Choosing an Electrolyser for Green Hydrogen.

Which Types Can Make Green Hydrogen?

All of them can.

“Green hydrogen” is not a particular type of electrolyser. It generally means hydrogen made by electrolysis using renewable electricity, with suitably low lifecycle greenhouse-gas emissions. An alkaline unit powered by wind can make green hydrogen; the same unit supplied by a fossil-dominated grid may not. The same principle applies to PEM, AEM and solid oxide technology.

A credible green-hydrogen claim therefore depends on more than buying an electrolyser. The project must establish where its electricity comes from, when it is generated, how associated emissions are counted and whether renewable generation is additional or merely redirected from other consumers.

Water treatment, compression, storage and delivery also consume energy and should be included in a full lifecycle assessment.

The four main technologies include:

• Alkaline
• PEM
• AEM
• Solid Oxide

Alkaline Electrolysis: Mature, Durable and Usually Lower-Cost

Traditional alkaline electrolysers circulate a concentrated solution of potassium hydroxide or sodium hydroxide.

At the cathode, water and electrons form hydrogen and hydroxide ions. Those negatively charged hydroxide ions cross a porous diaphragm to the anode, where oxygen is formed.

Alkaline electrolysis has been used industrially for many decades. It commonly uses nickel-based electrodes rather than large quantities of scarce platinum-group metals, and suppliers can build large modules with long operating lives. This maturity, established supply chain and relatively inexpensive stack materials often give alkaline systems a capital-cost advantage.

The trade-off is a more cumbersome liquid-electrolyte system. Pumps, gas-liquid separators and electrolyte concentration must be managed. The caustic electrolyte introduces handling, corrosion and workplace-safety requirements.

Traditional designs also tend to have lower current density, producing a larger plant for a given output, and generally make hydrogen at lower pressure than PEM systems.

Older alkaline machines prefer steady operation and may have restricted minimum loads because low current can increase hydrogen and oxygen crossover.

Modern pressurised alkaline designs are much more flexible than the stereotype suggests, and some can follow renewable generation well.

Even so, repeated cold starts, very low-load operation and abrupt cycling require careful attention to the supplier’s warranted operating performance.

Best Fit

Large, cost-sensitive projects with reasonably steady or deliberately smoothed power; industrial plants where space is available and operators are comfortable managing chemical process equipment.

PEM Electrolysis: Compact and Highly Responsive

A PEM electrolyser uses a solid acidic polymer membrane.

Water supplied to the anode is split into oxygen, electrons and positively charged hydrogen ions, or protons. The protons cross the membrane, then recombine with electrons at the cathode to form hydrogen.

PEM stacks operate at high current density, so they can be compact. They start quickly, offer a wide operating range and can change output in seconds or faster at stack level. They can also produce hydrogen at elevated pressure, potentially reducing the first stage of downstream compression. These attributes make PEM particularly attractive for variable wind or solar power and for sites where footprint matters.

Its principal disadvantage is material cost. The acidic, oxidising environment requires specialised membranes, iridium-based anode catalysts, platinum at the cathode and corrosion-resistant components that may include titanium. Iridium supply and cost are important scale-up concerns. PEM plant is consequently often more expensive to purchase than alkaline plant, although project-level differences depend heavily on pressure, purification, controls and the rest of the balance of plant.

PEM also needs very clean deionised water. Water-treatment consumables, filters and polishing equipment require routine attention, while membrane-electrode assemblies and stack seals eventually need specialist inspection or replacement. Rapid response does not mean unlimited cycling is harmless: starts, stops, standby conditions and operation at extreme loads can still affect degradation. The complete system—including dryers, chillers, pumps, compressors and power electronics—may constrain flexibility before the electrochemical stack does.

Best Fit

Direct or close coupling to variable renewable power, constrained sites, frequent turndown, grid-support operation, or applications valuing compactness and pressurised hydrogen.

AEM Electrolysis: A Promising Combination That Is Not Yet Equally Proven

An AEM electrolyser uses a solid polymer membrane that conducts negatively charged hydroxide ions.

In electrochemical terms it works in an alkaline environment, but it aims to obtain the compact architecture and responsiveness associated with PEM.

The attraction is compelling: AEM may operate with pure water or a dilute alkaline solution and use nickel, iron or other non-platinum-group catalysts. It can combine lower-cost materials with high current density, modular construction and flexible operation. These qualities make AEM particularly relevant to compact micro electrolysers and distributed onsite hydrogen production. The European Clean Hydrogen Partnership describes this as an attempt to combine the advantages of alkaline and PEM systems.

The difficulty is durability and scale. Membranes and ion-conducting polymers can degrade in alkaline operating conditions, carbon dioxide can contaminate the electrolyte, and long stack life at industrial current densities has not yet been demonstrated as widely as it has for alkaline or PEM. There are commercial products, but fewer suppliers, smaller reference fleets and less long-duration field evidence. Replacement stacks and specialist support may also be less readily available, which matters greatly in New Zealand.

