How a heat pump works

The function of a heat pump is very similar to that of a refrigerator. While a refrigerator extracts heat energy from the food, i.e. from the interior of the refrigerator, and conducts it outwards, a heat pump does the opposite: it extracts heat energy from the environment outside the building and makes it usable for heating indoors. In addition to the indoor and outdoor air, a heat pump is able to tap the heat energy from the groundwater and the ground. And because the temperature of this environmental heat is usually not sufficient to heat a building or the hot water, the thermodynamic process is used.

The refrigeration cycle in detail

Regardless of which heat source is used to generate heat, the cooling cycle, which takes place in four steps, is a fundamental part of how the heat pump works.

1. Vaporisation

In order to evaporate a liquid, energy must be supplied. This can be seen easily with water. When a pot of water is heated to 100 degrees Celsius (heat energy supplied), the water begins to evaporate. If further heat energy is then supplied, the temperature of the water does not rise any further. Instead, the water is completely converted to steam.

2. Compression of gas

If a gas, for example air, is compressed (the pressure increases), the temperature also increases. You can experience this if you close the opening of a bicycle air pump and squeeze the air - the cylinder of the air pump gets warm.

3. Condensation

Since energy cannot be lost, the heat energy previously used for evaporation is released again when water vapour condenses.

4. Cooling

If the pressure of a pressurised liquid is suddenly reduced, the temperature drops many times over. This can be seen, for example, on a liquid gas bottle in a camping gas cooker. If the valve is opened, ice may form on the valve of the liquid gas bottle even in summer. (Here the pressure is reduced from about 30 bar to 1 bar.)

Constant repetition of the process

These processes take place within the heat pump in a closed circuit. A liquid (refrigerant) is used for heat transport, which evaporates at very low temperatures. Heat energy, for example from the ground or outside air, is used to evaporate this liquid. Even temperatures of minus 20 degrees Celsius are sufficient for this. The cold refrigerant vapour, for example at -20 degrees Celsius, is then very heavily compressed. It heats up to a temperature of up to 100 degrees Celsius. This refrigerant vapour is condensed and transfers the heat to the heating system. The pressure of the liquid refrigerant is then greatly reduced. The temperature of the liquid drops to the initial level. The process can start again.

The process using the example of an air-water heat pump

The easiest way to explain this process is with an air-water heat pump: An air-water heat pump can consist of one or two units. In both cases, an integrated fan actively draws in ambient air and transfers it to a heat exchanger. A refrigerant that changes its physical state even at very low temperatures flows through the heat exchanger itself. In contact with the ambient air, the refrigerant heats up and gradually becomes a vapour. A compressor is used to increase the heat generated to the desired temperature. This compresses the steam and increases both the pressure and temperature of the refrigerant vapour.

A further heat exchanger (condenser) then transfers the energy from the heated vapour to the heating circuit (floor heating, radiator or heating buffer or hot water storage tank). The refrigerant still under pressure cools down and liquefies again. Before it can flow back to the circuit, the refrigerant is first expanded in an expansion valve. Once it has reached its initial state, the cooling cycle process can start again.

The picture shows the compression process as part of the operation of the heat pump

Compression requires electricity

An essential component of the refrigeration circuit is the compressor. Without compression, the output temperatures are too low to heat a building to a comfortable temperature - especially on very cold days with minus temperatures.

In practice, various compressors are used, including piston compressors or scroll compressors, which are electrically driven. The power consumption for compression depends on many factors. These include the heating requirement, the compressor technology and last but not least the temperature difference between the heat source and the heating system. In principle, the higher the temperature difference between the heat source and the flow temperature, the more the compressor must operate.

Heat pump energy optimises the environmental balance of a heat pump

For some years now, electricity suppliers have also been offering special heat pump tariffs with improved conditions for end customers. In this case, system owners benefit twice: On the one hand, such tariffs reduce heating costs to a minimum. On the other hand, the electricity is usually produced from renewable energies. It is clean, so to speak, which further improves the already positive ecological balance of a heat pump.