Technical explanation
The name already reveals something about the operation of a heat pump; it pumps heat. But where does the heat pump get its heat from and how is this heat released in a usable form? Heat pumps work on the principle of evaporation (boiling) and condensation. To understand this better, we first look at the evaporation and condensation of water.
Evaporation and condensation
At atmospheric pressure, water boils at a temperature of 100 ˚C. At the same pressure and a constant temperature of 100 ˚C, the water continues to boil, or evaporate, and turns into steam. During boiling (supplying heat), we can lose a lot of heat to the water/steam. If we now use a pressure cooker to boil and evaporate that same water, the pressure and thus also the boiling point are increased, the water only boils at 120 ˚C. Conversely, if we were to place a glass of water under a vacuum bell jar, the water would already boil at 20 ˚C, whereby heat must be supplied, for example from the surroundings. The boiling and condensation point of water is therefore related to the pressure in which the water is located. During boiling, molecules can escape from the water. The lower the pressure, the easier they can escape, and the higher the pressure, the more difficult. To make the molecules move faster, more energy/heat must be added. When heat is removed or pressure is increased, the vapor will condense, releasing the previously added heat and the steam will condense back into water, releasing heat.
- Evaporation (boiling) = adding heat (extracting heat from a source)
- Condensing = dissipating heat (transferring heat to the environment or installation)
Boiling water - heat pump explanation
A heat pump extracts heat from the environment according to this principle. Instead of water, the heat pump uses refrigerant, also called Freon. The type of refrigerant used boils under atmospheric pressure at -48.5 ˚C. This refrigerant is brought into a closed system as a liquid and placed in an evaporator under a pressure of 8.5 bar, causing it to boil at 5 ˚C. If we let outside air of 10 ˚C flow past this evaporator, warmer than the boiling point of the refrigerant, the refrigerant will boil and extract heat from the air. The cold air will cool down, and the refrigerant will evaporate. The resulting vapour is sucked in by a compressor, which increases the pressure to 30 bar. The gas moves under this pressure to the condenser, where it wants to condense at 50 ˚C. If we now let water of 30 ˚C flow past, below the boiling point of the refrigerant, the refrigerant will condense, release heat to the water and heat it up. After complete condensation, the refrigerant is depressurized again by an expansion valve and the process repeats.
In this cycle heat is transferred. The energy required to evaporate the refrigerant is obtained from the outside air. Electricity is required to drive the compressor. The added electrical energy is released in the condenser. The usable heat in the condenser is the heat extracted from the outside air to evaporate the refrigerant, plus the electricity consumed by the compressor.
Heat pump extracts heat from the outside air
Heat pump efficiency and COP
The only paid energy the heat pump needs is the electricity consumption of the compressor.
COP calculation
The majority of the energy required by the heat pump is extracted from the outside air. In modern heat pumps, the ratio of electricity to energy from the outside air is 1 to 4, which makes a total of 1 + 4 = 5 parts. The COP in this case is 5, with an efficiency of 500%.
COP and efficiency of a heat pump
In this explanation we extract heat/energy from the outside air, but it is also possible to use heat from the ground or a water source, resulting in a ground-source heat pump.
This is, in short, how a heat pump works.
Step by step - What exactly happens in the heat pump?
The heat pump consists of the following main components:
- Evaporator: In the low-pressure section, heat is extracted from a source to evaporate (boil) the refrigerant, changing its state of matter from liquid to gas.
- Compressor: This component uses electricity and ensures that the refrigerant can circulate under different pressures.
- Condenser: In the high-pressure section, the refrigerant condenses, releasing all the added heat. This heat is absorbed by water flowing past the condenser and is used to heat buildings.
- Expansion valve: An electronically adjustable constriction to reduce pressure. Before the expansion valve the pressure is high, after the expansion valve the pressure is low.
Heat pump applications
The heat pump is increasingly seen as the sustainable alternative to a central heating boiler for heating homes. This is due to advantages such as lower energy bills, reduced CO₂ emissions and a comfortable indoor climate. When using green energy or solar panels, fossil fuels are no longer used.
A heat pump, powered by electricity that can optionally be generated by solar panels, makes it possible to realise a zero-energy home, provided the overall concept is correct.
The heat pump is very efficient with electricity. For every kWh of electricity consumed, the heat pump produces 5 kWh of heat, sometimes even more. This is due to the clever use of heat from the outside air, ground or water. In comparison, a fully electric heater, such as an electric stove, produces only 1 kWh of heat for every kWh of electricity consumed. A heat pump is therefore 5 times more efficient.
A heat pump can be used for heating spaces and tap water, but also for cooling spaces. Applications can be found in homes, businesses, and even large hospitals and swimming pools.
In summary
By cleverly using physical principles, energy can be extracted from the environment around us. Unlike burning fossil fuels, this process does not emit any harmful substances. Electricity is needed to start this process. When this electricity is generated green, the emissions are zero.
