BollenG.ES@PARC-MAXC.ARPA (11/10/83)
Regarding Phil Karn's comment about the Extreme Cost of solar power generation: Solar Arrays for sattellites are constrained by both size, and mass, which a PowerSat would not have. PowerSats could use much cheaper technology to produce power, specifically, heat engines. In the orbittal vacuum, a shadowed area would radiate the heat absorbed on the sunward side of a barrier. This barrier could be extremely thin, as long as it remains opaque. This simple heat -cool cycle could very efficiently run a conventional turbine, and from there we go to our microwave transmitter. Such a structure could be built cheaply with lunar material, once we get out into space. No carefully processed Si is required, so it shouldn't cost $1,000 per watt. The technology is simple, so design and construction should be simple. The cost of the array should not be the determining factor in analysing the feasibility of Space Solar Power.
REM%MIT-MC@sri-unix.UUCP (11/12/83)
From: Robert Elton Maas <REM @ MIT-MC> There's a problem with using a simple barrier with sunward and darkward sides to drive a heat engine, rather than focussing the sunlight. If you focus the sunlight, you can approach the temperature of the Sun, i.e. 3000K, but if you merely face sunward you average the teensy Sun with all the black space around it and achieve only about "room temperature" of around 300K. On the dark side of course you get about 3K. But there's a temperature drop, analagous to the voltage drop in an electric circuit, because you are drawing (thermal) current between these two points, and there's not a direct contact between those points and the reference temperatures of 3K and 300K (or 3000K with focussing). Thus the sunward side may drop to 200K (or 2000K with focussing), while the darkward side may rise to 100K. The result is you're trying to run a heat engine with only a 3:2 ratio in temperatures, which gives a low conversion of energy-flow to useful power, in this case 3 units of incoming energy give 1 unit of useful power and 2 units of waste heat dumped out the darkward side. (The general rule is heat flow is proportional to temperature, and the difference is what you draw out in the form of useful power.) This is hardly what I'd call "very efficient". By comparison, with reflectors, and 2000K:100K = 20:1, you get 95% of your input energy converted to power instead of only 33%. Reflectors should be cheaper to build per unit area than conversion material, and the energy you produce doesn't have to be collected from a wide area if all light is focussed on a small convertor. The only main problem with reflectors is you have to carefully aim them toward the Sun all the time rather than just letting them face the general direction of the Sun. But all in all it seems reflectors are better than large conversion units. - Rebuttal anyone?