Effects of critical and boiling temperatures on system performance and fluid selection indicator for low temperature organic Rankine cycles

The critical temperature (Tc) and boiling temperature (Tb) of working fluids are important selection criteria for the organic Rankine cycle (ORC) system. In this study, the ratio of Tb and Tc (Tbr) and the vapor expansion ratio (VER) model based on Claussius-Claperyron equation are introduced to compare and explain their effects on the maximum net output power (Wnet,max) and VER of ORC. The investigation of 267 working fluids is done at four heat source temperatures (T5). Maximum vapor enthalpy method is proposed to determine the upper limit of the evaporation temperature, which is the optimization parameter for maximizing the net power output. At low T5 (423.15 and 473.15 K), the obvious relationships between Wnet,max and Tc are independent of Tbr. Therefore, Tc enables to select working fluids with high Wnet,max. However, at high T5 (523.15 and 573.15 K), Tbr is essential to exclude working fluids with optimum Tc (0.89 - 0.90T5) but low Wnet,max. Moreover, at a given Tc, high Tb or Tbr indicates high VER. Consequently, Tb or Tbr is suitable to be used as the second indicator. This paper proposes the optimal combinations of Tc and Tb and the developed composite indicator for selection of working fluids.