Thermodynamic Cycles Have Edges

In thermodynamic cycles the states of a system change in such a way that a final state is reached, which is equal to the initial state. We want to call such a system the machine. The machine takes up a certain form of energy, converts a part of this energy into another form of energy, and delivers the remainder of the energy taken up again. We want now to describe these general statements by the example of the paddle wheel. 

It is essential that the difference of the energy brought in and the energy exhausted is loss-free transformed. Of course, this is not possible in practice, but the efforts of engineers go all into this direction, e.g., by bearing the wheel as loss free as possible. 

We emphasize a further point For the execution of a cyclic process a machine is essential and an environment, which consists of three subsystems. From the subsystem (/ 2) energy is withdrawn and delivered into the other subsystem (Ri). The third subsystem (NA) serves to absorb the transformed energy. It collects the transformed energy from the machine and uses this energy in some way. It may be necessary that at some stage of the process, the subsystem (NA) will give back some energy to the machine. 

We also assume that both the energy transfer from the subsystem (R2) to the paddle wheel, the machine, and the delivery of energy from the paddle wheel into the other subsystem (NA), occurs loss free. The two mentioned subsystems R2), (Ri) serve as reservoirs and are idealized. So that we mean that the intensive variable associated with the energy form under consideration does not change, if we remove from or add energy into the reservoir. In the case of paddle wheel a reservoir is required that does not change the water level, if we take or add water. 

Actually, it is not of interest whether the energy is transformed free of loss or not into the subsystem (NA). It is important that the energy goes away from the machine. If we want to allow the process run in the reverse direction, then it is also important that the transfer into the subsystem (NA) takes place loss free from the paddle wheel and in reverse direction. We require thus with the machine works in both direction of the process equally thus we want a complete reversibility of the process. 

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