f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

Alkaline electrolysis

Alkaline electrolysis Electrolysis hydrogen Electrolyser power-to-X Industry power system

Alkaline electrolysis

Alkaline electrolysis is a process of electrolysis in which electrical energy splits water into hydrogen and oxygen. Its defining feature is a liquid alkaline electrolyte, usually aqueous potassium hydroxide solution. Hydrogen is produced at the cathode and oxygen at the anode. A porous diaphragm separates the electrode compartments and limits mixing of the product gases, while still allowing ion transport. Common abbreviations are AEL and AWE.

The process is one of the industrially established electrolysis technologies. Its strengths are many years of operating experience and the possible use of comparatively inexpensive, nickel-based electrode materials. However, this implies neither blanket cost leadership nor a restriction to steady continuous operation: design, balance of plant and operating profile are decisive.

Operating principle and design

Electrons move in the external circuit; in the electrolyte solution, ions carry the charge. At the cathode, water takes up electrons and forms hydrogen and hydroxide ions. These migrate to the anode, where oxygen and water are formed. The electrolyte solution serves as a conductive medium and is not consumed in the overall reaction. Typical operating temperatures are roughly between 60 and 90 °C.

  • Cathode: 2 H₂O + 2 e⁻ → H₂ + 2 OH⁻.

  • Anode: 2 OH⁻ → ½ O₂ + H₂O + 2 e⁻. Overall: H₂O → H₂ + ½ O₂.

Many cells form a cell stack, the stack. A complete plant also includes, for example, rectifiers, water treatment, an electrolyte circuit, cooling, gas separation and gas purification. Compression may be added. Product pressure and purity are therefore properties of the specific system, not of the electrochemical cell alone.

For an industrial off-taker, the interface is decisive: it needs a certain quantity of hydrogen at an agreed purity and pressure. An inexpensive stack does not yet fulfil this delivery task. When offers are compared, they must be based on the same delivery conditions and the same scope of auxiliary systems.

Power, output and electricity demand

The nominal electrical power is stated in kilowatts or megawatts. It describes power consumption, not a quantity of hydrogen and not an annual electricity consumption. Units such as kilograms per hour suit the production rate; tonnes per year suit annual production. For normal cubic metres, the reference temperature and reference pressure must be stated so that quantities are comparable.

Specific electricity consumption indicates how many kilowatt-hours are needed for one kilogram of hydrogen. As a rough orientation for many low-temperature plants, about 50 to 60 kWh/kg is plausible, but this is not a universally valid or guaranteed range. For traditional alkaline technology, the US Department of Energy gives, for example, 51 kWh/kg at the stack and 55 kWh/kg for the system as the 2022 status. The development targets listed alongside must not be read as market values already achieved.

A transparent calculation example: if a plant, including the auxiliary units considered, draws 100 MW and requires 55 kWh/kg, it produces, in calculation terms, around 1,820 kg of hydrogen per hour. At 4,000 full-load hours, this results in 400 GWh of electricity consumption and approximately 7,270 tonnes of hydrogen per year. Full-load hours are a calculated utilisation measure here; they need not match the actual operating hours.

Such calculations hold only if consumption is assumed to be unchanged. In a project comparison, the following should therefore be asked: Is alternating current measured at the plant connection or direct current at the stack? Are drying, cooling and compression included? Does the figure refer to the new condition, a specific load point or an annual average? Without this information, an apparently better consumption figure may merely reflect a narrower system boundary.

Efficiency figures also need an energy reference. Hydrogen has a lower heating value of around 33.3 kWh/kg and a higher heating value of around 39.4 kWh/kg. With an assumed electricity demand of 55 kWh/kg, this yields, in calculation terms, about 61 and 72 percent respectively. Both figures can describe the same plant. The difference arises from the treatment of the heat of condensation of the water formed during combustion, not from better technology.

Distinction from other processes

PEM electrolysis uses a proton-conducting polymer membrane instead of the liquid alkaline electrolyte. AEM electrolysis, by contrast, uses a hydroxide-ion-conducting polymer membrane. Solid oxide electrolysis works with ceramic electrolytes and hot steam; supplied heat can reduce its electrical energy demand. A comparison of electricity consumption alone would be incomplete here without taking the provision of heat into account.

