> In theory, a PC builder could try exchanging heat to a toilet tank, allowing for highly efficient cooling with each flush.
The future is here.
In an interesting way, the reverse is also here: some district heating plants use heat pumps to extract waste heat from sewage to provide efficient heat for district heating systems.
See https://www.eneco.nl/en/about-us/what-we-do/sustainable-reso... for a basic overview.
I dream of a future where there's an efficient energy recapture in the house. We waste a lot of heat in the shower, and washing clothes and dishes. We also run the fridge all winter when it's less than 5C outside, and the fridge then produces a lot of heat. And when we're done cooking, the oven is still really hot. Could the home heating system of the future use all this heat (and cold) more efficiently?