Glossary

The terms and abbreviations of offshore hydrogen, briefly explained.

Additionality: The EU requirement that electricity for renewable hydrogen comes from newly built installations rather than existing, already supported ones. Together with temporal and geographic correlation it determines whether hydrogen qualifies as an RFNBO.
Alkaline electrolysis: The oldest industrially used electrolysis process, with potassium hydroxide solution as the electrolyte. It is robust and comparatively inexpensive, but follows load changes more slowly than PEM electrolysis.
Ammonia: A compound of hydrogen and nitrogen that is easier to liquefy and ship over long distances than hydrogen itself. It is discussed both as a transport form and as an industrial feedstock.
AquaCampus: A research area of 3 square kilometres a few nautical miles off Helgoland, marked by eight yellow buoys, in water depths of up to 45 metres. Research institutions and companies test equipment above and below the waterline here.
AquaDuctus: The planned hydrogen pipeline from the German North Sea to shore, around 400 kilometres at sea and 100 kilometres on land. It is the only pipeline project foreseen for the German North Sea within the hydrogen core network; the operator is Gascade Gastransport GmbH.
AquaNavis: The AquaVentus project on decarbonising shipping. Around 90 per cent of world trade travels by sea; the project develops and tests hydrogen-based propulsion and supply concepts for ships.
AquaPrimus: The demonstrator for offshore electrolysis, with an electrolyser of 1 to 5 megawatts. It is the first to test the entire chain of hydrogen production at sea, from seawater treatment through to feed-in.
BSH: The Federal Maritime and Hydrographic Agency. It draws up the site development plan for German marine areas and runs permitting procedures for offshore projects.
Capital and operating expenditure: Capital expenditure arises once for construction, operating expenditure continuously for running and maintenance, abbreviated CAPEX and OPEX. For offshore installations the share of operating expenditure is high, because every maintenance visit means a trip out to sea.
Carbon contracts for difference: Support contracts that compensate a company for the gap between the cost of low-carbon production and the market price over many years. They are intended to enable investments that would not yet pay off on their own.
Centrally pre-examined site: A site that the BSH surveys itself before the tender, for example for wind conditions and seabed properties. Bidders receive the findings and need not repeat the work; for sites that are not centrally pre-examined, this falls to the bidder.
Combined connection concept: Connecting an offshore wind farm by two routes at once: power cable and hydrogen pipeline. Depending on electricity prices and demand, the farm can feed in power or produce hydrogen. Anchoring such concepts in law is AquaVentus' central regulatory request.
Compression: Raising the pressure of the hydrogen produced to the level the pipeline requires. The compressor is part of the offshore platform and itself consumes some of the energy produced.
Converter platform: A platform at sea that converts a wind farm's electricity for transport to shore, classically from alternating to direct current. With offshore hydrogen, a platform carrying an electrolyser takes its place or joins it.
Curtailment: Shutting down a generating installation even though wind or sun are available, because the grid cannot absorb the output. Electrolysis can capture that energy as hydrogen instead.
Do-tank: How AquaVentus describes itself in contrast to a think-tank: the point is not analysis alone but projects that get built. The work happens in working groups whose results feed into formal responses and ventures.
Efficiency: The share of the energy put in that is recovered in the hydrogen produced. It has a direct bearing on the cost of hydrogen, because electricity is the largest cost component of electrolysis.
Electrolyser: The technical plant in which electrolysis takes place. In offshore applications it sits on a platform at sea and additionally includes seawater treatment, compression and storage.
Electrolysis: A process that splits water into hydrogen and oxygen using electric current. The plant that does this is called an electrolyser; its size is given in megawatts or gigawatts of electrical capacity.
Energy island: An artificially built or installed island that collects and redistributes the electricity of several wind farms. Studies examine it as an alternative or complement to individual platforms, including for electrolysis.
European Hydrogen Backbone: An initiative of European transmission system operators for a cross-border hydrogen network. Via the backbone, AquaDuctus is to be connected to the Netherlands, Belgium, Denmark, the United Kingdom and Norway.
European Hydrogen Bank: An instrument of the European Commission that supports renewable hydrogen through auctions with a fixed premium per kilogram. Alongside national programmes it is the central European funding source for production projects.
Exclusive Economic Zone (EEZ): The sea area beyond the territorial sea in which a state holds economic rights, for instance for wind farms or pipelines. Germany's offshore hydrogen sites in the North Sea lie within the EEZ.
Federal Network Agency: Germany's regulator for electricity, gas, telecommunications, post and rail. In the hydrogen field it approves the hydrogen core network, among other duties.
Formal response: A written reply to a draft law or plan within an official consultation. AquaVentus uses them to bring the requirements of the offshore hydrogen sector into procedures such as the site development plan.
Full-load hours: The notional number of hours per year an installation would run at full capacity if its annual output were expressed that way. Offshore wind farms in the North Sea reach considerably more than installations on land, which makes electrolysis there more economical.
Gigawatt (GW): A unit of power, one thousand megawatts. The AquaVentus target of 10 gigawatts refers to offshore capacity in the North Sea, yielding around 1 million tonnes of green hydrogen a year.
Green hydrogen: Hydrogen produced by electrolysis of water using electricity from renewable sources. Using it releases no carbon dioxide. Other colour labels denote other production routes, for example grey hydrogen from natural gas.
