In Europe, where most of the research and development in this area is carried out, the trend for longer blades is driven by both economic and geographic motives. Long supported by public subsidies, Europe's wind industry is now under growing pressure to prove that it is cost-effective and competitive with other forms of electricity generation as fiscally constrained governments cut their public-spending programmes.
However, with turbine manufacturers also cash-strapped, many have been looking at doing as much as they can to improve their existing models' performance and cost effectiveness rather than developing new ones. The easiest way to do this, says Frank Virenfeldt Nielsen,chief technology officer at component supplier LM Wind Power, is to develop longer blades. "In the last couple of years we have seen quite a move towards larger rotors on existing turbines," he adds.
"In the wake of the financial crisis, manufacturers didn't have enough money and time to develop new turbine technology," says Virenfeldt Nielsen. "Instead, a way for a fast upgrade to make a turbine more profitable is a larger rotor. We see upgrades that produce 10-20% more energy on the same platform with not much more investment than a larger rotor."
Increased energy production
Jochen Birkemeyer, rotor blades director at German turbine maker Nordex, agrees. "The biggest target is to increase annual energy production," he says. "If you have a well-proven platform on the turbine it makes sense to increase its energy yield as much as you can by increasing its rotor diameter."
At a time when the cost of wind energy, particularly offshore, is being scrutinised - the UK government, for example, has decreed that the offshore wind sector in its waters must cut lifetime costs by 30% by 2020 - being able to increase the amount of wind power a turbine can capture and transform into electricity by 20% is a powerful selling point for those developing longer blades.
Virenfeldt Nielsen points out that the blades themselves will typically only improve performance by 3% - LM's target for each new blade. But increasingly a combination of new aerodynamics, more intelligent controls and utilising reserves in the turbine can help produce energy-yield increases of 10-20%. "Adapting the blade to compensate for any other component constraints in combination with improved controls can ensure we reach these significant improvement levels," he says. "This improvement comes because we are adapting older, more conservative designs with new improved rotor solutions. The development of new aerodynamics in combination with new control features and experience of turbine operations allows us to get more out of the turbines and reduce the cost of energy."
As well as needing to improve energy yield for cost reasons, because most of Europe's easily accessible high wind sites have already had turbines installed, developers need to look at low wind sites onshore or turn their attentions to offshore wind. Both options require longer blades - the larger the rotor diameter, the more wind a turbine can capture, delivering more generating hours in low wind sites and boosting energy yield for high wind but expensive offshore sites.
From an onshore perspective, Birkemeyer points out that the combination of Nordex's 58-metre blade for its N117 turbine (rotor diameter of 117 metres) with tall towers (141-metre hub height) means that German developers have been able to build wind farms in previously inaccessible Bavarian forests.
Outside Europe, Virenfeldt Nielsen says LM is seeing huge demand for turbines for low wind areas in China, where a delay to develop the grid to remote high wind class I areas in the west of the country has led to wind farms being built in less windy class II and class III areas closer to population centres and with better grid access. There is similar demand for low wind turbines elsewhere in Asia, Africa and Latin America.
However, the increased energy yield of longer blades comes at a price. Simply enlarging existing designs would make blades heavier, increasing the load on the turbine, making it operate less efficiently and rendering any gain made from lengthening a blade negligible. As blades become longer, they must also become lighter. This leads to longer, more slender blades, but when blades get thinner it is increasingly difficult to build in sufficient strength and stiffness to stop them bending under the load of the wind (see page 55). Such deflection can lead to blade tips hitting the turbine tower.
To prevent deflection, many blade manufacturers have switched materials from fibreglass to carbon fibre, which is much stiffer, but also more expensive.
Birkemeyer argues that the extra cost of carbon is more than covered by the improved energy yields of longer blades. Rob Sauven, Vestas' vice-president of wind capture systems, says that carbon is a cost-effective material as long as the design of the blade is integrated with the design of the turbine, because the rotor then puts lower loading on the rest of the turbine.
Henrik Stiesdal, chief technology officer of Siemens Wind Power - one of two major blade manufacturers, along with LM Wind Power, to so far resist using carbon in its blades - admits that the material is attractive for blade design. "It has a much higher thickness than fibreglass and a key design driver is the deflection on the blade," says Stiesdal. "But while carbon fibre is three times stiffer than fibreglass, it is also ten times the cost. You pay dearly for that extra thickness. We have found it more economically attractive not to use carbon. We may do in the future."
Both Siemens and LM have found alternative methods of protecting their longer blades against deflection. Siemens casts its blades in one piece rather than glue them together, which Stiesdal says creates inherently stronger blades. "When you have that advantage you are able to reduce weight to produce a given strength and give stiffness and therefore at the moment we don't need carbon," he says.
LM Wind Power pre-bends its blades forward to counteract deflection, and makes blade sections thicker than its rivals. This thickness, claims Virenfeldt Nielsen, gives them the necessary stiffness, yet they still perform as well under test conditions as thinner blades.
While length improves productivity, it also affects transport issues. When will blades become too large to transport as single pieces and need to be segmented and assembled on site?
Vestas is developing an 80-metre blade for its planned V164 offshore turbine. Transport is not an issue, says Sauven, as it is designed for the offshore market, built by the sea and transported by sea, so segmentation does not need to be considered.
For onshore, meanwhile, Birkemeyer says: "There will be a time for segmented technology." But that time has not yet come as there is no serious demand yet, he adds.Siemens is already developing its own segmented blade but is yet to put it into commercial operation, says Stiesdal. "Predictions for segmented blades have always been wrong," he says. "For our 52-metre blade we ran a segmented project but ditched it as we decided it wasn't necessary. That's in our technology portfolio, so we could produce it tomorrow if we wanted, but so far there has not been a business case that justified it - segmented blades will always be heavier and more expensive."
When onshore blades go beyond lengths of 75 metres may be the time for segmentation, says Birkemeyer. So, how long does the industry think blades can get? Sauven says that in his 25-years' experience, successive claims that the maximum length of a blade has been reached have always been proved wrong. "There's never been a technical limit, just purely the limit of what is the most cost-effective length for a blade."