O&M, Renewables

Record-setting solar-to-grid conversion efficiency

14 February 2008 — Sandia National Laboratories and Stirling Energy Systems set a 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 surpassed January 31 on SES’s “Serial #3” solar dish Stirling 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.

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 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 solar-to-grid conversion efficiency. 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. They focus as much as 94 percent of the incident sunlight to the engine package, where prior efforts reflected about 91 percent. The mirror facets 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 light is highly concentrated as it enters the receiver.

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

The weather also helped. January 31 was a very cold and extremely bright day, with sunlight 8 percent brighter than normal.

The temperature allowed the cold portion of the engine to operate at about 23 C, and the brightness meant more energy was produced while most parasitic loads and losses are constant. The test ran for two and a half hours and a 60-minute running average was used to evaluate the power and efficiency data to eliminate transient effects. During the testing phase, the system produced 26.75 kW net electrical power.