Despite the enormous promise of solar power, the drawbacks of the technology remain significant. People need electricity every day, around the clock, but there’s no part of the United States that is cloud-free 365 days a year - and no solar radiation at night. You have to find some way to store the energy for those sunless periods, and there’s not yet a large-scale way to do that.
Moreover, the best locations for solar arrays - the deserts of the American Southwest - are far from the centres of population, so even under the best of circumstances you’d have to send electricity many hundreds of miles through transmission lines that don’t yet exist.
But there is a way to tap into the sun’s energy 24 hours a day, every day of the year, and send it anywhere on the globe: launch solar panels into space and beam the power back to Earth.
The concept sounds far-fetched and wildly impractical, and when the Pentagon and space enthusiasts began talking about it back in the 1960s and 1970s, it was. Recently, however, the idea of space-based solar power, or SBSP, has begun to look less like science fiction and more like a technology whose time may be coming, with the Pentagon and private companies ramping up efforts to make space-based solar power a reality.
Two years ago, the Pentagon’s National Security Space Office (NSSO) issued a report recommending that the US “begin a coordinated national program to develop SBSP”. A year ago, engineers did a small but successful experiment using some of the technology that will be employed in SBSP, taking energy from solar cells, converting it to microwaves, and then beaming it 92 miles from Maui to the Big Island of Hawaii, where it was converted back into 20 watts worth of electricity.
And last spring, the California-based Solaren Corporation signed a contract with Pacific Gas & Electric (PG&E) to provide 200 megawatts of power - about half the output of an average coal-fired power plant - by 2016 by launching solar arrays into space. Several other companies have announced their intentions to put up solar satellites of their own.
Doubts abound that space-based solar power will come to pass anytime soon, and for good reason: The technology involves launching a series of large satellites into space, using robotic technology to assemble the solar arrays, transmitting the energy 22,000 miles to earth using microwave technology, and then converting that energy to electricity on the ground.
The fact is, however, that all of that is now feasible - if pricey - thanks to technological advances in recent years. These include cheaper and more reliable launch technology, lighter and stronger materials for solar stations, significant improvements in the robotic technology needed to assemble the solar arrays, far more efficient solar cells, more precise digital devices to direct that energy accurately to earth, and significantly smaller and more powerful microwave transmitters and receivers.
The big question is whether this engineering feat can be pulled off at a price competitive with terrestrial solar power. So far, the Pentagon’s estimate of what it will cost - $10 billion to put a 10-megawatt experimental solar station in orbit by 2016 - is five times higher than Solaren’s and would produce far less power.
A number of factors are driving the renewed interest in space-based solar power, including the push to cut greenhouse-gas emissions and growing interest from the military. But neither of these forces would mean much if the technology was outrageously expensive or too impractical.
It was a little bit of both when SBSP was first proposed in 1968 by an engineer named Peter Glaser, who worked for the consulting firm Arthur D. Little on a variety of space-related projects. The basic components - solar cells and microwave transmitters and receivers - already existed, and as the Apollo program began to wind down, NASA was trying to figure out what to do next.
In particular, says John Mankins, who became the manager for advanced concepts for NASA during the 1990s, “They were trying to figure out what to do with the space shuttle”. One idea was to begin launching space habitats - to get large numbers of people living and working in space. “These people would need something to do,” says Mankins, “so one idea was that they’d build solar-power satellites”.