On a perfect New Mexico winter day—with the sky almost 10% brighter than usual—Sandia National Laboratories and Stirling Energy Systems (SES) set a new solar-to-grid system conversion efficiency record by achieving a 31.25% net efficiency rate. The old 1984 record of 29.4% was toppled January 31 by SES’s “Serial #3” solar dish Stirling system at Sandia’s National Solar Thermal Test Facility (Figure 1).
3. For the record books. Sandia and Stirling Energy Systems set a new world record for solar-to-grid conversion efficiency of 31.25%, eclipsing the old record of 29.4% that had stood for 24 years. Courtesy: Sandia National Laboratory, Photo by Randy Montoya
The conversion efficiency is calculated by measuring the net energy delivered to the grid and dividing it by the solar energy hitting the dish mirrors. Auxiliary loads, such as water pumps, computers and tracking motors, are accounted for in the net power measurement.
“Gaining two whole points of conversion efficiency in this type of system is phenomenal,” said Bruce Osborn, SES president and CEO. “This is a significant advancement that takes our dish engine systems well beyond the capacities of any other solar dish collectors and one step closer to commercializing an affordable system.”
Serial #3 was erected in May 2005 as part of a prototype six-dish model power plant at the Solar Thermal Test Facility that produces up to 150 kW of grid-ready electrical power during the day. Each dish unit consists of 82 mirrors arranged in a dish shape to focus sunlight into an intense beam.
The solar dish generates electricity by focusing the sun’s rays onto a receiver, which transmits the heat energy to a Stirling engine. The engine is a sealed system filled with hydrogen. As the gas heats and cools, its pressure rises and falls. The change in pressure drives the pistons inside the engine, producing mechanical power, which in turn drives a generator and makes electricity.
Lead Sandia project engineer Chuck Andraka says that several technical advancements to the systems made jointly by SES and Sandia led to the record-breaking solar-to-grid conversion efficiency. SES owns the dishes and all the hardware. Sandia provides technical and analytical support to SES in a relationship that dates back more than 10 years.
Andraka says the first and probably most important advancement was improved optics. The Stirling dishes are made of a low-iron glass with a silver backing that make them highly reflective—focusing as much as 94% of the incident sunlight on the engine package, whereas prior efforts reflected about 91%. The mirror facets, patented by Sandia and Paneltec Corp. of Lafayette, Colo., are highly accurate and have minimal imperfections in shape.
Both improvements allow for the loss-control aperture to be reduced to 7 inches in diameter—meaning that light is highly concentrated as it enters the receiver. Other advancements to the solar dish–engine system that helped Sandia and SES beat the energy conversion record were a new, more effective radiator that also costs less to build and a new high-efficiency generator.
The temperature on the record-setting date, which hovered around freezing, allowed the cold portion of the engine to operate at about 73F, and the sky’s brightness meant that more energy was produced while most parasitic loads and losses were constant. The test ran for two and a half hours. A 60-minute running average was used to evaluate the power and efficiency data, in order to eliminate transient effects. During the testing phase, the system produced 26.75 kW, net.
SES is working to commercialize the record-performing system and has signed power purchase agreements with two major southern California utilities (Southern California Edison and San Diego Gas & Electric) for up to 1,750 MW, representing the world’s two largest solar power contracts. Collectively, these contracts require up to 70,000 solar dish–engine units.