Wind power has been marketed for many years as a solution to sustainable power generation. But NIMBY issues have meant many wind farms are limited in size and capacity. Now, large scale deepwater floating wind farms are being touted as the perfect solution. By Jeremy Torr

The appeal of offshore, sea-based wind farms is undeniable. Wind predictability and strength are usually better than on land; there is relatively little “not in my back yard” (NIMBY) opposition; and the access for maintenance and installation is easier than on many exposed hill or clifftop sites. But there has, to date, been just one fly in the ointment for offshore wind farms. They need relatively shallow water in order to install the retaining structures that tie them to the sea bed.

However, researchers are making a significant step forward with the introduction of large-scale, floating, wind farms. These use non-rigid, tethered pylons located by steel hawsers but using integral flotation chambers to maintain the pylons and rotors in the airstream. The benefit this approach brings is that a huge swathe of deeper water, even further offshore than presently accessible, is now quite practical for large installations.

Hywind Blowing

This year, Norwegian energy company Statoil is pioneering the world’s first commercial-scale floating wind farm called Hywind. It is situated in deep water off the Scottish town of Peterhead in the North Sea. The new farm will be made up of five turbines anchored between 95 and 130 metres depth in the Buchan Deep area. The farm, which will cover some 4 square kilometres in total, will put out up to 30 MW of power to be cable-linked to the mainland. Statoil says the output will be enough to reliably power up to 22,000 households some 25 kilometres away on the mainland.

“Our objective with the Hywind … park is to demonstrate the feasibility of future commercial, utility-scale floating wind farms,” said Leif Delp, Hywind project director. “This will further increase the global market potential for offshore wind energy, contributing to realising our ambition of profitable growth in renewable energy and other low-carbon solutions.”

European Power

In Europe, offshore wind has already been widely accepted as a power source, with some 10GW installed capacity to date – but only on shallow-water fixed sites so far. However, Statoil says the use of floating structures can boost that significantly by allowing siting of farms in more remote waters rather than the less common 20–50 metre depth needed for fixed installations. The Hywind project managers claim this can be expanded to a global capacity of around 100GW by 2030.

Following a small scale trial in 2009, Statoil has worked on a specially designed pitch motion sensor and controller system which utilises software and stabilising mechanisms to stop the pitching and bobbing that can make turbines less efficient on open water. This approach helps eliminate the loss of output due to both wind and water-induced movements and also maximises the power output from the blades.

Each 250m high, 12,000 tonne turbine, although freely floating, uses three mooring lines attached firmly to anchors on the seabed. The floatation element is a massive 80m deep floating tube which uses large weights to keep the towers stable and high up into the airstream. Statoil says its blade control software - which adjusts the pitch of the three 75metre, 25tonne blades in real time – helps damp and compensate movement of the installation caused by variations in wind, waves and currents. It claims the new technology will make the turbines as efficient as or even more effective than current technologies.

Opportunity Time

In the US, the University of Maine (UMaine) in association with Maine Aqua Ventus LLC, recently piloted a concrete-based floating offshore wind project called Volturnus. The pilot – currently on hold as a result of a change of Maine governor – used a 12 MW supply using two 6 MW turbines linked to the onshore Maine supply grid. It planned a floating concrete semi-submersible hull designed by UMaine for use in waters up to 90m deep.

“We have a tremendous opportunity before us to export this resource to our region, and met our energy needs. Deep-sea ocean wind energy is the future,” said previous governor, John Baldacci. Baldacci said he was convinced deep ocean wind will become a major resource of domestic renewable energy in the US.

Other American companies are also researching the alternative materials for the massive floating platforms required for deepwater wind turbine construction. These include conventional steel construction; spar buoy concrete floater; reinforced concrete and steel construction, or geopolymer cement with basalt rebar (GCBR).

Back in Europe, Statoil’s US$260 million Buchan Deep installation is being partly funded by a sustainable energy grant from the British government. But Statoil claims the price of energy from existing fixed offshore wind farms has dropped more than 30% over the last five years due to improvements in construction and control techniques.

The company said it is looking to use its floating software control system to establish a lead in the potential deepwater wind farm market, with new installations projected in both Japanese and American deep waters. And the capability for floating farms can only accelerate that trend, says Delp.

"[Deepwater] is a game-changer for floating wind power, and we are sure it will help bring [generation] costs down," Delp told the BBC in an interview.