Optimization of a Rankine Cycle integrated in a Long-Range Intercity Bus with expander sizing

In this work an optimization of a Rankine cycle, nested into a long-range intercity bus for the recovery of exhaust gas exergy is carried out. In order to optimize WHR-systems, it is of great importance to carry out a holistic optimization of the system in combination with all its subsystems and components. The applied working fluid is cyclopentane and the expander design is of scroll type. The main contributors to efficiency changes are the expansion machine and the heat exchangers. As can be seen in the literature, the process optimization is often carried out by assuming a constant efficiency of the expansion machine. If this is done, the relationship between the thermodynamic points of state applied to the in- and outlet of the expander and its efficiency is neglected. Thus, the full optimization potential of the process remains unused and leads to a non-realistic behaviour of the expander. In this paper a holistic approach to optimize the expansion machine and the thermodynamic process states is presented. The optimization is carried out by using accurate thermophysical properties of the working fluid. Part of this optimization is a novel approach to enable the scalability of the expander. The basis is a semi-empirical model well-known in the literature. This model allows the prediction of the expander performance in a wide operating range. To enable the optimization of the sizing of the expander, the model parameters are scaled based on the geometry and selected characteristic numbers. A stationary optimization is carried out. Thanks to its reasonable computational effort of the modelling approaches, an application in transient optimizations is feasible in future works.