In this article, I will show you how a heat pump works, how it can utilise heat from the environment and why a heat pump is often compared to a refrigerator. Below you will find a video to accompany this article:

Function of the heat pump – the most important basics

A heat pump utilises ambient heat sources for heating, even at temperatures far below 0 °C. To do this, it extracts heat from the outside air, the ground or groundwater and raises it to a higher temperature level using electrical energy. The heat obtained in this way is transferred to the heating system. The special feature: The heat pump supplies significantly more heat than it consumes in electricity.

If around one kilowatt of electrical power is required to operate the heat pump, it is possible to obtain a heat flow rate of three, four or five kilowatts for the heating system.

Note: As the heat pump delivers more heat to the heating system than it requires in electrical energy for operation, a performance indicator can be derived from the ratio of these two values: the coefficient of performance (COP). The less electrical energy the heat pump requires for a certain heat output, the higher the COP and the more efficiently the system works. In addition to the COP, there are other efficiency indicators, but more on this in an upcoming article.

The heat pump utilises heat from ambient heat sources such as air, soil, surface water or waste heat from industrial processes. The temperature of the heat sources is so low that it can rarely be utilised for other purposes. The energy flow diagram in Figure 1 illustrates this:

Using heat from the environment at -14 °C? Isn’t that actually cold?

Yes, it is possible! To utilise these ambient sources, a refrigerant runs through a cycle in the heat pump. The heat pump utilises simple physical principles. To understand these, let’s take a look at the terms heat, latent heat and refrigerant and then put them together in the refrigeration cycle to form an understandable puzzle.

What does “heat” mean for heating and cooling?

Heat is a form of energy transfer and occurs due to a temperature difference. This heat transfer ends as soon as both substances have reached the same temperature. The flow direction of the heat is decisive here.

Note: Heat always flows from a body or material at a higher temperature to a body or material at a lower temperature. Never the other way round! The following applies: Every body or substance with a temperature above 0 Kelvin (-273.15 °C) has heat. In physics, this temperature point is known as “absolute zero“. Only at this low temperature can no more particle movement be detected in a substance. This means that even at an outside temperature of -20 °C, the ambient air still contains usable heat, which is utilised by the heat pump.

Here is a simple example to help you understand: If you take a metal spoon out of your kitchen drawer and touch it, it will feel cold in your hand, but will get warmer over time and reach the temperature of your hand after a short time. The heat transfer (energy transfer) is then complete. This does not work the other way round! The spoon at room temperature has also stored heat, but cannot transfer it to your hand. If this were possible, your hand would get warmer and the spoon would get colder, which is physically impossible.

This means for building services engineering:

• Heating means that heat is supplied to a system.
• Cooling means that heat is removed from a system (i.e. heat is dissipated).

So when we talk about “heating”, we practically always mean the supply of heat, and when we talk about “cooling”, we mean the removal of heat.

Latent and sensible heat

We have now been able to establish a connection between heat and temperatures and, as a rule, the following statement can be made:

• If heat is added to a substance or body the temperature rises.
• If heat is dissipated from a substance or body the temperature drops.

This form of heat is called sensible heat because it can be “felt” or measured directly as a change in temperature.

However, there is one important exception and that is changes in the state of matter such as melting, solidification, evaporation and condensation.

Important: During a phase change, the temperature remains constant even though heat continues to be added or released. This amount of heat is called latent heat and is required for the dissolution or formation of bonds between the particles during a phase change.

The two examples below are intended to show you how much heat is required for a change of state of matter using one kilogram of water:

Example – Melting of water:
➔ To completely melt 1 kg of frozen water (ice) at 0 °C to liquid water at 0 °C, you need about 334 kJ (≈ 0.093 kWh).
➔ This corresponds approximately to the heat required to heat 1 kg of liquid water from 0 °C to 80 °C!

Example – Evaporising of water:
➔ To completely evaporate 1 kg of liquid water at 100 °C to water vapour at 100 °C, you need about 2,260 kJ (≈ 0.63 kWh).
➔ This corresponds approximately to the heat required to heat 1 kg of liquid water 5.4 times from 0 °C to 100 °C.

This means that a lot of latent heat is absorbed during a phase change such as melting or evaporation, whereby the temperature remains approximately constant. At the same time, this latent heat is released again during the reverse process, i.e. during solidification or condensation.

