Offshore Devices Get Bigger and Lighter

UK firms unveiled two innovative offshore turbines in July and August—one to reap the wind’s energy and the other, tidal power. Wind Power Ltd. made public the latest embodiment of its Aerogenerator project, a lighter 10-MW design, while Atlantis Resources Corp. unveiled and then deployed its mammoth AK1000 tidal turbine, which it says is the “largest and most powerful tidal power turbine ever built.”

Weeks after it was unveiled, the AK1000, a 1,300–metric ton tidal turbine, was successfully deployed on its subsea berth in 35 meters (m) of water in late August at the European Marine Energy Centre in Orkney, Scotland. About 22.5 m tall and with a rotor diameter of 18 m, the device is rated at 1 MW if water velocity is at 2.65 m/s (Figure 5).

5. Prince of tides. Atlantis Resources Corp. in late August deployed the AK1000 tidal turbine in the Pentland Firth. The device is 22.5 meters (m) tall with a rotor diameter of 18 m—which makes it the largest tidal power device on Earth. The AK1000’s blades churn about six to eight times a minute. Developers say it is rated at 1 MW if water velocity is at 2.65 m/sec. Courtesy: Atlantis Resources Corp.

Because of its slow rotation speed—the AK1000’s two sets of blades make six to eight revolutions per minute—the giant turbine is expected to be environmentally benign, say its developers. As Atlantis Resources CEO Tim Cornelius told the BBC in August, the device’s deployment in the Pentland Firth—reputedly the most dangerous stretch of sea on the planet—meant building a durable turbine. “In order to get a robust turbine we have had to make what we call ultimately the dumbest, simple but most robust turbine you could possibly put in such a harsh environment,” he said.

The 10-MW vertical-axis Aerogenerator X wind turbine, meanwhile, was unveiled at the end of an 18-month feasibility study undertaken by Cranfield University, QinetiQ, Strathclyde University, Sheffield University, and Wind Power Ltd. It was funded by a group including Rolls-Royce, Shell, BP, EDF, and the UK government. Company officials said at the unveiling that the new design sought to tackle increasing capital costs for deeper water installations.

At half the height of an equivalent horizontal-axis turbine, the radical V-shaped Aerogenerator X’s weight is concentrated at the base of the structure (Figure 6). According to Wind Power Ltd., unimpeded by the weight constraints of a normal wind turbine, the blades suffer little weight-induced fatigue. Designed for offshore deployment, vertical-axis turbines like the Aerogenerator are also able to harness wind energy from any direction.

6. X marks the spot. A UK consortium in July unveiled a 10-MW vertical-axis wind turbine at the end of an 18-month feasibility study undertaken by Cranfield University, QinetiQ, Strathclyde University, Sheffield University, and Wind Power Ltd. At half the height of an equivalent-size horizontal-axis turbine, the radical V-shaped Aerogenerator X’s weight is concentrated at the base of the structure. Courtesy: Wind Power Ltd.

“Upsizing conventional onshore wind turbine technology to overcome cost barriers has significant challenges, not least the weight of the blades, which experience a fully reversed fatigue cycle on each rotation,” explained Professor Feargal Brennan, head of Offshore, Process and Energy Engineering at Cranfield University. “As the blades turn, their weight always pulls downwards, putting a changing stress on the structure, in a cycle that repeats with every rotation—up to 20 times a minute.”

Brennan said that typically, to further reduce fatigue stress, the blade sections and thicknesses are increased—and consequently, drive-train mountings must be stiff enough to support the heavier components inside the nacelle on top of the tower. “Otherwise the systems can become misaligned and the support structure is also exposed to extremely large dynamic thrust and bending stresses, which are amplified significantly with any increase in water depth,” he said.

The UK consortium that unveiled the device admitted that it would be several years before full-scale prototypes of the devices are ready. By then, it could be possible to further economies of scale with 20-MW turbines, researchers involved with the project said. However, if proven, the designs could have major implications for the burgeoning UK offshore wind industry, where investments of up to £100 billion could be made by 2020.

—Sonal Patel is POWER’s senior writer.