Grid development plan: The transmission system operators' plan for which power lines must be built in the coming years; the Federal Network Agency confirms it. For offshore hydrogen it matters whether pipeline connections are considered on equal footing with cables.
Guarantee of origin: A certificate documenting which electricity was used to produce an energy carrier and under what conditions. For hydrogen it determines whether it can be sold as renewable and counted towards climate targets.
H2Global: A state-backed instrument that procures hydrogen through a double auction: bought at a high price from producers, sold on more cheaply to buyers, with the state covering the difference. It is meant to kick-start the market while supply and demand do not yet meet on price.
Hydrogen core network: Germany's starter network for hydrogen transport, approved by the Federal Network Agency, with a total length of 9,040 kilometres. The AquaDuctus pipeline has been part of this network since October 2024.
IPCEI: Important Project of Common European Interest. An EU procedure that allows member states to jointly support significant projects with state funding. AquaDuctus is IPCEI-notified.
Levelized cost of hydrogen (LCOH): The total cost of production over the lifetime of a plant, expressed per kilogram. The figure makes projects comparable because it combines investment, operation and the price of electricity.
Market ramp-up: The phase in which a new energy carrier grows from individual ventures into a functioning market with supply, demand and infrastructure. For hydrogen it depends on production, transport and offtake coming together in time.
National Hydrogen Strategy: The German government's strategy document for the production, import and use of hydrogen. Its 2023 update names at least 10 gigawatts of electrolysis capacity in Germany by 2030.
Offshore electrolysis: Electrolysis carried out at sea, on a platform close to the wind farm. The hydrogen produced is transported to shore by pipeline instead of sending the wind power through a subsea cable.
Offtake agreement: A long-term contract under which a buyer purchases fixed volumes on fixed terms. Without such agreements, banks generally do not finance production facilities.
Overplanting: Connecting more wind capacity to a grid connection than it can nominally absorb. Because wind farms rarely run at full output, this raises utilisation of the line; the rarer peaks are curtailed or converted into hydrogen.
Oxygen: The second product of electrolysis, which arises in significant volumes and today mostly goes unused. For offshore installations it is being examined whether it can be put to use, for instance in industry or to oxygenate water bodies.
PCI: Project of Common Interest. An EU status for cross-border energy infrastructure that enables faster permitting and access to EU funding. AquaDuctus holds this status.
Peak shaving: Deliberately curtailing generation peaks so that a smaller grid connection suffices. Instead of discarding the peaks, they can be converted into hydrogen.
PEM electrolysis: Electrolysis using a proton exchange membrane. It responds quickly to fluctuating power input and therefore suits wind electricity well, but requires precious metals as catalysts.
Power purchase agreement (PPA): A direct supply contract between an electricity producer and a buyer covering volume, period and price. For electrolysis it is the usual way to secure renewable electricity long term at a predictable price.
Reconversion to electricity: Turning stored hydrogen back into electricity, in a power plant or a fuel cell. It serves to bridge periods without wind or sun, but loses part of the energy in the process.
RED III: The third version of the EU Renewable Energy Directive. It sets binding quotas for renewable hydrogen in industry and transport and thereby creates the demand that production projects depend on.
Registered association (e.V.): The legal form in which AquaVentus is organised: a registered association with its seat on Helgoland and its office in Berlin. Its members are companies, research institutions and organisations; the association itself builds nothing but pools the work of its members.
RFNBO: Renewable Fuel of Non-Biological Origin. The term covers hydrogen and derived substances that meet the EU criteria for renewable production and may therefore be counted towards climate targets.
Salt cavern storage: An artificial cavity in an underground salt formation in which large volumes of gas are stored. For hydrogen, caverns are currently the most promising way to balance seasonal fluctuations.
Seawater treatment: The desalination and purification of seawater before it enters the electrolyser. Electrolysis needs very pure water, so at sea the treatment stage is part of every installation.
Sector coupling: The joint planning and linking of energy sectors, here primarily electricity and hydrogen. Offshore, sector coupling means connecting a wind farm both via a power cable and via a hydrogen pipeline.
SEN-1: The site in the German North Sea designated in the site development plan for the first large-scale production of hydrogen at sea. It is planned for 1,000 megawatts of electrolysis capacity, with the hydrogen carried away through AquaDuctus.
Site development plan (FEP): A planning instrument of the Federal Maritime and Hydrographic Agency that defines which areas of the German North and Baltic Sea are tendered when and for which use. Among other things, the plan designates the SEN-1 area for offshore hydrogen.
System costs: The costs of the overall system of generation, grids and storage rather than those of a single installation. Studies for AquaVentus work with them because the advantage of offshore sector coupling only appears at system level.
Temporal and geographic correlation: EU requirements that the renewable electricity used must have been generated in the same period and in the same region as the hydrogen. Offshore electrolysis directly at the wind farm meets both conditions by its nature.
WindSeeG: The German Offshore Wind Energy Act governs site allocation, tenders and grid connection for offshore wind in Germany. Whether and how offshore hydrogen appears in it determines whether offshore electrolysis is feasible.
Working group: A group of members focused on a key topic, at AquaVentus on technology, regulation, communications and international affairs. Joint positions and the groundwork for formal responses take shape here.