Investigation on the fluid selection and evaporation parametric optimization for sub- and supercritical organic Rankine cycle

• Evaporation parametric optimization and working fluid selection were conducted for both sub- and supercritical ORC.
• The effect of evaporation parameters on the system performance was investigated at various heat source temperatures.
• The suitable condition for supercritical cycle was summarized by comparing sub- and supercritical ORC.
• Working fluid selection criteria were summarized for sub- and supercritical ORC respectively.

ORC (Organic Rankine cycle) is a promising technology for recovery of low-grade heat. In the previous studies, different conclusions for working fluid selection criterion can be found and relatively few work for supercritical ORC has been made. Therefore, this paper investigates the net power output of ORC utilizing waste flue gas with various evaporation parameters and 12 working fluids in both subcritical and supercritical condition. The results indicate that the variation of the net power output with evaporation pressure is related to the heat source temperature, and the maximum net power output appears at supercritical condition rather than subcritical condition if the heat source temperature is about 25–40 °C higher than the working fluid's critical temperature. Besides, the parametric optimization is performed, and the most suitable working fluids for various flue gas inlet temperature of 150–250 °C have been found. It can be found that the most suitable working fluids have a critical temperature about 40–65 °C lower than flue gas inlet temperature, and the optimum condition is always supercritical. For subcritical ORC, it is better to adopt the working fluids with low evaporation latent heat and high liquid specific heat to pursue a high net power output.