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Solar dish breaks 20-year efficiency record

17 Mar 2008

A honeycomb structure of highly accurate and reflective mirrors boosts conversion of sunlight to electricity to beyond 31%.

A new solar-to-grid conversion efficiency record has been set by engineers in the US – thanks to an intricate arrangement of carefully aligned, highly reflective mirrors. The solar dish system, created by Sandia National Laboratories and Stirling Energy Systems (SES), enhances the conversion efficiency from the previous record of 29.4–31.25%.

"The old record was held by an excellent system, which has remained unbeaten for over 20 years, but the increase in efficiency we have achieved is a substantial improvement," Charles Andraka, lead project engineer at Sandia National Laboratories, told optics.org. "This is the highest system-level conversion efficiency recorded by any solar generation system."

The prototype design relies on key improvements to the optics, including the development and careful alignment of 82 highly accurate and reflective mirrors. The mirrors produce a peak concentration equivalent to 13,000 suns, which is converted into 26.75 kW of electrical power in a thermodynamic cycle.

"Each mirror is made from a thin, low-iron glass with a silver backing, which makes them highly reflective," explained Andraka. "The surface accuracy of each mirror is 0.8 mRad RMS compared with current commercial grade mirrors, which generally have an accuracy of 2–3 mRad."

The mirrors are then carefully arranged in a honeycomb structure to focus over 94% of the incident sunlight to the engine package, where previous designs reflected about 91%.

"We developed a mirror alignment strategy that provides a uniform flux at the engine receiver tubes," explained Andraka. "This results in an aperture diameter of just 7.5 inches compared with 8.6 inches on the prior record-holding system, which reduces thermal losses from re-radiation and reflection."

According to Andraka, the new design is not only more efficient, but as a pre-production prototype is ready for commercialization. "SES currently has power purchase agreements with Southern Californian Edison and San Diego Gas and Electric for up to 70,000 systems with additional contracts under negotiation," he said.

The next step for the group is to complete the design for manufacture stage with cost reduction in mind. "Approximately 50% of the cost is in the optical system, so careful design must be pursued to reduce this while maintaining a robust and accurate system," concluded Andraka.

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