Extension of dynamic operational range in alkaline water electrolysis process

Abstract

Currently, hydrogen is predominantly produced through the reforming of fossil fuels which emits the greenhouse gases failing to meet carbon reduction requirements. In contrast water electrolysis, a simpler technology that produce hydrogen with 99.9% purity. Long term energy storage, through energy-rich hydrogen, is considered promising as it allows for the use of surplus renewable energy generated during period of low demands. Water electrolysis, particularly alkaline water electrolysis has been applied in the industry for decades and is often discussed as a key technology for future hydrogen synthesis due to its fast dynamic response varying operating conditions. Renewable-powered alkaline electrolysis has the potential to play critical role in the transition to a low-carbon energy future. However, in this integration the variability in renewable energy supply can lead to frequent start-stop cycles and fluctuating operational conditions for electrolysis affecting their efficiency and longevity. Advanced control systems, integrating energy storage and dynamic load management can address these challenges.

This paper examines the extension of the dynamic operating range in alkaline water with respect to product gas purity. Specifically, it analyzes the gas crossover in product gases due to the dissolution of gas molecules in circulating electrolyte solution and diffusion through diaphragm and suggests methods to reduce it, including decreasing the pressure in the oxygen separator, controlling electrolyte circulation rate, and removing dissolved gases from the electrolyte. The study investigates variations in electrolyte flow rate (kg/h), capacity (MW) and gas composition (%). Simulation ware conducted in Aspen Plus using a 30 wt% KOH alkaline electrolyte solution at 80 oC and 16 bars pressure. The result indicates that for wide range operation of the plant and safe process shutdown, removing dissolved gases through the membrane wall in gas-liquid membrane module significantly affect gas composition, ensuring impurities remains below safety levels even when the load drops to 10% of maximum capacity. Moreover, control of the electrolyte circulation rate at low power inputs helps to keep gas composition at constant level reducing gas crossover.