Introduction#

The following sections present a brief summary of the thermodynamic theory we will apply to model the heat pump in the later sections. First, important terms and concepts are defined in the overview on thermodynamic systems. Then, the respective fundamentals are outlined in more detail. The theory is applied in a small exercise at the end of the chapter.

For in depth theory the following literature might be useful:

  • “Thermodynamik” (German) [1]

  • “Thermodynamics” [2]

Thermodynamic system#

A thermodynamic system is a distinct space which is defined by system boundaries. The system can have various attributes, for example it may be

  • open or close for mass transport over its boundaries,

  • open or close for transport of energy (e.g. by heat or work)

Fig. 1 illustrates these concepts.

Note

In this workshop we will only work with open systems with respect to mass transport. With respect to energy the systems may either be open or close. An important type of system is a system that does not allow transport of heat over its boundaries. This type of system is called adiabatic. Systems that do not transfer work over their system boundaries do not get a specific name, examples are heat exchangers.

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Fig. 1 Illustration of different types of thermodynamic systems#

If you take a snapshot of the system in time and describe its inner properties, you describe the state of the system. The state includes information like pressure, temperature, type of fluid or phase (liquid/gaseous) of the fluid. The state of a system is not fixed, it may change during a thermodynamic process. Fig. 2 shows a snapshot of a closed thermodynamic system before and after a process has taken place. The system changed its state.

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Fig. 2 Illustration of a thermodynamic system changing its state through a process#

Technical systems like a heat pump often consist of many connected components (each component can be considered an individual system), for example, heat exchangers, compressors, valves, pumps etc.. In these components different processes take place, that change the state of a fluid between the inlet of the component and its outlet. For example, Fig. 3 shows an electric heater, where a fluid flows through the system. The fluid changes its state between the inlet and the outlet of the system.

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Fig. 3 Illustration of a fluid changing its state from 1 to 2 while flowing through an open system#