Heat Pumps

A heat pump uses electricity to move heat from a cooler place to a warmer place, rather than producing all its heating output directly from electricity. Unlike resistance heating, which converts electrical energy into heat, a heat pump delivers both the energy drawn from its surroundings and the electrical energy used to operate it.

How the heat-transfer cycle works

In a typical vapour-compression heat pump, a refrigerant circulates through components that change its pressure and temperature, allowing it to absorb heat at one location and release it at another.

• In the evaporator, low-pressure refrigerant absorbs heat from a source such as outdoor air, the ground or water and evaporates.
• An electrically driven compressor raises the refrigerant vapour's pressure and temperature, making it hot enough to supply heat to the building.
• In the condenser, the refrigerant releases heat to indoor air or a heating-water circuit and condenses.
• An expansion device reduces the refrigerant's pressure before it returns to the evaporator, completing the cycle.

Why it outperforms resistance heating

The crucial difference is heat transfer rather than heat generation. Electrical work enables the pump to collect additional thermal energy from its surroundings.

• The heating-side energy balance is QH = QC + W, where QH is heat delivered, QC is heat absorbed from the source, and W is work supplied.
• Heating performance is expressed as the coefficient of performance, or COP: COP = QH/W.
• Resistance heating delivers approximately one unit of heat per unit of electricity at the point of use; its effective heating COP is approximately one.
• Illustratively, a heat pump with COP 3 delivers three units of heat using one unit of electricity while drawing two units from its surroundings.
• A COP above one does not violate energy conservation because the delivered heat includes transferred environmental energy, not just converted electricity.

Operating conditions and energy-transition relevance

The advantage depends strongly on the temperature lift, meaning the difference between the source temperature and the required heat-delivery temperature.

• A larger temperature lift generally lowers COP, so colder outdoor conditions or higher heating-water temperatures can increase electricity consumption.
• Good insulation, appropriate sizing and low-temperature heat distribution help improve system performance.
• Heat pumps support efficient electrification of heating, but their emissions advantage depends on electricity generation and the heating technology displaced.