Refrigerators and Heat Pumps

So far, every heat engine discussed has taken heat from a hot source and rejected some of it to a cold sink, converting the rest into useful mechanical work — heat flowing 'downhill' (from hot to cold) producing work as a by-product. Refrigerators and heat pumps are heat engines run in the opposite, backward direction: instead of producing work from a flow of heat, they consume mechanical work in order to force heat to flow the 'wrong' way — from a colder region to a hotter one — something that (as Section 4.10 will make precise) never happens on its own. …