IN DEPTH: The two-bladed revolution

If the designers’ calculations are accurate, three-bladed offshore turbines could soon be left in the shallows by a fleet of gigantic new two-bladed models now heading into final testing

Eccentric-looking they may be, but two-bladed turbines promise to offer generate at a levelised cost of energy (LCoE) that is as much as 50% lower than current offshore designs — a massive step towards making offshore wind competitive with other energy sources.

“If the improvement in cost of energy is of the magnitude we are calculating, major developers will be forced to see the commercial logic of two-bladers,” says Martin Jakubowski, chief executive of Italian turbine designer Condor Wind.

For almost 40 years, two-blade machines have lived on the fringes of the industrial wind farm landscape, marginalised in large part for being measurably louder than three-blade models and more likely to trigger the disconcerting visual effect know as “shadow flicker” as their propeller-like rotors turn.

But the rapid build-out of offshore wind — far from where noise and tricks of the light might upset Nimbys — has been a game-changer. Designers are now revisiting the concept for the coming generation of 6-20MW machines, lured by lower capital costs and cheaper, faster installation — two-blade rotors can be attached to nacelles quayside and installed offshore in a single lift.

The most hopeful calculations suggest two-bladers could flow power to the grid for under £60 ($100) per MWh — far below the UK government’s 2020 target of £100/MWh, and cheaper than the €95 (£77) per MWh expected by turbine giant Siemens in 2025. Currently, traditional three-bladed offshore turbines are producing at around £130/MWh.

Half a dozen two-bladed designs are in advanced stages of development, with analysts predicting they will steal a 2GW slice of the expected 10GW annual offshore market by 2020.

German turbine designer Aerodyn last year unveiled an 8MW offshore concept — the typhoon-class 8.0-168 — that is an evolution of operational smaller models, including a jacket-mounted 6MW unit that is soon to begin testing for Ming Yang at China’s Rudong offshore project, and is also in the frame for a first outing off Norway next year.

Ming Yang compatriot Envision, meanwhile, continues to trial a 3.6MW version of its two-bladed upwind concept — which is expected to be scaled up to an ultra-large class model — at its prototype testing site on the Danish coast, with an eye on its slow-burning home offshore market. And in Japan, Hitachi’s gigantic 20MW concept, set to be unveiled by mid-2015, is being modelled with a two-blade rotor.

In Europe, at least three designs are on the road to construction. Italian designer Condor Wind — which rose from the ashes of two-blade turbine pioneer Blue H — will have its 6.1MW machine running at a coastal test site in China next summer; Dutch outfit 2-B Energy’s 6MW downwind concept now has funding to be installed in Scottish waters by 2016; and the French Winflo project, backed by naval contracting giant DCNS, is inching ahead with development of a 2.5MW floating prototype off France’s coast.

The EU-funded Innwind project also has a two-bladed design on its drawing board, while in the US, start-up Nautica Windpower has a 4MW concept being advanced with support from the US Department of Energy, with a view to scale prototype tests next year.


Fleshing out its 8MW model, Aerodyn designed a lightweight carbon-sparred glass-fibre blade with a hydraulic pitch control system that allows each 82-metre unit to be adjusted individually, solving the “two dimensional problem” of complex bending moments presented by the interplay of the three blades on a conventional rotor star.

The 168-metre-diameter-rotor downwind turbine will also feature a kind of “teeter hub” — similar to that at the centre of a helicopter rotor — and a hydraulic yaw system to flex with class 4-5 winds to spread torque as it reaches the drivetrain. For the 8MW, Aerodyn is further evolving its super-compact drivetrain transmission system technology with the first of two planetary gear stages integrated into the rotor’s main bearing, saving space and smoothing load transfer through the nacelle.

Working with Ming Yang, Aerodyn has modelled the 6MW design to churn out 40GWh a year in the wind regime off China. Prototypes of its 3MW model have been turning onshore at a wind farm near Urumqi since 2010 and offshore since last year at Zhuhai, and the flagship 6MW is in line for installation this summer at Rudong.