Best Fit

Small, modular and distributed projects—particularly micro electrolyser systems—where compact construction, flexible operation and reduced dependence on precious-metal catalysts are valuable. Supplier warranties, operating history, stack replacement support and local service remain important procurement checks.

Solid Oxide Electrolysis: Very Efficient When Heat Is Available

SOECs operate with steam at approximately 650–850°C.

A ceramic electrolyte conducts oxygen ions from the steam electrode to the oxygen electrode. Because heat supplies part of the energy needed to split water, an SOEC can use less electricity per kilogram of hydrogen than a low-temperature electrolyser when a suitable source of steam and high-temperature heat is available.

That creates a strong potential fit with refineries, chemical plants, geothermal or other industrial sites that can integrate heat. Solid oxide systems may also co-electrolyse steam and carbon dioxide to make synthesis gas for fuels and chemicals.

High temperature is also their main operational challenge. Heating from cold takes time and energy. Thermal cycling stresses ceramics, seals and interfaces, while impurities in steam can damage the stack. Maintenance calls for specialist knowledge of hot process equipment, insulation, heat exchangers and brittle ceramic components. Stack durability and commercial track record remain behind established low-temperature systems.

An SOEC can modulate while it remains hot, but it is poorly suited to being heated and cooled with every passing cloud or shut down nightly. A thermal buffer, grid connection, battery or steady secondary load would be needed to isolate the hot stack from raw solar variability.

Best Fit

Continuously operated industrial plants with useful high-temperature heat or steam and a high value on electrical efficiency.

What Does Production Scale Mean in Practice?

Hydrogen projects are often described in megawatts, while customers usually think in kilograms per day.

A useful planning assumption for a complete low-temperature electrolyser is approximately 50–60 kilowatt-hours of electricity per kilogram of hydrogen before substantial downstream compression or liquefaction. Using a midpoint of 55 kWh/kg gives the following order-of-magnitude comparison:

These are energy-equivalent estimates, not equipment quotations. Real systems must allow for part-load efficiency, conversion losses, solar variability, downtime, water treatment, cooling, drying and compression.

Restricting production to daylight requires a much larger electrolyser and solar array than operating around the clock from a grid-backed renewable supply. A battery can smooth seconds and minutes of variation but storing enough electricity to run a large electrolyser all night is normally a very different—and much more expensive—design decision.

Around 10–30 kg per Day: Demonstrations and Small Onsite Users

This scale may suit research, backup-power fuel, small material-handling fleets or an early hydrogen user proving its demand.

Modular PEM and AEM units are practical choices at this scale because they are compact, responsive and suited to packaged systems. New Zealand Hydrogen’s micro electrolysers use modular AEM technology, making this route directly relevant to onsite projects producing up to 10 kg per day.

At this size, the electrolyser may not be the largest project challenge. A compliant installation still needs water treatment, ventilation, gas detection, controls, drying, compression, storage and dispensing or cylinder filling. Fixed safety and consenting costs are spread over very few kilograms. Specialist maintenance and imported spare parts can make each outage expensive in New Zealand. Unless the hydrogen has a high-value purpose, the cost per kilogram will usually be much higher than at a well-utilised industrial plant.

A small solar array could supply the daily energy in theory, but production would fall with weather and season. The project must either accept variable hydrogen output, maintain several days of hydrogen storage, use a grid connection, or add another renewable source.

Around 30–200 kg per Day: Depots, Farms and Light Industry

This range is relevant to a small fleet depot, a group of heavy vehicles, a remote-energy hub or a modest industrial gas requirement.

PEM is a strong candidate where the unit will follow onsite solar or produce hydrogen at useful pressure. Modular alkaline equipment can compete where demand is steady and electricity is smoothed or grid backed.

The project now needs deliberate coordination between production and demand. Vehicles may refuel in concentrated morning or evening windows even though hydrogen is made throughout the day. Buffer storage and compressor capacity must therefore be sized for the peak dispensing rate, not simply the daily average. Redundant compressor or electrolyser modules may be justified if one failure would immobilise a fleet.

Maintenance can still be provided through a supplier contract, but the owner should train local technicians for routine inspections, filters, water systems and safe isolation. Remote monitoring, stocked critical spares and a guaranteed service response become important procurement requirements.

Around 200–1,000 kg per Day: Commercial Transport and Industrial Hubs

At hundreds of kilograms per day, the electrolyser becomes a substantial industrial electrical load.

Both PEM and alkaline technology could be utilised. PEM may justify its higher capital cost if wide turndown, frequent renewable following, compact footprint or higher outlet pressure reduces other costs. Alkaline may win where land is available and a grid connection, hydro-backed contract, wind-solar portfolio or battery keeps operation relatively stable.

This is also the range where a hub can improve economics. Several fleets or industrial users can share production, storage, compression, safety systems and maintenance staff. However, shared demand creates a stronger requirement for availability. Multiple stack trains allow maintenance without stopping the entire plant, and adequate stored hydrogen can cover short production outages.

