Pumpa

Meet Pumpa, a heat pump who uses a little work to move a lot of heat from cold to hot.

Heat flows naturally from hot to cold. A heat pump runs it the other way. It uses work, usually from an electric compressor, to pull heat from cold outdoor air and deliver it indoors. Its performance is measured by the coefficient of performance (COP): heat delivered divided by work put in. The second law of thermodynamics sets the ideal maximum, the Carnot limit, at T_h/(T_h − T_c), with temperatures in kelvin.

Example: heating a house at 20 °C (293.15 K) from 0 °C outside gives an ideal maximum COP of about 293.15/20 ≈ 14.7 (computed). In other words, a perfect heat pump would deliver almost fifteen joules of heat for every joule of work.

Real heat pumps reach much less, typically COPs of 2 to 5 in practice, because of friction, finite temperature differences in the heat exchangers and other losses. Even so, they deliver several times more heat than the electricity they use, which is why they are efficient for heating homes. Refrigerators run the same cycle to move heat out.

The colder it is outside, the lower the COP.