PEM often offers high current densities, but usually requires, among other things, iridium and platinum. For alkaline technology, nickel-based solutions are common. This can explain material differences, but no generally valid statements can be derived about investment cost, service life or ramp rate.

For a procurement decision, guaranteed operating data are therefore more helpful than a blanket technology ranking. These include consumption curves, permissible load range, start-up conditions, maintenance scope and product quality. One technology may be favourable with steady off-take, another with frequent operational interruptions; what is decisive is the overall plant concept.

Flexible operation and role in the power system

Alkaline electrolysis can contribute to flexibility when it varies its power consumption. In doing so, rapid power changes of a warm plant must be distinguished from a start from a cold state. Load range, gas crossover through the separator, gas purity and thermal state limit operation. On-off cycles and dynamic operation must also be taken into account in service-life assessments.

A load profile could, for example, provide for increasing production in hours of high renewable feed-in and reducing it at high residual load. However, this does not automatically create a benefit for the local grid. A low exchange price does not reliably indicate whether free transmission capacity exists at the specific connection. Surplus electricity is neither available everywhere at the same time nor an unlimited, secured source of supply.

For project assessment, location, grid connection, electricity contracts and operating schedule must therefore be considered together. If the plant is to offer ancillary services, the respective technical proofs and market conditions must also be examined. The mere ability to change power consumption establishes neither market admission nor secure additional revenue.

Economically, a trade-off arises between favourable purchase hours and high utilisation. With unchanged annual fixed costs, halved production spreads these costs over half as many kilograms; the fixed-cost share per kilogram doubles. Conversely, in the example with 55 kWh/kg, a reduction in the electricity price of 10 euros/MWh saves exactly 0.55 euros per kilogram. These are arithmetic relationships, not hydrogen price forecasts.

Water, safety and industrial integration

The reaction equation implies a stoichiometric demand of around nine kilograms of water per kilogram of hydrogen, approximately nine litres. At the same time, around eight kilograms of oxygen are produced. This mass balance describes only the chemical conversion, not the total water supply of a site.

In addition, treatment losses and, where applicable, cooling water must be taken into account. The raw water demand depends on water quality and treatment; the cooling water demand depends strongly on the cooling system. Particularly important is the distinction between water withdrawal and actual water consumption: returned once-through cooling water must be assessed differently from evaporated water. Local availability, wastewater disposal and competing uses therefore remain separate site-related questions.

A sensible project assessment further covers the handling of caustic electrolyte solution, possible gas leaks, monitoring of gas separation and safe operational management. Which specific protective measures and permits are required cannot be derived from the term AEL alone. Decisive factors include, among others, location, pressure, plant size and the quantities of substances present.

The supply chain must also be closed. Hydrogen off-take, purification, compression, transport and, where applicable, storage belong in the concept. If an industrial process is to be supplied continuously while the electrolyser runs only part of the time, it must be clarified how the time gap is bridged. The hydrogen produced opens up a storage option; the electrolyser itself is not yet a complete storage system.

Renewable hydrogen and legal classification

AEL denotes the production technology, not automatically green hydrogen. For EU accounting as a renewable fuel of non-biological origin, RFNBO for short, Regulation (EU) 2023/1184 governs the allocation of electricity in a technology-neutral way. For the usual contractual purchase of grid electricity, additionality as well as temporal and geographical correlation are relevant; other routes of proof and exemptions exist.

According to the state of the rules reviewed on 11 November 2026, monthly matching is in principle sufficient under the relevant routes of proof until the end of 2029, and hourly matching thereafter; Member States may bring the latter forward from July 2027. For RFNBO plants put into operation before 1 January 2028, certain additionality requirements do not apply until 1 January 2038, except for later capacity expansions. Guarantees of origin alone do not replace this verification. These RFNBO rules are not a general operating obligation for every electrolyser and do not answer all certification or permitting questions.

The overall project remains decisive

Alkaline electrolysis is neither automatically the cheapest source of hydrogen nor automatically grid-supportive or climate-friendly. For a robust assessment, electricity origin, consumption and system boundary, utilisation, location, water supply and reliable off-take must fit together. Only this combined view shows whether the proven technology can play out its advantages in a specific application.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 13:58

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.