Pressurized green hydrogen from water electrolysis: Compression before or after electrolysis? A comparison among different configurations

Abstract

Storing the intermittent renewable energy in the form of green hydrogen is the key pathway toward achieving a complete shift to renewable energy dependence. This study compared and examined various settings to produce hydrogen from water electrolysis, considering pressure and temperature effects over a broad range that covers both liquid and steam phases. Pressurized and atmospheric electrolysis at low and high temperatures were investigated under both isothermal and multistage isentropic post-electrolysis gas compression. Modeling and analysis based on established correlations and equations of state were used to estimate the system’s overall power requirements for each scenario. Increasing the operating temperature significantly reduced the electrolysis work while the pressure had an opposite effect, especially at low pressures. The results conclude that compressing liquid water, heating, then electrolyzing requires less overall energy than atmospheric electrolysis followed by product gases compression. This was valid for isothermal or isentropic compression despite the fact that the isothermal required the least work. The difference between pre-and post-electrolysis compression scenarios becomes greater at high pressure and temperature values. For instance, to obtain hydrogen at 25 °C and 50 bar, the power saving from pressurized electrolysis was 2.5% compared with atmospheric electrolysis followed by gas compression, which increased up to 20% at 600 °C and 200 bar. This indicates that high pressure steam electrolysis integrated with renewable systems leads to producing green hydrogen ready to store, transport, or use at less cost and complexities, especially with a cheap heat source or waste heat utilization.