Unexpectedly Intriguing!
29 June 2005

Do you remember the electrical power crisis in California in 2000? At one point, then-Governor Gray Davis, after presiding over the state's Christmas Tree lighting on the capital lawn in Sacramento and ceremoniously flipping the switch to light the tree, felt he had to pull the plug just minutes later to conserve the state's then very limited supply of power.

The situation was bad enough that some really crazy ideas were advanced as serious solutions to the state's electrical supply problems. One of the more unique ideas involved launching satellites into space for the purpose of converting solar energy into microwave beams that could then be directed to a ground station on Earth and converted into electricity.

Solar Power Generating Satellite

As ideas go, it's a big one. Solar energy is clean, virtually free, and abundant - especially compared to other energy sources. In space, unlike ground-based solar power generating stations, the photovoltaic (PV) cells attached to an orbiting satellite would be completely unaffected by weather, smoke or dust in the air, or even the prolonged darkness of night. So, why hasn't this idea taken off?

The Dismal Science

The answer lies in the intersection of economics and engineering. Even with today's technology, which has greatly increased the electricity-generating capability of PV cells in recent years, the technology's lack of cost competitiveness with other energy sources limits its pursuit.

First, the cost of building such a solar power satellite must be considered. To generate enough energy to beam back to Earth to justify the expense of launching such a system into orbit would require constructing a very large satellite using available technology. Rocket launches aren't cheap - and it could take several launches to support the construction effort.

Second, the opportunity cost of the needed orbital real estate is substantial. To consistently provide power for a given area on Earth, the satellite would have to be positioned in geosynchronous orbit, some 22,300 miles above sea level, in order to remain in the same position in the sky above a ground station at all times. The number of geosynchronous orbit slots available for positioning a solar power satellie is limited, which means that a lot of satellites with other uses (communications being the primary one) would have to be potentially denied the use of the orbital slot designated to support generating power from space.

Third, such a satellite would also require substantial effort to keep it in its desired orbit given its size. In orbit, positioning thrusters on a satellite must be fired on semi-regular intervals in order to maintain the satellite's orbit. Without such orbital positioning maintenance, satellite orbits eventually decay - for a solar power generating satellite, this would mean eventually not being able to consistently supply power to its intended ground station as it drifts out of its desired position.

And then there's the problem of directly maintaining the satellite itself. What do you do when you need to fix the satellite? Or worse, when it's time to replace the satellite altogether? Is it really any wonder why this technology hasn't taken off already?

Where Would They Work?

Earth and Moon It does occur to me however that solar power satellites do have a role to play in supplying power to meet human needs. It's just not here on Earth! Instead, consider what it would take to supply power for a base on the Moon.

Fortunately, the solar power satellite it would take to provide power for a lunar base would be much smaller than one that would transmit power back to the Earth's surface. This means that the technical requirements for building and launching such a satellite for a lunar mission would be much less that for a mission that would transmit power back to Earth.

Also consider the extreme environment of the Moon. Daylight on the lunar surface lasts for two weeks, which is followed by darkness for another two weeks. A future lunar base powered by solar energy could get around this restriction by being located near the Moon's poles, where solar energy collection might be possible on a near-continuous basis, but that may not be the most desirable place to locate a base. Alternatively, a base could be located anywhere else on the Moon's surface, but would require significant energy storage capacity to be able to get through the lunar night.

A solar power satellite system would be more than capable of getting around this limitation. The satellite could provide power around the clock, making it possible to locate a base anywhere desired on the lunar surface. While it will still be necessary to have power generating equipment and battery storage for redundancy or safety requirements on the surface, the base would be able to get away with having a lot smaller energy generating and storage capacity, which brings up another good point: it would save weight!

For a lunar mission, the mass of what may be safely landed on the moon is just a tiny fraction of what is initially launched. For example, the Apollo 17 mission in 1972, the Saturn V rocket that launched the mission into space weighed some 6,444,965 lb (2,923,387 kg) at launch while the portion of the vehicle that was able to be safely landed upon the lunar surface weighed just 36,262 lb (16,448 kg), just 0.56% of the starting weight of the vehicle! When so much of the cost of supporting a lunar base is directly driven by the cost of launching and landing objects on the moon, the ability to not have to transport solar energy generating equipment to the lunar surface makes real economic sense.

Another application for solar power satellites is discussed at the Spacecraft blog.


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