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According to statistics from the European Wind Energy Association, in the first half of 2012 at least 82% of all new installed offshore wind turbines were in the 3.6MW "workhorse" class, dominated by German manufacturer Siemens. Within the same period, however, the average size of turbines installed and connected to the grid reached 4MW, a 14.2% increase on the first six months of 2011.

This trend towards installing bigger turbines is likely to continue over the next few years, and could accelerate once scores of new 5–6MW+ models enter the offshore market from 2014. Many of these new and commercially available larger turbines will be installed in deeper waters far from shore. Such locations are characterised by higher wind speeds and this, combined with other factors, favours bigger models.

Supplier statistics indicate that the bulk of new offshore turbines are in the 5–5.5MW and 6–7MW+ classes. At least one 8MW turbine and two 10MW machines are in development.

One 10MW turbine, development of which is readying for proof of concept in 2013, is a vertical-axis UK design called Aerogenerator X. This is characterised by a huge V-shape, Darrius-type rotor. Another pioneering 10MW turbine development in the UK, the Clipper Britannia, was discontinued in 2011. AMSC’s 10MW SeaTitan — a wind industry novelty — features a direct-drive (no gearbox) high temperature superconductor (HTS) generator, claimed to combine superior efficiency with compactness. It also boasts minimal component mass.

Most of the 5–10MW new turbine developments use a conventional horizontal-axis, three-bladed upwind rotor, but there are several two-bladed initiatives. They offer a mix of high-, low- and medium-speed geared systems, alongside direct drive.

Popular offshore turbine generators include the induction-type electric machines applied in Siemens’ 2.3MW and 3.6MW geared turbines, doubly fed induction (DFIG) used by, among others, Repower and Sinovel, and permanent magnet generators (PMGs). The latter enjoy increasing popularity, as they combine compactness and reduced mass with favourable partial-load efficiency and a claimed superior ability to cope with exacting future grid demands.

The wind industry is currently paying considerable attention to medium-speed turbines incorporating a two-stage planetary gearbox and a PMG, whereas two years ago, PMG-based direct drive seemed a clear race winner in drive technology. A key reason for this change in fortunes is that medium-speed systems, according to suppliers, require only 20–25% of the amount of magnets of direct drive — at similar power rating. Medium-speed proponents further claim comparable reliability and superior mass characteristics, combined with lower capital investment and optimised lifecycle-based cost of energy performance.

Driving ahead

There is wide variety in drive system approaches. Sinovel has introduced a 6MW high-speed turbine, while engineering consultancy Aerodyn’s high-speed 5MW turbine is licensed to two Asian clients, Gamesa 5MW medium speed, and XEMC 5MW direct drive. Alstom and Siemens have each introduced a 6MW direct-drive turbine with a rotor diameter in excess of 150 metres.

In the 7MW class, Vestas, Gamesa and DSME have announced medium-speed geared turbines, and Mitsubishi a full hydraulic drive solution. Three of these 7MW turbines offer a 165-metre rotor diameter, with Gamesa yet to provide a figure. Northern Power Systems is working on an 8MW direct-drive turbine with a 175-metre rotor.

Variable speed is now a state-of-the-art operating technology. Most of the turbines that use it incorporate a power electronic converter that converts generator power with variable frequency into grid-compliant 50Hz/60Hz power.

Mitsubishi’s SeaAngel 7MW drive system, by contrast, combines variable rotor speed and fixed generator speed, allowing direct grid connection while eliminating a power converter. Alternative solutions, with comparable variable rotor speed to generator fixed speed functionality, include a full hydraulic system developed by Norwegian company Chapdrive, and a mechanical variable speed gearbox developed by Israel’s IQWind.

Back in 2006, DeWind pioneered mechanical-hydraulic conversion systems based on Voith’s WinDrive. One of the latest drive system innovations in this category is a Dutch 9MW product called Hydrautrans, which claims to combine overall compactness and favourable head mass with high conversion efficiency.

The system comprises a double-sided crown-gear wheel, an angular gear drive principle best known from historic wooden Dutch windmills. This allows for the mechanical power to be split using six small patented "floating cup" hydraulic pumps, three at each gear wheel side. The system layout makes it possible to replace the pump without requiring readjustment of gears following assembly. This is needed with bevel-type gear drives that perform a similar function.

US giant GE has announced it is to develop a 10–15MW direct-drive turbine incorporating an HTS generator. A group of Spanish companies has announced a 15MW turbine, while the EU Upwind project has been exploring the feasibility of 20MW. All of these developments could happen by 2020.

Factors affecting yield

Turbine size is traditionally linked to rated power. However, pure megawatt figures do not tell the whole story; a 10MW turbine does not by definition generate twice the energy of a 5MW model. For a balanced picture, MW capacity should be viewed in relation to rated power, rotor-swept area and average wind speed at hub height.

Reliability and serviceability are determining factors in turbine availability; choice of drive system is no longer the key consideration of reliability.

With regard to specific power ratio in MW/m2, the initial three first-generation 5MW offshore turbines feature rotor diameters of 116, 122 and 126 metres. A recent 5MW Chinese turbine has a record 154-metre rotor. Similar trends can be observed in the 6MW class, where two new turbine models offer rotor diameters of more than 150 metres — exceeding even the 150 metres that was planned for the discontinued 10MW Clipper Britannia turbine.  

