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Netherlands

Windtech: Two-bladed concept claims 40-year life

NETHERLANDS: A radical downwind two-blade prototype has completed a six-month test in the Netherlands. Eize de Vries talks to 2-B Energy's founders on what's next for the 6.2MW 2B6

The radical two-bladed downwind turbine undergoing testing
The radical two-bladed downwind turbine undergoing testing

The 6.2MW 2B6 is a radical downwind turbine design, best characterised as incorporating a combination of proven technology with dedicated innovations.

Prominent in the latter category is an advanced load-based individual pitch control (IPC) system, which compensates for dynamic imbalances inherent to a two-bladed rotor, plus 2-B's specific choice for a rotor configuration with a rigid hub.

Another dedicated technology feature is the soft yaw system, which during normal operation allows the rotor to freely follow wind-direction changes with some degree of dampening.

And a third is a steel lattice-type truss tower or "full-jacket", which extends from the nacelle bottom to the seabed.

According to the company founders, Mikael Jakobsson and Herbert Peels, this structure has the highest share of turbine Capex.

"The damped free-yaw operating mode minimises tower torsional loading, enabling a wide-base 'open' tower design with comparatively few and lighter cross-bracings," says Peels.

"It also minimises welding quantity and effort compared with conventional jacket substructures for upwind turbines. The combination of favourable truss-tower loading and minimised material fatigue allows for 40-year design life at a competitive cost structure."

Two-blade experience

Reflecting on the project's launch in 2007, the founders say they did not have to start with a clean sheet of paper.

They could draw upon the extensive experiences of existing two-bladed research turbine developments, especially in Sweden and the Netherlands.

They give the Nordic 1MW and 3MW turbines at Nasudden and Maglarp in Sweden as prime examples.

"During this initial period we had roundtable discussions with international experts for evaluating design options on their potential for technology advancement," says Peels.

"Then, in 2008 the financial crisis hit, and we had to rely much more upon developing essential know-how in-house.

A positive side effect was that it gave us a real boost in quickly developing our overall capabilities and retain full ownership of key intellectual property in strategic areas.

For example, today we perform fully integrated designs of dedicated jacket foundations really fast."

Fixed-speed operation with active stall control was initially considered, but variable speed and IPC proved far superior, due to the reduction in rotational speeds provided by today's larger rotor diameters.

Incorporating IPC substantially reduces the high loads traditionally associated with two-bladed turbines, which was previously only possible with a teeter hinge.

Long life and reliability

The company's overall focus is on understanding and mitigating known offshore-related risks by applying proven technology whenever possible, following 2-B's slogan: "Where conventional solutions are good enough, stick to them".

It aims for the turbines to be reliable, easy to install and service, and offer a 40-year design life.

The service-friendly box-type nacelle has lower and upper inner levels, where up to eight technicians can work at the same time. Doorways on each side provide walk-around access to the hub in vertical rotor position.

A third outer helideck, not part of the prototype, will become a (semi-) standard design feature and an integral part of the upkeep strategy (see box, overleaf).

The lower nacelle level has a flat, open floor with largely undisturbed movability, including easy access to five internal yaw motors in front.

Another four external motors are arranged in pairs at the right and left-hand sides. During normal operation, these yaw motors provide the necessary nacelle-movement dampening.

In emergencies, such as extreme weather or turbine failure, eight motors are activated to bring the turbine into a safe non-operating position, with the ninth providing system redundancy.

A passive cooler platform, without moving parts, is located in the nacelle front, directly facing the approaching wind flow.

The lower level further contains a toilet unit and climate-controlled personnel room, the modular partial converter and MV-transformer.

The upper level incorporates a conventional non-integrated high-speed geared drivetrain with main shaft, two main bearings, a three-stage gearbox and a doubly fed induction generator (DFIG).

The 2B6 with 140.6-metre rotor diameter has a specific power rating of 399W/m2. This can be compared to the 331W/m2 of the MHI Vestas V164-7MW, and the 393W/m2 of the latest V164-8.0MW in 8.3MW power mode.

