Early installations of the technology have been operating for more than 75 years providing heating, cooling and hot water for residential, commercial and industrial purposes. Despite this history, the technology is still often deemed innovative.

Heat pump technology uses the refrigeration cycle. This principle was discovered in the 1850s by Sadi Carnot (and is now called the Carnot Cycle), and was described theoretically shortly thereafter by the famous Lord Kelvin.


Heat pumps use two simple principles: evaporation and compression. While this may sound very technical, these principles are easy to understand and have been known to humans for a long time.

Everybody benefits from evaporation on hot summer days. Swiping a damp cloth over one’s skin is refreshing because of the evaporation caused by the body and outside energy, which results in a cooling effect.

Anyone who has inflated the tire of a bicycle will understand the concept of compression. Mechanical energy from human muscular activity is used to compress the air in the pump before it can be released into the tire. The tip of the air pump actually gets warm, an effect that can be felt by your hand! Compressing air increases its temperature.

These two effects are what make heat pump systems work: similar to the skin example, a source of energy evaporates the refrigerant (by this process, the energy source is cooled down slightly). The result is a gas. In a second step – similar to the bike pump example – the gas is compressed thus increasing its temperature. Using a heat exchanger, the energy is then transferred to the distribution system of the building. Energy is usually distributed via (low temperature) radiators, floor heating system or fan coil units.

If the process is operated in reverse mode, cooling is provided. Energy can come from renewable sources: air, water, or the ground or from processes: exhaust air, waste energy stored in water/ground from buildings or industrial processes. Auxiliary energy – usually electricity or gas – is needed to run the compressor and the pumps. Heat pumps always provide heating and cooling, thus giving the same machine an additional economic advantage in cases where both services are needed. In heating mode, ambient energy is the heat source and the building is the heat sink. In cooling mode, the cycle is reversed: the building is cooled down using the outside as heat sink.

This article featured in Revolve’s special report on Heat Pumps  in association with the European Heat Pump Association on pages 8-9.