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Re: Heat instead of solar power



On Sat, 9 Mar 2002, Phil Karn wrote:

> >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.

Ah, but White actually has the same emissivity for infrared as black.
THus it radiates heat just as easily as black. So the delta-T comes from
the 3 times higher absorbtivity of black than white for the solar
spectrum.  In free space, insulated white will get to about +10C and Black
up to about +110 C.  But of course, when we connect them to get heat
flow, then the delta-T will be much less. So here would be my guess:

For a 1 foot cube satellite with 120 watts of solar energy impinging on
it, only half of that is hitting the Half-Black side so we have 60 watts
passing through the white/black system.  And if the efficiency of the
Peltier junctions could be as high as say 1%, then we might get
something less than a Watt.  (compared to 15 or more for Solar).

BUT it all depends on the DELTA-T.  And with all sides (just as you say)
"seeing" the earth, they are not going to get very cold.  My guess is that
the most we might get would be say about 20 deg C delta, and I bet that
the thermal efficiency of the junctions is so low at that low a value to
be useless.

Anyway, the half/black half-white sides is the only way we could figure to
come up with a constant delta-T with an unstabalized spacecraft..

Think of a solid WHITE cube (the coolest).  Now paint the center 1/2 area
of each side BLACK and thermally isolate the two, and internally bind all
the white parts together and the black parts and insulate them.  Then put
the Peltier junctions between the two...

Still, I agree that the energy is so small it is not worth doing unless we
did have an opportunity where we can have a Black side  always in SUN
and a White side always facing SPACE.

 de WB4APR@amsat.org, Bob

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