Technical explanation
The name already gives away something about how a heat pump works; it transfers heat. But where does the heat pump get its heat from, and how is this heat released in a usable form? Heat pumps operate on the principle of evaporation (boiling) and condensation. To better understand this, let's first look at the evaporation and condensation of water.
Evaporation and condensation
Under atmospheric pressure, water boils at 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 the boiling point, is 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 boil at 20°C, requiring heat to be supplied, for example, from the surroundings. The boiling and condensation points of water are therefore related to the pressure under which the water is placed. When boiling, molecules can escape from the water.
The lower the pressure, the easier they can escape, and the higher the pressure, the harder. To make the molecules move faster, more energy/heat must be added. When heat is removed or the pressure is increased, the vapor condenses, releasing the previously added heat and condensing back into water, releasing heat.
- Evaporate (boil) = supply heat (extract 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 using this principle. Instead of water, the heat pump uses a refrigerant, also known as Freon. This type of refrigerant boils at -48.5°C under atmospheric pressure.
This refrigerant is liquefied in a closed system and pressurized to 8.5 bar in an evaporator, causing it to boil at 5°C. If we allow outside air at 10°C, warmer than the refrigerant's boiling point, to flow past this evaporator, the refrigerant will boil and extract heat from the air. The cold air will cool, and the refrigerant will evaporate. The resulting vapor is sucked into a compressor, which increases the pressure to 30 bar.
The gas moves under this pressure to the condenser, where it aims to condense at 50°C. If we now allow water at 30°C, below the refrigerant's boiling point, to flow past it, the refrigerant will condense, transferring heat to the water and warming it. After complete condensation, the refrigerant is depressurized again by an expansion valve, and the process repeats.
Heat is transferred in this cycle. The energy required to evaporate the refrigerant is obtained from the outside air. Electricity is needed to drive the compressor. The additional 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 the heat pump requires is extracted from the outside air. Modern heat pumps have a ratio of electricity to outside air energy of 1 to 4, resulting in a total of 1 + 4 = 5. 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 part uses electricity and ensures that the refrigerant can circulate under different pressures.
- Condenser: In the high-pressure section, the refrigerant condenses, releasing all the heat it contains. This heat is absorbed by water flowing past the condenser and used to heat buildings.
- Expansion valve: An electronically controlled valve used to reduce pressure. The pressure before the expansion valve is high, and after the valve, the pressure is low.
Heat pump applications
Heat pumps are increasingly seen as the sustainable alternative to central heating boilers for heating homes. This is due to advantages such as lower energy bills, reduced CO₂ emissions, and a comfortable indoor climate. Using green energy or solar panels eliminates the need for fossil fuels.
A heat pump, powered by electricity that can optionally be generated by solar panels, makes it possible to realise a zero-on-the-meter home, provided the overall concept is correct.
The heat pump is highly 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. By 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 five times more efficient.
A heat pump can be used for heating rooms and tap water, but also for cooling rooms. Applications include homes, businesses, and even large hospitals and swimming pools.
In summary
By cleverly applying physical principles, we can tap into the energy already around us. Unlike burning fossil fuels, this process doesn't emit any harmful substances. Electricity is required to start this process. When this electricity is generated green, emissions are zero.
