Assessing the Performance of a Proton Exchange Membrane Green Hydrogen Generation System through Stack and Balance of Plant Modeling
Authors: Almpantis Diamantis , Davidsson Henrik , Andersson Martin 
Published: December 2024

The integration of hydrogen generation systems based on Proton Exchange Membrane (PEM) electrolyzers holds immense potential for sustainable energy solutions. From a sustainability standpoint, the sector coupling between hydrogen, electricity, heat, and renewable energy sources (RES) is particularly appealing. PEM electrolyzers offer a backup system for RES to mitigate intermittency, converting energy into hydrogen, due to fast ramp rate and load-following capability. However, commercialization hinges on reducing costs through the development of new materials while necessitating a deeper understanding of heat, mass, and electrochemical processes in the whole system.

This coupling, especially when electricity from the grid is absent, introduces challenges related to efficiency losses Addressing this challenge requires a multifaceted approach. While power electronics offers one possible solution, this paper delves into a preliminary step before advancing to power electronics optimization: a deeper understanding of the dynamic operation of PEM electrolysis systems. By dynamic operation, we mean the ability of PEM electrolysis systems, including both the PEM stack and the Balance of Plant (BoP), to adjust their operating parameters in response to changes in input conditions or external factors over time. Many existing studies tend to focus primarily on the PEM stack, often overlooking the significance of the Balance of Plant (BoP) and its dynamics. Despite constituting a significant portion of the overall investment cost, the BoP requires electricity from the power source for its operation, which is often not calculated in the levelized cost of hydrogen. By shifting the focus beyond the PEM stack to also include the BoP, we seek to gain insights into the operational dynamics that impact the overall system efficiency, focusing mainly on electrical sources. Through this approach, we aim to pave the way for more holistic and efficient PEM electrolyzer systems, contributing to the advancement of sustainable energy solutions.

Mathematical models serve as indispensable tools in bridging the gap between PEM electrolyzer systems and intermittent electrical sources, facilitating an understanding of their interconnected dynamics. While the focus remains on PEM electrolyzers, the comprehensive modeling includes consideration of both stack and BoP components, providing insights into system dynamics critical for real-world applications. Through Aspen Plus V14 software, we construct detailed models of the stack and BoP components to assess system performance comprehensively. The study focuses on leveraging simulations to analyze the behavior of the PEM electrolysis system under various operating conditions like electrical source profiles which depend on the solar radiation and solar cell output, providing valuable insights into its performance. Using ad-hoc developed models within Aspen Plus V14 (integrated as a subroutine using Aspen Custom Modeler), sensitivity analyses were conducted to examine the system's behavior. The modeling approach integrates a custom stack model developed based on semi-empirical equations describing cell voltage, Faraday efficiency, and gas purity as a function of current density. This stack model, along with standard operation units for other system components, allows for an accurate representation of the complete electrolysis system (e.g., stack plus BoP). The BoP modeling encompasses all necessary equipment for stack operation, including the water management subsystem, hydrogen production and purification subsystem, and system cooling. Moreover, a hydrogen compression and storage subsystem were introduced, and a simple control subsystem was created, to keep critical parameters within acceptable limits preventing fast degradation. Furthermore, the model enables detailed mass and energy balances of each subcomponent and the entire system, facilitating the analysis of dynamic system behavior and identification of energy consumption patterns and loss sources. The Aspen Plus model presented in this work serves as a valuable design tool for optimizing the efficiency and cost-effectiveness of PEM electrolyzer systems powered by renewable energy sources that can not simulated as a steady-stay power source, like solar power.

To sum up, the primary objective of this study is to enhance the efficiency of PEM electrolysis systems connected with renewable energy sources, especially solar energy. This involves not only optimizing the efficiency of the stack but also calculating the overall plant efficiency, accurately assessing electricity consumption, and identifying various types of system losses. By gaining a comprehensive understanding of these parameters, we can better manage the integration of power electronics for the direct or indirect coupling of renewable power sources with PEM electrolysis plants. This deeper insight into system dynamics lays the groundwork for more effective and sustainable energy solutions.

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