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Re: Peltier element in space ?



At 08:08 AM 2/24/2004, William Leijenaar wrote:
>With Peltier elements you can turn heat into DC electricity, maybe an 
>option to use in combination with solar panels that get hot, and to turn 
>this heat into electricity too...

You could.  However, when you talk about "in combination with solar panels 
that get hot", there is a difficulty.  We don't let solar panels get very 
hot.  They don't work well when they're hot, so we design 'em to stay a 
reasonable temperature by arranging for the heat to be transferred away.

Peltier devices require a high temperature DIFFERENCE between the two sides 
to be efficient.  If you were designing something to make a Peltier device 
work well, you'd have one thing that you intentionally made real hot, and 
another thing that you made real cold.  (Neither would likely be solar 
cells.)  It certainly is possible to make things very hot and very cold in 
space.

So one might imagine an arrangement where a satellite was designed with one 
face always pointing toward the sun, with the sun-facing-side that was 
designed to get very hot.  This face would have to be thermally isolated 
from the rest of the spacecraft, which we're trying to keep about "room 
temperature" so its electronics and batteries don't fry.  The face would 
then have to be thermally well connected to one side of the Peltier 
device.  The opposite face of the satellite would be the opposite.  Very 
interesting thermal design challenges conducting the heat where you want it 
and keeping it away from where you don't.  Probably would want to use heat 
pipes to conduct the heat to the Peltier device, and from there to the cold 
plate.  Might need to use different kinds of heat pipes on the two sides 
because of the significantly different temperature.

Of course this also has the difficulty that it seems to require a 
spacecraft with sophisticated enough attitude control that it can keep one 
face pointing toward the sun.  The big guys do it of course, but it adds 
complexity.

>Propably the efficiency is to low, but it would be a nice experiment I think,

That is also my first impression.  Although I've not done the calculations, 
I'll bet that if you take the approach described above, and calculate 
weight and power output, and then compare it to covering the same area with 
solar cells that the solar cells will win by a huge margin.

Doing a high level design and the associated calculations of expected power 
output would be a good class project for a thermodynamics or satellite 
design class.  Maybe a graduate level class.




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