The Homemade Heat Pump Manifesto

[BradC]

Originally Posted by randen

But if I understand what Vlad was speaking of its heating we are after and not the cooling side. We need to optimize the heating capacities of the unit and what occurs on the evaporation side is less of a concern as long as its picking up enough heat.

He's pretty right, although if you are talking about maximising heat then you want the evaporation temperature as high as you can practically get it.

If you look at a spec sheet for a medium temp fridge compressor, they specify capacity usually at 3 or 4 evaporation temperatures. As you would expect, the higher the evaporation temperature (or saturated suction temperature) the higher the capacity of the unit. Its a simple matter of mass flow through the pump. The denser the input, the more it can move per stroke. This is countered by the higher the output pressure, the less efficient the pump becomes as it has pumping losses to work against.

To maximise your heat *moved*, you want to evaporate as high as you can and condense as low as you can. More volume through the compressor. Although by increasing your condensation temperature you make the compressor work harder, generating more heat, and you also allow your condenser to reject more heat into the environment (as you increase the Td). You could keep doing that until you burst a pipe, burn a valve or toast the compressor. Better to make the condenser more efficient (or bigger).

If you really want to make a better heater, swap the R290 out for R22. It generates more heat while being compressed. I seem to recall something to do with the density and the shape of the molecules causing more friction during compression, but don't have a reference to hand.