As the industry scales up, turbines will get bigger, more efficient and further out to sea in deeper water, writes Tim Probert
The offshore wind sector is not just driving the revitalisation of the manufacturing sector in the UK, it is also deploying some cutting-edge technology.
The latest offshore wind farms employ turbines with a typical power capacity of 3 to 3.6 megawatts (MW) but, targeting the Crown Estate’s 33-gigawatt (GW) Round 3 programme where construction is set to start in the middle of the decade, the world’s leading turbine makers are bringing to market machines with double the capacity.
These turbines will be taller than The Gherkin building in London and have turbine-rotor diameters larger than the London Eye. They will stand in waters up to 60 metres deep and, in some cases, more than 200 kilometres from shore.
French power generation equipment manufacturer Alstom has designed its first offshore wind turbine – the 6MW Haliade 150 – with Round 3 in mind. Once serial production commences in 2014, Alstom plans to install 200 of the turbines, which are designed to operate at a depth of around 35 metres and a wind speed of 9.5 metres/second, every year in the UK and France, equivalent to 1.2GW of new capacity per annum.
Frederic Hendrick, Alstom’s vice-president of offshore wind, believes 6MW is the optimum size of turbines for Round 3. “We looked at various power ratings up to 10MW and what is required in terms of blade length, the weight of the nacelle and the implications for the turbine substructure, which can account for 30 to 40 per cent of the total cost. We found 6MW to offer the best value in terms of total cost of electricity.”
Turbines will be taller than The Gherkin and have rotor diameters larger than the London Eye
Siemens is also launching a 6MW turbine, the SWT-6.0. The German company has started building two test machines at Danish utility Dong Energy’s Gunfleet Sands array off the Essex coast. When they come on line in November, these will be the biggest turbines deployed in the UK.
Both manufacturers have opted for direct drive permanent magnet generators rather than gearboxes, which are prone to malfunction in the harsh, turbulent environment of an offshore wind farm. Henrik Stiesdal, chief technology officer of Siemens Wind, says direct drive generators also make the nacelle lighter, thus reducing stress on the substructure.
“So far, large wind turbines have always been heavier per megawatt than small ones,” he says. “The SWT-6.0 breaks this rule and has a weight per megawatt similar to many machines in the 2-3MW range.”
It is not clear that ever-larger turbines will be cost-effective, says Alstom’s Mr Hendrick. “It would be easy to make a 10MW turbine, but it would have a poor capacity factor. Our customers need simple, robust, reliable and efficient machines to minimise maintenance costs, which are a killer for offshore wind.” Alstom hopes the Haliade 150 will need to be maintained only once a year.
Larger turbines need larger foundations and at depths of 30 metres or more, low-tech, relatively low-cost steel monopiles hammered into the seabed start to become impractical. Steel jacket foundations, which resemble miniature oil rigs, will play a significant role in Round 3.
To exploit the deeper waters off the north-west coasts of Scotland and Cornwall, however, something more radical will be needed: floating turbines. The Energy Technologies Institute is looking at proposals to build a £25-million floating wind turbine demonstration project, which will see the installation of a 5-7MW prototype in waters 60 to 100 metres deep by early-2016.
Current floating designs envisage using horizontal-axis turbines that look similar to current models, but floating vertical-axis wind turbines are under active exploration, says Dr Simon Harrison, Mott MacDonald’s director of energy. “If they show promise, they are going to be the way forward for deep-water offshore wind farms.”