> the excess energy stored in the hot water is trivial compared to the capability of the heat pump
I don't know if I'd call it trivial, bringing 1 kg of water from 85C to 0C involves about 100W for a duration of one hour whereas a typical freezer removes heat at, what, 200W or 300W?
>bringing 1 kg of water from 85C to 0C involves about 100W
That looks like the amount of energy needed to heat that water.
But cooling things doesn't work the same way. Instead you want to extract that energy/move it somewhere else. The efficiency of some approaches, unlike when heating something, can hugely depend on ambient temperature. It's possible you'll be able to extract more useful energy cooling something than you'll have to expend!
I don't know if I'd call it trivial, bringing 1 kg of water from 85C to 0C involves about 100W for a duration of one hour whereas a typical freezer removes heat at, what, 200W or 300W?