*4.3. Summary*

The first set of calculations was related to the working fluid selection, which is based on the variation of isentropic expansion work vs. the pressure expansion ratio σP and characteristic (i.e., histogram) of the heat source. If the highest value of isentropic expansion work would be considered as the main working fluid selection criterion, R123 seems to be a promising working fluid reaching the highest values of the expansion work in a wide range of the studied conditions and for all characteristics of the heat source. For other working fluids studied, the obtained values of isentropic expansion work are lower than in the case of R123. However, if additional working fluid selection criteria (which are

related to the type of the applied volumetric expander and its design, such as pressure expansion ratio) are considered, then calculation results proved that other working fluids are also promising in the case of di fferent types of volumetric expanders. Di fferences are related to the heat source characteristics. For example, if the highest value of the isentropic expansion work would be considered as the working fluid selection criterion and the multi-vane expander (featuring the optimum pressure expansion ratio of 4–5) as the prime mover of the ORC system, R123 (for the heat source histogram presented in Figure 5a,b) and R134a (for the heat source histogram presented in Figure 5c,d) are the working fluids in which the application should potentially result in the highest obtained expander power.

The second set of calculations was related to the comparative analysis of the working fluids based on the defined rating parameters ( ψTC, ψTME, ψPH, ψV, ψTMC, ψC, ψL) together with pressure and the volumetric expansion ratio (<sup>σ</sup>P, σVEX), volumetric expandability (σV), e fficiency of the cycle (ηORC), and specific expansion work (lEX). R123 was selected as the reference substance based on the positive results of experiments on this working fluid, which were proceeded by the author using the test-stands and the results of the first part of calculations. The values of the rating parameters were calculated for 20 selected working fluids (see Table 2). Obtained results were then plotted in figures and compared. It was found that some of the analyzed working fluids have better thermal properties than the reference R123, i.e.,


were achieved for R152a and R600a. The value of this parameter can be minimized (if a smaller expander is needed) or maximized (if a larger expander is needed).


#### **5. Summary and Conclusions**

This study presents the comparative method of the working fluid selection for an ORC system powered by a low temperature heat source and employing a volumetric expander. This method is based on a comparison of the selected thermal properties of working fluids by the application of the rating parameters describing the following thermal properties of the working fluids: the working fluid thermal capacity, mean temperature of evaporation, mean temperature of condensation, pressure and volumetric expansion ratio, volumetric expandability, as well as the heat of preheating, vaporization, superheating, cooling, and liquefaction. Moreover, isentropic expansion work was considered as the rating parameter. The presented method can be used for selecting the working fluid for the ORC system, which is supplied by a heat source and heat sink featuring a specified thermal power and temperature. Moreover, using this method, it is possible to select the volumetric expander for the ORC system based on the comparison of the calculated parameters, the pressure expansion ratio, and specific expansion work. The proposed method is universal and can be easily adopted for di fferent working fluids, di fferent assumptions, and ORCs employing turbines as the expanders. Therefore, using this method, it is possible to compare many di fferent ORCs powered by di fferent heat sources. The method can also be applied for comparing di fferent ORCs to classical steam power plants. In order to illustrate this treatment, sets of calculations were performed in order to give an outlook for the selection of working fluids and a volumetric expander. Special attention was paid to a multi-vane expander. The calculation results showed the comparison of the working fluid candidates for di fferent operating conditions of an ORC system. These results can be helpful for scientists and engineers dealing with ORCs and volumetric expanders for making quick estimations and comparisons of di fferent ORCs.

**Funding:** This research received no external funding.

**Acknowledgments:** The author would like to thank Attila R. Imre for the invitation to publish this article.

**Conflicts of Interest:** The author declares no conflict of interest.
