Understanding Partial Load and Intermittent Operation of Alkaline Electrolyzers

Alkaline water electrolysis (ALK), as a mature and cost-effective technology for hydrogen production from electricity, can effectively produce hydrogen with a purity exceeding 99.9% when combined with drying and oxygen purification steps. However, this technology is primarily designed for steady-state operation at rated conditions or near full load, and its ability to adapt to partial load, especially intermittent operation, is limited. With the increasing proportion of renewable energy (such as wind and PV) in the power grid and the growing demand for utilizing energy storage systems (ESS) to absorb fluctuating renewable energy power, alkaline water electrolysis technology faces severe challenges in this field.

I. Main Challenges of Partial (Low) Load Operation: Gas Impurities and Safety Risks

During the electrolysis process, oxygen is produced at the anode and hydrogen at the cathode. Although the separator can significantly inhibit the permeation and migration of most gases (such as oxygen to the cathode side and hydrogen to the anode side), the generated hydrogen and oxygen are not absolutely pure and still contain trace amounts of cross-contaminants. Under rated conditions, these trace contaminants can be effectively removed through subsequent purification steps (such as oxygen removal from the hydrogen stream and hydrogen removal from the oxygen stream).

The core of the problem lies in the reduction of load:

Separator leakage rate remains basically constant: The physical permeation rate of gases through the separator is primarily determined by material properties and the pressure differential across both sides, and is approximately constant.

Gas production rate decreases: A reduction in load leads to a decrease in the total amount of H₂ produced at the cathode and O₂ produced at the anode per unit time (i.e., the denominator).

Relative concentration of contaminants increases: When (1) remains constant (the numerator) and (2) decreases, the relative concentration of the opposing gas in the product gas significantly increases, leading to excessive pollution levels.

This increase in contaminant concentration poses critical safety issues. When the hydrogen-oxygen mixture reaches a specific ratio (such as O₂ content ≥4% vol in H₂ or H₂ content ≥4% vol in O₂), it enters the explosive limit range. To ensure operational safety, alkaline electrolyzers are equipped with safety interlock mechanisms: once the contaminant concentration in either gas stream reaches 2% vol (a safety margin below the explosive limit), the system is shut down to allow for orderly purging under low-risk conditions.

Therefore, during partial load operation, the pollution level increases as the load decreases, and there is a minimum allowable load (lower limit) for the electrolyzer, corresponding to the 2% vol contaminant limit. For electrolyzers with different designs, the lower load limit generally falls within the range of 10%-40% of the rated load. This means:

When the intensity of power supply fluctuates below 40% of the rated value, it may lead to unplanned shutdowns.

The harm caused by shutdowns far exceeds that of hydrogen production interruption itself:

Long restart time: The restart process is complex and time-consuming, often far exceeding the time required for power fluctuation recovery.

Accelerated equipment aging: Each start-stop cycle causes stress damage to electrodes (such as nickel-based electrodes), significantly shortening their lifespan (typical electrode lifespan is only 5,000-10,000 start-stop cycles), and increasing maintenance frequency and equipment asset costs.

II. Causes of the lower load limit range (10%-40%): Differences in electrolyte circulation modes

The differences in the lower load limit range (10%-40%) of alkaline electrolyzers mainly stem from differences in their electrolyte circulation management modes: Separated Loop vs. Shared/Mixed Loop.

Challenges of the Shared/Mixed Loop: After the reaction, the gas-liquid mixture at the electrodes enters separate cathode and anode separators for gas-liquid separation. If the separated electrolytes are then mixed and recycled (Shared/Mixed Loop):

Increased gas cross-contamination: Trace residual gases dissolved in the separated liquid phases (a small amount of O₂ in the cathode liquid and a small amount of H₂ in the anode liquid) will transfer between each other with the mixing of electrolytes.

Elevated baseline impurity concentrations: This leads to higher background concentrations of O₂ impurities in cathode hydrogen and H₂ impurities in anode oxygen.

Forced shutdown at high load: To avoid triggering the 2% vol safety threshold at low loads, the system must shut down at a relatively high load (usually around 40% of the rated load), resulting in a poorer lower load limit (e.g., ~40%).

Advantages and costs of the Separated Loop: If the electrolytes separated from the cathode and anode are circulated independently (not mixed), it can effectively suppress the above cross-contamination, significantly reduce the baseline impurity concentration, and thus push the safe lower load limit closer to a lower level (e.g., ~10%).

