Wind power may be one of the cleaner, greener energy sources available, but turbine and blade failures point to dangers that were not anticipated, says Michael Connellan
David Campbell and his family were asleep in their farmhouse in Northern Ireland when the 16-foot blade from the wind turbine crashed through the roof of his home one windy night in January last year. "It was like a bomb hitting the roof," he told the Belfast Telegraph. "It shattered the tiles and the blade disintegrated itself."
Campbell was not the only person to see the direct effects of a turbine failure. Just over a year later, in February, a 200ft Vestas wind turbine near the Danish city of Århus disintegrated spectacularly in high winds when a blade came loose and smashed into the central tower, causing the whole structure to collapse. The incident was captured on video camera and footage has been viewed thousands of times on YouTube.
Just two days later a turbine close to the town of Sidinge, in Denmark, sent a blade flying more than 300ft before it hit the ground. Keld Boye, a farmer whose land is near the structure, told Danish television: "I drive my tractor and my wife rides horses out there. Just think if we'd been out there when it happened."
After Denmark's climate minister demanded an investigation, authorities found the blade loss was caused by a lack of maintenance: regular checks on bolts in the turbine had been neglected. The Danish government announced mandatory service checks for every one of the 5,000 wind turbines in the country.
Turbines in Britain - there are 2,000, almost all of which are onshore - aren't immune from failure. A 200ft turbine at a wind farm in Kintyre collapsed last November in a 50mph wind. Following that, 26 wind turbines across Scotland were shut down as a precautionary measure while the broken structure was examined. Then the following month in Cumbria, a 100ft steel turbine crashed to the ground.
Six months later, in June, as concerns over the incidents last year were receding, a blade cracked away from a 190ft turbine on a Sheffield University research park. Police evacuated the area while engineers allowed the 30ft blade to fall to the ground. The turbine was made by WES, a Dutch company. Earlier that month, the British government had published its renewable energy strategy - with plans to build around 4,000 new onshore wind turbines.
The turbines that failed in Cumbria and Scotland were built by Vestas of Denmark, the industry's leading name, which has installed 35,000 turbines globally. The Health and Safety Executive (HSE) is expected to shortly publish the results of its investigation into the collapses. Vestas said that the collapses were isolated incidents that did not appear to reflect any inherent construction problems.
But there is no clear safety standard for the strength of turbines, or their blades. We asked the HSE what legal requirements it has for turbines' strength, or their proximity to homes. It suggested we ask the Department for Business, Enterprise and Regulatory Reform ... which said it was probably an HSE issue.
So what's the risk of turbine failure? Mike Graham, professor of aerodynamics at Imperial College, says a modern turbine typically spins at 25rpm, which translates to a few million times a year. "Turbines have to face a lot of force," Graham says. "They are equivalent to the lift forces faced by aircraft in takeoff, and some blades are of comparable size now to the wing of a Boeing 747."
Unlike aircraft, however, wind turbines operate in "the lower part of the Earth's atmosphere, where it is very turbulent and wind is more interrupted. Repetitive loading of forces causes cracks. But the industry is well aware of this." Engineers calculate the forces and add a safety margin, which is then built into the design.
But crucially, manufacturers do not disclose how close to tolerance they make their products, according to Professor Leon Freris, a consultant who is also a member of the board of Ascot Renewco, an insurance company which underwrites policies for turbines. "The most critical components on a wind turbine in terms of fatigue loading are the blades and the gearbox," Freris says. "The majority of blades at present are made of glass-reinforced plastic." They are made to last 20 years, but accelerated fatigue tests, and two years of testing, are standard before new designs are included in products.
Errors can be made, though. Earlier this year, Suzlon Energy had to repair or replace almost its entire stock of 1,251 turbine blades, after many developed cracks. The refit is reckoned to have cost the company $25m (£14m).
Graham says most wind turbines require gearboxes to enable the slowly turning blades to power fast-running generators: "It's one of the major problem areas. Gearboxes are big heavy things based at the top of a tower and they are prone to failure. There is a lot of unsteady loading because they operate in wind."
In Germany - which, with 20,000, has the most wind turbines of any country - Der Spiegel reported last summer on a host of incidents in the previous year. They included one 230ft structure folding in half, two turbines burning down and another two turbines losing blades. Insurers expressed concerns about construction and maintenance, culminating in a report released last year by the German Insurance Association (GIA). It claimed many insufficiently strong turbines were being produced, and that faulty gearboxes and blades were having to be replaced with alarming frequency.
The GIA says insurers in the country now see the wind industry as risky: "Wind turbines are exposed to a high risk - storm, lightning, fire and ice. Also because of the extreme strain on the materials, there are often high damages. The insurance industry demands more time for the development and tests of new constructions, and higher standards for maintenance and repairs."
Warren Diogo, an underwriter for Ascot Renewco, believes the failed turbine incidents in the UK have been "insignificant from an insurance perspective due to their remoteness and relatively benign consequences". But he adds: "Should these occurrences escalate, causing more severe damage or injury, insurers are likely to more closely scrutinise them." Stuart Young, chairman of the Caithness Windfarm Information Forum, which opposes wind farms in north-east Scotland, says: "They pose unnecessary risks to the public."
But Charles Anglin of the British Wind Energy Association says: "In any field of development there are going to be health and safety issues. If there are issues the industry will learn from them. Any piece of equipment can fail. But we have some of the best industry safety records in the world. The UK incidents were due to very particular circumstances. The turbine in Cumbria was 20 years old."
He adds: "If the implication is that there has been some reduction in standards because of an increase in numbers, that's wholly unproved and that's not something we would tolerate as an industry. Manufacturers expect extremely high standards of safety. If you look at what happened in Denmark, there was monitoring in the equipment so they were able to clear the area and it was several hours before breakage. There was no danger."
Vestas, the Danish manufacturer, stresses that its turbines comply with international standards. And standards of construction are not falling, say manufacturers, despite a backlog of orders thought to be worth more than $3bn. Alfons Benzinger of Siemens says: "We do not see this as being correct and an extremely low number of accidents worldwide support this. Safety has been and will always have the highest priority."
Meanwhile, the great UK wind energy gold rush continues. Several schools across the country have submitted plans for smaller urban wind turbines of between 30ft and 80ft tall, following calls in March by Lord Turner, in his role as the government's climate change tsar, for turbines to be built next to motorways and outside every school.
Bradford Council in West Yorkshire recently received a planning application for the construction of a 400ft turbine about 300ft from a residential area. Jo Tremere, who lives within a few hundred feet of the proposed site in the suburb of Tong, has campaigned against it. She says: "We all know that clean energy is needed. But residents around here are really worried. The turbine they are proposing will be bigger than the Statue of Liberty. These things can be dangerous even if they are away from areas of housing. But if a turbine like this collapsed or snapped a blade in a built-up area, then people could die."