In order to be able to evaporate a liquid, you need to supply energy. This is easy to observe with water. When a pan of water is heated to 100 degrees Celsius (thermal energy is added), the water begins to evaporate. If further thermal energy is then added, the temperature of the water does not continue to rise. Instead, the water is fully converted to steam.
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How does a heat pump work?
A heat pump works in a similar way to a refrigerator – only in reverse. While a refrigerator extracts heat energy from the food, i.e. the interior of the refrigerator, and channels it to the outside, a heat pump does the opposite: It extracts thermal energy from the environment outside the building and makes it usable for heating indoors. In addition to the indoor or outdoor air, a heat pump is able to tap thermal energy from the groundwater and the earth. And as the temperature of the heat obtained is usually insufficient to heat a building or domestic hot water, thermodynamic processes are used to raise the temperature.
Refrigeration cycle process – the core of the heat pump principle
Regardless of which heat source is used to generate heat, the refrigeration cycle process, which has four steps, is always part of the heat pump's mode of operation.
If a gas, such as air, is compressed (the pressure is increased), the temperature also increases. You can experience this if you hold the opening of a bicycle air pump closed and squeeze the air – the cylinder of the pump becomes warm.
Since energy cannot be lost, when water vapour condenses, the thermal energy previously used for evaporation is released again.
If the pressure of a pressurised liquid is suddenly reduced, the temperature drops considerably. This can be observed, for example, on a liquid gas bottle in a camping gas cooker. If the valve is opened, ice can form on the valve of the LPG cylinder, even in summer (here the pressure is reduced from about 30 bar to 1 bar).
Continuous repetition of the process
These processes take place in a closed circuit within the heat pump. To transport the heat, a liquid (refrigerant) is used that evaporates at very low temperatures. Thermal energy, such as from the earth or outdoor air, is used to evaporate this liquid. Even temperatures of minus 20 degrees Celsius are sufficient to provide energy. The cold refrigerant vapour, for example -20 degrees Celsius, is then highly compressed. In the process, it heats up to a temperature of as high as 100 degrees Celsius. This refrigerant vapour is condensed and releases the heat to the heating system. Subsequently, the pressure of the liquid refrigerant is greatly reduced. This causes the temperature of the liquid to drop back to the initial level. The process can start from the beginning.
Principle of the heat pump using the example of an air source heat pump
The simplest way to explain this process is to use the example of an air source heat pump: An air source heat pump can consist of one or two units. In either case, a built-in fan actively draws in ambient air and channels it to a heat exchanger. Refrigerant flows through the heat exchanger, which changes its physical state at very low temperatures. When it comes into contact with the ambient air, the refrigerant heats up and gradually becomes vapour-like. A compressor is used to increase the resulting heat to the required temperature. This compresses the steam and increases both the pressure and the temperature of the refrigerant vapour.
A second heat exchanger (condenser) then transfers the energy from the heated steam to the heating circuit (underfloor heating, radiators, heating buffer and/or DHW cylinder). In the process, the refrigerant, which is still under pressure, cools down and liquefies again. Before it can flow back into the circuit, the refrigerant is firstly expanded in an expansion valve. Once it has reached its initial state, the refrigeration cycle can start again.
Compression requires electrical current
An essential component of the refrigeration circuit is the compressor. This is because without compression, the output temperatures are too low to be able to heat a building to a comfortable temperature – even more so on very cold days with double-digit minus temperatures.
In practice, a number of compressors are used, including piston compressors or scroll compressors, which are all electrically driven. The power consumption for compression depends on many factors. These include the heat demand, the compressor technology and, last but not least, the temperature difference between the heat source and the heating system. As a general rule: The higher the temperature differential between the heat source and the flow temperature, the more the compressor has to work.
Heat pump electricity improves the life cycle assessment of a heat pump
For some time now, electricity providers have offered special heat pump tariffs with improved conditions for end customers. In this case, system owners benefit twice over. These tariffs help to reduce heating costs to a minimum. At the same time, the electricity is usually generated using renewable energies. The energy is "clean", which improves the already positive life cycle assessment of a heat pump even further.