I don't have any good way to sketch and upload, but the idea that I'm thinking of is basically an insulated box that fits to the back side of your fridge with just enough room for air to move around the coils, at the top you would have your heat pumps, I'm personally a big fan of Peltier Junctions but others could work, the heat pump(s) would have fins on the fridge side and would be used for cooling the air in the enclosed box on the back of the fridge, the other side of the heat pump is going to get hot, this would have a water block heat sink and be connected with 3/8" or so pex water pipe and go to your thermal battery storage tank, that tank could then either have a coil that is used for preheating your domestic hot water or could again use a heat pump to heat your hot water up to full temp but that would require a better balance.
You would have a bit of loss from the heat pump but those losses would be turned in to heat an go in to your thermal battery and that energy/heat would offset what your water heater would otherwise be using.
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