How does a heat pump work?
The heat pump principle
FAQ 23.02.2022 | Karl KrollThe way a heat pump works is often compared to the reverse principle of a refrigerator. While a refrigerator extracts heat energy from its interior and transfers it to the outside, a heat pump does the opposite:
it extracts free heat energy from the environment and transfers it to the medium water, which is then used for heating, for example in the underfloor heating system, or for generating hot water.
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The big advantage: Although the heat pump uses electricity to harness environmental energy for heating and hot water, An air-to-water heat pump, for example, can typically draw up to 75% of the energy it needs from the environment—and only needs to consume 25% in the form of electricity. This makes it particularly efficient compared to heat generators that use heating oil or natural gas, for example.
Where does environmental heat come from?
Heat SourcesThe Most Common Heat Sources
In an air-to-water heat pump, the environmental heat comes from the outside air—and this works even at very low outdoor temperatures. A slightly more efficient option is to use thermal energy from the ground with a brine-to-water heat pump, also known as a geothermal heat pump.
But how does it work—how is energy extracted from the freezing cold winter air, relatively cool soil, or groundwater to heat our buildings? How is it possible to generate comfortable warmth without combustion or flames?
How Heat Generation Works
Step by stepHeat Pump Cycle
A refrigerant circulates in every heat pump. A refrigerant is a substance that can absorb heat at low temperatures and low pressures and release heat at high pressures and, consequently, elevated temperatures. The phase change of the refrigerant, which is hermetically sealed within the heat pump system, ensures the necessary transfer of energy. In heat pumps, the refrigerant follows the following cycle:
1. Evaporation: Regardless of the energy source, energy from the environment is transferred to the refrigerant in the evaporator. The liquid refrigerant gradually evaporates, turns into a gas, expands, and absorbs energy from the surroundings in the process. You can compare this to water, which turns into steam when heated—except that with refrigerant, this happens faster and at subzero temperatures.
2. Compression: Because the temperature of the refrigerant vapor is not high enough to heat a room, the vapor is drawn in by an electric-powered refrigerant compressor. Compression increases the pressure, causing the refrigerant vapor to heat up significantly. Think of a bicycle pump, where the compressed air in the piston becomes warmer.
3. Condensation: The hot, pressurized refrigerant vapor flows into the condenser. Since the energy previously absorbed from the environment cannot be lost, it is transferred here in the form of heat to the connected heating system.
4. Relaxation: As energy is released, the refrigerant returns to a liquid state and the pressure drops. It flows back through the expansion valve to the evaporator, and the cycle begins again
Incidentally, the refrigerant cycle can also be reversed to use heat pumps for cooling as well. With a properly equipped system—a so-called “reversible” heat pump—rooms can be cooled to comfortable temperatures during the hot months. If needed, this capability of a heat pump should be taken into account from the outset so that the system can be configured accordingly.
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FAQ: How does a heat pump work?
FAQA heat pump harnesses thermal energy from the environment and makes it usable for buildings. To do this, it extracts heat from the outside air, the ground, or groundwater and uses electricity to raise its temperature. Depending on the system, the heat is transferred to heating water or directly to the air in the room.
A heat pump works on the opposite principle of a refrigerator. While a refrigerator extracts heat from its interior and releases it to the outside, a heat pump absorbs heat from the surrounding environment and transfers it to the building. To do this, a refrigerant circulates in a closed loop.
This comparison helps explain the principle in simple terms. Both systems transfer heat from one place to another. In a refrigerator, heat is transferred from the interior to the outside. In a heat pump, heat from the environment is absorbed and used for heating inside the building.
Common types of heat pumps include air-to-water heat pumps, air-to-air heat pumps, ground-source heat pumps, and water-to-water heat pumps. They differ primarily in the heat source they use and how the heat is distributed throughout the building.
An air-to-water heat pump uses outdoor air as a heat source and transfers the heat to the heating water. An air-to-air heat pump also uses outdoor air, but releases the heat directly into the room air. A ground-source heat pump uses heat from the ground. To do this, a brine solution circulates through ground probes or ground collectors and transports the absorbed heat to the heat pump.
