Economical evaluation and optimization of subcritical organic Rankine cycle based on temperature matching analysis

This paper presents an economical evaluation and parametric optimization on subcritical ORC (organic Rankine cycle) system for recovering the low-temperature waste heat of flue gas which is released from industrial boilers. First, the effects of the PPTD (pinch point temperature difference) in evaporator and condenser, and evaporating temperature on the system performance are examined with the EPC (electricity production cost) as the evaluation criterion. The optimal evaporating temperature, PPTDs in evaporator and condenser, as well as condensing temperature corresponding to the minimum EPC are provided simultaneously for the subcritical ORC system under different heat source temperatures. Then, the optimal parameter values are compared for nine potential organic working fluids with the critical point temperature from 124 °C to 235 °C. Results show that there exists a possible relationship between the critical temperature of working fluids and the economical performance of the system. Furthermore, it is suggested that the working fluids including R123, n-pentane, R11 and R141b, whose critical temperature range from 180 °C to 210 °C, are preferable for recovering the low-temperature waste heat of flue gas.