Design of a Nonlinear, Dynamic Feedforward Part for the Evaporator Control of an Organic Rankine Cycle in Heavy Duty Vehicles

A model-based two degree of freedom (2DOF) controller for the evaporator outlet condition in an organic Rankine cycle (ORC) system is presented. The ORC is used in a heavy duty vehicle to convert exhaust gas heat into mechanical power which increases the fuel efficiency The evaporator outlet condition of the working fluid is a crucial variable for the ORC efficiency which thus has to be controlled very accurately However, the exhaust gas thermal energy which serves as heat source to the evaporator, is highly transient depending on the actual engine load. Therefore, excellent disturbance rejection is required with help of an efficient feedforward part. Its novel derivation is based on an exact inversion of a nonlinear, dynamic evaporator model, obtained by the finite volume method that is reduced subsequently. The resulting feedforward controller is therefore very close to the exact inverse dynamic model. Furthermore, online parameter adaption enhances the steady state accuracy of the feedforward part and compensates for model uncertainties. The improvements by the feedforward controller are shown in simulations and its applicability on the vehicle control unit is demonstrated by measurements on a Daimler prototype truck under real traffic conditions.