2011-07-07   Source: www.economist.com

http://www.economist.com/node/18864324?story_id=18864324&fsrc=rss

Harvesting solar power in space, for use on Earth, comes a step closer to reality

THE idea of collecting solar energy in space and beaming it to Earth has been around for at least 70 years. In “Reason”, a short story by Isaac Asimov that was published in 1941, a space station transmits energy collected from the sun to various planets using microwave beams.

The advantage of intercepting sunlight in space, instead of letting it find its own way through the atmosphere, is that so much gets absorbed by the air. By converting it to the right frequency first (one of the so-called windows in the atmosphere, in which little energy is absorbed) a space-based collector could, enthusiasts claim, yield on average five times as much power as one located on the ground.

The disadvantage is cost. Launching and maintaining suitable satellites would be ludicrously expensive. But perhaps not, if the satellites were small and the customers specialised. Military expeditions, rescuers in disaster zones, remote desalination plants and scientific-research bases might be willing to pay for such power from the sky. And a research group based at the University of Surrey, in England, hopes that in a few years it will be possible to offer it to them.

Heavenly power

This summer, Stephen Sweeney and his colleagues will test a laser that would do the job which Asimov assigned to microwaves. Certainly, microwaves would work: a test carried out in 2008 transmitted useful amounts of microwave energy between two Hawaiian islands 148km (92 miles) apart, so penetrating the 100km of the atmosphere would be a doddle. But microwaves spread out as they propagate. A collector on Earth that was picking up power from a geostationary satellite orbiting at an altitude of 35,800km would need to be spread over hundreds of square metres. Using a laser means the collector need be only tens of square metres in area.

Dr Sweeney’s team, working in collaboration with Astrium, a satellite-and-space company that is part of EADS, a European aerospace group, will test the system in a large aircraft hangar in Germany. The beam itself will be produced by a device called a fibre laser. This generates the coherent light of a laser beam in the core of a long, thin optical fibre. That means the beam produced is of higher quality than other lasers, is extremely straight (even by the exacting standards of a normal laser beam) and can thus be focused onto a small area. Another bonus is that such lasers are becoming more efficient and ever more powerful.

In the case of Dr Sweeney’s fibre laser, the beam will have a wavelength of 1.5 microns, making it part of the infra-red spectrum. This wavelength corresponds to one of the best windows in the atmosphere. The beam will be aimed at a collector on the other side of the hangar, rather than several kilometres away. The idea is to test the effects on the atmospheric window of various pollutants, and also of water vapour, by releasing them into the building.

Assuming all goes well, the next step will be to test the system in space. That could happen about five years from now, perhaps using a laser on the International Space Station to transmit solar power collected by its panels to Earth. Such an experimental system would deliver but a kilowatt of power, as a test. In 10-15 years Astrium hopes it will be possible to deploy a complete, small-scale orbiting power station producing significantly more than that from its own solar cells.

Other researchers, in America and Japan, are also looking at using lasers rather than microwaves to transmit power through the atmosphere. NASA, America’s space agency, has started using them to beam energy to remotely controlled drones. Each stage of converting and transmitting power results in a loss of efficiency, but with technological improvements these losses are being reduced. Some of the latest solar cells, for instance, can covert sunlight into electricity with an efficiency of more than 40%. In the 1980s, 20% was thought good.

Whether the Astrium system will remain a specialised novelty or will be the forerunner of something more like the cosmic power stations of Asimov’s imagination is anybody’s guess. But if it comes to pass at all, it will be an intriguing example, like the geostationary communications satellites dreamed up by Asimov’s contemporary, Arthur C. Clarke, of the musings of a science-fiction author becoming science fact.