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Re: Heat instead of solar power



>SO we figured to just make half of all 6 sides of our cube be WHITE and
>half be Black.  Then internallly thermally bind all the WHITE half-faces
>with the black-half-faces and put the Peltier junctions between.  It
>looked like a Rubix cube inside, but would be one way to get a one-way
>heat flow on an unstabalized spacecraft..

I don't think that will work. You have to dispose of nearly all of the
heat you absorb from the sun (minus the small fraction that is
converted into electricity) *and* you have to do it at a significantly
lower temperature to maximize the conversion efficiency. White paint
has low emissivity, so it doesn't make a good radiator.

Ideally you'd have attitude control that keeps a collector surface
facing the sun and a radiator surface facing dark sky. The collector
would have the highest possible alpha (absorptivity at optical
wavelengths) and the radiator would have the highest possible epsilon
(emissivity at long infrared wavelengths). You'd also want to keep the
radiator from seeing the earth, as the earth is much hotter than cold
sky. This may be hard to do at LEO.

I doubt you'll ever get efficiences from solar thermal electric
conversion that rival what you can now get from good (GaAs)
photovoltaics.  The information I can find on RTGs gives a 6%
conversion efficiency for a 700K temperature difference between the
nuclear fuel source and the radiators. The Pu-238 in each RTG on
Galileo produces 4,410W of heat, but only 285W comes out as
electricity.

To maintain a 700K temperature difference with solar energy, you'd
probably have to use concentration on the collector side. You might
also do better with a heat engine (e.g., Stirling) in place of the
thermal electric conversion, but we're already talking about something
far more complicated than a photovoltaic panel.

Phil

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