Heat flows naturally from a higher to a lower temperature.
Heat pumps,
however, are able to force the heat flow in the other direction, using
a relatively small amount of high quality drive energy (electricity,
fuel, or high-temperature waste heat). Thus
heat pumps can transfer
heat from natural heat sources in the surroundings, such as the air,
ground or water, or from man-made heat sources such as industrial or
domestic waste, to a building or an industrial application.
Heat pumps
can also be used for cooling. Heat is then transferred in the opposite
direction, from the application that is cooled, to surroundings at a
higher temperature. Sometimes the excess heat from cooling is used to
meet a simultaneous heat demand.
In order to transport heat from a heat source to a heat sink,
external energy is needed to drive the heat pump. Theoretically, the
total heat delivered by the
heat pump is equal to the heat extracted
from the heat source, plus the amount of drive energy supplied.
Electrically-driven heat pumps for heating buildings typically supply
100 kWh of heat with just 20-40 kWh of electricity. Many
industrial heat pumps can achieve even higher performance, and supply the same amount of heat with only 3-10 kWh of electricity.
Because
heat pumps consume less primary energy than conventional
heating systems, they are an important technology for reducing gas
emissions that harm the environment, such as carbon dioxide (CO2),
sulphur dioxide (SO2) and nitrogen oxides (NOx). However, the overall
environmental impact of electric
heat pumps depends very much on how
the electricity is produced.
Heat pumps driven by electricity from, for
instance, hydropower or renewable energy reduce emissions more
significantly than if the electricity is generated by coal, oil or
gas-fired power plants.