Hydrogen Glossary

European Commission rules setting out the criteria for qualifying hydrogen as an RFNBO (Renewable Fuel of Non-Biological Origin) under the RED II and RED III Directives. They establish three mandatory requirements: additionality of the renewable installation, temporal correlation and geographical correlation. They set the greenhouse gas (GHG) emissions reduction threshold at 70% compared with the fossil fuel comparator (94 g CO₂eq/MJ). Compliance is essential to access European public funding and for the hydrogen to count towards mandatory industry and transport targets.

Regulatory criterion under the RFNBO Delegated Act. It requires the renewable electricity generation plant supplying the electrolyser to be new (commissioned within the 36 months preceding the commissioning of the electrolyser) or not to receive production support. The aim is to prevent hydrogen production from diverting existing renewable energy that the grid needs for wider decarbonisation. Compliance is verified through power purchase agreements (PPAs) and the Guarantees of Origin registry.

Electrolysis technology based on the electrochemical splitting of water using a liquid alkaline electrolyte, typically an aqueous potassium hydroxide solution (25–30% KOH). It operates at temperatures of 60–90°C and current densities of 0.2–0.8 A/cm², with energy consumption of 50–70 kWh/kg H₂. It is the most mature technology at industrial scale. Its main advantages are the lower stack cost compared with PEM systems, the absence of precious metals in the electrodes and its long service life.

European Regulation (EU) 2023/1804 establishing mandatory requirements for the deployment of alternative fuels infrastructure, including hydrogen refuelling stations, across the Trans-European Transport Network (TEN-T). It sets minimum targets for the density of refuelling points on the core network by 2030. As a regulation, it is directly applicable in all Member States and requires the adoption of national policy frameworks with binding targets. It replaces Directive 2014/94/EU (DAFI).

Ammonia produced through the Haber-Bosch process using atmospheric nitrogen and renewable hydrogen instead of conventional grey hydrogen. Its production generates no CO₂ emissions. It is used as a marine fuel, a feedstock for carbon-free fertilisers and as a hydrogen carrier for long-distance international transport (1 kg of NH₃ contains 0.176 kg of H₂).

Spanish regulatory framework governed by Royal Decree 656/2017, which establishes the safety requirements for facilities used for the storage, loading, unloading and handling of hazardous chemicals, including compressed and liquid hydrogen. It sets out safety distances, ventilation requirements, detection systems and periodic inspections. Compliance is mandatory for the legalisation of any hydrogen installation in Spain. ARIEMA provides engineering and consultancy services for the processing of APQ projects. 

European Directives (Directive 2014/34/EU for equipment and Directive 1999/92/EC for the protection of workers) governing environments where there is a risk of explosive atmospheres. In hydrogen installations, areas are classified into zones according to the likelihood of gas being present: Zone 0 (continuously), Zone 1 (occasionally) and Zone 2 (infrequently or for short periods). This classification determines the type of mandatory technical certification required for electrical and mechanical equipment. 

The set of auxiliary systems required for the proper operation of an electrolyser or fuel cell. It includes water purification and circulation, gas separation, hydrogen purification and drying systems, cooling, power electronics (rectifiers), and control and safety systems. The BoP can account for between 40% and 60% of the total capital expenditure (CAPEX) of an electrolysis plant. 

The condition achieved by an investment project when it demonstrates a sufficiently low level of technical, financial and legal risk to attract bank financing on favourable market terms. In renewable hydrogen projects, key bankability factors include long-term sales agreements with creditworthy buyers (off-takers), secured electricity supply through competitive PPAs and technical performance guarantees from the electrolyser manufacturer. ARIEMA advises project developers on technical and regulatory optimisation to achieve bankability.

Port infrastructure dedicated to the storage and supply of hydrogen or green ammonia as fuel for ships (bunkering). The development of this infrastructure at major maritime logistics hubs is one of the strategic objectives set out in the European AFIR Regulation for the 2030 horizon.

The initial capital investment required to design, purchase, construct and commission a hydrogen production facility. It includes the cost of the electrolyser (stack and BoP), detailed engineering, civil works, electrical installation, and compression and storage systems. Reducing CAPEX through economies of scale is a priority for lowering the final cost of hydrogen. 

Voluntary European hydrogen origin certification scheme developed by an industry consortium with the support of the European Commission. It defines methodologies for assessing the carbon intensity of hydrogen and issues tradable Guarantees of Origin (GoOs). Although the EU Delegated Acts establish the mandatory legal framework for RFNBOs, CertifHy remains a reference for voluntary markets and international transactions. 

