Minimum Load (Turndown)

In plain terms

Electrolyzer minimum load is the floor of the operating window. Above it, the stack follows the power supply. Below it, the operator must idle the unit or stop it.

For alkaline stacks, the floor exists mainly for safety: gas impurity rises at low current density, which limits the part-load range (Brauns and Turek, 2020). Every membrane or diaphragm lets a small, fairly constant flow of gas leak across. At full load that leak is diluted by a large production flow. At low load, the same leak becomes a larger share of a smaller flow, and hydrogen concentration in the oxygen stream rises toward its flammability limit. The minimum load is set so that this gas crossover stays well clear of that limit.

The source cited here covers alkaline electrolysis. For PEM and other technologies, take the floor from the vendor.

Typical range

Minimum load depends on technology, pressure, stack design and vendor, so no single percentage applies. What the floor protects is the hydrogen-in-oxygen limit: the typical emergency-shutdown limit is 2 vol % (Brauns and Turek, 2020). Ask each vendor for minimum load as a percentage of rated stack power, at the stated operating pressure.

Why it moves LCOH

On a variable solar or wind supply, every hour where available power falls below the floor produces no hydrogen. That energy is either curtailed or sold elsewhere. A higher floor lowers the electrolyzer capacity factor, raises CAPEX per kilogram, and may require a battery or grid backup to keep the stack above minimum overnight or in low-wind periods.

Common mistake