Heat pumps warm homes by moving heat instead of making it
For every unit of electricity it uses, a heat pump can deliver between one and four and a half units of heat into a building. The trick is that it does not generate heat at all. Like a refrigerator running in reverse, it gathers warmth from cold outdoor air or ground and pumps it indoors.
Heat naturally flows from warm to cold, never the other way, unless work is done to push it. A heat pump does that work with a compressor. A refrigerant gas is squeezed until it becomes hot, then passed through a heat exchanger indoors, where it gives up warmth and condenses to liquid. The liquid travels outside, its pressure drops, and it evaporates and turns colder than the outdoor air, so it can soak up heat again before being compressed once more. Most units can run the cycle backwards in summer, working as air conditioners.
Physics sets the ceiling on efficiency. An ideal machine drawing heat from air at 270 kelvin, about minus 3 degrees Celsius, into a room at 280 kelvin could in theory deliver 28 joules of heat per joule of work. As the indoor temperature climbs to 300 kelvin, that figure falls to 10. Real devices do worse, for instance if the outdoor unit lacks airflow, but because the ratio stays above one they beat plain electric resistance heating.
The idea is old. Lord Kelvin described the theory, and Peter von Rittinger tried vapour compression in a pilot plant in 1857, aiming to save wood and coal in salt works. The first genuinely working system, with a two-stage piston compressor, went into the Bex salt works in Switzerland in 1877. In 1928 Aurel Stodola designed a water-source system for Geneva City Hall drawing on Lake Geneva.
Air-source models are the most common, alongside ground-source, water-source and exhaust-air types, and big units feed district heating networks. As electricity grids shed fossil fuels, heat pumps usually cut emissions. They could meet over 80 percent of the world's space and water heating needs with a smaller carbon footprint than gas condensing boilers, yet in 2021 they covered only 10 percent. Europe bought 3 million of them in 2023.
Source: Heat pump