Two blades cost 30% less than three, with only fractional drops in energy yields — 3% according to Aerodyn — while better-managed loads mean a less expensive nacelle and tower. The designer calculates that this could translate into a 20% LCoE advantage over three-bladed offshore machines.

“This means [two-bladed turbines] have big possibilities for onshore but even bigger for offshore, with the much larger rotor diameters and the steadier, higher speed winds — the economics are much better,” says Aerodyn co-founder Sönke Siegfriedsen.


Envision says that the rotor system of its two-blade offshore design should cost “significantly” less than like-rated three-blade turbines — partly because it can use cheaper carbon fibre, as the downwind orientation means there is no chance of tower strikes as the blades flex in high winds. The lighter nacelle also translates into fabrication savings of 25%, with a 3% reduction in the cost of the tower.

The Shanghai-based company has been testing an upwind 3.6MW demonstration turbine with “part-pitchable” blades outside the city of Thyborøn, Denmark, as it continues work on designs for larger models aimed at the European and Chinese markets. Engineered for the harshest offshore environs, the 128-metre-diameter rotor flies a pair of two-part LM prototype blades — made up of an “adaptable extender” inner section bolted together with a standardised outer length — that can be idled sideways in extreme winds to avoid overloading.

Last winter, the demonstrator rode out coastal storms with winds of more than 50 metres per second (m/s), vindicating modelling studies suggesting 50% load reductions in extreme conditions.

“This model will be particularly interesting for offshore environments where there are very high winds, even typhoons, which is a market that is not yet fully developed but is starting to take shape,” says Envision senior blade engineer Lennart Kühlmeier.

“We went from concept to operational demonstrator in 18 months to develop a turbine that includes many new features: it has been a fast-track process. But we have already seen in the performance of the Thyborøn demonstrator how well the two-bladed handles loads in extreme winds. It certainly looks different when it rotates, but difference is often how technology is progressed.”

Erected offshore, the overall price of Envision’s larger machines is expected to be 5-10% cheaper than three-bladers, depending on water depth, helped by an assembly process in which each unit — complete with blades fitted quayside — is taken out to site by barge and mounted on the tower in a single lift.

Condor Wind

Condor’s 6.1MW two-bladed design, which features a 125-metre-diameter rotor powering a “2.5-stage” geared drivetrain, has undergone simulation testing that suggests the machine could produce at least 5% more energy than like-rated three-blades spinning in similar wind conditions, churning out 31GWh a year in 12m/s winds.

At the heart of Condor’s design is an elastomeric teetering hinge, set between its interconnected blades and shaft, which filters out wind-driven gyroscopic loads to soften the impact on the drivetrain while allowing for power control to be finessed by yawing rather than by adjusting blade-pitch.

GL Garrad Hassan (now DNV GL Energy) reported that fatigue loads on Condor’s drivetrain were 80% lower than three-bladed turbines of a similar rating.

“Two-bladed turbines are easier to control and have simpler components [than three-bladed units] that are also easier to maintain offshore,” says chief technology officer Silvestro Caruso.

“There is no doubt the two-blade turbine technology can be improved further but the first years of operation will show this to be the right path for lowering the cost of energy [of offshore wind].”

Condor expects to sign off on a deal with a Chinese developer this month to erect an onshore prototype at a coastal location next summer. Deepwater demonstrators fitted on floating spar foundations, moored “most probably off Italy or in the North Sea and off the US, likely California”, are slated to start being built in spring 2016.

“We are finally, as an industry, moving away from simply marinising conventional onshore technology,” adds Jakubowski. “It just takes time to bring these ideas to reality. People are beginning to understand the huge advantages represented by two-bladed turbines in bringing the cost of offshore energy down to an economic level.”


Hitachi’s three-bladed downwind 2MW HTW2.0-80 machine — a floating version of which was switched on last autumn in 100 metres of water off Japan’s Kabashima island — has concentrated thinking on investigations into the development of a two-bladed 20MW model the Japanese company aims to unveil by mid-2015.

