Reading about the drawbacks of conventional Solar Power Satellites and the comments in response to Chris eventually triggered an idea, or perhaps a memory of something hinted at in something I read once. I’m somewhat less certain of complete originality of my ideas than I used to be.
The standard concept SPS has a few drawbacks that Chris brought out quite well. I hadn’t really given it that much thought and found the problems to be more interesting than the idea itself. There are several conversions in getting from sunlight to the terrestrial power grid. Each conversion has some efficiency loss which increases the required SPS size. The four or five conversions times efficiency of each jacks up the SPS size to several times the value that one would think of without doing the trades. The kilowatt per meter making a square kilometer a gigawatt facility becomes several square kilometers of SPS to net a gigawatt on the grid.
Another problem is the heat that must be disposed of to keep the solar cells and transmitters cool enough to work properly. The mass on orbit doubles again to net your gigawatt on the grid. A couple of unsettling problems if you happen to be an SPS fan.
In comments it was suggested that it would be better to simply orbit a mirror to reflect sunlight to the desired location. That doesn’t work because the sun is not a point source of light. Sunlight is converging at about 1% to a mirror in GEO and will diverge at the same 1% when reflected to Earth. The reflected sunlight would cover a disk of well over 300 kilometers on the ground so that a one km mirror would light the ground at 1/90,000 of solar power.
So my thought is to use one mirror to focus solar radiation on a hot spot that would then be a point source of radiation for a second mirror to send to Earth. The hot spot could be thought of as similar in intent to the tungsten filament in a light bulb in a flash light. This cartoon is not to scale and is meant to show the intent only.
This possibly could reduce beam spread to something reasonable at the expense of the beam being smeared across many frequencies. The visible light SPS could serve a few functions sometimes suggested by reflected sunlight advocates. If one km of sunlight could be focused such that 50% of the light was in a 10 km diameter, 1/200 of sunlight would be considerably brighter than a full moon. City lights for a large city without any conversion at all, and both storm and strike proof. Battlefield illumination as desired out of reach of interdiction. Operations lighting in the arctic for commercial and military uses. Night search and rescue. And so on for illumination as the beam would be too weak for power collection.
If a full sunlight focus is possible, then zenith solar cells would be considerably more productive than current usage.
The main advantage of a scheme like this, if feasible, is that it would be relatively light, cheap, and simple with a real likelihood of being implemented with mostly ET sources such as asteroids or the moon.
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