Effect Of Leakage On Performance And Environment Of Organic Rankine Cycle With Binary Zeotropic Mixtures

In recent years, The ORC(Organic Rankine Cycle) has rapidly developed and become the emphasis of researches in utilizing abundant low-grade thermal energy for its simple device and convenient operation, etc. Taking advantage of the temperature glide characteristic of mixture working fluids, can implement a better temperature matching between the mixtures with heat source and heat sink in the heat exchangers, which can decrease the irreversibility caused by temperature difference in the heat transfer process and improve the exergy efficiency. However, the organic working fluids have kinds of environmental impact including global warming potential, ozone depletion potential, etc. The changes of circulating composition and system performance caused by the leakage of zeotropic mixtures has been widely concerned and researched in the field of HVAC(Heating, Ventilation and Air Conditioning). As the working fluids in ORC system are almost the same as that in HVAC system, the influence of zeotropic mixtures leakage on ORC performance and environmental is worth taking into account and researching.This paper studies the effect of the zeotropic mixtures leakage on the circulating composition and cycle performance for ORC system by using binary zeotropic mixtures R245fa/R601a(0.6/0.4) and R114/R123(0.8/0.2), which leakage rate ranging from 0 to 30% in different position of the heat exchangers. And then, a new exergy balance model of ORC system with considering the environmental impact caused by the leakage of zeotropic mixtures has been built. By the quantitative analysis of environmental exergy cost, the evaluation method of levelized energy cost is corrected. Finally, according to actual requirement of the binary zeotropic mixtures ORC system, a solution to determine the composition of recharge zeotropic mixtures has been proposed. The main conclusions are as follows:(1) The leakage of zeotropic mixtures can change the circulating composition of the mixture, resulting in the deviation of ORC system performance from the design performance. The mass fraction of the higher boiling point component increases after vapor leak, whereas decreases for liquid leak. The change rate of mixture composition caused by vapor leak is greater than that of liquid leak.(2) The leakage of the zeotropic mixture has a great influence on the system performance, with analyzing the the exergy efficiency considering the environment impact. When the leakage rate is 10%, the exergy efficiency of R245fa/R601a(0.6/0.4) and R114/R123(0.8/0.2) are reduced by 3.9%and 30%, respectively. And the vapor leak near the condenser has the greatest influence on the performance of ORC system.(3) The leakage of the zeotropic mixture has a great impact on the levelized energy cost(LEC) considering environmental cost of leakage. When the leakage rate is 10%, the LEC increases 10.4%(vapor leak) and 8.3%(liquid leak) comparing with zero leakage for ORC system with R245fa/R601a(0.6/0.4), while for R114/R123(0.8/0.2), the LEC increases the 89.8%(vapor leak) and 81.6%(liquid leak).(4) For recharge progress of the zeotropic mixture ORC system, it is necessary to calculate the circulating composition of the mixture after leak. Take the R245fa/R601a(0.6/0.4) as an example, the recharge composition should be 0.38/0.62, 0.35/0.65, 0.64/0.36 and 0.66/0.34, respectively. Rather than using the original proportion 0.6/0.4.