Rankine cycle for waste heat recovery on commercial trucks: approach, constraints and modelling

With increasing oil price and growing interest in cutting green house gases emissions, waste heat recovery techniques appear as a very promising path to enhance engine thermal efficiency. Thermoelectricity and Rankine cycles are two possible ways to recover thermal energy. The Rankine cycle shows the highest potential due to
its higher cycle efficiency in comparison with the current state-of-the-art thermoelectric materials intrinsic conversion ratio. This paper will focus on the Rankine cycle system. The first part of the paper lists and describes the constraints of a heat recovery Rankine cycle system associated to a long haulage truck: limitation of heat available in the heat source, vehicle heat rejection constraints, safety and environmental issues for the working fluid, and backpressures occurring during the heat recovery process. The second part of the paper presents a 0-D
simulation model of a Rankine cycle system. It is shown how the model can be used to compare the performance achieved with several working fluids. Moreover, basic thermodynamic limitations of the system are underlined. Conclusions are drawn concerning the limitation of using only thermodynamic simulations. The performance of the condenser (and its influence on the overall performance) has been investigated more in details, which is shown in the third part of the paper. This was carried out based on a 1-D simulation model of the condenser.
Influence of the condenser size and ram air effect are discussed, and the impact on the Rankine cycle is assessed.