“During a cold start-up, the auxiliaries are switched on and the electrolyzer is driven through safety measures and several time-consuming steps including a pressurization stage, the activation of purification units and system heating, inducing additional power consumption and hydrogen losses," the report explains.

“The shut-down process involves the system depressurization and various safety measures related to crossover-related issues, which have to be launched quickly. An electrolyzer operating at high pressure will therefore take a longer time to reach a totally inert state than one operating at atmospheric pressure.

“Yet, it is necessary to ensure that the power supply needed for the quick launch of these operational steps is sufficient to ensure the system safety, which an intermittent energy source is not able to guarantee.

“Moreover, intermittent operation may lead to situations where the electrical setpoint drops below the minimum threshold. In this situation, the system automatically leaves the production state to initiate a shut-down. Pressurization hydrogen is then lost through the venting lines and additional energy required to complete each step of system shut-down is consumed.

“To minimize the energy consumption associated with frequent start-ups and shut-downs, most electrolyzers incorporate stand-by periods. These phases maintain pressure and keep the electrolyzer close to its nominal operating temperature, facilitating quicker system resumption from a hot state compared to cold start-ups.

“Nevertheless, the continuous operation of auxiliaries during stand-by increases energy consumption without producing hydrogen.

“Furthermore, due to pressure maintaining, gas crossover [ie, hydrogen and oxygen mixing after crossing over the membrane that is supposed to separate them], which still occurs during stand-by periods leading to decreased gas purity and greater demand on purification units, resulting in higher electricity consumption.

“Additionally, despite maintaining proximity to the required level, temperature may slightly drop below its nominal range, affecting efficiency.

“The intermittent nature of the operation therefore poses a challenge because frequent transitions between different operational states during system operation lead to reduced performance.”

The impact of intermittency on electrolysers and green hydrogen production: what we know and what we don’t