Radial turbine expander design for organic rankine cycle, waste heat recovery in high efficiency, off-highway vehicles

Although state-of-the-art, heavy duty diesel engines of today can reach peak thermal efficiencies of approximately 45%, still most of the fuel energy is transformed into wasted heat in the internal combustion process. Recovering this wasted energy could increase the overall thermal efficiency of the engine as well as reduce the exhaust gas emissions. Compared to other Waste Heat Recovery (WHR) technologies, Organic Rankine Cycle (ORC) systems are regarded favourably due to their relative simplicity and small back pressure impact on engine performance and fuel consumption. The key elements affecting the efficiency of the ORC system are the type of working fluid selected and the design of the expander. In this simulation study, a zerodimensional, design code has been developed to explore the impact of two, common, refrigerant working fluids on the design of a radial turbine expander. In addition, an off-design turbine analysis has been applied in order to evaluate the performance of the expander in the ORC cycle at various engine operating points. Moreover, the evaluation of ORC-diesel engine on improving fuel consumption, brake power, brake torque and exhaust gas emissions is investigated. Compared to a conventional diesel powertrain system, WHR showed an up to 5.7 % increase in brake torque and brake power and a 5.4% reduction in the brake specific fuel consumption (bsfc). The results also showed that the working fluid selection and the expander speed are critical parameters on the performance of the proposed hybrid powertrain configuration. Keywords— diesel engines, organic Rankine cycle, radial turbine design, waste heat recovery.