>How high of a COP would it take to make a heat pump a feasible way to make lower temperature waste heat a source? Or is this where the thermodynamics breaks do

>How high of a COP would it take to make a heat pump a feasible way to make lower temperature waste heat a source? Or is this where the thermodynamics breaks down? Yeah, there's no free lunch. You could use the low-temperature waste heat to pre-heat the working fluid for a combustion-based system, but using a heat pump to increase the temperature will cancel out the efficiency gain from having a higher temperature for the heat engine.