Study and application of the shift-temperature of heating fluid for zeotropic mixtures in organic Rankine cycle

The theoretical formulations of the shift-temperature of the heating fluid for zeotropic mixtures (Tshift) were derived when the pinch point was located at the evaporating bubble point and the working fluid inlet of the evaporator, respectively, which were applicable to dry, wet, and isentropic zeotropic mixtures. Tshift was corrected by the impact factor analysis and calculated exactly using the Dragonfly algorithm. The optimal zeotropic mixture was matched by comparing the heat source temperature(Tg, in) and Tshift. The results indicate that Tshift is a property of every zeotropic mixture and is related to the critical temperature, the evaporation bubble point temperature, and the condensation dew point temperature. When the pinch point is at the evaporation bubble point, the zeotropic mixture with Tshift lower than Tg, in is selected and then combined with the thermal efficiency for a comprehensive evaluation. When the pinch point is at the working fluid inlet of the evaporator, Tshift is also affected by the pinch point temperature difference and the temperature glide of the condensation process, Tshift is significantly lower than Tg, in, all zeotropic mixtures are applied. Among the matched zeotropic mixtures, R227ea/R22 with the lowest Tshift has the best thermal performance in the ORC system.