The problem is that that only really works if you've got that fridge running — and if you do whatever's powering it is a better heat source for your steam engine than waste heat from people.
The problem is that extracting much useful energy from waste heat is hard. If your waste heat is low temp (ie body temperature), moving that to a higher temp likely requires more energy than it gets. You can sometimes do some useful work with waste heat, but thermodynamics limits what you can do.
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