Green Hydrogen Electrolyzer Turndown & Crossover (2D)

Electrolyzer Technology

Load from Renewable Supply

Current density 100% of rated

Stack temperature 60 °C

Renewable duty cycle off

Safety Status

H₂ in O₂: 0.0% — SAFE

Live Readouts

Current density j

2.00 A/cm²

Cell voltage

1.98 V

H₂ output rate

—

Specific energy

—

How it works

Water electrolysis obeys Faraday's law: the molar rate of H₂ produced by a cell of active area A carrying current density j is

n(H₂) = n_cells · (j·A) / (2F)      F = 96 485 C/mol

Splitting a molecule of H₂O needs 2 electrons per H₂ molecule, so halving F is the "2" in the denominator. Cell voltage rises above the thermoneutral value (1.48 V) with ohmic and activation losses, roughly V(j) = V₀ + R·j — PEM's thin, low-resistance membrane gives a shallower slope than alkaline's thicker diaphragm.

Why turndown matters: renewable power is intermittent, so a green-hydrogen electrolyzer is constantly asked to run below its rated current. Gas crossover through the separator — H₂ diffusing into the O₂ stream — is driven mostly by a fixed concentration/pressure gradient, not by current. So as j falls, H₂ production falls but crossover barely does, and the *fraction* of H₂ contaminating the O₂ stream rises roughly as 1/j:

x_H₂,O₂(j) ≈ x₀ · (j_ref / j) · [1 + (T − 60°C)/100]

Industrial safety practice trips the stack well below hydrogen's ~4% lower explosive limit in oxygen — this simulator uses a conservative 2.0% shutdown threshold. PEM's thin membrane and high rated current density let it turn down to roughly 5–10% of full load before crossing that line; alkaline's thicker diaphragm but much lower rated current density means it typically can't safely go below ~20–25% load — exactly the "minimum turndown ratio" that limits how directly an electrolyzer can be coupled to wind or solar without a battery buffer.

PEM / Alkaline — switches the rated current density, voltage slope and crossover coefficients to each technology's typical values.
Current density slider — throttles load from 5% to 100% of rated, as a renewable source would.
Stack temperature — higher temperature speeds gas diffusion through the separator, worsening crossover at a given load.
Renewable duty cycle — animates the load slider itself as a noisy cloud-passing / gust pattern, so you can watch the safety margin swing in real time.
Turndown curve (bottom panel) — plots crossover fraction against load for the selected technology and temperature, with the safety line and your current operating point marked live.

H₂ bubbles (cathode) O₂ bubbles (anode) H₂ crossover into O₂ side

drag to pan · scroll to zoom

Turndown curve — H₂-in-O₂ crossover vs load