A review and future application of Rankine Cycle to passenger vehicles for waste heat recovery

Rankine Cycle (RC) is a thermodynamic cycle that converts thermal energy into mechanical work, which is commonly found in thermal power generation plants. Recently, there have been many studies focusing on applying Rankine Cycle to recover low-grade waste heat. On-road vehicles, which convert around one third of the fuel energy into useful mechanical energy for propulsion, are moving energy conversion systems that generate considerable waste heat. It is found from prior research that the Rankine Cycle has great potential in automobile waste heat harvesting applications. However, in contrast with other low-grade waste heat applications, vehicles have limited space for the RC system integration, and the waste heat is relatively unsteady. In this work, the efforts in the past few decades to apply RC to on-road vehicles, specifically passenger cars, are reviewed. Characteristics of the waste heat sources found in vehicles and the constraints put on the automotive RC application are identified. Rankine Cycle architectures, system components, and working fluids suitable to different applications are summarized, which provides a guideline for future RC system design in automobiles. Lastly, a new concept and case study into the future application of Rankine Cycle to vehicle waste heat recovery (WHR) is provided.