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Multi-purpose interconnector to maximise offshore wind efficiency
The newly-proposed interconnector could prove the start of a wider offshore transmission grid

Across northern Europe countries are looking to take advantage of the huge, renewable energy resource that blows across the North Sea and surrounding maritime area – offshore wind. The UK has adopted a target capacity of 40 GW by 2030, four times the amount installed at the end of 2019, while the Netherlands aims to have 11.5 GW by 2030 and a further 20-40 GW by 2050.

To achieve these ambitious targets wind farms will have to be built further from shore, and while offshore wind costs are on a clear downward trajectory, building further out to sea can raise average costs, in part because the transmission lines to bring power to land need to be longer.

Spare capacity

A wind farm needs a transmission line with capacity equal to maximum output, but wind conditions are not always optimal. Other factors, such as maintenance, also affect a wind farm’s output. As a result, there is spare capacity on the transmission line.

The average capacity factor of wind farms has increased by a third over the last decade and the newest offshore wind farms have a capacity factor of nearly 50%, according to the UK Crown Estate’s Offshore Wind Operational Report 2019. The other side of the coin, however, is that 50% on average of the transmission line’s capacity is still unused.

To address the issue, the UK and Dutch transmission system operators (TSOs), National Grid ESO and Tennet, have come up with an innovative idea — a multi-purpose interconnector between their two grids, which would also be connected to 4 GW of UK and Dutch offshore wind capacity. The line would provide transmission capacity for the wind farms, but also allow any spare capacity to be used for trading power between the UK and Dutch electricity markets.

Variability and flexibility

Wind is a renewable but variable resource and increased capacity, with a wider geographical spread, means there is a higher likelihood of having some wind power all the time, particularly as capacity factors rise.

In its Offshore Wind Outlook 2019, the International Energy Agency went so far as to label offshore wind a ‘baseload’ generation source. However, there is still substantial variability and, as capacity grows, that variability creates a need for greater system flexibility so that excess wind power is not wasted and, at times of low wind generation, electricity can be delivered from other sources – in this case from a different national grid. System flexibility plays a critical role in providing a secure and reliable flow of power.

There are many ways of providing flexibility and new forms are emerging as the energy transition progresses, such as battery storage and the production of green hydrogen as a means to store excess renewable energy. However, interconnectors are a tried and tested method. Power can be traded with no transformation and little transmission loss between different markets, with different generation profiles, helping to balance supply and demand in both.

The UK currently has four interconnectors, linking its electricity system to France, Ireland and the Netherlands, with a total capacity of 4 GW. The new project would provide an additional 2 GW of interconnector capacity and allow the efficient use of what would otherwise be spare transmission line capacity.

The Anglo-Dutch project would be centred on the Dutch side around the IJmuiden Ver Wind Farm Zone, about 40 nautical miles off the country’s north west coast – a far shore development. The IJmuiden Wind Farm Zone could potentially host 4 GW of capacity itself. Sites I and II are expected to be tendered in 2023 and sites III and IV in 2025. The two TSOs are exploring the potential for their multi-purpose interconnector and hope to have a concept defined by the end of 2021, with the aim of delivering an operational asset by 2029.

Offshore transmission grid?

However, the concept may be the start of something much bigger.

The growth of offshore wind will mean larger electricity surpluses at times and multi-purpose interconnectors would provide the infrastructure for a wider offshore transmission grid, which would use offshore wind energy with maximum efficiency. This, in turn, would reduce the cost of the total infrastructure spend and thus provide lower prices for consumers.

An offshore transmission grid would also work well with other innovative offshore ideas, such as the ‘energy island’ concepts being explored by countries like Denmark and the Netherlands.

It could also fit well with wider EU plans for the development of the European electricity grid, which are presented by ENTSO-E – the European body for TSOs – every two years in its ten-year development plan (TYNDP).  

The geographical distribution of renewable energy sources is uneven, with a massive offshore wind resource in northern Europe, huge solar potential in southern Europe and the majority of Europe’s hydroelectric power in the Alps. Each renewable energy source has different generation characteristics and the differences in their variability tend to complement each other, providing a more resilient and reliable power system — if they can be connected together.

A North Sea offshore transmission grid could feed into the long-distance transmission lines planned under the TYNDP, allowing the efficient use of different renewable energy sources across Europe.

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