In the cases where you are dumping heat, I believe these devices can have the opposite problem of effectively insulating the heat source.

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.

Wastewater comes to mind. A lot of it is heated, either directly (from e.g. showers, cooking) or indirectly because it was stored in toilets.

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.

Practical applications are circumstantial.

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.

This device exists, you replace a vertical section of your sewer pipe with a copper heat exchanger pipe. The HX heats up your incoming cold water when you take a shower. The HX must be vertical because in a vertical sewer pipe, surface tension makes the water spread out and wet the entire inner surface of the pipe for good heat transfer.

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 mostly depend on the temperature of the water coming into your house and the ease of install in your house. They are supposedly code on new construction in large parts of Canada now.

The economics were also better when copper prices were lower.

Some sources, you might want to move the heat away (ie, CPU), maybe these loads of pipes they have are the same. But yes, point taken, insulation first. Ockhams Razor: don't multiply entities beyond necessity.

But if you want to move the heat away from a CPU, wouldn't you want to pump more electricity into the Peliter element to move heat away faster?

On a High-End Desktop Chip? YES! Intel made such a cooler recently[1]. But on mobile/low power devices it might be worthwhile to have them run a little warmer (within spec) but take the heat energy back via a peltier element. Would be interesting to see such an element directly on die feeding back into the chip directly, sort of an electric rebreather for a chip.

[1] https://www.intel.com/content/www/us/en/support/articles/000...

Occam's Razor: no need for the K or H. Just double the C.

Ockham is a totally acceptable spelling which requires all three: C, K and H...

See: https://en.wikipedia.org/wiki/Occam's_razor https://en.wikipedia.org/wiki/William_of_Ockham

I suppose though in the case of the CPU the metric you want to maximise is the heat dispersal not electricity produced.

Could work well on things like exhaust pipes

If it works, I think the waste heat from server farms should be the first target.

Yes that'd be good. The figure I'd like to know (maybe I missed it) is how much it reduces the heat by

It seems like the devices would insulate the server-farm. That might not be worth it, because you'd need extra cooling to offset the higher insulation.

Even more effective: drain water heat recovery, preheating the incoming water with the outgoing waste heat.

Quick overview from one manufacturer here: https://renewability.com

That's just a heat exchanger. I wonder if it's worth it tho.

Perhaps only if you fill a large tub or a pool every day.

DoE says they breakeven is 2.5 - 7 years. That's in the US, imagine how much of a slam dunk it is in countries with carbon prices and higher energy prices.

https://www.energy.gov/energysaver/drain-water-heat-recovery

These devices actually require a similar amount of water to be moving through the drain pipe/heat exchanger and the cold water line that's refilling the water heater. So they're really only effective during showers.

This would be worse for efficiency than uninsulated pipes. It would be wrapping them in a conductor to more efficiently take away the heat that is supposed to stay inside.

Of course in the limited cases where the application requires the heat to be transferred to the destination, you wouldn't use this method. Those pipes are likely heavily insulated.

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.

> 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.

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.

Yup, and there's the same issue with using these on internal combustion automobile or truck exhausts - the density change of increased cooling could might change the back pressure/suction at the exhaust port and reduce the engine efficiency. It would surely be a consideration on highly tuned race motors, but I have no idea if the magnitude of the effect would be even noticeable on street cars & trucks. It is certainly fairly small, as you can drive through a puddle that soaks the exhaust and billows out clouds of steam, sucking out far more heat vs this thermoelectric system, and the engine doesn't even notice.

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.

There are many industrial processes that produce large amounts of waste heat. Capturing some of that can make sense. But I guess unless you're retrofitting existing installations you get better efficiency with different solutions than wrapping stuff around a pipe.

But isn't that "waste" heat already used to heat the office building and other buildings nearby. So there is a use case already.

Of course during the year there are times, when heating is not needed. Then it would make sense to use the heat otherwise.

I think most industrial waste heat is indeed wasted. You don't tend to build for example chemical factories close to where you would have offices or homes.

Aircon would be a great application for that, assuming this applies at all.

An air conditioner is a heat pump. It uses energy to pump heat from one end to another. One of these thermoelectric generator devices is basically the reverse, it generates energy when heat is pumped through it. There is perhaps an analogy here to a water pump, pumping water from one place to another, and a water wheel, generating energy when water is pumped through it.

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.

Not really : Peltier-based devices like this add (some) resistance to the heat flow. This added resistance means the air conditioner (or any heat pump, e.g. a fridge) gets less efficient. I think the energy captured by the device will always be lower than the extra energy needed for the heat pump.

A device like this is not useful in a system where you are actively pumping heat around.

Aircon needs to get rid of its heat to work well. Trying to capture the energy from the temperature difference makes it cool less well.