IN DEPTH: The 8MW era begins

Artist’s impression of Aerodyn’s two-bladed 8MW downwind turbine, which has its own helipad

Artist’s impression of Aerodyn’s two-bladed 8MW downwind turbine, which has its own helipad

The race to launch the world’s first 8MW wind turbine was won last month by Vestas when the 80-metre-long blades on its gargantuan V164 prototype began to arc through the winter air at the Danish national testing centre in Østerild.

The record-setting machine’s rotors, which reach 220 metres into the sky, sweep a circle larger than the London Eye, with a single turbine powerful enough to send electricity humming into 7,500 homes. Just to test the unit’s drivetrain at Vestas’ Aarhus factory required a new 42-metre-long rig.

The V164, originally a 7MW design, was unveiled in 2011 to great fanfare by former chief executive Ditlev Engel as an offshore turbine of “epic proportions to address energy challenges of a similar dimension”. Its three years in development coincided with one of the rockiest financial periods in the company’s history, which saw Engel and much of his management team replaced.

“Vestas is incredibly proud to be able to design, manufacture, install and service the first [V164] prototype,” intones Torben Hvid Larsen, Vestas’ chief project manager for the 8MW platform. “The turbine is truly an immense machine and seeing it assembled and turning has given everyone a big spark of motivation to carry on the good work we are doing in bringing it to the market.”

Assembling the unit at Østerild, 10km from the northern Jutland coast, was a project in itself. The blades were shipped from Vestas’ R&D facility on the UK’s Isle of Wight, the nacelle came from its Lindø factory and the hub and tower from Aarhus, with all the super-size components transported at night on flatbed trucks.

“Transporting 80-metre blades and a 390-tonne nacelle and so on was naturally a challenge,” says Larsen.

“The original schedule for installation and commissioning was actually the second quarter of 2014, but... the testing of the individual components went well and allowed us to proceed with the installation at an earlier stage.”

Erecting the prototype was nonetheless a test of Vestas’ mettle. Winds whipped up to over ten metres per second (m/s) — ironically the wind speed in which the turbine is expected to operate in the North Sea — and several heavy snowfalls “did pose certain challenges”, acknowledges Larsen.

Working round the clock in teams, all componentry for the prototype was fitted together “smoothly and on time”, with the three blades bolted on in the glare of industrial klieg lights during one exemplary 11-hour night shift.

All systems are go for the commercial roll-out of the V164 in 2015. Danish utility/developer Dong Energy has a framework agreement with Vestas to buy scores of the machines for its offshore projects under an “enhanced” co-operation deal. And last autumn, the turbine maker forged a tie-up with Japan’s Mitsubishi Heavy Industries, transferring development of the 8MW turbine to a new joint-venture company.

“The V164 prototype at Østerild will be tested in several different phases,” says Larsen. “Structuring the testing procedure ensures we are always in control of the process and we can document the performance of the turbine throughout thoroughly, before moving on to a new phase.”

Exhaustive trials of the drivetrain, including long stretches working under gut-wrenching torques of 18MNm — ten times that of an industrial rock crusher — will now be mirrored in real life.

The Østerild prototype recently passed its first kWh milestone as commissioning picks up pace during its “run-in” phase. This involves pitching the 35-tonne blades back and forth; connecting and disconnecting the machine from the grid; priming all on-board data-critical measurement systems; and ramping the turbine step by step to reach its full 8MW output.

IMG_1451.jpgEarly indications are that the wisdom of Vestas’ opting for a three-stage gearbox — a controversial choice given the growing appeal of direct-drive systems for ultra-large turbines — is being borne out.

“The V164 has been developed with a ‘semi-geared’ medium-speed drivetrain consisting of three planetary stages,” explains Larsen. “Internally in Vestas, our R&D department ran three parallel development projects to evaluate which drivetrain choice was to be used for the V164.

“The medium-speed concept was evaluated to produce the lowest cost of energy compared to the high-speed or direct-drive solution. Nothing we have seen since has changed our view.”

Another innovation is the construction of the blades, which were built with a structural shell that bears the wind loads on the outer surface, rather than any inner spar supports.

Vestas says it is on track to deliver zero-series turbines to the market by 2015, subject to firm orders. However, the V164 has yet to be allocated an offshore testing site. “At this stage no decision has been taken,” says Larsen.

