Waste power is heat to first order.

If there's a lot of it and you're not making it go elsewhere, assume that the system and it its environs are going to get a bit toasty.

Sometimes you also get things like radiation and vibration. These mostly show up in meaningful quantities in predictable places - nuclear systems, mechanical systems, high-power A/C or RF systems, high-power transducers.

You'll usually do okay if you guess that the system uses energy at around ~60% efficiency, which is enough to give you some idea of the heat to expect if you know the input power.

Or more generally, assume all input power goes to heat and plan to handle operation at that limit - while being aware that your expectations of power consumed by the system may well be low.

Is it possible to use thermocouples to convert waste heat to electrical energy?

Edit: Or maybe one of these bad boys? http://physicsworld.com/cws/article/news/2013/dec/09/new-gen...

Some of it, yes.

But they tend to have low efficiency, thermally-dependent efficiency, and the need for a temperature differential. They operate in response to a temperature gradient, not just heat.

Using Wikipedia for a quick-and-dirty figure, you can expect efficiencies to peak around 5-8%. And you can't just stack infinite thermocouples until system efficiency reaches 99.99% because of the thermal gradient issue.

Xenesys is a Japanese company that specialises in building these type of systems. http://www.xenesys.com/english/products/dtec.html

I thought there were infrared photovoltaic cells, but they aren't as common as the visible light ones.

Temperature differential should be easy to come by in interstellar space.

You need mass that's at a lower temperature. Vacuum is an insulator.

Ever used a Thermos vacuum bottle?[1]

[1] http://en.wikipedia.org/wiki/Vacuum_flask

Sadly, what you need is heat differential. Icy vacuum is next to useless here. Hence the large radiators...

Possible, but using the heat directly for industrial or heating purposes is probably the best bet - it's just awkward cause transporting heat over large distances is significantly more expensive than electricity, which is usually why you want some sort of co-generation.

Ultimately, you don't even need thermocouples, you just need more heat engines with ever decreasing working fluid temperatures, and then it all comes down to your capital costs.

Yes, but you'll need bigger radiators.

You can even make several layers of that, with thermal machines between the layers. As the number of layers increase, the efficiency of each one reduces, but the total work goes always approaching a constant assintote.