Dynamic Model of a Multi-Evaporator Organic Rankine Cycle for Exhaust Heat Recovery in Automotive Applications

This paper presents a dynamic model of an organic Rankine cycle (ORC) for exhaust heat recovery (EHR) in automotive applications. EHR converts the thermal energy of the exhaust gas into mechanical power. Dynamic components like evaporator, condenser, etc. are modeled based on physical conservation laws given as partial differential equations (PDE). The system involves two parallel evaporators coupled via an open t-piece which is handled numerically efficiently by introduction of algebraic constraints. An integral method improves the evaluation of the fluid properties which partly contain discontinuities. They can have negative effects on accuracy and simulation performance if evaluated inappropriately. The model is implemented by signal-based programming suitable for simulation in Matlab/Simulink. This enables convenient software-in-the-loop (SIL) engineering since many control unit producers provide rapid prototype programming via Matlab/Simulink. A simplified model is proposed for the increase of robustness and simulation performance or for application of controller design methods. Simulation shows both the entire ORC system compared measurements from a Daimler prototype truck, and comparison between complex and simplified model.