New Record Set for Solar-to-Grid Conversion Efficiency

Sandia National Laboratories and Stirling Energy Systems set the new solar-to-grid system conversion efficiency record by achieving a 31.25 percent net efficiency rate.

The old 1984 record of 29.4 percent was bettered on January 31, a perfect New Mexico winter day with the sky about eight percent brighter than usual.

"Gaining two whole points of conversion efficiency in this type of system is phenomenal," says Bruce Osborn, chief executive of Stirling Energy Systems, SES.

"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," Osborn said.

Mirrored Stirling solar dishes reflect the sky at the National Solar Thermal Test Facility (Photo by Randy Montoya courtesy Sandia)

The conversion efficiency is calculated by measuring the net energy delivered to the power 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.

The new record was set on SES's Serial #3 solar dish at Sandia's National Solar Thermal Test Facility at Albuquerque. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration.

Serial #3 was erected in May 2005 as part of a prototype six-dish model power plant that produces up to 150 kilowatts of grid-ready electrical power during the day. Each dish unit consists of 82 mirrors formed in a dish shape to focus the light to 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.

SES owns the dishes and the associated hardware at the Test Facility. Sandia provides technical and analytical support to SES in a relationship that dates back more than 10 years.

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.

Andraka says the first and probably most important advancement was improved optics. The Stirling dishes are made with a low iron glass with a silver backing that make them highly reflective, focusing as much as 94 percent of the incident sunlight to the engine package, where prior efforts reflected about 91 percent.

Osborn says that 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 megawatts of power, representing the world's two largest solar power contracts.

Collectively, these contracts require up to 70,000 solar dish engine units.

Osborn said SES is commercializing its SunCatcher system, preparing for mass production, adding that, "The demonstrated high efficiency means more energy is generated for the given investment, lowering the cost of the energy delivered. This exciting record shows that using these dishes will be a cost-effective and environmentally friendly way of producing power."

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