But opinions varied as to how soon a North Sea and Baltic Sea offshore transmission network is likely to become reality, what design it should adopt - and at what cost.
At national level, within the next ten years, only simple radial connections - which link individual wind farms directly to the mainland network - make sense, said Karsten Burges, manager for power systems and markets at renewables consultant Ecofys in Germany. "I am quite convinced it is wrong to make an offshore grid a pre-condition for further development of offshore wind power," he added.
A bigger challenge will be transmitting electricity generated from offshore wind farms deep into the onshore transmission network, Burges said. He pointed out that a key question for Germany at present is whether the government will press ahead with proposals for 10 GW of new nuclear power stations: "This affects the viability of offshore wind much more than the existence of an offshore network in the North Sea."
Thomas Trotscher of Norwegian research organisation Sintef Energy Research said that an interlinked - or meshed - grid would give more economic value to Europe than radial connections. Sintef has studied scenarios for a North Sea grid costing between EUR6.5 and EUR13.5 billion, he said. This could lead to savings of around EUR2 billion - some 25% of the investment cost - compared with radial connections.
He agreed with Burges that, over the next ten years, most connections will be radial. "But it is important to build those in such a way that later on you can hook up to them and build a meshed grid," Trotscher said. As developers go further offshore, a meshed solution will be cheaper, he said. And the cables would have a higher utilisation rate than radial wind farm connections, working out at around a 70% rate of use rather than 45%.
But there are many challenges to overcome. These include how to share the costs and benefits of a meshed network between the different national grid operators. Also, countries around the North Sea all have different levels of support for wind power and different legislation for permitting lines, different grid codes and different rules for system operation, he said. Trotscher concluded that some of these issues could be overcome by setting up a joint North Sea transmission system operator (TSO) and by harmonising support systems for wind energy.
Based on 20-40 GW of offshore wind, Europe's offshore grid infrastructure could cost a minimum of EUR10 billion by 2020, said Chris Veal, managing director of Transmission Capital, a potential investor in the UK grid. The British government and regulator estimate that the UK offshore transmission assets market will be worth EUR15-20 billion if its ambitions for over 33 GW of offshore wind are to be realised.
Joe Corbett, head of asset engineering at Mainstream Renewable Power, insisted that transmission charges for wind connected to a meshed offshore grid network are not expensive for wind farm operators. A study by Mainstream has calculated the charges to be EUR35 per megawatt hour, he reported. This is based on 21 GW of offshore wind connected to the network in stages, as well as a European "super regulator" and "super system operator", he explained. "Now we have a number we can hang our hat on," Corbett said. "And it will reduce when other players are given access to the network."
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