THE METHOD OF THE WORKING FLUID SELECTION FOR ORGANIC RANKINE CYCLE (ORC) SYSTEM WITH VOLUMETRIC EXPANDER

asme-orc2015 Tracking Number 79

Volumetric expanders are nowadays used in micro, small and medium power ORC systems. Tchanche (2011) indicated that most often spiral, screw and the rotary vane machines are applied. Volumetric machines have a number of specific features determining their operation. The most important are: the possibility of building expanders for small and very small capacities; small and moderate frequency of operating cycle - allowing for consideration of the processes taking place in the machine as a quasi-static; the ability to operate at high pressure drops in a single stage and ease of the hermetic sealing. The most important feature of the volumetric expander operation is the relationship of the expander power and the expansion ratio (the ratio of the inlet and outlet pressure). Each type of volumetric expander also has the optimum value of the expansion ratio. Unlike the turbines, volumetric expanders can operate at low working fluid flow rates and lower pressures. Thus, it is possible to apply volumetric expanders in ORCs powered by low-temperature heat sources, such as e.g. domestic waste heat. The task of a suitable working fluid selection to the ORC system with volumetric expander should be considered differently than in the case of the turbine-based systems. It is caused by low thermal parameters of the cycle and indicated earlier volumetric expander characteristic features. In this paper a new method of the working fluid selection to the ORC system working with volumetric expander was presented. The method is based on the dimensionless parameters useful for the comparative analysis of different working fluids. Dimensionless parameters were defined for selected thermal properties of the working fluids, namely the ability of heat absorption from the heat source, heat removal, mean temperature of the heat supply and the efficiency of the energy conversion. These comparative parameters were calculated for selected low-boiling ORC working fluids and selected temperature of the heat source and the heat sink. Basing on the values of these parameters the working fluids comparison was presented and applicable working fluids were selected.