I think it might technically be possible to have a very conductive heat-engine. But it seems to me like most options are still less conductive than leaving them out.
Edit: But then again, capturing and re-using the heat energy directly and saving the energy cost for heating via e.g. an electric heater would probably be even more efficient than extracting some of the heat as electricity.
Do you have a way of utilising or storing electricity/heat? If so, either are more efficient than not doing so. For example, if it's cold where you live and the heating is usually running... just expelling the heat from your fridge into the house is efficient. If you are cooling, it's not. Electricity would be better... or at least piping the heat out.
I looked into the economics at one point. Would be great for a laundromat or hotel, not so much for the average single family home.
The economics were also better when copper prices were lower.
Could work well on things like exhaust pipes
[1] https://www.intel.com/content/www/us/en/support/articles/000...
See: https://en.wikipedia.org/wiki/Occam's_razor https://en.wikipedia.org/wiki/William_of_Ockham
Quick overview from one manufacturer here: https://renewability.com
Perhaps only if you fill a large tub or a pool every day.
https://www.energy.gov/energysaver/drain-water-heat-recovery
But most cases, moving hot waste, the loss of heat is a side effect (the waste material just happens to be hot), and may in fact be an undesirable side-effect that squanders energy. Now, they don't need to squander that energy.
Easy example is a chimney - they're only trying to get rid of the exhaust gasses we don't want inside, and DGAF if it is cold or hot. In fact, heat exchangers are often installed to attempt to capture some of the lost heat and minimize losses.
Same for waste liquids going for treatment, or actual product exiting a hot process and going into a cold/ambient temp process -- they've got to dump the heat, far better to grab some energy on the way out.
Good example, except that you need a certain amount of heat to carry away the water (steam). I believe the rule of thumb for old-style furnaces is 80% efficiency with the 20% carrying away the water vapor.
So, sure if you did it naively, without checking & compensating for secondary effects, you might have a net problem. But I'd be highly confidant that a bit of R&D and engineering design changes would easily take care of it.
Of course during the year there are times, when heating is not needed. Then it would make sense to use the heat otherwise.
A device like this is not useful in a system where you are actively pumping heat around.
Take an ordinary house. The default, low-energy state is the one it tends to if we don't mess with it. The inside and outside of the house come to about the same temperature as energy leaks in through the insulation and holes in it, though slowly.
Now we put in a heat pump. This changes the system. With a little energy input, we capture the energy in the inside of the house, and we move it to the outside. The energy state of the system changes, it begins to gain energy as we put some in to create the heat differential.
Then we attempt to capture this heat differential. We place a heat engine, which one of these devices is, across our temperature differential. Assuming total efficiency for both processes, which we do not in reality have, our heat engine undoes the heat differential and produces for us the exact amount of energy we ourselves put in to create that heat differential. The inside and outside now come to some resting state, possibly with a constant heat differential upheld by the pump and actively sabotaged by the engine. Theoretically, we get net 0 energy and dysfunctional air conditioning, but in practice, we get a net loss of energy to inefficiencies.
The same applies to the water analogy. Imagine two tanks of water connected by a porous wall. The state it reaches naturally is that both tanks are at the same level through water seepage, though it is slow. If we place in a pump, we get a pressure differential across the tanks: one tank becomes higher and another lower. Then we place a wheel across the differential to take advantage of that pressure. Energy is generated, and at the same time the height difference is lost. Again net 0, though in reality you will experience a lot of inefficiencies.
If the point is to do air conditioning, you cannot recapture energy from the heat. Just running the air conditioner at a lower power achieves the same result more efficiently. Similarly with the water pump, it will in reality be more efficient to just not pump as much water up, and you will reach the same result.