The turbine concept, part-funded by Japan’s New Energy and Industrial Technology Development Organisation, owes an unlikely debt to the rugged landscape that dominates the northern highlands of the country’s two largest islands, Honshu and Hokkaido, where blustery “up-flow” winds churn over rough onshore terrain mirroring conditions offshore — and can cause energy-losing yaw misalignment of over ten degrees.

Downwind machines yaw passively, resulting in as much as a 7% boost to power output compared to like-size upwind designs as well as suffering less fatigue damage — including in typhoons where they can “free yaw” under gale-force winds.

2B Energy

In Europe, Dutch company 2B Energy recently set the seal on an investment package of about €26.5m from a group that includes a bank, venture capitalists and an oil company, to drive forward development of a two-bladed prototype that will start testing onshore next year before follow-on deployment in Scottish waters.

2B’s concept — purpose-designed for the 6MW turbine class with a 140-metre-diameter rotor, steel tower and integrated lattice structure jacket — promises to trim over one third off the current cost of offshore wind energy.

The two-blader’s offshore appeal has caught the attention of floating turbine designers with its tantalising prize of lower component and operations costs in an ultra-large-diameter rotor concept.


Plans by French naval contracting giant DCNS and compatriot floating turbine maker Nass&Wind to have a part-scale demonstrator model of their Winflo concept installed offshore next year have slipped slightly, with expectations now that they will have a 2.5MW unit in the water by 2017.

The two-bladed turbine by French manufacturer Vergnet, which will be set atop a lightweight semi-submersible free-floating platform fixed to the seabed using catenary cables, is to be moored in 35 metres of water at the grid-connected SEM-REV wave- and wind-energy testing facility, off Le Croisic on the Brittany peninsula.

The developers hope to have a pilot array turning by 2019-20, with full-scale commercial Winflo developments flowing electricity by 2023.

Nautica Windpower

By US crew Nautica’s reckoning, an LCoE below €75/MWh will be achievable by 2021-25 with floater-based technology — considerably cheaper than the current lifetime cost of a North Sea turbine.

The company has completed preliminary design of its 4MW Advanced Floating Turbine (AFT), with plans for a part-scale prototype with “significant innovation” in its rotor control system to begin testing in 2015.

The AFT is a slim-towered, spar-based floating design featuring V-shaped stabilising arms at water level. Digitally modelled to deal with high wind loads like “a palm tree bending in a hurricane”, the downwind concept is envisaged operating in water depths of 60 metres and more.

“We are very much committed to two-bladed rotors,” states Nautica chief executive Larry Viterna, a former Nasa engineer who was involved in the space agency’s pioneering work on industrial-scale onshore wind turbines in the 1980s.

“The potential benefits of reduced component costs, as well as reduced operations and maintenance costs, we believe will be significant to achieving cost of energy of under $0.10/kWh [based on a 500MW AFT development with average winds of 8m/s],” he says.

The company is on track to have a 4MW AFT in the show window by 2021, with models of “at least 10MW” ready for serial production by 2025.


The scale — and rate of growth — of the market for two-bladed turbines is hard to gauge, looking hugely promising yet founded on an emerging technology that will be in direct competition with tried-and-tested three-bladed models. The offshore machines expected to be installed in the water by the end of the decade will doubtless be dominated by three-bladed designs due to the lower risk of using a technology with such a long track record.

Key to the commercial future of the two-blader will be the performance of those first 6MW-plus models. If LCoEs of €75/MWh are field-proven, then mainstream take-up of the technology looks inevitable, potentially accounting for more than 20% of the expected annual deliveries into offshore waters by 2020.

On the far horizon, the market for two-bladers could be massive, when the combined global floating wind potential — over two terawatts, according to DNV GL Energy — is added to the mix.

The flagship two-bladers floated out to sea in the next few years will be the bellwether.