Performance investigation of transcritical and dual-pressure Organic Rankine Cycles from the aspect of thermal match

Abstract

Dual-pressure and transcritical Organic Rankine Cycles (DORC and TORC) all have advantages of increasing the thermal match between the working fluid and heat source. However, their temperature ranges of applicable heat source are different. This study focuses on the investigation and comparison of subcritical ORC (SORC), TORC and DORC using various working fluids for recovering energy of hot water which temperature is between 120 °C and 200 °C. Effects of evaporation pressure of SORC, two-stages pressures of DORC, and turbine inlet parameters (temperature and pressure) of TORC on the thermal match and the net power output have been studied. Then the thermal match and net power output comparisons of SORC, DORC and TORC under the optimal conditions for various temperatures of heat source (120–200 °C) have been presented. Furthermore, the maximum net powers of threes cycles using various working fluids are also optimized and compared when the heat source temperatures are given. The results indicate that for the DORC system, the thermal match improves as high stage pressure rises. And compared with the low stage pressure, the high stage pressure has a greater effect on the system performance. For the TORC system, the slop of temperature rising curve of supercritical working fluid is affected by the factors such as heat source inlet temperature, turbine inlet temperature and turbine inlet pressure of turbine. And it influences the thermal match in the vapor generator. There is a boundary temperature of heat source for the selection of DORC and TORC using various working fluids. If heat source temperature is less than the boundary temperature, DORC shows better system performance, otherwise, TORC possesses better performance. The boundary temperatures of R1234ze(E), R227ea, Isobutane, R245fa are 151.4 °C, 125.5 °C, 170.8 °C and 195.8 °C respectively. And the analyzed results also present that there exists suitable cycle layout and appropriate working fluid to achieve best thermal match and produce maximum net power when the heat source temperature is given.