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Short answer: an offshore energy island is a collection and transmission hub. Subsea cables gather electricity from several offshore wind farms, electrical equipment on or near the island consolidates and converts it, and high-voltage cables carry the power to onshore substations. Those substations connect the offshore route to a national transmission network. If the hub has links to more than one country, it can also route electricity between national grids, combining an offshore wind connection with an interconnector.
What is an offshore energy island?
An energy island does not generate wind power by itself. It is a strategic electrical hub placed offshore, on an existing island, or on a purpose-built artificial island. Wind turbines remain distributed across one or more wind farms. Their electricity travels through array cables to shared offshore electrical equipment, where the output is collected and prepared for transmission to shore.
The word island therefore describes the hub’s physical or planned location, not a particular machine or technology. Bornholm is an existing natural island whose electrical facilities are planned to serve nearby wind farms. Princess Elisabeth Island is a planned artificial island 45 kilometres off Belgium, intended to connect new offshore wind farms and interconnectors to the onshore transmission system. The European Investment Bank describes Princess Elisabeth as a first-of-a-kind artificial-island electricity connection hub, so its planned role should not be mistaken for completed or operating infrastructure (European Investment Bank project description).
How does an offshore wind farm connect to the national grid?
The physical chain has five main stages. Exact equipment layouts differ by project, but the sequence below shows how electricity gets from turbines to consumers.
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Wind turbines produce electricity
Turbines convert the wind’s kinetic energy into electrical power. A wind farm contains many turbines spread over a large sea area, so the first task is to gather their separate outputs.
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Array cables collect turbine output
Subsea array connections run between turbines and an offshore substation or hub. The cables progressively combine the output of individual machines into larger circuits. This collection network is different from the export system: array cables serve the wind farm, while export cables carry aggregated power toward land.
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A hub or offshore substation aggregates the power
Switchgear, transformers, protection systems and control equipment manage the collected electricity. A hub can bring together several wind-farm circuits, reducing the number of separate landfall routes that would otherwise be needed. On an artificial island, these systems are installed on a purpose-built platform or reclaimed structure; on a natural island, they can be installed on land or in associated offshore facilities.
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Converters prepare power for long-distance transmission
In the Bornholm design, the collected electricity is converted to high-voltage direct current (HVDC). The project describes 525 kilovolt HVDC sea and land cable systems. Converter equipment changes the electrical form at the sending end and changes it back where the cable meets the onshore transmission system. The 525 kV value is a Bornholm project specification, not a universal voltage for every energy island.
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Export cables reach onshore substations
Subsea cables come ashore and continue through land cables to a high-voltage substation. That substation is the interface with the national grid: it controls switching and protection, transforms voltage as required by the transmission network, and hands power to the country’s existing lines. From there, electricity follows the normal transmission and distribution system to demand centres.
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Why a hub can connect more than one country
A conventional interconnector is a transmission link between two electricity systems. An offshore hybrid asset combines that cross-border function with an offshore wind connection. National Grid’s explanation of the North Sea concept describes offshore hybrid assets that connect wind-farm clusters with multiple countries, an approach associated with a more meshed offshore grid (National Grid explainer).
In practical terms, a multi-country hub has several possible routes. When wind output is high in the connected area, the hub can send electricity through one or more export links toward available demand. At other times, an interconnector may transfer electricity between national systems even when the local wind farms are producing less. Actual dispatch depends on grid conditions, market rules, operating agreements and the control systems approved for the project; the hub is not an automatic guarantee that every connected country receives power at all times.
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Bornholm: a natural-island hybrid connection
Bornholm shows how an existing island can function as an offshore collection and cross-border transmission point. The project is being developed by Danish transmission operator Energinet and German transmission operator 50Hertz. Its project description says electricity from wind farms off Bornholm will be centralised at the hub, converted to HVDC and transported through 525 kV HVDC sea and land cables.
The cables are planned to reach onshore substations in Zealand, Denmark, and Mecklenburg-Western Pomerania, Germany. Those two landfalls give the wind-farm connection access to both national transmission systems rather than a single national landing point. The European Commission reported on 26 January 2026 that Denmark and Germany agreed to develop the project to connect 3 GW of offshore wind to the two grids. That figure is the project capacity cited in that announcement, not a general capacity for energy islands (European Commission announcement).
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Bornholm’s official project wording is unusually clear about the sequence: electricity generated by wind farms off the island is centralised at the hub, converted into HVDC, and transported via 525 kV sea and land cable systems to substations in Zealand and Mecklenburg-Western Pomerania (Bornholm Energy Island). This is a project design, not a standard blueprint that all future hubs must copy.
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Princess Elisabeth Island: an artificial offshore hub
Princess Elisabeth Island represents the purpose-built version of the concept. The planned artificial island is located 45 km off the Belgian coast and is intended to connect new offshore wind farms and interconnectors to Belgium’s onshore transmission grid. The European Investment Bank’s project page presents it as an electricity hub for the planned offshore wind zone and future cross-border links (EIB project page).