At an offshore conference in Hamburg earlier this year, Repower’s vice-president of business development offshore, Axel Birk, revealed that offshore wind plants in Europe had cost of energy (CoE) peak values averaging €170/MWh. He said the aim of bringing this down by 40% to around €120/MWh by 2020 required a number of specific actions to unleash their cumulative effects.

A key requirement is a new generation of typically larger and faster installation vessels. Other contributing factors are advanced transport logistics, lower capital investment costs and more efficient turbines.

Assuming an unchanged turbine investment cost level, Birk offered two examples for driving down CoE. Rotor diameter enlargement from 120 to 150 metres would yield 15–18% extra a year, he said, while increasing power rating from 6MW to 8MW would offer a further 15%.

The latest Siemens SWT-6.0-154 turbine, with 154-metre rotor diameter, increases the rotor-swept area by 65% compared with the earlier SWT-6.0-120. The enlarged rotor is expected to generate 20–24% extra energy at 9–10m/s average wind speeds, which are common in the North Sea offshore environment.

Repower has upscaled and optimised its initial 5MW 5M turbine type into a 6.15MW 6M successor with unchanged 126-metre rotor. The company quotes 12–15% extra annual yield at 10m/s mean wind speed for the 6M, 10–13% more at 9m/s, and 8–12% at 8m/s.

These figures show that a large rotor-swept area is of key importance for boosting yield and driving down CoE, even at high-wind offshore sites. The Repower example illustrates that finding the optimal balance between power rating and rotor in relation to wind speed can also be beneficial.

Rated power is a function of rotor torque and rotor speed or P = f (T x n). If a 5MW offshore turbine is upscaled to 15MW, rotor speed might halve. Rotor torque in this example increases by a factor of 6 (3 x 2). That in turn necessitates thicker shafts, larger bearings and gears, and heavier complex castings. Stronger materials are also needed, together with design solutions to limit turbine loads and avoid excessive mass increase.

The newest turbine developments are likely to hit temporary component limits. For instance, the Siemens SWT-6.0-154 incorporates a single rotor bearing with an outer diameter of 4 metres. This is thought to be close to the maximum size the industry is capable of delivering in serial production.

Similar problems with manufacturing quality have been reported for 6–8MW low-speed planetary gearbox stages. However, limitations on quality due to size constraints have always proven solvable over time.

Ongoing industry progress allows increasingly bigger components while maintaining stringent quality standards, through a combination of improved design, better materials and overall technological advancement. In parallel, alternative solutions might make it possible to circumvent existing technology and/or manufacturing-related bottlenecks and challenges.

Blade design innovation

One huge design challenge has been to develop today’s long slender blades — and the even longer ones likely in the future — combining the lowest-possible blade mass with sufficient built-in stiffness and strength. Limiting blade deflection under load is essential to prevent blade tips hitting the tower at high-wind operation and during yaw actions.

One option is to integrate carbon fibres into a blade’s load-carrying structural parts, combining optimised stiffness and favourable mass characteristics. But several industry experts remain sceptical about carbon, as the fragile material is expensive, much harder to process and more difficult to integrate compared with failure-tolerant, glass fibre-based composites. Today’s longest blades, made by Siemens and LM/Alstom, do not contain carbon.

A radical approach that, among other things, allows the use of long blades, is to mount the rotor behind the tower. With these downwind turbines, the approaching wind now ‘pushes’ the blades away from the tower.

German engineering consultancy Aerodyn has developed a 6.5MW two-bladed, medium-speed SCD 6.5 downwind turbine with a 140-metre rotor diameter. The company says it is especially suited to typhoon conditions like those encountered along the Chinese east coast.

The downwind principle allows passive or free rotor yawing with the yaw system released, enabling the rotor to follow frequent changes in wind direction. During typhoons, the combination of downwind and the rotor locked in horizontal position is said to minimise rotor loads and overall stresses on the turbine.

Dutch company 2-B Energy’s 6MW downwind turbine, currently in development, features a 140-metre rotor diameter placed atop a "lattice-type" truss tower. The structure itself extends from the seabed to the bottom of the nacelle and replaces a traditional tower-foundation combination.

Development initiatives for two-bladed machines are on the increase but because of limited overall offshore operating experience, this technology principle — rightly or wrongly — faces ongoing scepticism across the industry.

Price volatility

This raises the wider question of which drive technologies are poised to emerge as ultimate winners. Market wisdom suggests that high prices kill demand. A key driver for the current medium-speed trend is the requirement for much lower quantities of rare earth materials for PMG magnets compared with direct drive. This is directly linked not only to high prices but even more to the perceived risk of future price volatility.  

The same wisdom could explain the emerging renaissance of classic generator solutions not requiring permanent magnets, perhaps HTS-type in future, or new variable rotor to fixed generator speed drive solutions.

What will happen when — sooner rather than later as some are predicting — turbines reach 10, 15 and even 20MW, is also hard to predict. The fact is that for the immediate future, a substantial number of new 5–7MW+ turbines will enter the market from 2014, representing a key market focus and the beginning of many product life cycles.

The few existing suppliers of 5–6MW+ turbines also depend largely on future turnover for their return on investment, while in all cases building a supply chain takes time and effort. The same reasoning could apply to a large share of existing and new-built installation vessels.

Each new size leap will present substantial challenges regarding design, manufacturing, transport and logistics, and supply chain demand.

Wider factors within the economy, the trend for large-scale volume production of semi-standardised products and the optimisation of technology beyond turbine size alone look likely to dictate what pace of upscaling is feasible and desirable for the turbine market and the offshore wind industry.