Fluid selection and advanced exergy analysis of dual-loop ORC using zeotropic mixture

Abstract

The comprehensive performance of a dual-loop organic Rankine cycle (DORC) is mainly influenced by working fluid pairs and key components. Based on multi-objective optimization, we conducted the working fluid selection and advanced exergy analysis for the DORC system driven by flue gas at 300℃. The exergy efficiency, payback period (PBP), and annual CO2 emission reduction (AER) were selected as the objective functions. Operating parameters and mass fraction of mixtures were optimized by non-dominated sorting genetic algorithm-II (NSGA-II). A suitable fluid pair for the DORC system was selected from 13 candidate fluid pairs. Then, the improvement potential of each component was estimated based on advanced exergy analysis. The results show that there is an optimal mass fraction for different zeotropic mixtures. When cyclohexane/cyclopentane (0.2/0.8) and butane/pentane (0.65/0.35) are used for the high-temperature (HT) loop and low-temperature (LT) loop respectively, the DORC system can achieve the best performance. Its exergy efficiency and AER increase by 14.7% and 19.1% respectively, while PBP only increases by 2.6%, compared with the fluid pair of cyclohexane-butane. There is 261.9 kW endogenous avoidable exergy destruction in the DORC system, accounting for 53.5% of the total exergy destruction. The HT turbine (HTT) is the first component that needs technical modifications, followed by the HT evaporator (HTE), intermediate heat exchanger (IHE), and LT turbine (LTT).