Refrigeration cycle in a heat pump? – Function and basic principles

It sounds contradictory: What does a refrigeration cycle do in a heat pump? Resolving precisely this apparent contradiction is the key to understanding the fundamentals and functioning of refrigeration cycles – and heat pumps.

Manuel Gräber

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June 30, 2021

© franconiaphoto / 123RF.com

In contrast to most other resources available in Internet, we do not jump directly into the description of single refrigeration cycle components but start with the essential thermodynamic basics. That means we clarify "why" and "what for" before we get to "how".

Heat flows from warm to cold

It is perhaps the most important principle in Thermodynamics: Heat flows by itself only from high to low temperatures.

Intuitively, this connection is clear to everyone from their own everyday experience. If I don't actively do anything about it, my hot coffee will cool down over time until it reaches ambient temperature. However, it will never get colder than the surroundings, no matter how long I wait. For a refreshing iced coffee at summer temperatures, I or my refrigerator would have to do something.

This principle can also be vividly described with an analogy to water. With heat flow, the driving potential is temperature, and with flowing water, it is pressure. On its own, water only flows from high to low pressure. To move it against this natural direction, e.g. in a heating system from the basement to the 2nd floor, I have to do something actively. In heating systems, this is done with a circulation pump.

Just as a circulating pump pumps water from a low-pressure level to a higher one, a refrigeration cycle pumps heat from a low-temperature level to a higher one. And that's where the term heat pump comes from.

Refrigeration cycle or heat pump?

The short answer is: they are both the same thing. Both a refrigeration cycle and a heat pump are pumping heat against the natural direction of flow. Only the use case is different. In both cases, as with the analogy to water and a circulating pump, external energy must be supplied.

You can simply picture a refrigeration machine or heat pump as a box with electrical energy input, and one side gets cold (heat flow into the box) and  the other side gets warm (heat flow out of the box).

Depending on the application, the hot or cold side is used. In a refrigerator, the cold side is used to cool the interior, while in a heat pump the hot side is used to heat heating water. Inevitably, the other side also exists. In the case of a refrigerator, the backside becomes warm on the outside, so it gives off heat to the environment. With an air-source heat pump, on the other hand, the cold side is outside in the garden, for example, and absorbs heat from the environment.

By the way, manufacturers of heating systems often speak of a refrigeration cycle in their heat pumps rather than a heat cycle. And in fact, the refrigeration circuits in heat pumps and chillers are almost identical.

How the refrigeration cycle works

The principles described so far are laws of nature and apply to all conceivable technical implementations. There are many different technologies for refrigeration, but by far the most widely used is vapor compression. This means when people speak about refrigeration cycles, they usually mean a vapor compression cycle.

To understand this process, you only need three key physical relationships:

  1. A liquid that evaporates absorbs heat.
  2. A condensing gas releases heat.
  3. Evaporating/condensing temperature increases with increasing pressure.

Let's go back to the thought from the previous sections: We want to pump heat from low temperature to high temperature. According to the three physical laws mentioned above, this is quite simple. We look for a suitable fluid, the so-called refrigerant, and repeatedly perform the following changes of state:

  1. Evaporate liquid at low pressure and temperature.
  2. Bring the resulting vapor to a higher pressure level.
  3. Condensate vapor at high pressure and temperature.
  4. Return the produced liquid to the low-pressure level.

And this is how a refrigeration cycle works.

Practical implementation and components

The four changes of state described above are still quite abstract. To build up a functioning refrigeration cycle in a heat pump or chiller, we need concrete components.

The schematic of a simple refrigeration cycle with four components corresponding to the four changes of state looks like this:

The evaporation (1) and condensation (3) process works with heat exchangers, in which heat is transferred between the refrigerant and a secondary medium. These heat exchangers (evaporators and condensers) come in different designs, depending on the secondary medium. For example, it makes a big difference whether the heat of evaporation transfers from ambient air or a water circuit and a geothermal borehole. For the highest possible efficiency of a heat pump or chiller, it is important to design evaporators and condensers according to the specific application.

The refrigerant compressor (2) drives the entire process. External energy must be added here. Depending on laod demand and other application requirements, various technologies exist either based on the displacement principle or as turbomachinery. The simplest principle is that of a reciprocating compressor, as used, for example, in mechanically driven car air-conditioning systems. Just like an air pump, the gas is compressed by pushing a piston back and forth.

The expansion of the refrigerant (4) is the simplest step. All that is needed is a narrow point in the refrigerant line where a high-pressure drop occurs. Usually, very thin long tubes (so-called capillary tubes) are used for this purpose or adjustable valves to be able to react to changing boundary conditions.

These four components are connected with tubes so that a closed cycle is created. The refrigeration cycle is filled with a refrigerant via a valve until a certain filling quantity and pressure level are reached. We completed the refrigeration circuit.

Another blog article describes a common extension of this simple refrigeration cycle and discusses the issues of the refrigerant charge, receiver, and separator.

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Dr.-Ing.

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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