Performance analysis of a dual-loop organic Rankine cycle (ORC) system with wet steam expansion for engine waste heat recovery

A dual-loop organic Rankine cycle (ORC) system is designed to recover the waste heat of a diesel engine. The high-temperature (HT) loop utilizes the heat load of the engine exhaust gas, and the low-temperature (LT) loop uses the heat load of the jacket cooling water and the residual heat of the HT loop sequentially. These two loops are coupled via a shared heat exchanger. Water is selected as the working fluid for the HT loop and wet steam expansion, which can be implemented through screw expanders, is exploited. The dryness fraction of the wet steam at the inlet of the expander can be adjusted to attain a suitable evaporation temperature and provide a better temperature match with the heat source. The working fluid candidates for the LT loop are chosen to be R123, R236fa and R245fa. The influence of the HT loop parameters on the performance of the LT loop is evaluated. The simulation results reveal that under different operating conditions of the HT loop, the pinch point of the LT loop occurs at different locations and therefore, results in different evaporation temperatures and other thermal parameters. The maximum net power output of the dual-loop ORC system reaches 115.1 kW, which leads to an increase of 11.6% on the original power output of the diesel engine.