A switching Moving Boundary Model for the simulation of ORC plants in automotive applications

Waste heat recovery is a promising approach to improve fuel economy and emissions of thermal engines for stationary and mobile applications. Among recent solutions, Organic Rankine Cycles (ORC) seem to join effectiveness and technological readiness for the application to Internal Combustion Engines (ICE), both Spark Ignition (SI) and Diesel. Significant reductions in fuel consumption have been reported, but – especially in automotive applications – further improvements in the ORC plant matching and performance in transient operations are required. This paper presents a lumped-parameter model of an ORC system for exhaust waste heat recovery in automotive engines. The heat exchangers dynamics is accounted for by modeling the behavior of the working fluid through the Moving Boundary Method (MBM), which is based on a lumped-parameter representation of the conservation laws for single and two-phase fluid flows. An original switching technique has been implemented to account for variations in the fluid properties and heat transfer during transient operations of the ORC plant. Grey-box models for the pump and expander have been developed starting from steady-state characteristic maps. The behavior of the comprehensive model in transient operating conditions has been significantly improved also during start-up process, usually a threatening situation for mathematical models.