Fukushima Forward: The technology

The installation of two 7MW Mitsubishi SeaAngels at Fukushima will mark not only the christening of the world’s largest offshore turbines, but also the construction of the world’s first floating wind array.

With a hub height of 110-120 metres and a rotor diameter of 167 metres, the SeaAngel is among the biggest beasts heading offshore, alongside Samsung’s 7MW S-171-7.0 (now in commissioning in the shallows off Scotland) and Vestas’ 8MW V164 (just erected for testing onshore in Denmark).

But under the bonnet, the SeaAngel is a breed apart. Where other manufacturers have opted for either direct-drive or medium-stage geared transmission systems powering permanent-magnet generators, the SeaAngel is being built around cutting-edge hydraulics technology — known at Mitsubishi as digital displacement transmission (DDT) — to turn an off-the-shelf brushless synchronous generator.

Hydraulic transmissions are common in industrial machinery such as rock crushers and pulping mills, converting high-speed engines’ rotation into powerful low-speed turning motions. For wind turbines, the equation is reversed, with the SeaAngel’s hydraulic drivetrain designed to accelerate low-speed irregular input of 6-11rpm to spin a 1,000rpm electrical generator.

Mitsubishi’s DDT concept employs a radial arrangement of pairs of pistons and cylinders that open and close low-pressure and high-pressure spring-shaped valves using lightning-fast computer-controlled electronic latches that can change the flow of hydraulic fluid on a stroke-by-stroke basis to pumps powering twin high-pressure motors. These in turn crank the synchronous generator.

Only the low-pressure valves need to be controlled. If one remains open, the pump does no work because the hydraulic fluid flows from the piston cylinder to and from a reservoir in an almost lossless cycle. If it is closed, any fluid in the cylinder is pressurised, causing it to flick open a high-pressure valve through which the fluid is then pumped.

This way a DDT drivetrain can instantly adapt to shifting wind conditions. By working at anything from full displacement (where every cylinder’s upstroke pushes high-pressure oil through the system) to low-pressure “idling” (where the valves are not energised), the generator is always powered at a steady rate, even in gusty conditions.

Going forward Mitsubishi will base its offshore deployment plans around Vestas’ 8MW offshore turbine, as part of the joint venture agreement signed by the two companies in September last year. But the Japanese company, which is erecting a prototype at Scotland’s pseudo-offshore testing facility at Hunterston, says that it will continue to develop the DDT technology used in Sea Angel with the possibility that the technology could be incorporated in future machines built by the joint venture.

Mitsubishi says that hydraulic drivetrains are some 20% lighter than comparative geared models. And they have been shown to run at better than 95% efficiency using system architecture that cuts out the historically problematic step-up gear, which ramps up a rotor’s rotations from 10rpm to the 1,000rpm needed to power the generator, as well as the turbine’s power electronics, so there is no need for frequency converters or transformers.

Mitsubishi’s buy-in to DDT — a concept hatched by Scottish engineer Stephen Salter and fine-tuned by Edinburgh’s Artemis Intelligent Power — is based in large part on its economic scaleability for even larger turbines than are currently being built.

The Fukushima project could go some distance towards proving up a technology that until now has been widely regarded as more a conceptual technology for the next-generation 10MW-plus offshore models.

The twin Mitsubishi turbines are being mated to two distinctly different floating foundations. One is a V-shaped inverted semi-submersible that is moored with its upper hull submerged and columns bobbing above the water line; the other is an “advanced” spar concept that takes the traditional cylindrical steel design and adds three buoyancy discs for extra stability.

The Fukushima Forward SeaAngels will both be flying 81.6-metre Eurus M-EU167 blades — a low-weight 32.5-tonne design built around ultra-stiff carbon-fibre reinforced-plastic spar caps for high load-bearing and durability.