Alkanes as working fluids for high-temperature exhaust heat recovery of diesel engine using organic Rankine cycle

Study on recovering waste heat of engine exhaust gas using organic Rankine cycle (ORC) has continuously increased in recent years. However, it is difficult to find out appropriate working fluids to match with exhaust gas waste heat due to high temperature. In this work, several tentative attempts and explorations are made in selecting Alkanes as working fluid owing to their excellent thermo-physical and environmental characteristics. Parameters optimization of the combined system of diesel engine with bottoming ORC (DE-ORC) is performed on Alkane-based working fluids with six indicators, including thermal efficiency (η), exergy destruction factor (EDF), turbine size parameter (SP), total exergy destruction rate (IORC), turbine volume flow ratio (VFR) and net power output per unit mass flow rate of exhaust (Pnet). Afterwards, the impact of molecular complexity on the indicators of VFR and SP is analyzed. Furthermore, the energy distribution of engine exhaust gases and the improvement of fuel economy, after integrating the bottoming ORC with diesel engine, are also discussed. Finally, the performance comparison between Cyclohexane-based ORC and steam cycle with relative pressure is carried out. The results show that optimized working fluids are not always constant subject to different indicators and operation parameters. However, cyclic Alkanes, Cyclohexane and Cyclopentane are considered as the most suitable working fluids when taking into account of all comprehensive indicators. The maximum improvement of 10% in brake specific fuel consumption (BSFC) is obtained for DE-ORC combined systems with Cyclohexane used as working fluid. In addition, although steam has more advantages in thermal efficiency in the current conditions, from a technical and economic point of view, Alkane-based ORCs may be more attractive than conventional steam cycles, specifically for DE waste gas heat recovery.