The effects of composition ratios and pressure drops of R245fa/R236fa mixtures on the performance of an organic Rankine cycle system for waste heat recovery

Abstract

The aim of this study is to investigate the effects of composition ratios of working fluids and pressure drops in heat exchangers on the performance of an organic Rankine cycle (ORC) system employing R245fa/R236fa for waste heat recovery (WHR). Thermodynamic analysis, heat transfer calculation, and the cost-evaluation of apparatus and maintenance of the ORC system are applied to assess the levelized energy cost for the economic performance. Moreover, exergoeconomic factors of the components in the ORC system are calculated. The results indicate that the ORC system cycled with R245fa/R236fa mixture has higher thermodynamic and economic performances than ORC system with either pure-component of R236fa or pure-component of R245fa. At optimal mixture ratio, R245fa/R236fa has higher net power output than R245fa by 7.07% and lower levelized energy cost than R236fa by 3.07%. Both the saturated temperature of liquid working fluid and the proportion of R236fa in R245fa/R236fa mixture increase as pressure drops in the heat exchangers of the ORC system increase. As pressure drop ratio equals to 0.16, R245fa/R236fa with optimal composition ratio is thermodynamically superior to R236fa and R245fa by 2.16% and 9.07%, respectively. Similarly, at the optimal composition ratio, R245fa/R236fa mixture also performs better than R236fa by 2.37% or R245fa by 1.5% in economic analysis. As for exergoeconomic factors, relatively high values for the pump and expander and moderate values for the heat exchanger devices are obtained in the system. On the whole, the values of exergoeconomic factor increase with composition ratio of R245fa/R236fa mixture. Compared with R245fa and R245fa/R236fa mixtures, R236fa has the highest value of exergoeconomic factor for each component of the ORC system. Finally, the correlations of the optimal thermodynamic and economic performances of the ORC system with R245fa/R236fa are proposed for WHR.