Performance Analysis of Zeotropic Organic Rankine Cycle for Marine Diesel Exhaust Heat Recovery

Abstract

Abstract Marine diesel engine plays the dominate role on the merchant ship power supply field, which is responsible for 3.1% of global CO2 emissions. As one third of the energy is wasted along with the exhaust gas, heat recovery of marine diesel exhaust is an effective way of improving the thermal efficiency of entire power system. The temperature of marine diesel exhaust ranges from 250°C to 550°C, and this medium/low-grade heat source can be recovered by thermodynamic cycle technology. The Organic Rankine Cycle (ORC) is most commonly used in medium/low-grade waste heat recovery because of its simple structure. However, ORC utilizes the pure organic compound as the working fluid, and the evaporation of the pure organic compound experiences with isothermal and isobaric process, resulting in a relatively high temperature difference of heat transfer between the heating source and working fluid, which ultimately limits the thermal efficiency of ORC. In comparison, the Zeotropic Organic Rankine Cycle (ZORC), which utilizes the zeotropic mixture and working fluid, is proved competitive for the temperature slip phenomenon in its evaporation process. Besides, the heat transfer in the condenser will get worse due to the liquid accumulating on the condensation wall, leading to a larger heat exchange area requirement. Therefore, the liquid-separation condensation method is investigated to reduce the heat exchange area of condenser. In this study, both the multi-pressure evaporation and liquid-separation condensation processes are introduced to improve the ZORC performance. Working fluid selection for the ORC and ZORC with a wide range of marine diesel exhaust temperature is conducted, with corresponding evaporation temperature ranges between 100°C and 200°C. The proportion analysis for different kinds of zeotropic organic mixture is made to obtain the optimized working fluid under the given evaporation temperature by 200°C. Cyclopentane-toluene (0.55–0.45) can reach the highest efficiency with 19.76%. Moreover, thermodynamic analysis based on the first law and second law of thermodynamics are made to reveal the thermal efficiency, output work and exergy efficiency of these cycle, as well as the exergy destruction in each component. In addition, the dryness of mixture at the liquid-separation point in the condensation process, which reveals the position of the liquid-separation outlet in the condenser, is discussed. The results show that the major exergy destruction occur in the evaporation and condensation processes. With the assistance of multi-pressure evaporation and liquid-separation condensation processes, the exergy destruction in the heat exchangers is decreased by 17.09%∼21.61%.