All heat pumps use the principle of Boyles law – that the temperature and vapour pressure of a substance are directly and proportionately related at a constant volume.
Boyles Law: PV/T = constant
With most heat pumps the overall heat pump itself has a fixed volume hence as pressure is changed the temperature changes proportionately and vice versa.
The Vapour Compression Cycle
By far the most common form of heat pump uses a vapour compression cycle and hence has some form of compressor installed in the circuit which is most usually powered by electricity but can be driven by a gas internal combustion engine.
Gas Absorption Heat Pumps
These use the principle of applying heat – most usually direct gas fired – to create temperature and thus pressure differentials in a mixed compound substance such as ammonia/water or lithium bromide/water.
All heat pumps have the basic need for an evaporator and condenser which perform the function of drawing heat in from a source and expelling it (respectively).
They also need a form of circulator which is a compressor in vapour compression cycle or a 'solution pump' in an absorption process.