What is a heat pump?

It is difficult to understand intuitively why a heat pump pumps heat at all. Thermodynamic concepts such as heat and energy cannot be directly experienced in everyday life. Therefore, we explain the basic principle of a heat pump with an analogy to the much more tangible water pump.

Manuel Gräber

Manuel Gräber


May 17, 2023

Heat pump

123rf / eakrinr

A heat pump is a technical device that transports heat from a low temperature level to a higher one. There are different operating principles according to which heat pumps can be built. By far, the most common operating principle is a refrigeration cycle - just like in an air conditioner or fridge.

Why are heat pumps energy efficient?

The simple way to heat a house is to burn fuels (gas, oil or wood) and use the resulting heat directly in the heating system. This is never particularly efficient from a thermodynamic point of view. A heat pump is much more efficient.

The crucial point here is: relatively low temperatures (50°C) are sufficient to heat a house. When burning gas, however, much higher temperatures could be reached (well over 1000°C). Not using this potential of high temperatures is a waste. A high-grade form of energy (fuels) is used to convert it 1:1 into low-grade energy (heat at 50°C).

Heat pumps, on the other hand, also consume a very high-quality form of energy: electricity. But they make much better use of the available potential than a combustion heating system. In fact, they generate 3 to 4 times the amount of heat with the electrical energy used. This ratio is called COP (Coefficient of Performance).

1 becomes 3 sounds like magic? No, the physical laws of nature such as energy conservation are all fulfilled. The additional energy is taken from the freely available ambient heat (air or soil). In order for this to work, you need this temperature lift that I mentioned. The easiest way to understand this is to make an analogy with an ordinary water pump.

The analogy to the water pump

Let us imagine a house idyllically situated at the foot of a mountain. About 80 m above this house there is a crystal clear mountain lake with sufficient drinking water. A very simple way to supply the house with water would be to lay a pipe directly from the mountain lake to the house. You wouldn't even need a pump, because of the difference in altitude the water would flow into the house by itself. Only a valve to throttle the pressure would be useful.

Without a doubt, it is a simple and inexpensive solution. But is it also energy-efficient?

Let's look at a second technical solution. We install a small water turbine next to the house and run it with water from the mountain lake. With the generated electricity, we drive an electric water pump that pumps water from a nearby well.

Was ist eine Wärmepumpe? Erklärung anhand einer Analogie.

The difference in height between the groundwater and the house is 10 meters. Sounds cumbersome at first, but let's look at the energy efficiency

We ultimately use the potential energy of the mountain lake water 80 m higher up to pump well water up from 10 m below. The respective amounts of energy are proportional to the differences in altitude. That means there is a factor of 8 in between. In addition, there are conversion losses, which we estimate using the efficiency of the well pump (50%) and the turbine plus generator (50%). The ideal factor of 8 becomes the real achievable factor of 2 with the efficiency chain. This means: to supply the house with the same amount of drinking water from the well, we only need half of the precious mountain lake water. Or to put it another way: we can supply two houses with the same amount of mountain lake water, which is admittedly a technically more complex solution.

But if we assume that well water is available in almost unlimited quantities and free of charge, while mountain lake water is a precious and limited resource, then this costly method of supply is clearly more efficient.

Wärmepumpen Effizienz erklärt anhand einer Analogie.

A heat pump is nothing else. The limited resource of mountain lake water is equivalent to fossil fuels such as gas. All fuels are a valuable form of energy that can be converted into all kinds of energy. You can simply burn gas and convert it into heat at a low temperature level (50°C). Just like we supplied the house with mountain lake water through a simple throttle valve. It works, but it works better.

Just as we only need a relatively low pressure to supply water to houses, a relatively low temperature is sufficient to heat houses. Due to the difference in altitude of the mountain lake, the mountain lake water is available in the valley at a much higher pressure than we would need for the water supply. And even when gas is burnt, temperatures of 1000 °C and more prevail, which are much higher than we would need for heating. With direct water supply (the first variant) and direct heating by combustion, we do not use the available potential, but waste it uselessly.

Viewed holistically, it is much more efficient to take the diversions via electrical energy (second variant). As shown in the (quite realistic) numerical example above, converting the high water pressure into electrical energy makes a lot of sense. In exactly the same way, it makes sense not to burn gas directly for heating, but to convert it into electricity in a modern gas-fired power plant.

The electrical power generated can then be used to operate a heat pump in order to pump free, available heat from an even lower temperature level (e.g. from the ambient air) to the temperature level in the house. Just like the water pump example, this is a factor of 2 more efficient overall and we could heat two houses with the same amount of gas.

Can it be more efficient?

Yes, it can! Why don't we also use the mountain lake water after it has flowed through the turbine? It has less pressure than before the turbine, but it's too good to throw away. We can supply a third house with this water without any problems.

We can also apply this idea to heat supply. Just as water is removed at a low pressure level after the water turbine, heat is removed at a low temperature level in a gas power plant. This is basically the heat that is left over from burning gas after generating electricity. This is often uselessly discharged into the environment in cooling towers. But this warmth is also too good to throw away. The temperature level is sufficient to heat buildings. This is done via a district heating network and is called combined heat and power. Thermodynamically, like the heat pump, this is a very efficient way of heating.

Wärmepumpe und Fernwärme Analogie
Analogy between water supply and provision of heat.
Manuel Gräber


Manuel Gräber

Managing Director

TLK Energy

Dr.-Ing. Manuel Gräber works on modeling, optimization and controlling of thermal systems since 2008. He received his PhD at TU Braunschweig after finishing his thesis on “Energy-Optimal Control of Refrigeration Processes”. As research assistant at TU Braunschweig and employee of TLK-Thermo GmbH, Manuel Gräber carried out numerous research and development projects with various partners from industry. His focus is the combination of a broad theoretical knowledge base in different disciplines with practical experience of concrete engineering projects.

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