Assessment of Waste Heat Recovery Options in Passenger Car Applications by Various Rankine Cycles

Abstract

In power plant applications it is state of the art to recover waste heat in combined-cycle power plants. Hence, overall efficiencies are increased from 35% to more than 45%. Actually, in marine applications as well as in commercial vehicle applications, similar technologies are developed to recover waste heat. The common basis of all these applications is running only or mostly at steady-state conditions. With passenger cars the operation conditions are completely different, as passenger cars are operated under highly transient conditions. In the work presented, the characteristics of a passenger car-based internal combustion engine are analyzed. From exhaust gas temperatures and exhaust gas mass flows, the characteristic of available waste heat over load and speed is estimated. Based on this characteristic an ideal, a water-based Rankine cycle is designed and compared with different organic Rankine cycles. Finally, the expected waste heat recovery at typical passenger car operation conditions is determined by weighting the waste heat recovery characteristics with the operation conditions of the new European driving cycle (NEDC). Based on NEDC scenario, only about 0.2 kW of power could be recovered. By moving toward range extender conditions, between 2.5 and 4.0 kW would be recoverable.