Solar Power from Space

Proposed space solar power systems utilise well-known physical principles -- namely, the conversion of sunlight to electricity by means of photovoltaic cells. Giant structures consisting of row after row of photovoltaic (PV) arrays could be placed either in a geostationary Earth orbit or on the Moon. A complete system would collect solar energy in space, convert it to microwaves, and transmit the microwave radiation to Earth where it would be captured by a ground antenna and transformed to usable electricity.

According to an April 2000 article in the Electric Power Research Institute (EPRI) Journal, photovoltaic arrays in a geostationary Earth orbit (at an altitude of 22,300 miles) would receive, on average, eight times as much sunlight as they would on Earth's surface. Such arrays would be unaffected by cloud cover, atmospheric dust or by the Earth's day-night cycle.

When the idea was first proposed more than 30 years ago, PV technology was still in its infancy. The conversion efficiency rate -- the fraction of the sun's incident energy converted into electricity -- was only 7 to 9 percent.

"We now have the technology to convert the sun's energy at the rate of 42 to 56 percent," said Marzwell. "We have made tremendous progress."

Even so, launching thousands of tons of solar arrays into space will be expensive. But there may be a way to reduce the needed area of the arrays -- by concentrating sunlight.

"If you can concentrate the sun's rays through the use of large mirrors or lenses you get more for your money because most of the cost is in the PV arrays," said Marzwell.

A drawback to concentrated sunlight is that it is hot. Focused radiation that's not converted to electricity turns into heat -- enough to damage the arrays if there's too much excess warmth. Marzwell and his colleagues at JPL are studying ways to capture waste heat and convert it to electricity by means of thermal voltaic processes. Special coatings on the mirrors and lenses can also reject portions of the sun's spectrum that PV arrays don't use, further reducing excess heat.

Once the Sun's energy is captured in space, what do you do with it?