originally posted by: EartOccupant
What i still don't understand about "space power generation" is why they don't use the sun side and the shadow side of an object, or a designed object
itself as a thermal generator.
With a temperature difference of 500F or around 280 C between te sunside and the backside a Stirling type or Peltier type generator would be roaring.
Even a closed loop steam turbine would be no problem.
When your orbit is within the sunlight you will have a continues difference between the back and front.
You can solid state, or rotate the exposure sides.
When you experience shadow periods you can store heat, within molten salt for example.
Wikipedia: Without thermal controls, the temperature of the orbiting Space Station's Sun-facing side would soar to 250 degrees F (121 C), while
thermometers on the dark side would plunge to minus 250 degrees F (-157 C).20 mrt. 2001
I googled a little and there seems to be something like this:
A flight-proven capable source of power is the Radioisotope Thermoelectric Generator (RTG)–essentially a nuclear battery that reliably converts heat
into electricity. NASA and the Department of Energy (DOE) have developed a new generation of such power systems that could be used for a variety of
But i don't understand why you need the nuclear .
Other systems use overengineered antenna's like:
But why isn't the whole object an "antenna" ?
Not an expert here, so ther must be some obvious reason i'm missing.
I'm a spacecraft designer(semi-retired). For the last three years I worked at Planet Labs, helping to design their next generation Earth-monitoring
small spacecraft. Before that, I worked at NASA, designing Lunar, Mars, and deep space missions.
Your basic question is "why can't you use a solar dynamic converter to make electricity, since there is a temperature difference between the hot side
and the cold side of a spacecraft?"
The answer is, you could, but it wouldn't necessarily be better than using solar cells. Most spacecraft that are intended to operate at approximately
Earth distance from the Sun are usually designed to have a hot side (pointing to the Sun) and a cold side (pointing to deep space). So heat naturally
flows from the hot side to the cold side. Solar cell panels are designed to make use of this temperature gradient the same way that a fluid dynamic
system would; they just do it with no moving parts. The optimum efficiency of typical space-rated solar cells is around 30% when they are operating
at their optimum temperature. As the cell temperature increases, their quantum efficiency decreases; they are little thermodynamic systems, by
themselves. So, the normal practice is to point the front of the solar array as directly to the Sun as possible and either allow the back side to
point directly to dark space, or heat sink it to a cold structure.
If you were to use the same amount of collection area to provide electrical power by using a thermal dynamic converter of some kind, you would have to
have a solar concentrator of some kind, heat exchangers for the working fluids, insulated pipes from the concentrator to the expander, an expander of
some kind connected to a generator, heat rejection radiators, and probably some kind of pump. For normal sized spacecraft, that amount of complexity
usually weighs more and costs more than simply using solid state solar cells.
Another factor is that any dynamic system that is pumping fluids around and has rotating or reciprocating machinery imposes vibrations and torques on
the spacecraft structure. If your spacecraft is intended to make precision observations with telescopes, etc. this can seriously complicate the
pointing and control problem. Also, a system with no moving parts is almost always more reliable than one with moving parts.
The best solution along these lines for normal sized spacecraft that I have seen is the Advanced Stirling Radioisotope Generator (ASRG). It was
originally designed to use the decay heat from Plutonium 238 to power a little reciprocating Stirling engine connected directly to a reciprocating
alternator. A Stirling engine has a hot side and a cold side, so it is easy to place the hot side in contact with any heat source and point the cold
side to space. They were intended to be operated in opposing pairs, so that the vibrations would mostly cancel each other. A pair would put out
about 100 W, or so. They were also designed with very high reliability, redundant electronics so that they could expect a 10 year lifetime, or so.
The motivation for designing this kind of thing in the first place was to extend the supply of Plutonium. The standard Radioisotope Thermoelectric
Generator (RTG) has a conversion efficiency (thermal to electric) of about 5%. The ASRG has a conversion efficiency of around 26%, so it can use 1/5
as much Plutonium and still supply the same electrical power. But, the thermodynamic efficiency of the ASRG is still not as good as aerospace grade
solar cells, so there is no advantage to using one of these converters when you are in near Earth space.
When you get to outer planet missions, solar powered missions (of any kind) become pretty infeasible and you have to go to nuclear power. For
example, Saturn is 10 AU from the Sun, so the solar power available is 1% as intense as it is at Earth.