A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using the refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing heat from the enclosed space and rejecting it outside. Units that only provide cooling are referred to as air conditioners.
When in heating mode, a refrigerant at outside temperature is being compressed. As a result, the refrigerant becomes hot. This thermal energy can be transferred to a central heating system. After being moved outdoor again, the refrigerant is decompressed. It has lost some of its thermal energy and returns colder than the environment. It can now take up the surrounding energy from the air or from the ground before the process repeats. Compressors, propellers and pumps run with electric energy.
Common types are air source heat pumps, ground source heat pumps, water source heat pumps and exhaust air heat pumps. They are also used in district heating systems.
The efficiency of a heat pump is expressed as a coefficient of performance (COP), or seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is and the less energy it consumes. When used for space heating, heat pumps are typically much more energy efficient than simple electrical resistance heaters.
Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps can play a key role in electrification, the energy transition, and climate change mitigation. With 1 kWh of electricity, they can transfer 3 to 6 kWh of thermal energy into a building. The carbon footprint of heat pumps depends on how electricity is produced. Heat pumps could satisfy 90% of global heating needs with a lower carbon footprint than gas-fired condensing boilers