Estimation and Predictive Control of a Parallel Evaporator Diesel Engine Waste Heat Recovery System

This paper proposes a real-time capable augmented control scheme for a parallel evaporator organic Rankine cycle (ORC) waste heat recovery system for a heavy-duty diesel engine, which ensures efficient and safe ORC system operation. Assuming a time constant separation between the thermal and pressure dynamics, a nonlinear model predictive control (NMPC) is designed to regulate the mixed working fluid (WF) outlet temperature and the differential temperature between the two parallel evaporator outlets. Meanwhile, the evaporator pressure is regulated by an external PID control. The NMPC is designed using a reduced order, moving boundary control model of the heat exchanger system. In the NMPC formulation, state feedback is constructed from the estimated state via an unscented Kalman filter based on temperature measurements of the exhaust gas and WF at the evaporator outlet. The performance of the proposed control scheme is demonstrated in simulation over an experimentally validated, high fidelity, and physics-based ORC plant model during a transient constant speed and variable load engine drive cycle. The performance of the proposed control scheme (NMPC plus PID) is further validated via comparison with a conventional, multiple-loop PID controlling both the mixed evaporator outlet WF temperature, and the evaporator pressure. The simulation results demonstrate that the proposed control scheme outperforms a multiple-loop PID control in terms of both safety and total recovered thermal energy by up to 12% and 9%, respectively.