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Author Topic: How can you cool things down in space?  (Read 4685 times)

Offline Pumblechook

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How can you cool things down in space?
« on: 31/08/2008 12:57:15 »
Trying to settle an argument.  I reckon this a problem in satellites.   Someone claims it isn't as space is cold??.  No cooling air you see.   
« Last Edit: 05/09/2008 09:12:16 by chris »


 

Offline LeeE

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Re: How can you cool things down in space?
« Reply #1 on: 31/08/2008 14:15:00 »
Space is really neither hot or cold.  Whether something is hot or cold comes down to how energetically the molecules are moving in whatever it is that's hot or cold.  As space is a near vacuum, there just aren't any molecules to be moving around and thus make it hot or cold.

If cooling is needed on a satellite, and because it can't be done by conduction - "No cooling air you see",  it can only be done by radiation, mostly in the Infra Red bands.

Something to remember though, is that generating excess heat, which would then need to be dumped by a satellite, is wasted energy and wasting energy as excess heat is something that most satellites can't afford to do - they don't generally have an excess of energy but run on quite a tight energy budget and so are designed to run while generating as little excess heat as possible.
 

lyner

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Re: How can you cool things down in space?
« Reply #2 on: 02/09/2008 11:18:39 »
Satellites are energy self sufficient (apart from the fuel used for station keeping). The energy  produced by and wasted in the electronic components is from the EM energy  converted in the solar cells to electricity.
The average temperature of a uniform sphere with a perfectly black surface at the Earth's distance from the Sun would be about 300K (same as us). Equilibrium is reached when the surface temperature is high enough to radiate as much energy as is being absorbed. The rate of absorption of energy is the same as the rate of emission for any surface, for a given wavelength. To keep coolest, you need a surface which is reflective (poor absorber) to the majority of sunlight (the visible bands, mainly) and non reflective (good radiator / absorber) at IR. The equilibrium temperature will be much lower than for an object with the converse characteristics. The problem is that you need lots of power to drive the equipment - so the solar panels catch a lot of the Sun's energy.
There is a complicated thermal transfer system in satellites to keep temperatures down in would-be hotspots. There is an available power supply of a few kW (for a broadcast satellite with many square metres of solar array) and, with the efficiency of even the best transmitting devices being fairly low, most of the electrical energy 'gathered' by the solar cells ends up as heat rather than as transmitted RF signal so it has to be disposed of. And it all has to be 'radiated'.

 

Offline Pumblechook

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Re: How can you cool things down in space?
« Reply #3 on: 02/09/2008 12:51:59 »
From a website...

""Electronic components utilized throughout
the various subsystems experience thermal stress resulting from high
temperature effects from the sun, from low temperatures occurring during
eclipse, and from heat dissipated internally by components located aboard
the satellite. Thermal devices, such as radiators, are commonly used to
dissipate excess heat and to protect the electronic equipment from thermal
stress. Radiators used on satellites typically include sheets of a highly
thermally conductive material with a high thermal emissibility
characteristic. To provide maximum heat radiation to space, high power
dissipation components are commonly mounted directly to the radiator
panels.""

The original thread was about huge solar panels in space beaming energy back to Earth with RF.   My main point was that energy conversions Solar-DC-RF-Transmission-RF-DC-AC can never be very efficient and lots of waste heat (hundreds of millions of Watts) couldn't be got rid of easily in space.  Apart from the huge size of the panels and transmitting arrays which would require thousands of space flights even for a relatively small station (one GigaWatt say).  Even wires carrying the DC would have to be thick and heavy with no cooling air.  One for science fiction or for a hundred years or more in the future.
« Last Edit: 02/09/2008 12:54:02 by Pumblechook »
 

lyner

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Re: How can you cool things down in space?
« Reply #4 on: 02/09/2008 13:05:34 »
I totally agree.
I never understood why it would be worth while collecting energy out in space for terrestrial use. We have plenty of it arriving on the surface. You'd have to look into the 'carbon footprint' of such a venture, too.
« Last Edit: 02/09/2008 13:07:46 by sophiecentaur »
 

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Re: How can you cool things down in space?
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