Public-private body of the European Commission responsible for co-funding research, development and innovation (R&D&I) projects in hydrogen and fuel cell technologies under the Horizon Europe programme. As the successor to the former FCH JU, it prioritises technological maturation from laboratory stages through to industrial demonstrators (TRL 4 to 8). ARIEMA has a well-established track record of participating in projects funded by this body. 

A process required to increase the energy density of gaseous hydrogen and facilitate its storage or transport by road (tube trailers) or pipeline. The main commercial technologies are piston, diaphragm (ideal for high-purity applications) and ionic compressors. For mobility applications, standard dispensing pressures are 350 bar for heavy-duty vehicles and 700 bar for passenger cars. The process consumes between 2 and 5 kWh per kilogram of H₂, a factor that directly impacts operating costs.

Criterion under the RFNBO Delegated Act requiring the electrolyser and the associated renewable generation plant to be located in the same bidding zone or in adjacent zones where electricity prices demonstrate that there is no congestion in the transmission grid. It prevents hydrogen production from exacerbating technical bottlenecks in the electricity grid. 

Criterion under the RFNBO Delegated Act requiring the electrolyser’s electricity consumption to coincide in time with the electricity generated by the associated renewable energy plant. European regulations allow monthly matching until December 2029; from January 2030 onwards, correlation will become strictly hourly. This affects the electrolyser’s annual operating hours and requires advanced energy management systems. 

Operating parameter that measures the electric current applied per unit of active surface area of the electrolyser electrode or membrane (A/cm²). A higher current density increases hydrogen production within a smaller space, reducing the size of the stack and its CAPEX. However, it increases losses due to internal resistance and reduces the overall energy efficiency of the equipment. Alkaline systems typically operate at 0.2–0.5 A/cm², while PEM systems exceed 1–2 A/cm².

Progressive loss of the electrochemical performance of electrolyser cells due to operational use (corrosion, wear of coatings or loss of membrane properties). It is usually measured by the increase in voltage required to maintain constant gas production (mV/1,000 hours). It determines the stack’s service life (between 60,000 and 100,000 hours in industrial alkaline systems) and requires its replacement or refurbishment to be planned within operating expenditure (OPEX). 

Direct reduction process for iron ore using green hydrogen as the reducing agent instead of syngas or coal. The chemical reaction produces sponge iron and water vapour as the only by-product, eliminating CO₂ emissions from the traditional steelmaking process. The resulting iron is subsequently melted in electric arc furnaces to produce green steel. It is the main technological pathway for decarbonising the heavy steel industry.

Renewable synthetic aviation fuel produced from renewable hydrogen and captured carbon dioxide (from biogenic sources or direct air capture). It is a drop-in fuel, meaning that it has properties identical to those of fossil kerosene and can be used in existing engines and logistics infrastructure without requiring modifications to aircraft. The European ReFuelEU Aviation Regulation imposes mandatory minimum quotas for the use of this fuel on flights operated within the European Union.

The ratio between the energy content of the hydrogen produced and the electrical energy consumed by the complete system (stack and BoP). It is commonly expressed in kWh/kg H₂ or as a percentage. Current industrial systems consume between 50 and 70 kWh/kg H₂, representing an efficiency of 47% to 67% when calculated on the basis of the Lower Heating Value (LHV) of hydrogen. Optimising this parameter is the most important factor in reducing the cost of hydrogen production, after the price of electricity.

European technical panel specialising in hydrogen safety, established by the Clean Hydrogen Partnership. Its mission is to identify regulatory gaps, draft best-practice guidelines, assess risks and compile data on incidents in European projects. ARIEMA serves as the Technical Secretariat of the EHSP, contributing its expertise to the oversight of hydrogen safety across the European sector. 

Technological system that splits water molecules (H2O) into hydrogen (H2) and oxygen (O2) gases by applying direct current electricity. It consists of an electrochemical unit (stack), mechanical and electrical auxiliaries (BoP), and an automated control system. Electrolysers are classified according to their technical principle as alkaline (AEL), polymer electrolyte membrane (PEM), or solid oxide (SOEC). ARIEMA Enerxía manufactures alkaline electrolysers in Spain under the AMEly programme. 

Electrolysis technology that uses a proton exchange membrane based on solid polymers as the electrolyte. It operates at high current densities, offers a very fast dynamic response to renewable energy intermittency and enables the direct production of high-purity hydrogen under pressure. The catalyst layers applied directly to the membrane require platinum-group metals (iridium at the anode and platinum at the cathode), which affects the cost of the stack compared with traditional alkaline technology. 

