Transient modeling and control of a small-scale and self-pressurized electrolysis system

Abstract

Zero Emission Fuels B.V. (ZEF) is a start-up company developing a fully autonomous methanol synthesis micro-plant that will be energy driven by solar panels. The process implements an alkaline water electrolyzer, which supplies the methanol synthesis reactor with hydrogen. The electrolysis system includes a small-scale stack of cells and is designed to operate at 90 ^oC and the high pressure of 50 bar, with a 30% potassium hydroxide electrolyte. The system is also self-pressurized through the continuous accumulation of the electrolysis gases in the closed flash separation vessels. To control the process, the company has designed a novel system that aims to maintain the gas pressure at 50 bar and the liquid electrolyte level inside the flash separators at a fixed point. 
The present work has two main objectives. The first is to characterize the transient dynamic response of the company's current experimental electrolysis setup under the effect of the operating conditions and control. The second objective is to predict the level of gas crossover that is induced during the system's operation and evaluate the risk of explosive mixtures formation. Two different models were developed to fulfill the research targets.
The first model is based on a 1-d transient hydraulic network analysis. Simulations were conducted using Simulink for a current density range of 500-5000 A/m². The model predicts the electrolyte flow and pressure response in the various elements and locations of the network respectively, indicating also the oscillatory behavior induced by the operation of the valves. A key finding is the high differential pressure between the stack anodes-cathodes that is caused when the valves open, posing a danger for the integrity of the separators between the half-cells.
The second model was developed in MATLAB and uses the predicted flow response of the first model to estimate the crossovers by solving numerically the unsteady 1-d advection-diffusion equation in the corresponding elements. The effect of the supersaturated electrode boundary layer was also considered as an enhancing parameter of the mass transport through the separators of the cells and a sensitivity analysis was conducted. The main finding is that this parameter has an important effect, especially on the hydrogen crossover inducing 1% higher hydrogen impurity when it is increased by a factor of 10. The crossovers of both hydrogen and oxygen are found to gradually increase and reach an almost stable value under the effect of the control system. A comparison of the model with preliminary experimental data indicates a supersaturation intensity between 5 and 15 times higher than the solubility of the components. In this range, the system will safely operate above 2000 A/m² without the formation of explosive mixtures.