This is very relevant for building services engineering, as latent heat plays a central role in heat pumps, refrigeration machines and also in condensing boilers. In these machines, heat is absorbed and released particularly efficiently through phase changes (evaporation/condensation). However, we will take a closer look at this later in the heat pump cycle. In the next step, we will continue with the refrigerant, the working medium in a heat pump.

Refrigerant: The working medium in the heat pump

Refrigerants are special fluids (liquid/gaseous) that are used as working mediums in vapour-compression cycles (of heat pumps and refrigeration machines). The working medium refrigerant has the task of transporting heat. How does this work? Many refrigerants have excellent thermodynamic properties and boil at very low temperatures, for example.

The refrigerant propane (R290), for example, has a boiling point of around -42 °C at atmospheric pressure and can absorb latent heat (evaporate) at very low temperatures and change its physical state from liquid to gaseous. The heat is then stored in the gaseous refrigerant and can be transferred to a heating system in a heat pump, but more on this later.

Important: Refrigerants are not antifreeze. Antifreeze (e.g. in brine loops) lowers the freezing point. A refrigerant, on the other hand, is the heat transport medium in the refrigeration cycle.

We have now learnt the basics of understanding heat pumps (heat, latent heat and refrigerants). In the next step, we will look at how these are used in the refrigeration cycle and how heat from the environment can be utilised for a heating system.

Heat pump working principle: In four steps

The function of a heat pump can best be described using an idealised cycle (reverse (counter-clockwise) Carnot cycle). The Carnot cycle is a thermodynamic cycle and describes the constantly changing state of matter of a working medium, which in the case of a heat pump is the refrigerant.

The refrigerant circulates in a closed cycle, absorbing heat from the environment at a low temperature level (it evaporates) and releasing the heat to a heating system at a high temperature level (it condenses). The refrigerant has no direct contact with the heat source (air, soil, water) or the heating water.

There are four components in the refrigeration cycle with clearly defined tasks for this cycle: Evaporator, compressor, condenser and expansion valve. Figure 4 shows an example of the refrigeration cycle of a heat pump with four steps. We will now take a closer look at these.

Note: The following thermodynamic changes of state are based on the ideal cycle of a heat pump and are a simplified representation of the real processes.

Note: The sketches show $dot Q$ (Q-dot) stands for the heat flow rate: It describes how much heat is transferred or utilised per unit of time.

Step 1 – Evaporation of the refrigerant (heat absorption from the environment)

①Evaporation → ② Compression → ③ Condensation → ④ Expansion

The refrigerant changes its state of matter from liquid to gaseous, whereby the temperature of the refrigerant remains almost constant, and absorbs heat. This process is called isothermal expansion in the reverse (counter-clockwise) Carnot cycle.

Step 2 – Compressing the refrigerant (generating temperature lift)

① Evaporation → ② Compression → ③ Condensation → ④ Expansion

Note: The smaller the temperature difference (temperature differential) between the heat source and the flow temperature of the heating system, the less the refrigerant has to be compressed in the compressor and the more efficiently the heat pump works. This is why surface heating systems (such as underfloor heating) with low flow temperatures (approx. 30-35 °C) in combination with a heat source at a constant temperature level (such as geothermal energy or groundwater) are ideal for high heat pump efficiency. We take a closer look at this in the article on heat pump efficiency.

Step 3 – Condensing the refrigerant (heat transfer to the heating system)

① Evaporation → ② Compression → ③ Condensation → ④ Expansion

The gaseous refrigerant releases its heat in the condenser and changes its state of matter from gaseous to liquid. Condensation transfers the latent heat stored in the refrigerant to the heating system, whereby the temperature remains almost constant. At the end of this step, the refrigerant is once again a liquid, but is still under high pressure and at a high temperature. This process is referred to as isothermal compression in the reverse (counter-clockwise) Carnot cycle.

Step 4 – Expansion of the refrigerant (reduce temperature and pressure)

① Evaporation → ② Compression → ③ Condensation → ④ Expansion

Heat pump and refrigerator – what do they have in common?

Even if it doesn’t look like it at first glance, heat pumps and refrigerators work according to the same physical principle and have the same components in a refrigeration cycle: evaporator, compressor, condenser and expansion valve. A refrigerant circulates in both appliances, absorbing heat, transporting it and releasing it elsewhere. The drive for this is provided by an electrically powered compressor, which compresses the refrigerant and thus raises it to a higher temperature level.

