IN DEPTH: Fleet fit for a revolution

An energy revolution is looming on the high seas. And Innwind is designing the EU’s armada.

The €20m ($27.3m) European Commission-funded project, headed by Danish research institute DTU, is looking at the next generation of offshore turbine technologies — gigantic jacket-mounted and floating machines of up to 20MW — that will carry the industry out into the rich winds of the world’s deep waters, starting with the northern seas.

A follow-on from the 2006 Upwind project, which worked up a reference model for a best-in-class 5MW concept, Innwind is forecasting the future. It envisages 100-metre modular blades reading the wind with active flow control and adaptive ultra-light rotors, superconductive and “pseudo direct-drive” high-efficiency transmission systems, towering concrete and steel foundations and, far offshore, farms of moored hulls topped with 10MW two-bladed or vertical-axis turbines.

“Going beyond the 10MW rating and beyond 200-metre rotor diameter in the near future is possible,” says Peter Hjuler Jensen, DTU deputy head of wind energy. “The drive in recent years to develop more efficient turbine technology all the time has created a culture of innovation that is making this size of design possible — and larger, up towards 20MW.

“This itself has required moving far away from the first principle of taking an onshore machine and marinising it. We can’t do it like that any more [for 10-20MW turbines]; we need innovation that is offshore-specific.”

Innwind has lofty aims, none more so than beating the turbine-defining square-cube law — which says power production rises exponentially with rotor diameter but is checked by the physical stress limits of ever-heavier blades — by devising a supersize machine that is cost-effective offshore.

But the researchers, pulled in from 27 companies and R&D institutes, including Siemens, Gamesa and Fraunhofer, are also staying sturdily anchored in the engineering, with component-by-component work packages from the rotor hub through the nacelle and down-tower into the foundation.

“The perennial question is, ‘How big can turbines be?’ And the answer is: ‘With existing technologies you could build these [10-20MW] turbines but they would not be economic without a range of new innovations which will drive the cost down,” Jensen says.

The redesign begins with the blade. Innwind is developing a slender, high-tip-speed design, as well as

exploring alternative tip shapes and “multi-element”, flat-back aerofoil concepts, with more sensor-driven active and passive load-control technology.

Different aerodynamic rotor concepts have been hatched too: in the first instance, lightweight, adaptive designs with passive built-in geometrical and structural couplings and active distributed smart sensing, as well as intelligent stall and pitch control. In the longer term, Innwind is looking at flexible downwind rotors and two-blade designs.

Two transmission system concepts have been scrutinised for answers to a key challenge in scaling up turbines to above 10MW: fashioning a drivetrain that can cope with 10MNm of torque — think industrial rock crusher.

Demonstrations of a 3-6MW superconducting direct-drive generator, headed by Siemens, and a small-scale 100-220kW magnetic geared drivetrain, led by Magnomatics, were conducted in phase one of Innwind.

An ultra-large superconducting drivetrain design, which has the generator mounted in front of the rotor using a “king-pin” layout, is next. The system is calculated to be feasible at €84 per kW for a 10MW machine. That is well below the €300/kW threshold where Innwind reckons the technology would be economic.

“Superconducting designs are already creating a lot of interest, and the concept with magnetic gearing has very promising potential,” says Innwind work package leader Takis Chaviaropoulus, renewable-energy director of Greek R&D body CRES.

Current cryogenically cooled superconducting transmission system designs, such as GE’s 10MW Hydrogenie concept and AMSC’s early blueprints for its 10MW SeaTitan, have opted for high-temperature or low-temperature field windings. Innwind, however, is going for a medium-temperature model, running at 39° Kelvin (-234°C), which it reckons has a better chance of being low-maintenance in the field.

The coming generation of jackets is being readied for mass production. Innwind is designing a standardised integrated tower and foundation that “simplifies and unifies” a turbine’s dynamic structural behaviour at water depths of 50 metres — out of reach for monopiles and borderline shallow for many floaters.

“A great deal has been done in the field of alternative jacket designs which we have looked at, which goes to show how closely the wind industry is connected to our project,” says Jensen.

Anand Natarajan, a DTU Wind senior researcher seconded to Innwind, adds: “Reduce the cost of the substructure and — as it is around one third of the total cost — you are straightaway that much closer to current targets [for offshore wind, of around €130 per MW].”

Along with next-generation jackets, Innwind has been turning its hand to floating foundation designs, including spar and semi-submersible models for deepwater regions and “flexible” designs for medium depths.

Jensen says Innwind aims to support more efficient rotor, transmission and converter systems and foundations and floaters, with manufacturing, installation and operation integrated to optimise the turbine’s life cycle.

Without such advances in technology, a turbine’s cost of energy would balloon by 20% as it was scaled up from 5MW to 20MW, according to Innwind calculations. By taking these strides, offshore machines could be flowing at a levelised cost of energy 30% lower by the end of the decade and 50% lower by 2030.

“In the post-subsidy era, after 2020, offshore wind turbines will have to become more cost-competitive with conventional generation. The only way to do that is to proceed with novel, larger, more efficient designs that can be installed further and further offshore to harvest the greater energy potential,” says Jensen.