[lambertsimnel]

I share your enthusiasm about heat pumps, but I wonder what the efficiency of using waste heat is. Couldn't it be competitive with heat pumps? As it's a waste product, isn't it reasonable to also expect it to be more than 100% efficient?

[matt-p]

As a rule of thumb (obviously it varies) you spend about 1% pumping water round a heat network. So your CoP is around 99 if you consider heat truly free. It's actually higher as pump energy largely is converted to friction/heat.

[Nevermark]

You can’t extract energy from heat by itself. Only from a heat delta.

Think of heat like flowing water or charge. Only an altitude or voltage delta creates the flow needed to harvest energy.

You get no useful energy from heat you are already trying to shed because you have no delta to work with. (The entire problem exists because there is no surrounding environment with high heat capacity and lower heat.)

[CorrectHorseBat]

What is waste heat depends on your usecase. Using waste heat from industrial processes for district heating is done in some places.

[Nevermark]

Yes, because there is a heat delta. A heat difference.

Using higher heat to raise lower heat is just the most simple case.

But purpose isn't relevant to this constraint, it is a physics constraint. Regardless of purpose, you can't extract useful energy from heat without a heat difference to work with. (And without a heat difference, even "heating" with heat doesn't do anything.)

[CorrectHorseBat]

Yes you can, that is exactly what heat pumps do. As long as the total entropy increases it is not in violation of the laws of thermodynamics.

But I don't really see how that is relevant to the question of using waste energy to heat homes. We don't have ideal Carnot machines so there's always energy wasted, which most of the time is still good enough for residential heating.

[Nevermark]

Agree with your characterization.

The conversation was about harnessing energy, from heat, in orbit.

Heat pumps take energy to move energy. But you can't power the heat pump from the heat it is already pushing against the heat gradient.

Waste heat can be used, if there is a difference in heat to work across, but not if there isn't. A datacenter in Antarctica could recover energy from waste heat, against the freezing outdoor temperatures.

In orbital systems, the problem is getting rid of heat, so there isn't some cold place to use to create a heat gradient and harvest energy. Space is cold, but particles are so diffuse they have little heat energy capacity, so essentially a heat insulator, and not useful to create a gradient. Thus the use of radiators.

[CorrectHorseBat]

Much more than 100% since the only energy you need to put in is for pumping the hot water around.