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Wind firms look to supersize turbines

In a difficult investment climate for the wind sector, technological developments aimed at generating more power from turbines and reducing costs are increasingly important

THE BIG challenges for turbine manufacturers are improving reliability, cutting costs and enhancing energy output, says Walt Musial, principal engineer for ocean renewable energy at the US National Renewable Energy Laboratory in Colorado. There are a number of ways to achieve these objectives, such as improving the pitch system, which enables blades to catch the wind better, or using stronger and cheaper materials.

Increasing the size of turbines, in an effort to produce economies of scale, is another area of interest. The largest turbines in widespread use, both onshore and offshore are 2.5-3.0 megawatt (MW) machines, although 5 MW turbines – REPower's prototype of which made its debut in Germany in 2005 – are becoming more common.

Testing, testing

The New and Renewable Energy Centre (Narec), an independent UK test facility at Blyth, northeast England, is testing blades for a 7.5 MW machine being developed by California's Clipper for offshore use. A single turbine of this size could potentially meet the electricity needs of more than 5,500 homes and offset more than 32m tonnes of carbon dioxide, according to Narec, which is funded by UK and EU regional development agencies. Clipper says the turbine, which would be the world's largest to date, could be adapted to achieve a generating capacity of 10 MW.

Turbine engineers are also looking at the practicalities of building 20 MW turbines, although the engineering challenges of this remain daunting. A 5 MW turbine has fibreglass blades of 60 metres or more in length, while those of a 10 MW machine would be over 80 metres. A 20 MW machine would require blades well in excess of 100 metres.

If the technological hurdles involved in increasing blade size to these lengths can be overcome, the benefits in terms of output should be substantial, given that the power generated by turbine blades rises in proportion to the square of the increase in blade length.

However, there is a downside to increasing the length of blades, as a turbine's volume rises in proportion to the cube of its dimensions and that means bigger turbines would cost much more to build than their smaller counterparts, if made of the same materials, as well as being considerably heavier and less manageable. Much effort is being directed towards the development of blades made from lighter materials. Strength is also an issue, as 80-metre plus blades under considerable pressure need to be rigid enough not to flex into the supporting towers. The towers themselves are also costly, given the large amount of steel involved in their construction

Larger turbines, if the technology can be mastered, are likely to be of most use at sea. Transporting 100 metre-long blades and even longer mountings would be less of a challenge on the open sea than it would be on land, where obstacles such as bridges, and bends in roads could hinder movement. Also, the installation cost per unit of power would be lower, given that the footprint of a 10 MW turbine would be not be much bigger than that of a 3 MW device.

That is particularly important if wind power is to be exploited in deeper offshore waters, where installation costs rise considerably. In waters shallower than around 15 metres, a monopile column can be driven into the seabed relatively easily and cheaply and a turbine placed on top of it. However, in deeper water, technology more akin to that used for transporting and installing deep-water oil and gas production platforms is required.

"Because the equipment used to install turbines offshore is so expensive, it makes sense to use it to its capacity. The issue is more a question of how much the vessel costs, rather than how much the foundation costs, so once the vessel is out there it might be more economic to put in the biggest foundation you can," says Musial.

Floating solutions

But in deeper water, dropping expensive foundations into the seabed may not prove to be the most cost-effective option. Floating platforms tethered to the seabed, similar to those used for deep-water oil rigs, might provide a solution, but the costs of locating dozens of turbines – rather than just one oil platform – on either floating or fixed mountings could stack up, especially given variables such as sea conditions and weather, which can slow down installation and maintenance operations.

These problems were reflected in the slow pace at which two prototype deep-water 5 MW turbines were erected in 40 metres of water on the Beatrice oilfield in the UK North Sea in 2006/07. The REPower-manufactured turbines were installed by the disused Beatrice Alpha oil platform at a cost of £45m ($65m) by Scottish and Southern Energy and Canadian oil firm Talisman. But the job took around a year to complete, partly because of delays caused by bad weather.

Engineers say the process will improve as the sector gains more experience of deep-water installations, but with wind power poised to play a significant role in the global push towards greater renewable energy use, the hunt is on for cheaper, faster ways to install offshore wind turbines. In one effort to find a solution, the Carbon Trust, a UK government body, and a group of companies launched an initiative last year to stimulate new technologies that would cut cost the of offshore wind power by at least 10%. The Offshore Wind Accelerator award is worth up to £30m over the next five years. 

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