heat pump

A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a 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 called 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 an indoor unit. After being moved outdoors again, the refrigerant is decompressed — evaporated. 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, fans, and pumps run with electric energy.
Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps and exhaust air heat pumps. Large-scale heat pumps are also used in district heating systems.The efficiency of a heat pump is expressed with 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 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 generated, but they usually reduce emissions in mild climates. Heat pumps could satisfy over 80% of global space and water heating needs with a lower carbon footprint than gas-fired condensing boilers: however, as of 2021 they only meet 10%.

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