In a typical heat pump cycle, heat is extracted from the heat source by the evaporator, which is at a low temperature. In it, the heat is transferred to a working fluid, changing it from liquid to gas. A compressor then increases the gas pressure, causing the temperature to rise. The hot gas flows to the condenser, where heat is given off as the working fluid changes back from gas to liquid. The working fluid, which is still at a high pressure, is then passed through an expansion valve, which lowers the pressure. In doing so, the fluid becomes cold and returns to its liquid state before once again entering the evaporator.
In refrigeration and air-conditioning applications, the process is reversed, supplying useful cooling instead.
Two ratios are used to evaluate the performance of an heat pump.
The simplest one is the COP (coefficient of performance), the ration between the released heat and the energy input of the running device.
A more complex one is the SPF (seasonal performance factor), which, firstly refers to the performance over an entire season, and secondly incorporates the complete system including collectors, pumps and auxiliary heaters if it is the case.
Presently, State of the art SPF are 3.5 for air/water Heat Pumps, 4 for ground-coupled HP, but it is expected than SPF of 6 could be achieved in the future.
COP or SPF increase as the temperature difference decreases between the heat source and the heated destination. They can be optimized by choosing a heating system requiring only a low final water temperature (e.g. underfloor heating), and by choosing a heat source with a high average temperature (e.g. the ground).