Peter Hope’s job is to break the blades of wind turbines. Standing in a huge shed on the coast of north-east England, he describes how he uses a set of heavy-duty winches to bend them until they snap with a loud bang. He tests the designs to breaking point to ensure they can withstand the wildest gales. Once he tugged the tip of a 42.5-metre blade 11 metres off its axis - bending it by 15 degrees - before it failed. "We've broken every blade we've tested," he says
Hope is head of one of the world's most advanced turbine blade testing facilities, run by the UK New and Renewable Energy Centre (NaREC) in Blyth, near Newcastle. Next year he is looking forward to starting tests on what will be the longest blade yet made - a 75-metre monster being developed by the California wind power company Clipper, designed to generate 10 megawatts (MW) of electricity.
After that, the blades are likely to get even bigger. The European Union is funding research into 20-MW machines, which could have 130-metre blades. In theory blades could be larger still but economic factors and the practical problems of construction and installation will come into play long before that limit is reached.
The time and money being spent on wind power is perhaps not surprising when you consider that, based on global average annual wind speeds, worldwide there is the potential to generate 106 million gigawatt-hours of electricity per year from wind - five times the total amount of electricity generated globally today. Recent estimates put the cost of generating electricity from wind at €0.04 to 0.08 per kilowatt-hour, comparable with nuclear power, and electricity from gas turbines with natural gas at today's prices.
The dramatic increase in the length of turbine blades - mostly for offshore wind farms - mirrors the rapid expansion of wind farms worldwide. Between 2006 and 2007 global capacity leapt by over 25 per cent to 94 gigawatts (GW) - equivalent to around 90 average-sized coal-fired power stations - that's a ninefold increase on the 10.2 GW generated from wind just 10 years ago. By comparison, total global electricty generation from all sources increased by just 30 per cent.
All indications are that this booming growth in wind energy capacity will continue. The Global Wind Energy Council (GWEC), based in Brussels, Belgium, predicts that the global wind market will grow by over 15 per cent from its current size to reach 240 GW of total installed capacity by the year 2012. By then, wind energy will be producing more than half a million gigawatt-hours of electricity a year, pushing its share of the global total from 1 per cent in 2007 to 3 per cent. How has wind gone from being a quaint afterthought to such a significant contributor to global electricity generation in such a short space of time?
The biggest influence on wind generation has been the huge investment in a few key European countries that are rich in wind resource and driven by ambitious targets to cut CO2 emissions. Germany leads the world, with 19,460 turbines in 2007 capable of generating 22.25 GW, which can supply 7 per cent of the country's electricity. To encourage the shift to renewables, all suppliers of electricity from renewable sources are paid a premium "feed-in" tariff for the first five years they supply power to the grid.
But Germany may soon be toppled from its wind power throne. The GWEC predicts that in 2009 the US will overtake it and become the world's biggest producer of wind-powered electricity. China, which has succeeded in doubling its capacity every year since 2004, is not far behind. The Chinese Renewable Energy Industry Association forecasts it will reach around 50 GW by 2015.
A leading US expert on wind power, Walt Musial from the National Renewable Energy Laboratory in Golden, Colorado, compares the state of wind technology today to that of the car industry in 1940. There are many ways to improve the technology, he says, which could make turbines even more efficient, more reliable and more powerful.
But he warns that these improvements come at a cost. For example, using carbon fibre to make lighter turbine blades would mean turbine makers having to compete with aircraft makers for the raw material. Making the blades longer will make torque on the drivetrains among the largest of any piece of rotating equipment ever constructed, putting immense strain on the materials used.
Clipper's answer to the torque problem has been to develop a gearing system it claims reduces the load from the blades to one-quarter that of an equivalent conventional turbine.
To identify and overcome potential pitfalls for the next generation of giant 10-megawatt turbines, in 2006 the European Union launched a five-year €22.3 million research project involving over 40 partners from 14 countries. Known as UpWind, it has already come up with some useful ideas.
A team led by Martin Kühn, from the University of Stuttgart in Germany, has suggested attaching big turbines to the seabed via tripods at depths of between 35 and 50 metres. For water deeper than 50 metres, they suggest floating turbines on platforms anchored to the seabed.
Another group, led by aerospace engineer Harald Bersee from the Delft University of Technology in the Netherlands, has been investigating "smart blades". Inspired by research into helicopter rotors, he envisages sensors controlling a series of wing flaps along the trailing edges of turbine blades that would unfold in light winds to expand the blade's surface area and so improve its performance.
UpWind researchers are also studying the possibility of a 20-MW turbine. They have concluded that although technological barriers could be overcome, it is doubtful whether such large machines are economically viable. This is because of what wind engineers call the "square cube law".
A turbine's power output is proportional to the square of the length of its blades, making it attractive to lengthen them. But its volume and weight are proportional to the cube of its dimensions, meaning the price of a turbine climbs faster than its power output as its size increases. This suggests there will be an optimum size for a wind turbine, though so far no one has calculated what that will be.
There are those who believe size isn't everything. Peter Jamieson from the wind consultants Garrad Hassan in Glasgow, UK, says the advantage of scaling up has "rarely been demonstrated". Instead, he suggests it would be more economic to build lots of small cheap, lower-power turbines. There is also growing interest in high-altitude wind power in which kites attached to winches tap the higher wind speeds available up there.
But Jamieson is in a minority. For now, when it comes to turbines, bigger is broadly accepted as better. Back in Blyth's big shed that is certainly the way Peter Hope expects things to go. He's clearly looking forward to bending 100-metre blades until they break. "They'll make a big bang," he says.