Jakobsson and Peels call the 2B6 a good starting point for future scaling. "We continuously review alternative drive concepts and turbine configurations beyond 6.2MW. In parallel, we ask ourselves what could be done to make our high-speed drivetrain concept even more reliable," says Peels.

Another ongoing discussion is whether to continue with a DFIG-based electrical layout operating with partial converter and 10kV stator voltage, or switch to full converter solutions.

Acceptance test

"We have sold the prototype to a customer, and it qualified for conventional project debt financing by a leading bank," says Peels.

"Starting in March, the unit met all technical and performance conditions set by the customer in early April, following the successful completion of a six-week acceptance test.

"Considering that conceptual differences and new systems normally cause at least some issues, we were pleased that all testing went smoothly and above expectations, with over 97% availability. This has boosted our confidence in the 2B6 platform for the next steps and future scaling."

Platform rollout will start with a demonstration project comprising two largely identical turbines. It will be built by 2-B Energy UK subsidiary Forthwind off Scotland's east coast in the Firth of Forth.

The turbines will again feature truss-type towers, and, for demonstration purposes, one will be fitted with a helicopter landing platform. Commissioning is planned for early 2018, and 2-B's main goal is to demonstrate overall turbine performance under demanding marine conditions.

A number of steps are envisaged for serial ramp-up. One is expanding the Scottish project with another seven 2B6 models, two of which would be mounted on floating foundations. "For the floating project, we will re-apply our universal truss-tower concept," says Peels.

"Its wide base is expected to positively impact system behaviour by reducing floater-turbine inclination angles and acceleration forces.

"We are also considering deploying our 2B6 with universal jacket solution in typhoon-prone markets, and parallel market segments characterised by very soft soils, like the Yellow Sea. For these plans, discussions with Chinese certification bodies continue."

Extended lifespan

Finally, further product demonstration, optimisation and platform scaling to the next size are high on the agenda.

Jakobsson and Peels point to the huge levelised cost of energy (LCOE) reductions achievable when extending 2B6 operating life to 40 years instead of knocking them down after 20 years.

"Keeping turbines running for another two decades is perfectly feasible for structural key components when combined with a mid-life overhaul programme and condition-based turbine exchange," says Peels.

"Even compared to the current lowest LCOE levels, such lifetime extension offers further LCOE reduction opportunities up to 70%, making it impossible for any new and larger-scale technology to compete."

INTEGRATED HELICOPTER-BASED SERVICE APPROACH

Each future 2-B-Energy turbine in large wind projects will incorporate a helicopter landing platform. This can accommodate a common Eurocopter EC135 T2 with up to six service technicians.

"At wind farms with three-bladed turbines and helicopter-hoisting possibilities, the aircraft must carry extra fuel for reaching a secondary landing deck beside the primary deck, like a hotel-service vessel or offshore high-voltage station.

In our integrated helicopter-based service approach, each turbine serves as secondary landing platform," says Mikael Jakobsson

The company's turbines further have a remote-controlled hatch for receiving tools and parts from a helicopter. A small EC135 can take over 800kg on its centre hook, while larger helicopters, such as the EC224, can take more than four tonnes.

"Conventional offshore turbines require redundancy capabilities and standstill mitigation strategies," says Jakobsson. "With our service, any turbine can be reached most of the year within ten minutes. Components exchange by helicopter can be conducted in up to 20m/s of wind speeds," he adds.

More efficient servicing

Conventional offshore vessel-based service access in a 12-hour shift system gives on average less than five effective working hours at a turbine. The rest is waiting, transfers, breaks, tools hoisting, and climbing and descending.

"With helicopter-based service, effective working time in similar 12-hour shifts is around 8.4 hours. Taking weather delay differences between the access methods into consideration, a helicopter-operated offshore project only needs half the technicians for similar service activities, and helicopters can operate and land in wind speeds up to 50 knots (around 26m/s).

"This potential is enormous and we will further explore processes and safe approach issues with experts," he says.

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