Abstract
The working fluid selection is one of the most important issues faced when designing Organic Rankine Cycle (ORC) systems. The choice of working fluid is dictated by different criteria. The most important of them are safety of use, impact on the environment, and physical and chemical parameters. The type of ORC system in which the working fluid is to be used and the type of expander applied in this system is also affecting the working fluid selection. Nowadays, volumetric expanders are increasingly used in ORC systems. In the case of volumetric expanders, in addition to the aforementioned working fluid selection criteria, additional parameters are considered during the selecting of the working fluid, such as the range of operating pressures and geometric dimensions (determining the volume of working chambers) affecting the achieved power and efficiency of the expander. This article presents a method of selecting a working medium for ORC systems using volumetric expanders. This method is based on the dimensionless rating parameters applied for the comparative analysis of different working fluids. Dimensionless parameters were defined for selected thermal properties of the working fluids, namely 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. In this article, in addition to the working fluid selection method, computational examples related to the selection of the working fluid for the ORC system fed by a heat source featuring specified temperatures are presented. The results of calculations of rating parameters and their comparison gave an outlook on the selection of appropriate working fluids.
Highlights
The most important design and scientific problems connected with the Organic Rankine Cycle (ORC) systems design are suitable working fluid and expander selection [1]
Isentropic expansion work was considered as the rating parameter
Superheating is not applied, Temperature of the heat source ranges between 40 ◦ C and 150 ◦ C, Temperature of the heat sink is equal to tc = 20 ◦ C, The reference substance is R123, which was selected based on the positive results of experiments on this working fluid, which were conducted by the author using the test-stands described in [29,30], ORC system is operating as the power plant, Heat losses in the ORC system are negligible
Summary
The most important design and scientific problems connected with the Organic Rankine Cycle (ORC) systems design are suitable working fluid and expander selection [1]. Turbo expanders are mainly applied in large power (1 MW and more) ORC systems harvesting the energy from the heat sources that have high thermal power and temperature (150 ◦ C and more). Each type of volumetric expander feature specified the range of the pressure expansion ratio For this reason, only selected working fluids can be applied in ORC systems employing a volumetric expander. The examples of working fluids that can be used in a low-power ORC system employing a volumetric expander are listed in Table 2 together with their basic thermal properties, which were obtained using REFPROP [28] and CoolProp [29].
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