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Comment by joeyo | Hacker News Reader

David MacKay discusses this in Sustainable Energy Without the Hot Air. He concludes that, while it's on first glance appealing to want to use waste heat from power generation, heat pumps are strictly superior to combined heat and power except for in a few specialized circumstances (e.g. industrial uses that require high temperatures). The book is fifteen years old now, so I suspect the math even more strongly favors heat pumps than when it was published.

See the section, "Heat pumps, compared with combined heat and power": https://www.withouthotair.com/c21/page_147.shtml

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But district heating is a distribution mechanism, not a heat generation mechanism. Cogeneration plants are one way to power it, but you can also use waste heat from industrial processes, very large scale heat pumps, or burning household waste. All of these are in common use in places where district heating is common. The public district heating utility in Stockholm claims to have the world's largest heat pump installation, which has the capacity to extract 225 MW of heat from treated sewage, in the process generating both heat that's sent into the district heating network, cooling that's sent into the district cooling network, and finally a small amount of electric power by releasing the treated water into a lake via a turbine. This facility opened in 1986. The utility also has various other facilities, both cogeneration plants that burn biofuel (mostly byproducts from the lumber industry) or household waste, as well as other heat pumps that extract energy from seawater.

District heating/district cooling has three advantages: you can change the heat source centrally without having to refit every single dwelling, there are economies of scale in the heat generation, and you can take advantage of heat or cooling that's just in the wrong place and transport it to where it's needed.

5th generation district heat systems can transport water at roughly the same temp as the underground and use distributed heat pumps to bring them up to the required temperature at point of use. This prevents heat losses along the way.

https://5gdhc.eu/5gdhc-in-short/

I’ve often wondered if late if rather than a “wet wall”, if houses should have a “hot wall”. Could I run a thermal loop past the fridge and a couple other appliances and either shunt it outside in the summer or distribute it around the house.

Total BTU aren't that great. Would (somewhat) work for a very small apartment, for any big house you would have a better experience making a proper thermal insulation.

Given that people are pointing out that heat recovery from washers doesn’t really work because they alternate between filling and draining (so there is no counterflow except incidental from other house activities), I’m not so sure that would be true.

Particularly if you use heat pumps instead of basic plumbing to achieve the transfer. Dump heat into a glycol line with a reservoir.

Seems to me like the electric power grid is an already existing, efficient energy distribution mechanism. Converting waste heat to electricity is probably a lot cheaper to retrofit than adding a new distribution network to places that don't already have one.

To my knowledge there's no really effective way of converting low grade waste heat into electricity. Basically all heat-based electricity generation today relies on being able to boil water into steam to spin a turbine. But with heat pumps and district heating, you can recover useful amounts of energy even from quite small thermal gradients. That's why things like room temperature sewage and waste heat from data centers can be useful to these systems even though they couldn't be used to generate electricity.

Also, if you rely on local heat pumps, you still need to take the energy from somewhere. Taking it from the air is fine in warmer climates, but in colder climates the efficiency of air-source heat pumps starts to become an issue when it gets colder. You can extract heat from deep underground, but that doesn't universally work in densely populated areas because the ground doesn't contain infinite amounts of energy and it's possible to extract more than it can sustain. So, in cold climates there's definitely a use case for distributing heat using water pipes, with or without local heat pumps in every building (see the sibling comment about 5th generation district heating systems).

Thermodynamics prevent you from turning low grade heat into electricity with anything resembling reasonable efficiency. Moving the heat around on the other hand it reasonably simple.

Arguably, decentralized approaches have other advantages when it comes to redundancy, gradual upgradability and expandability.

Yeah; if just used for hot water, this doesn’t make much sense compared to a hybrid heat pump water heater. Here in California, with a well insulated/partially passively heated/cooled house, our hot water is well under 10% of our electricity bill (ignoring our EV).

In places where it’s really hot or cold, it would be an even lower percentage. The money that it would cost to trench and maintain the hot water distribution lines for a house would probably be better spent on heat pumps, architectural features or upgraded insulation (Even just $2000 of extra insulation does an amazing amount of good vs. bare minimum code insulation, and I doubt you can get district hot water installed for less than that.)

Heat pumps move heat so the heat from the waste water is still useful to them.

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