Selection of refrigerants for a modified organic Rankine cycle

Abstract The use of waste heat in many branches of industry is limited due to temperature in the range of 30–100 °C. One of the methods of using waste heat are devices that implement the Organic Rankine Cycle (ORC). In currently used ORC systems, the heat source temperature is at least 80 °C, while the low temperature heat source (usually atmospheric air) has a temperature of 30 °C. The work analyzes the influence of the organic fluids properties on the performance of the novel, proposed installation with electric power output 1 kW driven by the waste heat and working based on the ORC. In comparison to a commonly used ORC installation the examined installation is characterized by one essential difference: expansion process. The expansion device used in a typical ORC structure, is replaced by two connected tanks filled by secondary fluid, between which the hydraulic turbine is installed. The basic operation parameters in nominal conditions were determined for ten selected refrigerants: R134a, R152a, R227ea, R236fa, R245fa, R290, R600a, R717, R1234yf, R1234ze(E). The boiling point 80 °C and the condensing temperature 30 °C were used as nominal conditions. Analogical calculations were also done for conventional ORC system working with the same parameters and providing the same electric power output. As a result of the analysis, it turned out that for the proposed device, the best refrigerant is ammonia, for which the 5.68% efficiency was obtained. Comparing the results with the achievements for a standard ORC system, it is almost twice lower (for ammonia ORC systems, the efficiency is 9.93). However, in the experimental installation it was decided to use R245fa refrigerant, because unlike ammonia it is not flammable and is not toxic. According to the analysis, the R245fa was one of the best refrigerants in terms of efficiency and, what is the most important, it did not mix with the glycerol used in the tanks as a secondary fluid. Theoretical analysis for fluid R245fa has shown that the total energy efficiency coefficient for whole proposed system will be at level 4.72%, while as a result of the experiment, it managed to achieve an efficiency level of 2.51%.