(469c) Heat Pump Cycles to Generate Electricity From Low Temperature Waste Heat

This paper presents an extensive study of heat pump cycles and associated working fluids to generate electricity from low temperature industrial waste heat. These heat pump cycles have been used to increase the efficiency of thermochemical cycles for hydrogen production from water. An Aspen Plus simulation has been developed to evaluate the effect of various working fluids on the net heat pump efficiency over a wide range of heat exchanger/evaporator output temperatures between 50°C and 250°C. There are 45 refrigerants as working fluids under investigation from the environmental classifications of HydroChloroFluoroCarbons (HCFC), HydroFluoroCarbons (HFC), ChloroFluoroCarbons (CFC) and HydroCarbons (HC) with boiling points between -88.65°C and 110.65°C. The simulation results reveal that the net efficiency increases with the increase of the inlet temperature of the waste heat source except with the azeotropic blends, R503 and R508B, since both are very low temperature refrigerants. The net efficiency obtained from this work ranges from 2% to 46%. Some of the results of the present study demonstrate that the working fluid R503 delivers the maximum efficiency among the evaluated working fluids for temperature <100°C; the zeotropic blend R410A shows the best efficiency for temperature between 100°C and 150°C; and for the temperatures between 150°C and 250°C, the azeotropic blend R500 exhibits the best efficiency. The highest net efficiency has been achieved with R503 at 50°C by varying the azeotropic composition of R23:R13 to 85:15 weight percent.

Key words: heat pump, working fluid, refrigerants, net efficiency, waste heat, thermochemical cycles, hydrogen production