The ratio between the energy or volume of hydrogen produced by an electrolyser over a given period and the amount it would have produced if operating continuously at its rated capacity. It is expressed as a percentage. In installations isolated from the grid (off-grid), the capacity factor is limited by the available hours of solar or wind generation (20–40%). In grid-connected installations with diversified PPA contracts, it can be increased substantially, allowing the system’s CAPEX to be recovered more quickly.

European Union legislative package designed to achieve the binding target of reducing net greenhouse gas emissions by at least 55% by 2030 compared with 1990 levels. It includes the reform of the Emissions Trading System (ETS), the revision of the Renewable Energy Directive (RED III), and the introduction of specific mandates to promote the uptake of RFNBOs in refineries, chemical plants, steelworks and heavy-duty transport.

Public body responsible for managing Spain’s subsidised vocational training system for companies. It enables companies to use their Social Security contributions to fund technical training for their employees. ARIEMA offers specialised training in hydrogen technologies (basic, advanced and specialised courses) that is eligible for funding through this system, directly managing the application process for its corporate clients.

Pipeline infrastructure designed for the large-scale, long-distance transport of gaseous hydrogen. It requires the use of specific steels and welds resistant to hydrogen embrittlement (a phenomenon whereby hydrogen atoms penetrate the metal structure, reducing its ductility). The transport of pure hydrogen by pipeline is regulated at European level by the Internal Markets for Renewable Gas, Natural Gas and Hydrogen Regulation, facilitating the development of backbone networks such as the H2Med corridor.

Electronic certificate confirming that a specific amount of energy (usually 1 MWh) has been produced from renewable sources. In the case of hydrogen, GoOs serve as a commercial traceability tool to demonstrate to regulators and customers compliance with the strict sustainability criteria required by European Union regulations. In Spain, the CNMC is the body responsible for managing this registry. 

Priority European energy infrastructure project designated as a Project of Common Interest (PCI). It will connect the hydrogen transport networks of the Iberian Peninsula with France and Germany, including a subsea section between Barcelona and Marseille (BarMar). Its aim is to transport renewable hydrogen produced in south-western Europe to the major industrial consumption centres of Central Europe from 2030 onwards.

Hydrogen produced through steam methane reforming (SMR) or autothermal reforming (ATR) of natural gas, incorporating carbon capture and geological storage (CCS) systems. These systems can capture between 85% and 95% of the CO₂ generated during the process. Although it significantly reduces the carbon footprint compared with grey hydrogen, it is not legally classified as an RFNBO under the European framework and is instead categorised as a low-emission fuel for the transition phase. 

Hydrogen produced from fossil fuels, primarily natural gas through steam methane reforming (SMR), without the use of emissions capture technologies. It is currently the predominant production method in the chemical and refining industries. It generates a carbon footprint of between 10 and 12 kg of CO₂ for every kilogram of H₂ produced, representing the volume of hydrogen consumption that European regulations require to be replaced as a priority with renewable hydrogen. 

Hydrogen produced through water electrolysis using electricity generated by nuclear power plants. As it does not use renewable primary energy sources according to the criteria of the RED III Directive, it does not count towards RFNBO fuel quotas. However, it has a very low life-cycle carbon footprint and provides a stable supply as it does not depend on weather variability.

Commercial term for hydrogen produced through electrolysis using exclusively electricity from renewable sources. Under European Union legislation, it officially falls within the RFNBO category. It is a clean energy carrier whose combustion or use in fuel cells produces only water vapour, making it a key technology for decarbonising industrial thermal processes and heavy-duty transport. 

Solid-state hydrogen storage technology. It uses metal alloys capable of reversibly absorbing hydrogen atoms into their crystalline structure at moderate or low pressures and at room temperature, subsequently releasing them through the application of heat. It offers high volumetric density and excellent safety characteristics, making it particularly suitable for long-term stationary energy storage applications. 

International collaboration programme of the International Energy Agency (IEA) focused on coordinating research, technological development and market policy analysis relating to hydrogen. It brings together experts from government ministries, research centres and global companies. ARIEMA performs Technical Secretariat functions within this global programme, enabling it to maintain direct access to the latest international industry data and trends.

European Union State aid and funding mechanism that allows national governments to grant public subsidies to highly strategic and innovative cross-border industrial projects involving risks beyond those that can be managed by the private market. Successive waves of hydrogen IPCEIs (Hy2Tech, Hy2Use, etc.) are funding the deployment of the first economies of scale in electrolyser manufacturing and transport infrastructure.

Benchmark economic metric used to calculate the total cost of producing one kilogram of hydrogen over the useful operating life of a facility. It incorporates CAPEX (initial investment), OPEX (fixed operating, maintenance and water costs, as well as the variable cost of electricity), the stack degradation rate and the cost of capital (WACC). The price of renewable electricity is the determining factor, typically accounting for around 80% of the operating cost of green hydrogen.

