In recent years, wind power has grown from a quirky sideline to a key player in Europe's energy mix. Today's wind towers can reach 200 metres high, with rotor diameters of more than 100 metres, generating up to 7.5 megawatts of power. They are 10 times the size of those built 30 years ago, generating 100 times the power. As the towers get bigger and stronger, new challenges arise, particularly how to make the tower and blades robust enough to cope with the forces gusting their way.
© Fotolia, 2012
A European Union (EU) research project is addressing this problem by devising new ways to build the steel towers to cope with winds that can spin the tips of the rotors at race car-like speeds of up to 300 kilometres per hour. The project, High Strength Steel Tower for Wind Turbines (HISTWIN), was launched in 2006 and backed by €836,000 in EU funding from the Research Fund for Coal & Steel.
By bringing together universities, tower designers, turbine manufacturers and a certification body, HISTWIN was able to pool knowledge and resources on how to build bigger and more cost-efficient towers. "Wind towers are getting more powerful all the time, and our project looked at ways to make them strong enough to support the extra load," says HISTWIN's project coordinator Milan Veljkovic, from Sweden's LuleÃ¥ Technical University. "We have been able to create a new way to join the sections of a wind turbine tower together that is not only much stronger, but also much cheaper."
The new method dispenses with the previous round, collar-style sections with flanges, the rims or ridges on the edge that are used to attach the pieces. On these connections, the worn-out bolts had to be regularly removed and replaced in costly, tricky and even dangerous operations, particularly for off-shore wind farms.
However, the new method uses a slotted friction-grip connection with high strength bolts (TCBs) to join the parts. By exploiting the friction between the tubular walls of the connection, the only load carried by the bolts is the pre-stressing for contact pressure. No welding is required in the joint and the structure has much better fatigue resistance. "The new joint has at least double the fatigue endurance and almost no limits to the connection strength," says Veljkovic.
The newly designed towers are up to 10% cheaper to build, and the cost of materials used in the joint is around 80% less. The HISTWIN bolted connections are already being used by one leading manufacturer and as industry awareness increases, more are expected to join.
The impact of this innovation could be huge. The wind energy sector contributes around €32 billion to EU GDP, accounting for around 240,000 jobs. Veljkovic estimates that if the method if accepted by all European tower makers, it could lead to annual savings of €240 million.
The research has already led to new design recommendations that comply with both the Eurocode design rules and Germanischer Lloyd WindEnergie's Guidelines for the certification of Wind Turbines.
Although HISTWIN was initially only scheduled for three years, a three-year follow-up project, HISTWIN 2, was launched in 2010 with over €720,000 of EU funding to look at how best to lay foundations and transport the tower sections. Given the way things are blowing in Europe's energy market, the project could represent a massive boost for the wind sector.