NASA Scientists Push PV To Its Limits

NASA scientists are improving the output of solar PV cells to keep spacecraft powered up as they go further from the sun.

CLEVELAND, Ohio, US, 2002-01-22 [] “A 1 metre square solar array producing 400 watts at a distance of 1 AU would have to be 25 square meters in size out at Jupiter and almost 2,000 square meters at Pluto to yield the same power,” notes Geoff Landis of the Glenn Research Centre in the latest edition of NASA science news. The term ‘AU’ means an astronomical unit, and is the mean distance between earth and the sun, or 150 million kilometers. Glenn’s Photovoltaics & Space Environment Branch is exploring new ways to harness the sun’s power, including more efficient solar cells, laser-beaming energy to distant spacecraft, and solar power systems for the moon and Mars. “The use of solar power is a complex field of study,” explains Landis. “Finding solutions requires that we balance such factors as distance, weight, the energy of different light bands, and the actual materials available to us.” “Using today’s technologies, the ‘edge’ of sunshine we can use is about four astronomical units away from the sun, where the sunlight is about one-sixteenth as bright as it is near the earth.” To date, the farthest that any solar-powered spacecraft has ventured from the sun is 2.35 AU, set last October by NASA’s Stardust probe. Stardust will extend its record until this April, when it will reach a maximum distance from the sun of 2.72 AU on its way to Comet Wild 2. Stardust’s solar arrays are producing more energy than expected, perhaps because PV cells operate more efficiently in the cold of deep space. NASA’s experimental spacecraft Deep Space 1 recently tested a ‘solar concentrator’ of 720 lenses that focused sunlight onto 3,600 solar cells. It was the first solar-powered probe to rely on triple-junction multi-bandgap cells. The system generated 2.5 kW, enough to energize three microwave ovens and to power the craft’s ion engine. “In the long term, solar arrays won’t have to rely on the sun,” says Landis. “We’re investigating the concept of using lasers to beam photons to solar arrays. If you make a powerful-enough laser and can aim the beam, there really isn’t any edge of sunshine. With a big enough lens, we could beam light to a space-probe halfway to alpha-Centauri!”
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