Offshore power is entering a new era with the development of ‘energy islands’ designed and built to produce and distribute green energy for power-hungry European networks.

Northern European economies have fostered grid interconnections allowing cross-border excess energy sales. But many states still lack the grid infrastructure to transmit energy even internally.

In 2025, the UK, Germany, France and the Netherlands together curtailed over 13.9 Terawatt-hours (TWh) of renewable energy – enough to power Slovenia for a year.

This reality is driving the new notion of the offshore energy islands, capable of supporting onshore turbines and solar, as well as experimental wave and tidal.

Princess Elisabeth Island, the first energy island, is under construction 45km off the coast of Belgium and is due to be completed in 2028.

The six-hectare island is being constructed from 23, 32m-high concrete armoured caissons, poured layer by layer over three months. These structures are then towed into place by tug, sunk, and filled with sand.

The pouring of the final concrete caisson completed at Vlissingen shipyard in January. Once a perimeter is established, dredgers will fill the structure with sand and pour concrete over the top to create the surface.

The island will feature a flood wall to protect its infrastructure, and several structural features along its sides, designed to encourage marine growth and support habitats of local seabirds, fish and mussels.

“Rather than disrupting local marine environments, energy islands are designed with nature-inclusive features,” explained Tjerk Suurenbroek, sector manager offshore, Maritime & Offshore NL. “They are constructed using massive concrete structures and stone materials that actively foster biodiversity, providing safe havens for marine life and birds to flourish.”

Operating infrastructure this far offshore will also require substantial fleet support, with year-round servicing in demanding North Sea conditions.

Supplying the islands will necessitate the use of low-draught service operation vessels (SOVs), capable of accessing the shallow artificial harbour, but robust enough to maintain station in rough seas.

Princess Elisabeth is being equipped with a helipad and is designed to be mostly uncrewed. However, fast-crew transfer vessels will be needed to move technicians, inspectors and maintenance crews to the island quickly in the event of power failures.

Designers are increasingly focused on hull forms and motion-compensation systems suited to the short, aggressive wave patterns of the southern North Sea, where weather windows for maintenance are often narrow.

Market conditions are forcing mitigations to the project with transmission system operator Elia Transmission Belgium dropping the high-voltage direct current (HVDC) transformer element, and the Belgian federal government dropping the UK Nautilus connection, as well as one of the three Princess Elisabeth Zone wind farms.

"The international tender that Elia has set up for these direct current components shows an overheated supply chain with significant price increases,” the group stated in February 2025. “Although the terms from the supplier involved are comparable to those of other European grid operators, they are – despite our efforts – much higher than our initial estimates."

Two more projects have been mooted, this time in and off Denmark. One is an ambitious project involving the construction of a new artificial island some 100km off the West Coast of Denmark.

A great deal of wind resource lies at these further reaches, where there is nothing to block the sun, wind and tides. But as evidenced by Princess Elisabeth’s significant cost overruns, the main challenge lies in connecting with the mainland, and Denmark’s North Sea energy island has faced major delays.

Most recently, the project was put on hold for three years from 2024, “with an estimated completion date… pushed to 2036 or later.”

The other, “Energiø Bornholm”, has met with more success. It involves the transformation of the existing island of Bornholm, in the Baltic Sea, known to some as ‘Europe’s Greenest Island’.

Bornholm has long been powered by onshore wind turbines and incorporates other green measures. New HVDC infrastructure is being built on the south of the island, near the town of Aakirkeby.

As with Princess Elisabeth Island, cost estimates for the Bornholm project have spiralled to DKK61 billion (US$9.5 billion) – around double the original cost by February this year – of which Germany has agreed to pay 70%.

Cheaper batteries are gradually altering the economics of energy, with ‘dispatchable solar’ – panels combined with batteries to make grid energy available round-the-clock – as low as US$76/MWh in some economies, according to Ember.

But physical and material constraints underlie the market, ensuring that large-scale battery storage will remain costly in the medium term.