Agreed! I am not an expert, sorry, but I understand just enough to be dangerous.
All normal heatpumps use a closed loop system with a compressor, condensor, pressure drop, evaporator. The working fluid is operated in the region where it is changing from gas to liquid and back -thats where the serious energy carrying capacity is.
I’ve added a pic showing water temperature versus enthalpy (heat, energy transfer, it’s what you need lots of). You can see it takes way more heat to actually boil water to vapour than it does to merely heat it up all the way from freezing to boiling. I picked water, as we’re used to this phenomenon with it.
I think you could use air, but it phase changes at much colder temperatures, so the efficiency will be awful for normal use. It will give hot air, but hardly any heat before it cools down too much. Somebody could perhaps use a nitrogen pressure-enthalpy diagram to help guesstimate efficiency.
Pick a better gas! CO2, propane, ammonia, they work at normal temps and pressures.
