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Making a mesh of grid connections for offshore wind

Offshore grids, connecting North and Baltic Sea wind farms to electricity consumers, will be substantially cheaper to build than expected, according to the latest industry study

Finding a cost-effective solution for offshore grid connection is vital
With hundreds of gigawatts of offshore wind farms planned, finding a cost-effective solution for grid connection and integration to the onshore network is vital

Offshore wind could generate more than 500TWh of electricity a year by 2030, enough to meet 15 per cent of Europe’s annual consumption. But to do so, “a reliable, modernised and efficient grid, both onshore and offshore” is required, says Geert Palmers, chief executive officer of global renewable energy consultancy 3E.

“Onshore, this means significant investments to strengthen current infrastructure, which faces strong public opposition and lengthy project lead times,” he says. “Offshore, the challenge is to more efficiently connect power harvested at sea with the onshore transmission system, while, at the same time, building a system which can actively contribute to stability and security of supply by enabling further integration of the European power market.”

So “a coherent European long-term vision for both the onshore and offshore electricity grid is a prerequisite to make the required steps in an optimal way,” he says.

The OffshoreGrid project is co-financed by the European Commission under the EU's Intelligent Energy Europe programme. As well as 3E, project participants included the German Energy Agency (dena), the European Wind Energy Association (EWEA), the Polish Institute for Renewable Energy (IEO). The renewable energy sources unit of the National Technical University of Athens, the Norwegian Energy Research Institute (SINTEF), the German Centre for Wind Energy Research (ForWind) and the UK based global energy services Group Senergy Econnect, are all participants.

Hub connections

On the offshore side, the technical solution deemed most viable by the OffshoreGrid partners is a “meshed grid” consisting of linked “hub connections”. This will involve building hub connections at sea for 114 of the 321 offshore projects planned. The idea is by effectively connecting up projects close to each other, they then share a single transmission line to shore, instead of using cables to connect single offshore wind farms individually to the shore.

It will result in investment costs to 2030 of EUR 69bn. But this is EUR 14bn lower than the EUR 83bn it is estimated to cost to connect them individually. This is according to the findings of the OffshoreGrid project, published in its October final report, OffshoreGrid: Offshore Electricity Infrastructure in Europe.

Hub connections generally become economically viable for distances above 50km from shore, when the sum of installed capacity in a small area (20 km around the hub) is relatively large, and standard available HVDC voltage source converter (VSC) systems can be used, the report explains. Wind farms situated closer than 50km to an onshore connection point are virtually always connected individually to shore.

“Apart from the costs savings, offshore hubs can also help to mitigate the environmental and social impact of laying multiple cables through sensitive coastal areas and allow for more efficient logistics during installations,” the report adds.

Moreover, if these hub connections were combined with an even more interconnected “meshed grid”, the necessary additional costs of EUR 5bn to 7bn would be compensated of billions worth of additional benefits over 25 years of grid operation. The cost for creating the complete meshed offshore grid, including wind farm connections, would amount to about EUR 0.1 per KWh consumed in the EU 27 over the project lifetime.

Two design options

The OffshoreGrid project final report outlines two potential cost-efficient grid designs - one dubbed the direct design and the other a split design - to connect North and Baltic Sea wind farms to electricity consumers. Although it says a new regulatory framework is needed to enable its construction.

In the direct design, interconnectors are built to promote unconstrained trade between countries and electricity markets as average price difference levels are high. Once additional direct interconnectors become non-beneficial, tee-in, hub-to-hub and meshed grid concepts are added to arrive at an overall grid design.

Tee-in connections are when a wind farm or hub is connected to a pre-existing or planned transmission line, or interconnector between countries, rather than directly to shore. Hub-to-hub connections involve the interconnection of several wind farm hubs to form transmission corridors between various countries. The wind farm hubs belonging to different countries are connected to shore, but are also connected to each other.

The split design is essentially designing an offshore grid around the planned offshore wind farms. Not only are direct interconnectors investigated, but also interconnections are built by splitting the connection of some of the larger offshore wind farms between countries.

These split wind farm connections establish a path for constrained trade. As in the direct design, these offshore wind farm nodes are then further interconnected to establish an overall meshed design where beneficial.

The overall investment costs are EUR 86bn for the direct design and EUR 84bn for the split design. This includes EUR 69bn of investment costs for the most efficient connection (hub-connections where beneficial) of the 126GW of offshore wind farms to shore. Also, around EUR 9bn for interconnectors planned within the ten year network development plan of the European transmission system operator association.

The rest of the investments that make up the EUR 84bn or EUR 86bn for this further interconnected grid are EUR 7.4bn for the direct design and EUR 5.4bn for the split design. These relatively small additional investments generate system benefits of EUR 21bn (direct design) and EUR 16bn (split design) over a lifetime of 25 years. These are benefits of about three times the investment, says the report.

With the forecast offshore wind energy produced over 25 years amounting to 13,300TWh, translating into a market value of EUR 421bn when assuming an average spot market price of EUR 50/MWh, “the infrastructure costs represent about a fifth of the value of the electricity that is generated offshore”, adds the report.

Which connection concept is best depends on several factors. This includes the distribution of the offshore wind farms, such as whether there is more than one farm planned in the vicinity, and the wind farms’ distance to shore. In the case of interconnecting several wind farms and/or countries, the distance of the farms to each other and the electricity trade between the countries is also a factor.

But “as a concrete roadmap for further grid developments, the OffshoreGrid report sends a credible and urgent call for action to our policymakers and regulators,” says 3E’s Palmer.

“It should help trigger and steer timely investments, and deepen the international exchange and collaboration that are necessary for an efficient Europe-wide grid.”

Stephan Kohler, chief executive officer of German energy agency dena adds that the compatibility of grid connection policies and support mechanisms has to be pursued with “high priority”.