Bottoming Organic Rankine Cycles for Passenger Cars

Organic Rankine Cycles (ORC) are very efficient and flexible conversion systems with a high degree of maturity. They can be used with different heat sources, mainly using exhaust heat from different processing also with low temperature level sources. They have been mainly considered suitable for stationary power plants. Furthermore, the limitations of layout and size are less stringent if compared with road transportation propulsion systems, in particular when passenger cars are considered. In this paper, the authors numerically investigate an ORC system as a bottoming solution for energy recovery from exhaust gases for internal combustion engine (ICE) passenger car. A passenger car was tested over a Real Driving Emission (RDE) cycle. Exhaust gas mass flow rate and temperature have been sampled allowing calculation of the thermal power available for the ORC plant at realistic driving conditions. The engine operational range was discretized using engine torque and speed values. As a result, a grid of 10 points was set up in the operational plane and the running conditions assigned to the closest discretized point in the grid, each one characterized by a residence time. The ORC recovered power resulted between 0.5 and 2.5 kW, the Rankine cycle efficiency ranged from 11 to 12% while engine efficiency increase varied from 2.5 to 12%. By considering the permanence time in each discretized operating condition the engine efficiency increment resulted slightly higher than 6%.