Owners need planned stack overhaul or replacement budgets, power-quality studies, industrial water treatment, spare-parts strategy and formal process-safety management. Electricity contracting becomes central: at 500 kg per day, a one-cent-per-kilowatt-hour change in electricity price changes annual electricity expenditure by roughly NZ$100,000, assuming 55 kWh/kg and year-round production.

More Than 1,000 kg per Day: Multi-Megawatt Hydrogen Production

One tonne per day requires approximately 55 MWh of electricity each day at the midpoint assumption—an average load of about 2.3 MW if operated continuously.

A solar-only plant producing within eight good hours would require an electrolyser approaching 7 MW before allowances for weather, seasonal output and losses. Projects of several tonnes per day quickly become major generation, grid-connection and infrastructure developments.

Alkaline electrolysis often has a strong economic case at this scale because of its maturity, long life and relatively low stack-material cost. PEM remains highly credible where renewable variability, constrained footprint or pressure has high value, and modular PEM trains can provide useful redundancy. SOEC deserves consideration when the project has continuous high-temperature heat or steam and an industrial operator able to manage thermal integration. AEM should be treated as a project-specific technology choice until more bankable large-scale operating history exists.

Large plants can lower unit costs by spreading engineering, controls and staffing across more hydrogen, but they expose the project to utilisation risk. A cheap electrolyser that has no contracted buyer—or operates for only a small portion of the year—can make expensive hydrogen. Long-term electricity supply, dependable offtake, storage, transport and replacement-stack agreements should be developed together.

These are tendencies, not procurement specifications. Product design is advancing quickly, and two machines with the same chemistry can have different minimum loads, start times, efficiencies, output pressures and maintenance intervals. A project should compare warranted system performance, not just technology labels.

Which Electrolyser Works Best with Fluctuating Solar Power?

For a project that intends to follow an unsmoothed solar profile directly, PEM remains a well-proven choice, while suitably designed modern alkaline and AEM systems should also be evaluated against the required duty cycle.

Its fast start, wide turndown range, high current density and rapid response allow it to capture changing solar production without the thermal constraints of an SOEC. The US Department of Energy specifically identifies PEM’s sub-second response and broad load capability as compatible with variable solar and wind power.

That answer changes with the project architecture:

Direct, Off-Grid Solar with Frequent Clouds

PEM is usually the safest and most bankable starting point. A small battery can keep controls, pumps and safety systems alive, absorb very fast transients and reduce damaging start-stop cycles.

Grid-Connected Solar or a Hybrid Solar-Wind Supply

Modern alkaline equipment may offer the lowest-cost result if the grid, battery or mixed generation keeps it above its warranted minimum load. It should not be dismissed merely because older alkaline plants were inflexible.

A Smaller Modular or Micro Electrolyser Project

AEM can provide PEM-like flexibility without the same precious-metal burden and is already used in modular micro electrolysers. At this scale it can be a strong choice where the supplier warrants performance for the expected solar duty cycle and provides dependable stack-replacement and service support.

A Solar-Powered Industrial Site with Continuous Steam or Thermal Storage

SOEC may deliver excellent electrical efficiency, but the hot stack should be kept thermally stable rather than follow raw photovoltaic output.

The International Renewable Energy Agency reports that alkaline, PEM and AEM systems can all offer substantial flexibility, whereas SOEC flexibility is constrained by thermal conditions. This supports a technology-neutral tender: define the solar profile, minimum hydrogen output, required pressure, start-stop pattern and lifetime, then require suppliers to warrant performance against that exact duty cycle.

A Practical Selection for New Zealand

For New Zealand, technology choice should follow the energy resource, production scale and hydrogen customer rather than precede them.

PEM is likely to be attractive for distributed production from roughly 10 kg per day upward, renewable microgrids and sites following variable generation. AEM is also directly relevant to modular, distributed production and is the technology used in New Zealand Hydrogen’s micro electrolyser systems.

Alkaline becomes a particularly strong contender at hundreds or thousands of kilograms per day with high utilisation, especially where renewable electricity can be contracted through the grid or balanced across wind, hydro and solar. SOEC could become relevant at industrial scale where process or geothermal heat can be integrated. AEM can be a practical and attractive choice at smaller modular scale, provided the selected equipment has suitable performance warranties, stack support and operating evidence.

New Zealand’s hydro-dominated electricity system can help smooth variable generation, but grid electricity is not automatically surplus, cheap or emissions-free in every trading period. A sound project will schedule production when renewable power is genuinely available and economic, while ensuring the electrolyser runs often enough to repay its capital cost.

The best answer is therefore not simply “PEM” or “alkaline.” For raw fluctuating solar, PEM currently has the operational advantage. For the lowest delivered cost of green hydrogen, the winner is the complete system that best balances renewable supply, utilisation, efficiency, durability, storage and dependable local support.