Parametric study and performance evaluation of an organic Rankine cycle (ORC) system using low-grade heat at temperatures below 80 °C

A parametric study and performance evaluation of an organic Rankine cycle (ORC) system that uses low-grade heat at a temperature below 80 °C and with a cycle power of less than 10 kW was undertaken to analyze the effects of the operating conditions on the performance and efficiency. R245fa was used as the working fluid, and a scroll expander as the power generation device. A model of the system was developed and its predictions were verified by the results of experiments, which were also used to determine the appropriate charge of the working fluid for maximizing the system power output. Owing to the difference between the variations of the power generated by the expander and the power consumed by the pump, the performance of the system was found to initially increase and then decrease with increasing refrigerant charge. The parametric investigation of the ORC system was conducted by varying the pressure ratio and mass flow rate of the working fluid. Increases in both parameters were found to increase the system performance by improving the torque generation and expander speed. The variation of the system performance with the heat sink temperature, which was varied between 20 and 35 °C, was also examined. The system power and efficiency were observed to decrease with increasing heat sink temperature owing to the decrease in the pressure ratio resulting from the increased condensation temperature. Based on the study findings, we propose a performance map for a low-temperature low-cycle-power ORC. The power and efficiency are shown to be proportional to the heat source temperature and capacity, which varies in a manner similar to the expander performance. The findings of this study highlight the feasibility of an ORC system that uses low-grade heat at a temperature below the boiling point.