Multilinear modeling and controller design for a plate heat exchanger of a PEM electrolyzer

Abstract

The increasing share of renewable energy (RE) in the German electricity grid has led to the curtailment of RE plants due to potential risks to the existing transmission infrastructure caused by the intermittent nature of RE. To address this issue, the production of green hydrogen through proton exchange membrane water electrolysis (PEMWE) systems has emerged as a vital solution, serving as a chemical energy storage and grid operation support to prevent the curtailment of RE plants. To ensure the long-term reliable operation of these electrolysis assets, temperature control of the feed water for electrolysis is of vital importance. This thesis focuses on the practical design of a plate heat exchanger (PHE) and a PID controller for controlling the feed water temperature of a proton exchange membrane (PEM) electrolyzer. The research involved developing a tailored PHE model for the cooling system of a 1 MW PEM electrolyzer and investigating its first principles (FP), linear time-invariant (LTI), and multilinear time-invariant (MTI) modeling approaches. The designed PHE coupled the electrolyzer's feed water with the cooling water pumped circuits, enabling control of the electrolyzer's feed water temperature using a PID controller to adjust the flow rate of the cooling water pump. The results show that the PHE MTI model accurately captures the dynamics of the FP model, consistently outperforming the LTI model. Furthermore, the PHE MTI model exhibits a 74 % reduction in simulation time compared to the FP model for this application. Suitable PID controller parameters were identified for effective feed water temperature control. Leveraging the structure of the electrolyzer's cooling system model developed, an assessment of power consumption in the balance of plant (BOP) components associated with the electrolyzer's cooling system is conducted. This assessment aids in gaining a comprehensive understanding of energy consumption patterns in the electrolyzer, a crucial step toward optimizing power usage in BOP components. The estimation and breakdown of power consumption associated with the variable flow rate in the cooling water pump and the constant flow rate in the feed water pump reveal that the feed water pump accounts, on average, for 97 % of the power consumption in the cooling system. Overall, the consumption related to operating the electrolyzer's cooling system represents 1 % to 5.4 % of the total power consumption of the electrolyzer. This thesis provides a solid framework for analyzing and modeling thermal dynamics and power consumption patterns in PEM electrolyzers' cooling systems. It can be used as a basis for further research on optimizing power usage in BOP components, implementing advanced control strategies for feed water temperature regulation, and modeling of PHEs.