Good, we're going to need lots of good whisky when the shit really starts to hit the fan.
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It'll take the edge off.
If we take the edge off of the fan, it'll be much less energy efficient.
I think there's a lot of opportunity for reducing wasted energy in many buildings. Even the term "waste heat" is indicative that energy is typically exhausted when it could be used for space or water heating. Obviously mechanical modifications would be needed, sometimes extensive, but it's a good option for reducing energy use.
It's actually a really cool concept honestly. I do this on a very tiny scale at home. I run a small server cluster largely as a playground but also for things like Plex and Vaultwarden. The waste heat from that is pushed out the back of the rack and a heat pump water heater a few feet away uses that to help heat water.
As you said, seems like there are many opportunities to do this on a much lager scale.
Nice, I have much the same setup in my house!
I visited the recently Passive House certified student residence buildings at the University of Victoria, and the heat recovery there is quite interesting. Passive House requires a very low heating load, so they recover all the heat they can from the commercial kitchen (the presence of which is rare in a Passive House because of high ventilation requirements) processes such as ventilation hoods and refrigeration systems and put it into the DHW system.
They had to get a bit creative with the design, but it's really not that complicated. More just not doing things the way they've always been done.
One huge use I can see, but isn't being used as far as I'm aware, is data centers. They use a ton of energy, but when it turns to heat it's only seen as a negative to expel. Why not capture it and use it to heat houses/water or something? It turns the massive energy use into effectively near zero, since heating is needed anyway in a lot of places.
Waste heat reclamation for water heating purposes seems like a no brainer. For small buildings as well as large ones, but no one talks about it.