Dish/engine concentrating solar power uses a mirror in the shape of a dish to collect and concentrates the sun's heat onto a small area where a receiver is located. The receiver transfers the sun's energy to a heat engine, usually a Stirling cycle engine, that converts the energy into power. Of all the solar technologies that hve been demonstrated on a practical scale the solar dish has the highest efficiency, 30%. The electricity needs of the entire U. S. could theoretically be met by such a system, in the desert, in an area 100 miles on a side.
The six dish system, shown at left, was installed by Stirling Energy Systems (SES) at Sandia National Laboratories in early 2005, the largest array of dish/Stirling systems in the world. Each unit operates automatically. Without operator intervention or even on-site presence, it starts up each morning at dawn and operates throughout the day, tracking the sun and responding to clouds and wind as needed. Finally it shuts itself down at sunset. The system can be monitored and controlled over the Internet. Experimental models of the Stirling dish technology have undergone more than 26,000 hours of successful solar operation. The cost for each prototype unit is about $150,000. Once in production SES estimates that the cost could be reduced to less than $50,000 each, ($2.00 per watt) which would make the cost of electricity competitive with conventional fuel technologies.
Southern California Edison (SCE) and Stirling Energy Systems have announced an agreement that could result in construction of the world’s largest solar facility, capable of producing more electricity than all other U.S. solar projects combined. The SCE-SES project represents the first major application of Stirling dish technology in the commercial electricity generation field. Initially, Stirling would build a one-MW test facility using 40 of the company’s 37-foot-diameter dish assemblies. Subsequently, a 20,000-dish array would be constructed near Victorville, Calif. The 20-year power purchase agreement signed August 9, 2005, which is subject to California Public Utilities Commission approval, calls for development of a 500-megawatt (MW) solar project 70 miles northeast of Los Angeles using innovative Stirling dish technology. The agreement includes an option to expand the project to 850 MW.
Development of modern dish/engine systems began in the late 1970s and early 1980s. This technology used directly illuminated, tubular solar receivers, a kinematic Stirling engine developed for automotive applications, and silver/glass mirror dishes. Systems, nominally rated at 25 kWe, achieved solar-to electric conversion efficiencies of around 30 percent. Eight prototype systems were deployed and operated on a daily basis from 1986 through 1988.
Current dish/engine efforts are being continued by three U.S. industry teams - Science Applications International Corp. (SAIC) teamed with STM Corp., Stirling Energy Systems (SES), and WG Associates with Sunfire Corporation. SAIC and SES together have five 25kW systems under test and evaluation at utility, industry, and university sites in Arizona, California, and Nevada. WGA has two 10kW systems under test in New Mexico, with a third off-grid system was developed in 2002 on an Indian reservation for water-pumping applications.
Dish/engine systems utilize concentrating solar collectors that track the sun in two axes. A reflective surface, metalized glass or plastic, in the shape of a dish, reflects incident solar radiation to a small region called the focus. The thermal receiver, located at the focus, is the interface between the dish and the engine/generator. It absorbs the concentrated beam of solar energy, converts it to heat, and transfers the heat to the engine/generator. A thermal receiver can be a bank of tubes with a cooling fluid, usually hydrogen or helium, which is the heat transfer medium and also the working fluid for an engine. The most common type of heat engine used in dish-engine systems is the Stirling engine.
A Stirling engine uses heat provided from an external source (like the sun) to move pistons and make mechanical power, similar to the internal combustion engine in your car. The mechanical work, in the form of the rotation of the engine’s crankshaft, is used to drive a generator and produce electrical power. Working gas temperatures of over 700ºC (1292ºF) and as high as 20 MPa are used in modern high-performance Stirling engines. In the Stirling cycle, the working gas is alternately heated and cooled by constant-temperature and constant-volume processes. Stirling engines usually incorporate an efficiency-enhancing regenerator that captures heat during constant-volume cooling and replaces it when the gas is heated at constant volume. The best of the Stirling engines achieve thermal-to-electric conversion efficiencies of about 40%.
The collectors track the sun in two axes. In larger systems the the dish rotates in a plane parallel to the earth (azimuth) and in another plane perpendicular to it (elevation). This gives the collector left/right and up/down rotations. In the polar tracking method, used in some smaller systems, the collector rotates about an axis parallel to the earth’s axis of rotation at a constant rate of 15º/hr to match the rotational speed of the earth. The other axis of rotation, the declination axis, is perpendicular to the polar axis. Movement about this axis occurs slowly and varies by +/- 23½º over a year.
According to a 2001 "Sun-Lab Snapshot" the Boeing/SES system, for which SES has obtained rights and is very similar to the module being marketed, incorporated the following components:
- Collector: 82 facet mirror made with 0.7 mm thin glass with 87.7 m2 area, relectivity 0.91
- Engine: Kockums 4-95, 4 cylinder stirling engine 380 cc, working temp 720ºC (1328ºF), with variable pressure control
Rating and performance:
- Module rating: 25 kW at 1000W/m2 solar input, Electrical: 480 v, 3 phase 50 or 60 cycle
- Module performance: Peak power 24.9 kW, peak efficiency 29.4%, Annual efficiency 24%
There may have been changes since that time, but that is the best information available.
Solar/Concentrating Solar Power/Dish Engine Systems, US DOE Energy Efficiency and Renewable Energy
"Solar Dish Engine", solarpaces.org/solar_dish.pdf (very slow link)
"Sandia, Stirling to build solar dish engine power plant", Sandia press release, 10/9/04
"Major New Solar Energy Project Announced by Southern California Edison and Stirling Energy Systems, Inc.", Edison International press release, 8/9/05
Stirling Energy Systems Inc., Phoenix, AZ, www.stirlingenergy.com/
"The Boeing/SES DECC Project", Sun Lab Snapshot, August 2001