Aerodyn

German turbine designer Aerodyn is pursuing a more experimental path.

Its on-paper offshore concept, the 8.0-168 TCIB, revolves around a two-bladed downwind rotor powering an in-house-designed super-compact drive (SCD) transmission system with a novel electrically excited synchronous generator.

The typhoon-class turbine, which will feature pitch-controlled 82-metre carbon-sparred glass-fibre blades, was unveiled last November, with Chinese partner Ming Yang now scouting for prototype installation sites in the South China Sea.

“The two principal arguments against two-bladed concepts in the past — visual disturbance and noise emissions — disappear once you are offshore,” states Aerodyn founder Sönke Siegfriedsen.

“There are many cost-saving advantages [over three-bladed designs]. In particular, two blades cost 30% less than three and you have the same energy yield; and you can assemble the whole turbine, including the blades, in the harbour, take it out to site by barge and install it in a single lift onto the tower.

The 8MW builds on the Hamburg-based company's 6MW design, a prototype of which has been assembled at a shipyard in Nantong on the Yangtze River, with another slated to be erected this month on a six-legged jacket as part of China’s shallow-water Rudong project.

“We have our 3MW machines being installed at the Zhuhai offshore project [near Zhuhai Guishan island off China] and they have been performing very well — I have put my hands on the tower of one of these turbines in 20m/s wind and it is clear it is running very smoothly,” notes Siegfriedsen. “We expect the 6MWs will operate as well [as the 3MW turbines].”

For the 8MW, Aerodyn’s “reduce to the max” SCD technology — in which the first of two planetary gear stages is integrated into the rotor’s main bearing, saving space and streamlining load transfer — translates into a drivetrain 2.5 metres long and 3.5 metres in diameter, with a tower head mass of only 380 tonnes, including blades and a helipad.

Pitch control is crucial for turbines having to cope with highly turbulent offshore wind conditions. Aerodyn’s design uses a hydraulic single-blade system that, according to Siegfriedsen, simplifies the “two dimensional problem” faced by three-blade rotors.

“We control the two blades independently — and it is much easier to create the algorithm for this because you are dealing with only one dimension for bending moments because you have a straight line between the two blades.

“Extensive evaluations have been carried out — CFD [computational fluid dynamics] simulations of the wake behind the tower as the blades pass by — to understand the loads.”

The 8MW model will also feature a kind of “teeter hub” — similar to that used on a helicopter rotor — and a hydraulic yaw system to flex with the gusts to spread torque on the drivetrain.

“This will be particularly important for the huge projects coming up offshore China in the [typhoon-prone] Taiwan Strait,” says Siegfriedsen. “There you will need turbines that can handle class 4-5 winds.”

The other key benefit of downwind designs such as Aerodyn’s is that they perfectly fit the bill for mounting on a floating spar, as the ballasted cylindrical foundation counterbalances the horizontal loads on the rotor.

A feasibility study is under way with an unnamed “German marine technology institute” for a concept suited to water depths of around 150 metres that can be towed to site fully assembled.

“Downwind turbines, we feel, will be much easier to integrate into a floating spar concept than upwind ones,” says Siegfriedsen.

“We would hope to have a first unit installed off Europe, but I think Japan could be an interesting area for deepwater wind turbines.”

Areva

First announced in November for two GDF Suez-led bids in France’s second offshore tender, Areva’s 8MW contender is still shrouded in some mystery. 

The design — which is slated for a prototype in 2015 with a move into serial production three years later — would feature near-90-metre blades powering a medium-speed geared drivetrain and permanent-magnet generator based on a concept "extrapolated" from the technologies used in its 5MW M5000 model.

With a rotor diameter approaching 190 metres, the turbines would be even larger than Vestas’ V164.

Details “are currently being kept for clients and partners”, says Areva. It does reveal, however, that the machine’s output would enable a 40% reduction in the number of present-day turbines needed for an offshore wind farm, with “more space” for commercial fishing operations.

Discussions are "ongoing" with suppliers for the blades, gearbox and generator, according to Areva chief commercial officer Rémi Coulon.

Areva’s planned four-factory complex at the French port of Le Havre is seen as the likeliest site for a large-scale test bench on which to test its 8MW drivetrain.

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