Analysis of the quantitative correlation between the heat source temperature and the critical temperature of the optimal pure working fluid for subcritical organic Rankine cycles

Organic Rankine cycles can use various working fluids. Different heat sources and working conditions lead to very different optimal working fluids. This paper presents a new quantitative criterion for selecting the optimal working fluid for open type heat sources with temperatures from 150 to 350 °C. The heat source recovery exergetic efficiency is used as the evaluation indicator. Over 30 working fluids with critical temperature from 71.87 to 357.65 °C are under evaluation. The working fluid critical temperature's influence on the heat source utilization, cycle efficiency and exergetic loss in the evaporator and condenser is analyzed to show that the critical temperature is the key parameter for selecting the optimal working fluid. The critical temperature of the optimal working fluid is determined by the heat source temperature, the pinch point temperature difference and the condensation temperature. When the cooling fluid outlet temperature is between 30 and 60 °C, which covers the most common natural cooling condition and the requirement of combined heat and power system, the correlation between the critical temperature of the optimal working fluid and the heat source temperature with different pinch point temperature difference is given. This correlation provides concise guidance for the optimal pure working fluids selection and mixture design.