TIL "district heating" (a heat distribution system) is a thing. It does make sense to use waste heat from industrial processes to heat homes. I'm not a thermo

TIL "district heating" (a heat distribution system) is a thing. It does make sense to use waste heat from industrial processes to heat homes. I'm not a thermodynamics expert, but I think you probably need some density of heat sinks so it only really works in cities. I wonder if in the summer, it could work in reverse? I speculate there are two options for summer operation: (a) a centralized AC plant sends cold fluid into the pipes, or (b) a decentralized AC heats hot fluid. With option (a) I suppose it's not very backward compatible with a heat-only system, if you have a bunch of customers who built their factory expecting "this pipe will always be hot" they might have a bad time if you make it cold half the year. With option (b) it's more backward compatible but I think customers might not like it. I'm pretty sure AC gets less efficient based on temperature gradient so pumping AC into a heat transportation pipe would be more expensive for customers (electricity, wear-and tear on AC, takes longer to get to the commanded temperature when you reduce the set-point). So you'd have an incentive to unhook your AC from the system and just send heat into the air instead.