Liquid organic compounds capable of storing and releasing hydrogen reversibly through chemical hydrogenation (charging) and dehydrogenation (discharging, which requires an external heat supply) processes. They enable the use of existing logistics infrastructure, including road tankers, storage tanks and pipelines designed for liquid hydrocarbons, facilitating the safe long-distance transport of hydrogen at atmospheric pressure and ambient temperature.

Central component of the electrochemical unit of a PEM electrolysis cell or fuel cell. It integrates into a single structure the proton exchange membrane, the catalyst layers where the chemical reactions take place, and the gas diffusion layers (GDL). It is the most critical component in terms of performance, durability and cost within stacks based on polymer membrane technology. 

Liquid synthetic fuel produced by combining renewable hydrogen with carbon dioxide captured from biogenic sources or directly from the atmosphere. It is emerging as one of the most commercially viable alternatives for replacing fossil fuels in long-distance maritime transport (regulated by FuelEU Maritime) and as a key feedstock for the sustainable chemical industry.

US safety code recognised internationally as a reference standard, establishing fire protection, system design and operational safety requirements for the production, storage, piping and use of hydrogen. ARIEMA incorporates the criteria and safety distances established by the NFPA into its advanced technical training programmes to support projects with an international scope or developed under Anglo-American investment standards.

Recurring costs arising from the day-to-day operation and maintenance of a hydrogen production plant. These include renewable electricity consumption, the supply of demineralised water, insurance, plant personnel and scheduled maintenance (including financial provision for the future replacement of the stack due to degradation). The cost of the electricity consumed is by far the largest component of OPEX in any electrolysis project. 

Configuration in which an electrolyser is connected directly to a dedicated renewable generation plant (solar photovoltaic or wind) without being connected to the public electricity transmission or distribution grid. This architecture inherently ensures compliance with regulatory additionality criteria, although it limits the electrolyser’s operating hours to the local weather profile, affecting the project’s design and LCOH.

European Directive 2014/68/EU applicable to the design, manufacture and conformity assessment of equipment subject to an internal pressure greater than 0.5 bar. In hydrogen plants, it applies to gas storage vessels, interconnecting pipework, safety valves and compressors. All these components must bear the CE marking after successfully completing the relevant tests carried out by the notified body. 

Fuel cell that uses a polymer proton exchange membrane to carry out the reverse process of electrolysis: it combines hydrogen with oxygen from the air to generate direct-current electricity, heat and water as the only by-product. Due to its excellent power density, compact size and rapid start-up capability, it is the leading technology for zero-emission mobility in heavy-duty vehicles, buses and trains. 

Electrochemical device that converts the chemical energy contained in a fuel (primarily hydrogen) directly into electrical and thermal energy through a controlled reaction with an oxidising agent (oxygen), without undergoing a thermal combustion cycle. This eliminates the efficiency limitations of the Carnot cycle and avoids local pollutant emissions. Fuel cells are classified according to their technology into PEMFC, SOFC (high-temperature) and alkaline (AFC) systems.

Long-term bilateral agreement signed between a renewable energy developer and a consumer (the electrolyser operator) for the supply of electricity at an agreed price. Structured PPAs are the standardised financial instrument used to secure competitive pricing, energy volumes and compliance with the regulatory additionality and hourly correlation requirements established by the European Union.

Administrative mechanism created by the Spanish Government to channel European recovery funds (NextGenerationEU) towards strategic industrial sectors. The PERTE ERHA (Renewable Energy, Renewable Hydrogen and Storage) is the main national public incentive mechanism for funding technology development projects, production clusters and hydrogen valleys through to 2030.

European Renewable Energy Directive. Its third revision (RED III – Directive (EU) 2023/2413) raises the European Union’s overall clean energy targets for 2030 and introduces specific, binding targets for the consumption of renewable hydrogen (RFNBO) in industry (42% of the hydrogen used must be renewable by 2030) and in the transport sector, thereby driving the mandatory development of the green hydrogen market. 

European Commission strategic plan designed to accelerate the energy transition and eliminate Europe’s dependence on imported fossil fuels. This plan doubled the EU’s initial ambitions for green hydrogen, setting a target for 2030 of 20 million tonnes of renewable hydrogen (10 million tonnes produced within the Union and a further 10 million tonnes through guaranteed international imports). 

