TenneT Proposes Central Island Hub for North Sea Electricity Interconnection

Dutch power grid operator TenneT on June 10 unveiled plans for a large-scale island transmission hub in the North Sea that could connect numerous offshore wind farms and transmit their generated power to the Netherlands, the UK, Belgium, Norway, Germany, and Denmark via direct current (DC) cables. Those cables, or “spokes” could also serve as “international electricity highways” that would connect the countries’ energy markets, TenneT said.

The “hub and spoke” plan is based on an artificial island with a modular structure, with each module covering about six square kilometers—large enough to connect roughly 30 GW of offshore wind capacity, TenneT said (Figure 4). The island will also be expandable by adding one or two modules.

north sea

4. An island transmission hub. Dutch power grid operator TenneT’s “hub and spoke” proposal unveiled this June foresees the installation of a modular island, about six square kilometers in size—which can be expanded—that will connect countries on the North Sea via a number of “international electricity highways.” The direct current cable interconnections between those countries’ energy markets would also connect numerous offshore wind farms. Courtesy: TenneT

TenneT, which has developed and currently operates Germany’s and the Netherland’s offshore grids, says the vision will enable relevant cooperation needed to achieve Europe’s environmental targets. A June 6 political declaration on energy cooperation between the North Sea countries recognized that the success of the energy transition depends largely on the extent of a coordinated joint effort in Europe, noted TenneT CEO Mel Kroon.

The vision is “realistic and achievable,” Kroon added, though he noted the countries will have to agree to share their offshore power. “It will be very important for the six European North Sea countries to be willing, in due course, to make their targets independent of national borders, which means agreeing that the electrons generated offshore must not necessarily be transmitted to their own country,” he said.

The island would have to be built in a centrally located area with a lot of wind and relatively shallow waters. Dogger Bank is an ideal site, Kroon said.

One reason the hub is necessary is that Europe will soon be forced to look far out to sea to develop enough offshore wind power to reach its goals. “The disadvantage here is that the costs will be significantly higher,” TenneT said. “The construction and maintenance of the wind farms are higher and these must be connected via many relatively expensive, single direct current connections.” Alternating current technology cannot be used for connecting distant offshore wind farms because of an unacceptably high loss of electricity during transmission to the onshore grid, it said.

Building the island surrounded by wind farms closer to land would enable wind energy obtained further out to assume the cost benefits of near-shore wind. “The smaller distance will allow use of the far cheaper alternating current (AC) connections,” it said. The AC power from the wind turbines would be converted to DC on the island for transmission to shore. Thus, the multiple converter stations and long-distance DC lines that are currently used for offshore wind farms would not be necessary.

“Further considerable [cost] benefits can be derived from an island, as it offers a permanent place for people and resources,” it added. The island, for example, could house joint storage of components such as turbines, rotor blades, pylons, and transmission equipment, as well as provide a permanent base for builders of wind farms and infrastructure. Using the concept as an international connection would also boost utilization to 100% from the current 40% for a typical connection between a wind farm and the mainland.

TenneT said it will enter talks with the EU and its member states to see whether the required European cooperation can be set up, anticipating that an island could perhaps be built on Dogger Bank between 2030 and 2050. It is not clear how Brexit (the United Kingdom’s vote on June 23 to exit the EU) will affect these plans, though the operator is keen to continue a joint examination of connecting the Dutch offshore areas of IJmuiden Ver to a UK wind energy area, possibly East Anglia, by 2030.

Sonal Patel, associate editor