Its engineering challenge is different from Bornholm’s. A purpose-built island must provide foundations, buildings, cable approaches, maintenance access, protection against the marine environment and connections for equipment before it can perform its electrical role. The cited project description establishes the planned connection function and location; it does not establish that the island is already operating.
What happens inside an onshore substation?
The onshore substation is the boundary between the offshore project and the national transmission operator’s network. It performs several jobs that are easy to overlook:
- Voltage transformation: transformers match the incoming transmission circuit to the voltage used by the national grid’s connected lines.
- Switching and isolation: breakers and disconnectors allow operators to take a cable, converter or line out of service safely.
- Protection: relays detect faults and isolate only the affected section where possible, limiting disruption.
- Measurement and control: sensors and control systems report power flows and help operators coordinate the offshore link with the wider grid.
- Grid interface: the substation hands electricity into the country’s transmission network, where it can move onward to regional substations and distribution systems.
For a two-country project, each national landing station has its own grid-code requirements, permits and operator responsibilities. The offshore hub and control system must coordinate those requirements without treating the two national networks as a single electrical system.
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HVDC in energy-island projects
High-voltage direct current is the named transmission method in the Bornholm project. In an HVDC link, converter stations change the electricity’s form for transmission and convert it again at the receiving grid. The cable route can include a subsea section and a land section, as Bornholm’s 525 kV design does.
It is important not to overgeneralise from one project. The available project material does not provide a complete comparison of alternating-current and direct-current costs, losses or distance thresholds, so there is no defensible single rule saying that every energy island must use HVDC. The choice depends on the project’s generation layout, cable distances, landing points, network requirements, converter design, permitting and cost model. Treat 525 kV as Bornholm’s stated design figure only.
Artificial island or natural island?
| Question | Natural-island hub | Artificial-island hub |
|---|---|---|
| Physical example | Bornholm, an existing Danish island | Princess Elisabeth, planned 45 km offshore from Belgium |
| Where equipment can be installed | Planned onshore facilities and associated cable connections | Purpose-built offshore structures designed for electrical equipment and cable landings |
| Connection role | Collects nearby wind power and routes it to Danish and German grids | Planned to connect offshore wind farms, interconnectors and Belgium’s onshore grid |
| Project maturity stated by cited source | In development; construction timing is a forecast | Planned first-of-a-kind artificial-island hub |
The distinction affects construction, access, environmental assessment and cable routing, but neither form changes the basic electrical chain: wind-farm collection, hub equipment, export cables and onshore grid substations.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare offshore hub proposals
Headline capacity alone can be misleading. Compare projects using the same phase and measurement, then examine the infrastructure behind the number.
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- Island type: existing natural island, artificial island, offshore platform or another arrangement.
- Generation scope: which wind farms are included and whether the stated capacity covers a complete build-out or one phase.
- Grid destinations: one national landing route or multiple national substations.
- Cable and converter design: export voltage, HVDC or another configuration, sea and land sections, and the number of circuits.
- Interconnector function: whether the link is intended only to bring wind ashore or also to transfer electricity between countries.
- Project maturity: political agreement, permitting, detailed design, procurement, construction and commissioning are different milestones.
- Network reinforcement: whether additional onshore lines or substations are required before the national grid can accept the power.
Do not compare a project’s eventual ambition with another project’s currently contracted phase. Capacity, route length and completion dates need the same definition and status before they can support a fair comparison.
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Current status and schedule cautions
Energy-island plans can change as costs, permits and grid needs are reassessed. The Danish Energy Agency’s North Sea energy-island page warns that its displayed information does not reflect the latest political decision to postpone the tender and analyse more cost-effective options. Its older procurement schedule should therefore not be presented as a current delivery timetable (Danish Energy Agency project page).
A Bornholm project update says construction was expected to begin in 2027 after detailed design and planning. That is a forecast in the update, not a guaranteed start or commissioning date (Bornholm construction contract update). The cited material does not establish one authoritative completion schedule for both Bornholm and Princess Elisabeth. For a current date, check updates from Elia, Energinet and 50Hertz rather than relying on an older announcement.
Common misunderstandings
“The island is a giant battery.”
No. The core role described for these projects is collection, conversion and transmission. Storage may be considered alongside an energy island, but the hub itself should not be described as storing electricity unless a specific storage installation has been confirmed.
“Every energy island is artificial.”
No. Bornholm is a natural island. The term covers a function and project concept, not one construction method.
“One voltage applies everywhere.”
No. Bornholm’s project material specifies 525 kV HVDC. Other projects may select different voltages or cable arrangements after engineering and grid studies.
“A cross-border hub guarantees simultaneous supply to every country.”
No. Multiple connections create routing options. Actual flows depend on wind production, available cable capacity, grid security, market rules, outages and operator decisions.
Further technical reading
Readers who want an engineering-level treatment can consult Wiley’s 528-page hardcover reference HVDC Grids: For Offshore and Supergrid of the Future, which covers offshore wind-farm integration and HVDC systems (Wiley publisher listing). It is specialist further reading rather than a consumer guide; availability should be checked with the publisher or booksellers.
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