
Hydrogen Mitigation
NREL is investigating how to mitigate the increase in receiver heat loss associated with the diffusion of hydrogen from the heat-transfer fluid into the receiver vacuum space. Industry identified this as a key need to maintain long-term field performance. Researchers are testing different gas combinations to counteract the presence of hydrogen, as well as barrier coatings on the receiver tube. They are also modeling the occurrence of hydrogen within trough power plants to fully characterize the hydrogen generation process and implement a method to remove hydrogen from the heat-transfer fluid. This will eliminate significant migration of hydrogen into the receiver's annulus, which is the space between the receiver's metal tube and glass sleeve.
Power-Cycle and Total Plant R&D
Trough fields provide high-temperature fluid for boiling water and providing steam for Rankine steam power-cycles. DOE-supported R&D improves the performance and reduces the cost of electricity from trough systems by improving the power cycle and better integrating the field and power cycle.
One project in this area focuses on parallel heat-rejection systems. Researchers found that hybrid cooling systems (air-cooled condensers in parallel with water-cooled condensers) can reduce water use by 80% compared to a water-cooled plant, with only modest performance and cost penalties. These systems have undergone various analyses—for example, comparing the performance of air-cooled condensers in parallel with different-sized water-cooled condensers, and water-cooled condensers in parallel with different-sized air-cooled condensers. Further analysis will include simultaneously varying the sizes of both types of condensers to determine the optimum overall configuration for different locations.
A 2009 DOE Report to Congress provides more information on reducing water consumption of CSP systems (PDF 1.1 MB). Download Adobe Reader.
Industry Support
Solar companies are increasingly proposing and building parabolic trough power plants. And NREL and Sandia experts are providing technical support to industry to ensure that these new plants are built at minimum cost and maximum operational success. This technical assistance includes analytical studies, optical measurements, and field support.
Some of DOE's technical activities support the awardees of financial opportunities for concentrating solar power. The complete list of DOE-funded CSP projects is available on the Concentrating Solar Power Industry Projects page that specifically includes the linear concentrator industry projects.