Modeling and Optimization of Organic Rankine Cycle for Waste Heat Recovery in Automotive Engines

In the last years, the research effort of the automotive industry has been mainly focused on the reduction of CO2 and pollutants emissions. In this scenario, concepts such as the engines downsizing, stop/start systems as well as more costly full hybrid solutions and, more recently, Waste Heat Recovery technologies have been proposed. These latter include Thermo-Electric Generator (TEG), Organic Rankine Cycle (ORC) and Electric Turbo-Compound (ETC) that have been practically implemented on few heavy-duty applications but have not been proved yet as effective and affordable solutions for passenger cars. The paper deals with modeling of ORC power plant for simulation analyses aimed at evaluating the opportunities and challenges of its application for the waste heat recovery in a compact car, powered by a turbocharged SI engine. A grey-box modeling approach has been applied to simulate the ORC plant components (i.e. pump, heat exchangers, scroll expander); model identification and validation have been carried out against literature experimental data, showing good agreement with published results. The ORC plant model has been integrated into a dynamic vehicle-engine model to estimate the recovered electric energy as function of hot side (exhaust gas) and cold side (ambient air or coolant) temperature as well as exhaust gas mass flow. Several simulations have been carried out to explore different driving conditions (e.g. NEDC, WLTC) and optimization analyses have been performed on ORC operating conditions and components to maximize the output power and reduce the packing. Results show that significant improvement of fuel economy can be achieved by suitable ORC operation, with average CO2 savings up to 4% on standard driving cycles for the analyzed vehicle model.