Comment by Nevermark
17 hours ago
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.)
What is waste heat depends on your usecase. Using waste heat from industrial processes for district heating is done in some places.
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.)
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.
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