[greesil]

Ideal thermodynamic efficiency is 1-Tcold/Thot in an absolute temperature scale. This means given a perfect mechanism to turn heat into work this is the upper bound of how much of the heat can be turned into useful work.

Say ambient air is 20 deg C, with your 90 deg C heat source then you get 1-293/363 which is about 20%. So it's not to say you can't get work out of this system, but it's not great compared to having something nice and not.

And even then, you can't push a turbine with a hot liquid so your energy extraction technique now gets complicated.

[this_was_posted]

would it be possible to use a heat pump to increase the temperature to a more efficient range? Or would all of the efficiency gained be lost by operating the heat pump?

[seiferteric]

I have wondered about this as well... If your heat pump uses say 1kW, and they usually provide 4x-5x heat out, so say 5kW of heat out and your able to extract 50% of that as energy using say a turbine, you would get 2.5kW out? or maybe 1.5kW (subtracting the initial 1kW used to run it?) so maybe not great but maybe possible?

[rcxdude]

given perfect heat pumps/engines in that circumstance, you would get exactly as much energy out as you put in. Theoretically perfect heat engines are called 'reversable' for that reason: they give you the best possible exchange between heat and work. Said another way: heat pumps can give you 4-5x the heat out as work you put in precisely for the same reason you can't get much work out of that heat: the temperature increase is small. If you were to heat something up to a temperature where you could more efficiently extract work from it, it would require more energy to do so.

[greesil]

Yes you can use a heat pump, but that requires work to run. Think compressors and whatnot. So even with a perfectly efficient heat pump that limit still holds because you are using some of the energy gained from your new hot temperature to run the heat pump.