Green Hydrogen Production: Process Design and Technology Selection Guide

Navigating the complexities of electrolyser selection and process balance-of-plant.

Green hydrogen has moved from research topic to active project pipeline across India and globally. But “green hydrogen” is a narrow slice of a much larger hydrogen production landscape — and choosing the wrong technology at the design stage locks in cost and performance problems that are expensive to fix later.

This article covers what makes hydrogen “green,” the electrolysis technology options, and the process design considerations that determine whether a green hydrogen project is actually viable.

What Makes Hydrogen “Green”?

Hydrogen itself is colourless. The colour classification refers to the production method and its carbon footprint.

• Grey hydrogen — produced from natural gas via steam methane reforming (SMR), with no carbon capture. This is the majority of hydrogen produced globally today.
• Blue hydrogen — produced the same way as grey hydrogen, but with carbon capture and storage (CCUS) applied to the CO₂ generated. Lower carbon footprint than grey, but still fossil-fuel-based.
• Green hydrogen — produced by electrolysis of water, using electricity from renewable sources (solar, wind, hydro). This is the only hydrogen production route with a near-zero carbon footprint at the point of production, provided the electricity supply is genuinely renewable.

This article focuses on green hydrogen — specifically, the electrolysis technologies and process design decisions that determine project viability.

How Is Hydrogen Produced? A Quick Overview of All Methods

Before focusing on electrolysis, it’s worth knowing the full landscape of hydrogen production methods, since project teams often evaluate multiple routes before committing to green hydrogen:

• Steam Reforming — natural gas + steam, catalytic reaction, produces grey hydrogen (or blue, with CCUS).
• Partial Oxidation — sub-stoichiometric combustion of hydrocarbon feedstock.
• Autothermal Reforming — combines steam reforming and partial oxidation.
• Biomass Gasification — thermochemical conversion of biomass feedstock.
• Coal Gasification — hydrogen from coal, highest carbon footprint of common methods.
• Electrolysis of Water — the only route capable of producing green hydrogen when powered by renewable electricity.

Electrolysis is the technology this article focuses on, because it is the only pathway that qualifies as green hydrogen production.

The Principle of Electrolysis

Electrolysis splits water into hydrogen and oxygen using electrical current:

2H₂O → 2H₂ + O₂

An electrolyser consists of an anode and cathode separated by an electrolyte or membrane. Water is oxidised at the anode, releasing oxygen. Hydrogen ions or hydroxide ions migrate across the electrolyte, and hydrogen gas is produced at the cathode.

The efficiency and cost of this process depend almost entirely on which electrolyser technology is used — and this is the single most important process design decision in a green hydrogen project.

Electrolyser Technology Comparison: PEM vs Alkaline vs Solid Oxide

Why Electrolyser Choice Depends on Power Supply Profile

This is the process design decision most green hydrogen projects get wrong.

If the plant is coupled directly to solar or wind power — where electrical input varies significantly through the day and is intermittent — PEM electrolysers are generally preferred because they can ramp up and down quickly without efficiency loss or degradation. Alkaline electrolysers, historically, degrade faster under frequent cycling, though newer alkaline designs are improving in this area.

If the plant has access to stable, continuous power — grid-connected with firm renewable power purchase agreements, or baseload renewable supply — alkaline electrolysers often make more economic sense due to lower capital cost and decades of proven commercial operation.

Solid oxide electrolysers are best suited to applications where waste heat is available to supply the high operating temperature requirement — for example, co-located with an industrial process that generates high-grade waste heat. Without that waste heat integration, the energy penalty of heating the electrolyser to operating temperature can offset the efficiency advantage.

Selecting an electrolyser technology without first defining the power supply profile is one of the most common and costly mistakes in green hydrogen project design.

Water Quality Requirements for Electrolysis

Electrolysers require high-purity water — typically deionised water meeting strict conductivity and impurity limits. Feed water quality directly affects electrolyser membrane life and performance.

Water pretreatment for a green hydrogen plant typically includes:

• Reverse osmosis (RO) for bulk desalination (if source water is not already fresh water)
• Ion exchange or electrodeionisation (EDI) for final polishing to electrolyser-grade purity
• Continuous conductivity monitoring

Water pretreatment system design must be sized based on the specific electrolyser manufacturer’s feed water specification — this varies between PEM and alkaline systems, and between manufacturers.

Process Design Scope for a Green Hydrogen Plant

A complete green hydrogen process design covers considerably more than the electrolyser itself:

1. Feasibility Study: Renewable power resource assessment, water availability assessment, hydrogen offtake and demand analysis, and preliminary technology screening.
2. Electrolyser Selection and Sizing: Technology selection based on power profile, capital cost, and operational requirements. Sizing based on required hydrogen production rate and electrolyser stack capacity.
3. Balance of Plant (BOP) Design: Everything around the electrolyser stack itself — water treatment, power electronics and rectification (AC to DC conversion), hydrogen purification and drying, gas compression, and cooling systems.
4. Hydrogen and Oxygen Separation: Electrolysis produces both hydrogen and oxygen streams that must be separated, cooled, and purified. Separator sizing for gas-liquid separation at the electrolyser outlet is a standard process equipment design task.
5. Renewable Energy Integration: Design of the interface between the renewable power source and the electrolyser’s power electronics — including handling of variable power input and battery storage integration for load smoothing where needed.
6. Hydrogen Storage Design: Compressed gas storage, or integration with downstream conversion (ammonia synthesis, methanol synthesis) that consumes hydrogen directly without requiring large-scale storage.
7. Safety Assessment: Hydrogen has a wide flammability range and low ignition energy, requiring specific area classification, ventilation design, and leak detection philosophy distinct from conventional hydrocarbon processing.
8. Process Simulation: Steady-state modelling of the full plant — from power input through electrolysis, separation, purification, compression, and storage — typically performed in Aspen Plus or Aspen HYSYS, validated against electrolyser manufacturer performance data.