Legal category defined in European directives covering liquid and gaseous fuels whose energy content comes from renewable sources other than biomass. Renewable hydrogen and its synthetic derivatives (e-methanol, e-SAF, green ammonia) are considered RFNBOs provided that they can demonstrate, through documentary evidence, a 70% reduction in GHG emissions over their life cycle by complying with the additionality and correlation criteria established by the Delegated Acts. 

Mandatory technical regulations in Spain (Royal Decree 2267/2004) defining the fire prevention and protection requirements that industrial facilities must meet. They classify establishments according to their building configuration and assess their intrinsic risk level based on the fire load of the materials stored (such as hydrogen), determining the mandatory fire suppression systems, compartmentation measures and structural fire resistance requirements. 

A set of engineering practices, regulations and technological solutions aimed at mitigating and controlling the specific risks associated with hydrogen arising from its unique physicochemical properties: a wide flammability range in air (4–75%), low ignition energy, an invisible flame in the visible spectrum, high diffusivity and a tendency to cause embrittlement in common steels. Strict compliance with safety distances and industry-specific certifications ensures that industrial hydrogen plants operate at risk levels equivalent to or lower than those associated with conventional hydrocarbons. 

Industrial chemical process in which natural gas (methane) reacts with steam at high temperatures (700–900°C) in the presence of a catalyst to produce a mixture of hydrogen and carbon monoxide (syngas). It is the most widely used conventional method worldwide for producing low-cost industrial hydrogen. It generates high direct CO₂ emissions (grey hydrogen), unless costly carbon capture systems are incorporated (blue hydrogen). 

High-temperature electrolysis technology that uses a solid ceramic electrolyte made of stabilised zirconium oxide. It operates at temperatures ranging from 600 to 900°C. Its main competitive advantage is its low electrical energy consumption (35–40 kWh/kg H₂), as part of the energy required to split the water molecule is supplied in the form of high-temperature industrial waste heat, significantly reducing electricity-related operating costs. It also enables the co-electrolysis of water and CO₂ to produce syngas directly. 

The central and essential component of an electrolyser or fuel cell. It consists of the physical arrangement in series of multiple individual electrochemical cells in order to achieve the electrical voltage and gas production levels required for the scale of the project. In ARIEMA’s alkaline electrolysis stack, each cell contains the electrodes and separation diaphragm immersed in the liquid electrolyte, forming the core of the company’s patented technology. 

Financial analysis methodology that calculates the total cost of acquiring, operating, maintaining and decommissioning a technological asset or vehicle fleet over its entire useful life cycle. In the field of transport and heavy-duty freight mobility or urban buses, TCO studies show that hydrogen fuel cell technology can be competitive with battery-electric solutions on long-distance routes or in applications with high daily utilisation, where lengthy electric charging times and battery weight could reduce vehicle productivity.

International scale consisting of nine levels, used to describe the maturity of a technology in a standardised manner, from basic laboratory principles (TRL 1) through to full commercial validation in real-world industrial operating environments (TRL 9). Commercial alkaline and PEM electrolysers are firmly positioned at TRL 8–9, while experimental developments and new components fall within intermediate ranges (TRL 4–7). ARIEMA development programmes encompass the technological maturation of advanced systems across this range of the scale.

A semi-trailer truck specially equipped with an array of high-strength steel cylinders or composite material cylinders (carbon fibre), designed for the road transport of compressed gaseous hydrogen at high pressures (from 200 to 500 bar). It is the standard logistics solution used for the commercial distribution of hydrogen from centralised production plants to small- and medium-scale end-use points, such as hydrogen refuelling stations or local industrial facilities.

A range of industrial and mobility applications in which green hydrogen directly replaces fossil fuels or conventional grey hydrogen. Key applications include its use as a chemical feedstock (fertiliser production and fuel refining), direct reduction of iron ore (green steel), long-distance heavy-duty road transport, rail transport on non-electrified lines, maritime transport (via ammonia or e-methanol), aviation (via e-SAF), and large-scale seasonal energy storage. See: DRI-H₂, e-SAF, PEMFC.

An integrated geographical and industrial ecosystem that brings together the production, storage, logistics distribution and end use of renewable hydrogen by multiple companies from different sectors within the same area. This physical proximity optimises investment in shared infrastructure, generates economies of scale and minimises gas transport and distribution costs. The development of these industrial hubs is a priority co-funded by the European Union and channelled in Spain through PERTE funding.

A financial rate that calculates the average cost of a project's financial resources, weighting the return required by shareholders (equity) and the interest cost of bank debt. As green hydrogen projects are capital-intensive (high CAPEX), a low WACC is crucial to reducing the levelised cost of hydrogen (LCOH). This can be achieved by mitigating project risks through secured purchase agreements (off-take) and technologies from manufacturers that provide comprehensive guarantees.