Sun seekers: advances in space exploration technology could herald the answer to the global energy crisis and the search for emission-free sources to replace fossil fuels. Helen Knight and Jon Excell report

Engineer: The Professional Bulletin for Army Engineers, March 11, 2005 by Helen Knight, Jon Excell

IN JULY a team of engineers from Europe and Japan will launch two small crawling robots into space. This humble experiment may come to represent a major step in the development of an emission-free energy source to meet all of Earth's requirements.

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The robots, one made in Europe and one expected to be from Japan, will be launched from Japan in a small test rocket. When the rocket reaches a height of 220km three daughter satellites will be separated from their mother satellite to stretch out a 40m X 40m Furoshiki net. A microwave antenna will then be switched on and begin transmitting signals to a receiver on the ground, and the robots will spend three minutes crawling across the net, under the eye of several cameras.

Later this month five robots, including two developed by Vienna University of Technology with funding from ESA, will undergo aircraft micro-gravity experiments in preparation for the rocket launch. The experiment could have huge implications for telecommunications, Earth observation and satellite navigation. If it is successful such robots could in future crawl along a large mesh of up to 1km X 1km in space, deploy an inflatable structure to create a rigid frame for the net, before assembling themselves into huge structures such as antennas.

But perhaps the most intriguing application for this concept is the part it could play in harvesting solar power from space and transmitting it to large receivers on Earth using microwave or laser beams. The huge structures needed to build solar power satellites would make construction difficult and expensive--a microwave transmitting antenna alone would be around 1km in diameter. So using robots to build the structures in space could cost less.

The concept of solar power from space (SPS) was first proposed by Czech-US engineer Peter Glaser in 1968, but is now receiving renewed attention amid fears over climate change and the search for emission-free energy sources. Unlike terrestrial solar power plants, a space-based system could collect energy 24 hours a day, with no atmospheric conditions limiting its exposure to the sun. Plus solar flux, the energy or photon flow rate from the sun, is around eight times higher in space than it is on Earth.

According to ESA's Advanced Concepts Team (ACT), if we could come close to the theoretical power transmission efficiencies via electromagnetic waves of 50 to 60 per cent, the electricity grid could be continuously supplied with 75-100W/[m.sup.2] of space photovoltaic panel, about three or four times the amount from the same-sized terrestrial PV surfaces.

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The technology could potentially generate up to several hundred GW of power, whereas a modern nuclear power plant generates around 1GW. Europe alone is expected to need around 500GW of electricity by 2020.

ACT is investigating solar power from space under its SPS Programme Plan. Its first phase, assessing the general viability of recent concepts for supplying power to Earth from space, was completed last year. Concepts include the European Sail Tower, which would consist of a 15km central tether with 120 power-generating solar sails attached in pairs down its length. The tower, which could generate 450MW (170W/[m.sup.2]) of power in space, resulting in 275MW (100W/[m.sup.2]) of electricity output on Earth, would be launched in individual sail modules into low Earth orbit, from which point electrical thrusters would propel each module to geostationary orbit, to be constructed into a tower robotically.

The second phase of ESA's SPS programme, due to start in spring, will identify technology areas requiring further research before such systems become a possibility, and establish roadmaps for research priorities. The team will also investigate the integration of SPS concepts into future energy networks, in particular the use of space solar power for hydrogen generation.

The technologies needed to make SPS a reality are not yet mature enough for it to be seen as a serious option, said Dr Leopold Summerer, deputy head of ACT. Further development is needed in areas such as photovoltaic cell efficiency to ensure that the concept is ready to be taken seriously at the next big energy discussion after the current one, expected in 2020-25, he said. 'SPS is too attractive not to do anything, but too immature to embark on anything really substantial for the moment.'

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To ensure the concept is taken more seriously, ESA has brought together SPS experts and independent energy consultants under umbrella research groups. As a result of these discussions the SPS experts have convinced their terrestrial counterparts that there's more to the concept than simply a way to build up the European launcher industry, said Summerer.

'Using its own models, the terrestrial community has had to acknowledge that the energy payback time, or the time necessary to get as much energy out of the system as you put into it when you construct and maintain it, is lower for the space system than for the ground system, despite the launchers.'