Green hydrogen glossary

AACE estimate classes rank capital cost estimates from Class 5 (concept screening) through Class 4 (feasibility) to Class 1 (check estimate), each with a typical accuracy range.

RFNBO additionality requires the power behind EU-compliant hydrogen to come from new, unsubsidised renewables, in operation no earlier than 36 months before the electrolyzer (Art. 5, Reg. 2023/1184).

Alkaline water electrolysis (AWE) splits water into hydrogen and oxygen in an alkaline electrolyzer filled with liquid potassium hydroxide; it is the longest-established electrolysis technology for large plants.

Electrolyzer capacity factor is the hydrogen a plant actually produces in a year divided by what it would produce running at full rated power every hour; it is one of the strongest levers on LCOH.

Curtailment in green hydrogen is renewable electricity a solar or wind plant could produce but the electrolyzer cannot absorb, because power is above its rating or below its minimum load.

Cycles of concentration (COC) in a cooling tower is the ratio of dissolved solids in the circulating water to those in the make-up water; it sets how much blowdown is needed.

Delivered vs produced hydrogen is the gap between hydrogen leaving the stack and hydrogen reaching the offtaker after plant losses; a credible LCOH divides cost by delivered kilograms.

A deoxo unit removes residual oxygen from electrolytic hydrogen by reacting it with hydrogen over a catalyst to form water; in deoxo hydrogen purification, the dryer then removes that water.

Gas crossover is hydrogen leaking through the electrolyzer separator into the oxygen side; hydrogen in oxygen crossover sets the electrolyzer's safety trips and minimum load.

RFNBO geographic correlation requires contracted renewable power to come from the electrolyzer's own bidding zone or a qualifying interconnected one (Art. 7, Reg. 2023/1184).

Hydrogen compression raises hydrogen from electrolyzer outlet pressure to storage, pipeline or process pressure; hydrogen compressor types include reciprocating, diaphragm and centrifugal machines.

Hydrogen purity grades are minimum quality specifications set by end use; for hydrogen purity, ISO 14687 defines them, with Grade D for PEM fuel-cell road vehicles requiring at least 99.97 %.

The levelized cost of hydrogen (LCOH) is the constant price per kilogram at which a hydrogen plant recovers all its discounted costs over its life; LCOH calculation uses discounted cash flow.

LHV vs HHV hydrogen efficiency is a choice of energy basis: 33.3 kWh/kg (lower heating value) or 39.4 kWh/kg (higher heating value), which changes one electrolyzer's quoted efficiency.

Loss of power supply probability (LPSP) is the share of demand an energy system fails to meet over a period; in hydrogen plants, it measures how much of the offtake goes unserved.

Electrolyzer minimum load is the lowest share of rated power at which the stack can run safely; below it, the unit must go to standby or shut down.

P50 and P90 solar and wind yield are annual energy estimates exceeded with 50 % and 90 % probability; lenders typically size project debt on the P90 value.

PEM electrolysis splits water using a solid proton exchange membrane as electrolyte; a PEM electrolyzer responds in seconds to power changes but relies on iridium and platinum catalysts.

A polarization curve in electrolysis plots cell voltage against current density; it shows how much extra voltage, and so extra energy, each step up in hydrogen output costs.

A pre-FEED study and FEED are the front-end engineering stages that take a hydrogen project from a chosen concept to a design and cost estimate firm enough for investment.

Renewable fuels of non-biological origin (RFNBO) are the EU category for fuels, such as green hydrogen, whose energy comes from renewables other than biomass; only RFNBOs count toward EU targets.

Solid oxide electrolysis (SOEC) splits steam at high temperature on a ceramic electrolyte; an SOEC electrolyzer uses less electricity per kilogram than AWE or PEM when external heat is available.

Electrolyzer specific energy consumption (SEC) is the electricity, in kWh/kg, needed to produce one kilogram of hydrogen, measured either at the stack or for the whole system.

Stack degradation is the gradual loss of electrolyzer cell performance over operating time, usually given as an electrolyzer degradation rate in %/1,000 h or µV/h of cell voltage rise.

RFNBO temporal correlation requires hydrogen to be produced in the same period as its renewable power: monthly until 2029, then hourly matching from 2030 (Art. 6, Reg. 2023/1184).

TSA drying removes water vapour from hydrogen on adsorbent beds that are regenerated by heating; TSA vs PSA hydrogen drying is a trade-off between heat demand and hydrogen loss.

Ultrapure water is the deionised feed an electrolyzer needs; water treatment for electrolysis typically chains reverse osmosis (RO) and electrodeionization (EDI) to remove salts that would damage the stack.

Water intensity is how much water it takes to produce 1 kg hydrogen; electrolysis needs about 9 litres per kilogram for the reaction, and several times more once cooling is included.

WACC is the blended return a project must pay its lenders and equity investors; in a hydrogen project, WACC is the discount rate that turns upfront CAPEX into a cost per kilogram.

Working inventory is the hydrogen a storage system can deliver between its maximum and minimum operating pressures; hydrogen buffer storage sizing is based on it, not on total gas held.