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I would have thought that you would want to conserve as much energy as possible - or is energy/electricity not a problem?

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    $\begingroup$ Some space probes have electronics kept next to heat sources so they don't go lower than their minimum operating temperature, but 'waste heat' is called 'waste' mostly because it is either difficult to repurpose it to providing useful energy, or that it would cost more mass than it is worth (than could be produced with solar cells or a reactor of the same mass). $\endgroup$ – Andrew Thompson Jun 17 '16 at 8:31
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    $\begingroup$ ..also note that space probes are in 'noon' sunshine 24/7, and they cannot be cooled using thermal convection (which cools anything in air or water significantly faster than it would cool by thermal radiation). It is typically easier in space to keep something warm, than to keep it cool. $\endgroup$ – Andrew Thompson Jun 17 '16 at 8:34
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    $\begingroup$ The second law of thermodynamics, maybe? $\endgroup$ – a CVn Jun 17 '16 at 9:08
  • $\begingroup$ What's with the downvotes on the answers? Neither of them is wrong. $\endgroup$ – Hobbes Jun 17 '16 at 13:30
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Yes, you would like to conserve the energy, but sadly that is not possible. The problem is: The energy you have is heat.

The second law of thermodynamics states that entropy always increases, and that means that while every process in the spacecraft produces heat, going in the opposite way is impossible. You can produce energy from heat if you have a temperature gradient, but if you have that on board, you would only be moving the heat around in any case.

In short, everything going on produces heat, and you can not magically transfer that back to useful energy. The only solution is to dump it.

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  • $\begingroup$ I'm not sure what you are trying to say here, but it's factually wrong. There are devices that convert heat into electricity and they are already used in radioisotope thermoelectric generators for spacecraft $\endgroup$ – GdD Jun 17 '16 at 11:42
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    $\begingroup$ @GdD RTGs are no exception. They create a high local temperature, and have passive radiators to create an area of lower temperature. They do not magically convert heat into electricity, they only use a temperature gradient. $\endgroup$ – Hohmannfan Jun 17 '16 at 12:07
  • $\begingroup$ I'm saying you need to clarify that. $\endgroup$ – GdD Jun 17 '16 at 13:30
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    $\begingroup$ @GdD: Okay, let's say you can recover some energy back using a thermocouple generator (and let's make this Carnot cycle efficient). Where would you use it? Inside the spacecraft of course. There it would do work and dissipate as heat, that goes back through the thermocouple and so on… Congratulations, you just made a perpetuum mobile. Eventually you're ending up with maximum entropy heat and you can't gain any energy from it. The only thing left to do is to dump it. $\endgroup$ – datenwolf Jun 17 '16 at 20:12
  • $\begingroup$ This could still use some improved wording. There is a thermodynamic limit, but of course if you start with 1000 Joules of heat, and you produce enough electricity from that to do 200 Joules of work, then you better have lost 200 Joules of heat and only have 800 Joules of heat left. You would not just be moving the heat around, some of it would be something else - maybe chemical energy in a battery or a packet of microwaves towards Earth. You can replace "Joules" with "Watts" and adjust "do work" to "provide power" as well. "Heat" is not a conserved quantity the way entropy is. $\endgroup$ – uhoh Nov 30 '16 at 2:27
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Although there is technology which can convert waste heat to electricity you cannot do it economically.

Energy is precious in a spacecraft, the more you need the bigger solar panels you need to generate it, the bigger batteries you need to store it, or the bigger nuclear thermal generator you need. Bigger means heavier, and heavier means more expensive to launch. When electricity is converted to motion, or used in computers/sensors waste heat is generated, the amount of energy lost can be limited by using more efficient materials. There's a limitation to how efficient you can get, so you will always have waste heat.

Keep in mind that you want some heat generation in most spacecraft so it doesn't get too cold to operate. Waste heat is one way, some spacecraft have heaters to convert electricity into heat when in shadow. So waste heat is actually useful.

There are devices that can convert heat into electricity, called thermo-electric generators. These are used in spacecraft already in nuclear thermal generators, aka radioisotope thermoelectric generators which are used in spacecraft going to places where there isn't enough sunlight for solar panels. As radioisotopes break down they produce heat which is converted into electricity to power the spacecraft.

Thermoelectric generators could in theory be used to convert waste heat back into electricity, but you'd lose more than you'd gain as the weight you'd gain from them would not be offset by less solar panels and batteries - they are heavy and inefficient, with 5-8% typical energy conversion. This means you'd still have 92% of the heat to radiate. Newer materials are being developed that may bring this to around 20% efficiency but that would still not be enough to make them worthwhile.

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