An air-to-water heat pump extracts thermal energy from the outside air. This heat is transferred to the refrigerant in the evaporator, raised to a higher temperature in the compressor, and then transferred to the heating water. This water can then be used for heating or hot water production.
An air-to-air heat pump extracts heat from the outside air and transfers it directly to the indoor air. In heating mode, a reversible split-system air conditioner operates like a heat pump: The outdoor unit absorbs heat from the outside air, and the indoor unit releases it into the room as warm air.
A brine-to-water heat pump utilizes the heat stored in the ground. To do this, a brine solution circulates through ground probes or ground collectors, where it absorbs heat. Inside the heat pump, this energy is brought to a usable temperature level via the refrigerant circuit and transferred to the heating water.
A geothermal heat pump harnesses heat from the ground. These are often brine-to-water heat pumps. Since the temperature in the ground remains relatively constant, this heat source can be utilized particularly efficiently.
Heat pumps can extract heat from the outside air, the ground, or groundwater. Air-to-water and air-to-air heat pumps use outside air. Brine-to-water heat pumps use geothermal energy. Water-to-water heat pumps use groundwater as a heat source.
Even cold outdoor air still contains thermal energy. A heat pump can absorb this energy and make it usable through the refrigerant cycle. The refrigerant evaporates even at low temperatures and is then heated to a higher temperature in the compressor.
The refrigerant transports heat within the heat pump. It absorbs heat from the environment, evaporates in the process, and turns into a gas. It is then compressed, heats up significantly, and releases the heat. Afterward, it expands, and the cycle begins again.
In the evaporator, heat from the environment is transferred to the refrigerant. The refrigerant absorbs the heat and evaporates. This step marks the beginning of the refrigerant cycle.
The compressor draws in the gaseous refrigerant and compresses it. This causes the refrigerant's pressure and temperature to rise. Only then does the absorbed heat from the environment reach a temperature level that can be used for heating.
In the condenser, the hot refrigerant releases its heat. Depending on the type of heat pump, this heat is transferred to the heating water or directly to the indoor air. In the process, the refrigerant returns to a liquid state.
The expansion valve reduces the pressure of the liquid refrigerant. This causes it to cool down, allowing it to absorb heat from the environment again in the evaporator. The cycle then begins anew.
A heat pump primarily requires electricity to power the compressor, the control system, and other technical components. The electricity is not used to generate heat directly, but rather to absorb heat from the environment, compress it, and make it usable for the building.
A heat pump draws a large portion of the energy it needs from the environment. Electricity is used primarily to raise this environmental heat to a usable temperature. As a result, a heat pump can generate several kilowatt-hours of heat from a single kilowatt-hour of electricity.
Yes, many heat pumps can provide hot water in addition to space heating. This is especially true for systems that transfer heat to a water-based heating system, such as air-to-water or ground-source heat pumps.
Yes, heat pumps designed for this purpose can also be used for cooling. In reversible systems, the cycle is reversed: The system extracts heat from the room and discharges it outside.
A reversible heat pump can reverse the refrigerant cycle. In heating mode, it absorbs heat from the environment and releases it into the building. In cooling mode, it removes heat from the room and transfers it outside.
They are used in existing buildings. In new construction, the building envelope, heating surfaces, and heat pump can be directly coordinated with one another. In existing buildings, the key factors are how well the building is insulated, what heating surfaces are available, and what flow temperatures are required. Depending on the building, air-to-water, brine-to-water, or air-to-air heat pumps may be suitable options.
In water-source heat pumps, the supply temperature refers to the temperature at which the heating water is fed into the heating system. The lower this temperature is, the more efficiently the heat pump can operate. This is because the environmental heat absorbed then needs to be heated less before it is transferred to the heating system. This works particularly well with properly sized heating surfaces, such as underfloor heating or suitable radiators.
With air-to-water heat pumps, a distinction is often made between monoblock and split designs. In a monoblock heat pump, the refrigerant circuit is located in the outdoor unit. Heat is transferred from there to the water-based heating system in the building. In a split-type heat pump, the outdoor and indoor units are separate: the outdoor unit absorbs heat from the environment, and the indoor unit transfers it to the heating or hot water system. Both designs use the same basic principle but differ in their construction and installation.
