Performance analysis of organic Rankine cycles using R600/R601a mixtures with liquid-separated condensation

Organic Rankine cycle (ORC) is a promising technology for power generation from medium to low temperature heat sources (<350°C). Although a zeotropic mixture can improve ORC efficiency, its large condenser heat transfer area is a crucial factor that impedes the broad application of ORCs using zeotropic mixtures. The liquid-separated condensation method is the process of separating the condensed fluid from the two-phase flow during condensation. This method has potential to effectively reduce the condenser heat transfer area of an ORC using a zeotropic mixture by increasing the condensation heat transfer coefficient and reducing the pressure drop. This study introduced the liquid-separated condensation into ORCs using R600/R601a mixtures. The effects of vapor quality at the liquid-separated unit inlet (xLSI) on the net power outputs, condensation heat transfer coefficients and heat transfer areas were analyzed. Results show that a xLSI below 0.3 maximizes the average condensation heat transfer coefficient for the R600/R601a mixture. The liquid-separated condensation can increase the average condensation heat transfer coefficient by 23.8% and reduce the condenser heat transfer area by 44.1% compared to the conventional condensation. Although the liquid-separated condensation may slightly reduce the maximized net power output for a low xLSI, the decrease of total heat transfer area is larger than that of the maximized net power output. Thus, the ratio of the total heat transfer area to the net power output can be reduced by 28.3%. The R600/R601a (0.9/0.1) mixture with xLSI=0.3 obtains the lowest electricity generation cost for the R600/R601a mixtures with single-stage liquid-separated condensation.