Naked Science Forum
Non Life Sciences => Technology => Topic started by: Riki on 26/01/2010 21:58:45
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Solar panels are used in space because the sun is always shining. One side of the craft, facing the sun would be hotter than the other side. Can't this potential heat difference be used to run a stirling engine generator?
A Stirling engine setup would be smaller, more efficient than huge solar panels, surely?
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The near vacuum of orbital space doesn't have a temperature; temperature in the sense that you're referring to only applies to the temperature of a medium. Sure, if you stuck a Sterling Engine in space, so that one half was in sunlight and the other in shadow, the side in sunlight would absorb solar radiation and heat up, but then you'd have a problem dissipating the heat from the side in shadow as it could only do so by re-radiating it - there's no medium to conduct the energy away.
To radiate the energy away, the 'cold' side of the engine would have to get very hot indeed before it became effective, and probably hotter than the 'hot' side actually becomes from the absorbsion of the solar radiation in the first place.
If you tried to connect the engine to the walls of the craft itself, you'd not only have to ensure that the same walls always faced towards and away from the Sun but you'd still have the same problem re radiating the energy away from the walls of the space craft/station, not to mention issues with conduction of heat from the hot side to the cool side of the craft/station, reducing the effectiveness of the engine.
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I may be hallucinating, but I seem to remember that Stirling engines are actually used on spacecraft. Unfortunately, I've no idea where I saw that (if I ever did!). I seem to remember the energy source was nuclear fission rather than solar, but my memory is probably playing tricks on me.
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I am certain they are, built somewhat like moving coil loudspeakers.
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Here's something from NASA on the subject. This one runs on plutonium, but it also says something about a solar version.
http://sbir.nasa.gov/SBIR/successes/ss/3-115text.html
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Radiation does not need a medium. Heat is lost very effectively from a radiator in space, so this is no obstacle to the success of a heat engine in space.
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Radiation does not need a medium.
True enough, but no one is disputing that.
Heat is lost very effectively from a radiator in space, so this is no obstacle to the success of a heat engine in space.
Ah - in terms of space craft, you're exactly wrong. The total amount of radiation follows T4 (where T is the absolute temperature of the radiator), so a radiator on a space craft would need to be very hot indeed before it became efficient enough to maintain the heat gradient required for a Sterling Engine. Like I said, the 'cold' side of the Sterling engine, which has to radiate the heat away quickly enough to maintain the heat differential, would have to be hotter than the 'hot' side would get from absorbsion of solar radiation before it became efficient enough to work.
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Radiation does not need a medium.
True enough, but no one is disputing that.
Heat is lost very effectively from a radiator in space, so this is no obstacle to the success of a heat engine in space.
Ah - in terms of space craft, you're exactly wrong. The total amount of radiation follows T4 (where T is the absolute temperature of the radiator), so a radiator on a space craft would need to be very hot indeed before it became efficient enough to maintain the heat gradient required for a Sterling Engine. Like I said, the 'cold' side of the Sterling engine, which has to radiate the heat away quickly enough to maintain the heat differential, would have to be hotter than the 'hot' side would get from absorbsion of solar radiation before it became efficient enough to work.
Oi! Another anglification. Like most of the World's greatest inventions, the Stirling Engine was invented by a Scotsman [;D]
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Och! sarry aboot tha' Jimmy.
Actually, it seems that my family is of Scottish descent; we have a clan and a tartan etc. Heh - I was walking around Glasgow City center, on a visit with a friend who'd grown up there, and when I pointed out my family name related stuff, like the tartan and heraldic stuff, he had to sadly concede that I was more Scottish than he was [:D]
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Ah - in terms of space craft, you're exactly wrong.
Thanks, LeeE, I've never before been complemented for being so precise!!
Seriously though, how does any spacecraft (with or without Stirling engine) get rid of the heat collected by it's sunward side? I know that spacecraft are always white or shiny, probably to reflect as much as possible of the energy collected, but surely this will only reduce the rate of energy collection, not the final temperature? (unless 100% reflection is achieved).
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Well they'll heat up until they reach an equilibrium where the energy radiated is equal to the energy absorbed
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Well they'll heat up until they reach an equilibrium where the energy radiated is equal to the energy absorbed
Perhaps I should re-phrase my question:
How does a spacecraft in full sunlight limit it's internal temperature to something suitable for occupants and equipment?
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By reflecting most of it away and not absorbing it.
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Hi, i was just going through the same course and found this post..
As for the temp problem you were discussing: Sun is at 5000K temp and those radiations are received by satellite. Satellite temp (on the solar side rises around 350K - 500K and 190K on the eclipsed side). So energy will be emitted by eclipsed side. This may make whole engine works.
Now consider the system, 'hot' side of the engine of the side of electrical components producing heat/solar panels and cold side to sink radiator (one cannot attach the both side of the engine to the panel, they have almost same temp on both the sides). This engine have to work on the temp difference of about 20K-30K, which has been already done.
Also, japan has developed the stirling cooler for their Astro-H/SXS. I don't the working of the cooler, but it works.