Boffins break solar energy conversion record

Scientists at Sandia National Laboratories and Stirling Energy Systems have set a new record for solar energy conversion with a solar-to-grid system net conversion efficiency of 31.25 per cent.

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

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, president and chief executive at Stirling Energy Systems.

"This significant advance takes our dish engine systems well beyond the capacities of any other solar dish collectors and one step closer to commercialising an affordable system."

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.

Chuck Andraka, lead Sandia project engineer, explained that "several technical advancements" to the systems made jointly by Stirling Energy Systems and Sandia led to the record-breaking solar-to-grid conversion efficiency.

The first, and probably most important, was improved optics. The Stirling dishes are made from a low-iron glass with a silver backing that makes them hig hly reflective.

The dish focuses as much as 94 per cent of the incident sunlight to the engine package, whereas prior efforts reflected about 91 per cent.

The mirror facets, patented by Sandia and Paneltec Corp, are highly accurate and have minimal imperfections in shape.

Both improvements allow for the loss-control aperture to be reduced to seven inches in diameter, meaning that light is highly concentrated as it enters the receiver.

Other advances were a cheaper and more effective radiator and a new high-efficiency generator.

The perfect weather conditions on the New Mexico winter day when the record was set also played a part.

"It was a 'perfect storm' of sorts," said Andraka. "We set the record on a very cold day which was eight per cent brighter than normal."