While searching about solar power I came across an interesting piece on NASA website.Though a bit old it is still relevent.
Try this: Close your eyes for a moment and imagine the International Space Station (ISS), sunlit and gleaming as it circles our planet.What did it look like? The lingering image in your mind is probably dominated by broad, beautiful wings — the station’s awesome solar arrays.
It’s no accident that solar panels dominate the station’s profile. On the ISS (as on the earth below) solar energy ultimately powers everything that happens. Our Sun, a star named Sol, radiates enormous power: a constant output of 4 x 1023 kilowatts (kW), which is a 4 followed by 23 zeros! Photovoltaic cells, which convert sunlight to electricity, need only intercept a tiny fraction of that total to energize the station.
But not all spacecraft linger near Earth where sunlight is plentiful. Many NASA probes travel far beyond our planet’s orbit. And as they do, the Sun grows more distant and dim. Somewhere out there, solar power ceases to be a useful source of energy for spacecraft. But where?
That’s what NASA spacecraft builders want to know: Where is the edge of sunshine?
The Space Station’s solar cells, developed decades ago, convert 14% of the Sun’s energy that hits them into electricity, and modern multibandgap cells, which convert light in multiple parts of the spectrum into electric power, reach efficiencies of 30% or so. Such devices work well enough in the brightly-lit inner solar system, but more efficient cells and larger arrays will be needed as spacecraft travel to places where solar photons are scarce. In the outer reaches of the solar system, for instance, the ability to convert even single photons into electricity would be important.
“Sunlight decreases in intensity over distance by a factor of 1/r2, where r is the distance from the Sun,” explains Geoff Landis, a scientist at NASA’s Glenn Research Center. “This means a 1-meter-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.” (Note: An astronomical unit or “AU” is the mean distance between Earth and the Sun. 1 AU equals 150 million kilometers.)
Left: The Sun as viewed from distant Pluto is just another star in the night sky, albeit the brightest one. Space artist (and space physicist) Dan Durda painted this view from the 9th planet.
Landis and his colleagues at Glenn’s Photovoltaics and Space Environment Branch are 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,” says 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.”