Liquid level balance management: Ensure stable liquid levels on both sides to avoid siphoning or pumping issues.

Concentration adjustment capability: Maintain the electrolyte operating within the optimal concentration window (for example, NaOH has a peak conductivity of ≈65 S/m at <20 wt% and ~50°C; KOH has a peak conductivity of ≈95 S/m at >30 wt%), which is crucial for ensuring low energy consumption and high efficiency.

Challenges faced: The Separated Loop itself presents new problems.

Complexity of maintaining balance: This requires the system to have precise

Increased concentration in the cathode area: Water consumption in the cathode reaction leads to a local increase in KOH/NaOH concentration.

Diluted concentration in the anode area: Water generation in the anode reaction leads to a local decrease in KOH/NaOH concentration.

III. Superimposed Effects of Pressure Operation

Although high-pressure operation can reduce the energy consumption of subsequent hydrogen compression, it exacerbates the issue of gas cross-contamination at low loads:

Enhanced separator diffusion: Increased pressure may elevate the permeation rate of gases through the separator.

Increased gas solubility: The solubility of gases in the electrolyte rises with increasing pressure, leading to an increase in the amount of impurity gases carried by dissolution.

Conclusion: Under the most demanding operating condition combination of partial load and pressurized operation, a bypass loop design must be adopted, accompanied by the implementation of efficient strategies for liquid level and concentration balance to maintain impurity concentrations within safe limits. Research indicates that the adaptability of shared electrolyte circulation to this combined operating condition is highly sensitive and unstable.

IV. Power Outage and Low Voltage Management

Under the premise of maintaining impurity concentrations within safe limits, the system can temporarily tolerate operation below the lower load limit, but electrode voltages must be strictly controlled:

Cathode protection voltage: When the cathode voltage drops below approximately 0.25 V, the deterioration (such as corrosion and dissolution) of electrode materials significantly accelerates.

Safety redundancy: Even if impurity concentrations are controllable, once the voltage approaches this threshold, immediate shutdown is required to protect the electrodes.

Response strategies:

Capacitive effect buffering: Modern multi-layer composite electrode structures can exhibit a certain capacitive effect (equivalent to a large internal capacitor). Experiments have demonstrated that after a complete power outage, this effect can slow down the rate of electrode voltage drop, providing the system with time for restart. Research data indicates that if power can be restored within 10 minutes after a power outage, it may be possible to avoid shutdown and maintain continuous operation, significantly enhancing the system's ability to cope with transient fluctuations.

V. Complexity of Temperature Management

Partial load operation also poses significant challenges to system temperature control:

Narrow optimal efficiency range: Alkaline electrolysis typically achieves optimal efficiency (high conductivity without significant material deterioration) between 50-80°C.

Insufficient heat at reduced loads: As the load decreases, the reaction heat (ohmic heat and reaction enthalpy) correspondingly decreases. Under low ambient temperatures or inadequate insulation, the system struggles to maintain temperatures above the minimum of 50°C (compromising efficiency and safety).

Heat dissipation required at high loads and high temperatures: Under full load or high ambient temperatures, an effective cooling system is necessary to prevent temperatures from exceeding the upper limit (typically 80-90°C) to avoid accelerating material degradation or exacerbating corrosion.

VI. External Strategies for Coping with Partial Load Fluctuations

To effectively cope with power fluctuations below the lower load limit (such as 10%-25%) and avoid frequent startups and shutdowns, external strategies are often required to keep electrolyzer submodules operating at higher loads:

Power fluctuation buffering: Integrate energy storage systems (such as batteries, supercapacitors, or flywheels) to smooth out rapid fluctuations in renewable energy and provide a stable DC input.

Grouped operation of electrolysis stacks: Divide a large electrolysis system into multiple independent sub-modules. When the total required power decreases, some sub-modules can be shut down (put into shutdown or standby mode), while maintaining the remaining operational sub-modules close to their rated load operation.

Technical challenges:

Load distribution algorithm: Efficiently and flexibly distribute power fluctuations to different sub-modules.

Thermal state management: Manage the temperature rise, fall, and insulation requirements when starting and stopping different sub-modules.

Operational history recording and analysis: Accurately track and record the start-up times, operating durations, load curves, etc., of each sub-module for assessing aging status, predicting lifespan, formulating precise maintenance plans (such as electrode replacement), and optimizing operational strategies.