Solar record

On a perfect New Mexico winter day - with the sky almost 10 per cent 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 per cent net efficiency rate.

The old 1984 record of 29.4 per cent was toppled on January 31 on SES’s 'Serial 3' solar dish Stirling system at Sandia’s National Solar Thermal Test Facility.

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.

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 to150kW 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.

Lead Sandia project engineer Chuck Andraka said that several technical advancements to the systems made jointly by SES and Sandia led to the record-breaking efficiency.

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 per cent of the incident sunlight to the engine package, where prior efforts reflected about 91 per cent. The mirror facets, patented by Sandia and Paneltec of Lafayette, Colorado, are highly accurate and have minimal imperfections in shape.

Other advancements were a new, more effective radiator that also costs less to build and a new high-efficiency generator.

While all the enhancements led to a better system, one aspect made it happen on a beautiful New Mexico winter day - the weather. The temperature, which hovered around freezing, allowed the cold portion of the engine to operate at about 23^oC, and the brightness meant that more energy was produced.

The test ran for two and a half hours, and 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 26.75kW net electrical power.

Osborn said that SES is working to commercialise the system and has signed power purchase agreements with two major Southern California utilities (Southern California Edison and San Diego Gas & Electric) for up to 1,750MW of power, representing the world’s two largest solar power contracts. Collectively, these contracts require up to 70,000 solar dish engine units.