Working fluids for high-temperature organic Rankine cycles

Alkanes, aromates and linear siloxanes are considered as working fluids for high-temperature organic Rankine cycles (ORCs). Case studies are performed using the molecular based equations of state BACKONE and PC-SAFT. First, “isolated” ORC processes with maximum temperatures of 250 °C and 300 °C are studied at sub- or supercritical maximum pressures. With internal heat recovery, the thermal efficiencies ηth averaged over all substances amount to about 70% of the Carnot efficiency and increase with the critical temperature. Second, we include a pinch analysis for the heat transfer from the heat carrier to the ORC working fluid by an external heat exchanger (EHE). The question is for the least heat capacity flow rates of the heat carrier required for 1 MW net power output. For the heat carrier inlet temperatures of 280 °C and 350 °C are considered. Rankings based on the thermal efficiency of the ORC and on the heat capacity flow rates of the heat carrier as well as on the volume and the heat flow rates show cyclopentane to be the best working fluid for all cases studied.

Research highlights
► ORC systems with alkanes, aromates and siloxanes as working fluids are modelled.
► Maximum working fluid temperatures range from 180°C to 300°C.
► Thermal efficiencies of the cycles are about 70% of the Carnot efficiency.
► Optimizations of the net power output from a given heat carrier are performed.
► Rankings based on power output and design show cyclopentane as best working fluid.