Fluid Selection and Thermodynamic Analysis of an Electricity-Cooling Cogeneration System Based on Waste Heat Recovery from Marine Engine

The environmental issues combined with the rising of crude oil price have attracted more interest in waste heat recovery of marine engine. Currently, the thermal efficiency of marine diesels only reaches 48~51%, and the rest energy is rejected to the environment. Meanwhile, energy is required when generating electricity and cooling that are necessary for vessels. Hence, the cogeneration system is treated as the promising technology to conform the strict environment regulation while offering a high energy utilization ratio. In this paper, an electricity and cooling cogeneration system combined of Organic Rankine Cycle (ORC) and Absorption Refrigeration Cycle (ARC) is proposed to recover waste heat from marine engine. ORC is applied to recover exhaust waste heat to provide electricity while ARC is used to utilize condensation heat of ORC to produce additional cooling. Four typical high-temperature working fluids (benzene, toluene, cyclohexane and cyclopentane) are selected as ORC working fluid while ammonia/water is applied as working pair for ARC. Simulations were performed at different evaporating pressure and condensation temperature of ORC. Results show that the highest primary energy ratio of WHR system can be obtained when the condensation temperature of ORC is 135°C. Among the four fluids considered, from view of efficiency of cogeneration system, the most promising candidate are benzene and toluene. The former is suitable for high evaporation pressure system with a highest exergy efficiency of 50.8%, and the latter is potential for low evaporation pressure system with a highest exergy efficiency of 48.3%.