Spacebased Solar

With panels in orbit, where the sun shines brightest— and all the time—solar could really take off. But there's a catch

When Peter Glaser proposed in 1968 that city-size satellites could harvest solar power from deep space and beam it back to the earth as invisible microwaves, the idea seemed pretty far out, even given Glaser's credentials as president of the International Solar Energy Society. But after the oil crises of the 1970s sent fuel prices skyrocketing, NASA engineers gave the scheme a long hard look. The technology seemed feasible until, in 1979, they estimated the "cost to first power": $305 billion (in 2000 dollars). That was the end of that project.

Solar and space technologies have made great strides since then, however, and space solar power (SSP) still has its champions. Hoffert cites two big advantages that high-flying arrays could lord over their earthbound brethren. In a geostationary orbit well clear of the earth's shadow and atmosphere, the average intensity of sunshine is eight times as strong as it is on the ground. And with the sun always in their sights, SSP stations could feed a reliable, fixed amount of electricity into the grid. (A rectifying antenna, or "rectenna," spread over several square kilometers of land could convert microwaves to electric current with about 90 percent efficiency, even when obstructed by clouds.)

"SSP offers a truly sustainable, global-scale and emission-free electricity source," Hoffert argues. "It is more cost-effective and more technologically feasible than controlled thermonuclear fusion." Yet there is minimal research funding for space-based solar, he complains, while a $10-bil-lion fusion reactor has just been approved.

NASA did in fact fund small studies from 1995 to 2003 that evaluated a variety of SSP components and architectures. The designs took advantage of thin-film photovoltaics to create the electricity, high-temperature superconductors to carry it, and infrared lasers (in place of microwave emitters) to beam it to ground stations. Such high-tech innovations enabled SSP engineers to cut the systems' weight and thus reduce the formidable cost of launching them into orbit.

But here's the catch: the power-to-payload ratio, at a few hundred watts per kilogram, has remained far too low. Until it rises, space-based solar will never match the price of other renewable energy sources, even accounting for the energy storage systems that ground-based alternatives require to smooth over nighttime and poor-weather lulls.

Technical advances could change the game rap idly, however. Lighter or more efficient photovoltaic materials are in the works [see "Nanotech Solar Cells," on page 110]. In May, for example, researchers at the University of Neuchátel in Switzerland reported a new technique for depositing amorphous silicon cells on a space-hardy film that yields power densities of 3,200 watts per kilogram. Although that is encouraging, says John C. Mankins, who led NASA's SSP program from 1995 to 2003, "the devil is in the supporting structure and power management." Mankins sees more promise in advanced earth-to-orbit space transportation systems, now on drawing boards, that might cut launch costs from more than $10,000 a kilogram to a few hundred dollars in coming decades.

JAXA, the Japanese space agency, last year announced plans to launch by 2010 a satellite that will unfurl a large solar array and beam 100 kilowatts of microwave or laser power to a receiving station on the earth. The agency's long-term road map calls for flying a 250-megawatt prototype system by 2020 in preparation for a gigawatt-class commercial SSP plant a decade later.

NASA once had similarly grand designs, but the agency largely halted work on SSP when its priorities shifted to space exploration two years ago.

1 Incoming sunlight is concentrated by a thin-f ilm reflector covering 2.9 square kilometers

2 A solar panel converts the light to electric current

3 Cables carry the current to a phased array of microwave generators

Show|toppers

□ Large teams of robots will have to work together to assemble the giant arrays.

□ The microwave beams could cause interference with communications systems.

□ Space agencies will have to boost their launch rates by a factor of about 80.

□ Rectennas will occupy large swaths of land.

REALITY FACTOR

4 Afocused microwave beam delivers the energy to an antenna on the ground

▲ Giant solar collector in geosynchronous orbit would work day and night, in any weather. A pilot plant of the size above would intercept four gigawatts of sunlight, convert it to 1.8 GW of microwaves, and deliver 1.1 GW of electricity to the grid.

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Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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