Hydrogen Storage: The Often Underestimated Design Challenge

Hydrogen storage deserves specific attention because it frequently becomes the most technically and economically challenging part of a green hydrogen project.

Storage technology options:

• Compressed gas storage — the most common near-term approach, typically at 200–700 bar depending on application, requiring multi-stage compression.
• Liquid hydrogen storage — cryogenic storage at -253°C, high energy penalty for liquefaction, used mainly for high-value applications like mobility or aerospace.
• Metal hydride storage — hydrogen absorbed into a metal lattice, safer and more compact than compressed gas, but currently higher cost and lower technology maturity.
• Underground storage (salt caverns) — being explored for large-scale, long-duration storage, similar to natural gas storage practice.

Key challenges in hydrogen storage design:
Hydrogen has very low volumetric energy density compared to natural gas, meaning storage systems must be significantly larger for equivalent energy content. Hydrogen also causes embrittlement in certain steel grades, requiring careful materials selection for storage vessels and piping. And hydrogen’s small molecular size makes it prone to leakage through joints and seals that would be adequate for other gases.

Transportation of green hydrogen — whether by pipeline, compressed gas trailer, or conversion to ammonia for easier transport — adds another layer of process design decisions that must be evaluated early, since it significantly affects the storage and compression design at the production site.

Global and Indian Green Hydrogen Trends

Globally, green hydrogen project announcements have accelerated sharply, driven by declining renewable electricity costs, falling electrolyser capital costs, and policy support in the EU, US, Middle East, and Asia-Pacific.

In India specifically, the National Green Hydrogen Mission has established production targets and incentive frameworks, driving significant project development activity — particularly in states with strong solar and wind resource availability. Indian green hydrogen projects increasingly focus on export-oriented production as well as domestic industrial decarbonisation, particularly for fertiliser and steel applications where hydrogen is a direct feedstock or reducing agent.

Common Mistakes in Green Hydrogen Process Design

• Selecting electrolyser technology before defining the power supply profile — leading to a technology mismatch between electrolyser ramp capability and actual power variability.
• Underestimating balance of plant complexity and cost — the electrolyser stack itself is often less than half of total capital cost; water treatment, compression, and storage frequently exceed electrolyser cost.
• Inadequate water availability assessment — particularly in water-stressed regions, where electrolysis water demand can be a project-limiting factor that is overlooked during early feasibility.
• Underestimating hydrogen storage and compression costs — storage system cost scales significantly with pressure and duration requirements, and is frequently underestimated in early feasibility studies.
• Not integrating safety assessment early — hydrogen’s distinct safety profile (wide flammability range, low ignition energy) requires area classification and safety system design specific to hydrogen service, not simply adapted from natural gas practice.

Frequently Asked Questions

How is green hydrogen produced?

Green hydrogen is produced by electrolysis — splitting water into hydrogen and oxygen using electrical current — where the electricity comes from renewable sources such as solar or wind power. This distinguishes it from grey hydrogen (produced from natural gas via steam reforming without carbon capture) and blue hydrogen (same process as grey, but with carbon capture applied).

What are the types of electrolysis for green hydrogen?

The three main electrolyser technologies are PEM (Proton Exchange Membrane), alkaline, and solid oxide. PEM offers fast response suited to variable renewable power and high efficiency but at higher capital cost. Alkaline is the most mature and lowest capital cost technology, best suited to stable power supply. Solid oxide offers the highest efficiency but requires high operating temperatures, typically best paired with available waste heat.

Which electrolyser technology is best for a solar-powered green hydrogen plant?

PEM electrolysers are generally preferred for solar-coupled projects because they can respond quickly to the variable and intermittent power output characteristic of solar generation, without significant efficiency loss or accelerated degradation. Alkaline systems can also be used but historically require more careful design to handle load variability well.

What is the biggest cost driver in a green hydrogen project besides the electrolyser?

Balance of plant costs — water treatment, power electronics, hydrogen compression, and storage — frequently represent more than half of total project capital cost. Hydrogen storage in particular is often underestimated in early feasibility studies due to hydrogen’s low volumetric energy density, which requires larger and higher-pressure storage systems compared to other gases.

What water quality is required for hydrogen electrolysis?

Electrolysers require high-purity deionised water, with specific conductivity and impurity limits that vary by electrolyser manufacturer and technology type (PEM systems are generally more sensitive to water quality than alkaline systems). Typical pretreatment includes reverse osmosis for bulk desalination followed by ion exchange or electrodeionisation for final polishing.