Heat pump and refrigeration cycle

In heating, ventilation and air conditioning (HVAC) applications, the term heat pump usually refers to easily reversible vapor-compression refrigeration devices optimized for high efficiency in both directions of thermal energy transfer.


Heat spontaneously flows from warmer places to colder spaces. A heat pump can absorb heat from a cold space and release it to a warmer one, and vice-versa. "Heat" is not conserved in this process, which requires some amount of external high grade (low-entropy) energy, such as electricity.


Heat pumps are used to provide heating because less high-grade energy is required for their operation than appears in the released heat. Most of the energy for heating comes from the external environment, and only a fraction comes from electricity (or some other high-grade energy source required to run a compressor). In electrically powered heat pumps, the heat transferred can be three or four times larger than the electrical power consumed, giving the system a Coefficient of Performance (COP) of 3 or 4, as opposed to a COP of 1 of a conventional electrical resistance heater, in which all heat is produced from input electrical energy.


Heat pumps use a refrigerant as an intermediate fluid to absorb heat where it vaporizes, in the evaporator, and then to release heat where the refrigerant condenses, in the condenser. The refrigerant flows through insulated pipes between the evaporator and the condenser, allowing for efficient thermal energy transfer at relatively long distances.