Modified hierarchical strategy for transient performance improvement of the ORC based waste heat recovery system

Abstract

Organic Rankine Cycle (ORC) system outperforms in the low-grade waste heat recovery and has found its wide application in engineering practice. Due to the strong coupling multivariable nature of the ORC power unit, the traditional hierarchical control scheme, which is composed of real-time optimization (RTO) in the upper layer and model predictive control (MPC) in the lower one, is usually utilized in the control of the ORC system for good economic performance. The upper RTO solves an economic objective function in real time and obtains the optimal steady-state setpoints which are sent to the lower MPC layer for tracking. However, the disadvantage of the traditional hierarchical control scheme lies in its neglection to the transient characteristics with the evolution of the ORC system, which will finally lead to poor overall control performance. To address this problem, in this article, a modified hierarchical control strategy which is denoted as RTO-PFFDMC grounded on pseudo feedforward dynamic matrix control (PFFDMC) algorithm is proposed. The novel PFFDMC presented in this article is applied to replace the traditional dynamic matrix control (DMC) in the lower layer and thus achieve the closed-loop reference trajectory of ORC system expected by field engineers as closely as possible without cumbersome adjustment of tuning parameters. Simulations are performed to verify the effectiveness of the proposed strategy on transient performance in comparison with the traditional one.