Thermodynamic heat pump and refrigeration cycles are the models for heat pumps and refrigerators. The difference between a heat pump and a normal air conditioner is that a heat pump can be used to heat a home as well as cool it. Even though the heat pump can heat, it still uses the same basic refrigeration cycle to do this. In other words a heat pump can change which coil is the condenser and which the evaporator. In cooler climates it is common to have heat pumps that are able only to heat the house, making the pumps simpler and cheaper, since cooling is rarely necessary.
A heat pump has the ability to pump heat two ways. This allows the heat pump to both bring heat into a house and take it out. In the cooling mode a heat pump works the same as your typical A/C. It uses an evaporator to absorb heat from inside the house and rejects this heat outside through the condenser. The refrigerant is flowing in the normal direction to where the condenser and compressor are outside of the space to be conditioned, while the evaporator inside. The one key component that makes a heat pump different from an A/C is the reversing valve. The reversing valve allows for the flow direction of the refrigerant to be changed. This allows the heat to be pumped in either direction.
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Nowadays, the vapor absorption cycle is used only where waste heat is available or where heat is derived from solar collectors.
When the working fluid is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a gas cycle. Air is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas heat exchangers in gas cycles.
