Dynamic Operation and Control of a Multi-Stack Alkaline Water Electrolysis System with Shared Gas Separators and Lye Circulation: A Model-Based Study

An emerging approach for large-scale hydrogen production using renewable energy is to integrate multiple alkaline water electrolysis (AWE) stacks into a single balance of plant (BoP) system, sharing components such as gas-lye separation and lye circulation. This configuration, termed the $N$-in-1 AWE system, packs $N$ stacks into a modular system, reducing land requirements, the complexity of plant topology, and overall capital costs. However, the coupling of these stacks through the shared BoP introduces challenges in dynamic operation under varying energy inputs, making their performance unclear compared to traditional 1-in-1 systems. To address this, we develop a state-space model of the $N$-in-1 AWE system, capturing the dynamic behaviors of lye circulation, temperature, and HTO impurity, and their impact on energy conversion efficiency. We then propose a nonlinear model predictive controller (NMPC) to coordinately optimize inter-stack electrolytic current distribution, lye flow, and cooling, enabling the system to dynamically track varying load commands while maximizing efficiency, stabilizing temperature, and limiting HTO impurity accumulation. Simulation studies on a 4,000 Nm$^3$/h-rated 4-in-1 system verify the proposed controller under dynamic operation. Comparison with 4 independent 1-in-1 systems reveals that, with proper control, the $N$-in-1 configuration offers comparable flexibility in accommodating real-world wind power inputs. The average differences in the root-mean-square errors (RMSEs) for load-tracking and stack temperature stabilization, and specific energy consumption are below 0.014 MW, 2.356 K, and 0.003 kWh/Nm$^3$.