The difference is therefore not in the technology, but in the objective: a refrigerator is technically a refrigeration machine and is designed to extract heat from a substance (food or drink). When you put food in your fridge, the food is in a closed space. Heat is extracted from the food inside the refrigerator and this is released into the kitchen as waste heat via the back of the appliance. If you hold your hand against the back of a running fridge, you can feel this effect.

The purpose of a refrigeration machine is therefore to extract heat from a room or a refrigerated item. The resulting waste heat is only a by-product.

With a heat pump, it is exactly the opposite: it extracts heat from the environment, i.e. the outside air, the ground or the groundwater, and transfers it to the heating system. In this case, the heat output is the purpose and the cooling of the environment is the by-product. You can see this in Figure 9.

• Heat pump: Heat source $dot{Q}_0$ is heat from the environment (air, ground, groundwater), heat output $dot{Q}_C$ is the heating system
• Refrigeration machine: Heat source $dot{Q}_0$ is heat from the area to be cooled (refrigerator, building), heat rejection $dot{Q}_C$ is the environment

And this is where it becomes particularly interesting for building services engineering: many modern heat pumps can also cool in summer. The refrigeration cycle itself remains unchanged; a reversing valve simply swaps the function of the heat exchangers so that the heat pump extracts heat from the rooms and dissipates it outside. This is known as active cooling.

Important: The cycle is not reversed, even though this is often described as such. The four steps (evaporation, compression, condensation, expansion) take place in the same order. The only thing that changes is the allocation of the heat exchangers: a reversing valve reverses the direction of flow of the refrigerant after the compressor so that the previous condenser works as an evaporator and vice versa.

With brine-to-water heat pumps (geothermal or groundwater), there is also the option of passive cooling, in which the low temperature of the ground or groundwater can be used directly for cooling without compressor operation. The heat pump thus becomes an appliance that can heat and cool.

The following pictures show two typical heat pump configurations: a split heat pump with an indoor and an outdoor unit connected by a refrigerant line, and a monobloc heat pump with all components inside.

Frequently asked questions about the function of the heat pump (FAQ)

Conclusion

A heat pump utilises a simple physical principle: a refrigerant absorbs heat from the environment, is compressed and releases the heat at a higher temperature level to a heating system. To do this, the refrigerant goes through four steps: evaporation, compression, condensation and expansion in a closed refrigeration cycle.

The latent heat is crucial here: during evaporation, the refrigerant absorbs large amounts of energy without its temperature rising. During condensation, this energy is released again and transferred to the heating system. As a result, a heat pump can release significantly more heat than it requires in terms of electrical energy for operation.

The principle also works at low outside temperatures, as the refrigerants used boil at temperatures well below 0 °C. How efficiently a heat pump implements this process in practice is described by performance indicators such as the coefficient of performance (COP) and the seasonal performance factor (SPF), but more on this in an upcoming article. You can also find out there what this means in concrete terms for electricity consumption and operating costs.

You can find out which heat sources are suitable for heat pumps and what advantages and disadvantages they have in this article: Heat sources for heat pumps.

I hope this article has helped you to better understand the function of a heat pump. If you have any questions, suggestions or criticism about the article, please feel free to use the comments function.

Best regards! Martin

Further links and sources:

• Source 1: Martin Schlobach, from: “Haustechnik für Dummies”, Wiley-VCH 2023, used with permission.
• Source 2: Midjourney AI
• Wikipedia – Carnot cycle
• Wikipedia – Heat Pump
• Wikipedia – Thermal expansion valve
• Pocketbook for Heating and Air Conditioning Technology 05/06, Recknagel, Sprenger, Schramek – 72nd edition, Oldenbourg Industrieverlag Munich, 2006
• Amendment of the EU F-Gas Regulation 2023-2024
• Erdgekoppelte Wärmepumpen – Geschichte, Systeme, Auslegung, Installation, Dipl.- Geol. Burkhard Sanner, IZW- Berichte 2/92, November 1992, (german)
• Handbuch der Gebäudetechnik – Planungsgrundlagen und Beispiele Band 2 Heizung/ Lüftung/ Energiesparen, Prof. Dipl.-Ing. Wolfram Pistohl, Architekt 7th edition, Werner Verlag, Cologne 2009 (german)
• Cover image created with Midjourney AI