Furthermore, EWEA’s scenario report states, cumulative offshore wind capacity in Europe may reach 40 GW by 2020 and 150 GW in 2030. However, despite the importance of a rapid increase in offshore capacity, much more relevant than adding megawatts is a shift in focus towards optimising power generation potential per installed MW for the lowest lifecycle costs. Several examples of leading wind turbine suppliers that have developed dedicated solutions for specific (environmental) circumstances emerged at the European Offshore Wind event.
Among many milestones that have taken place in the wind industry during 2009 was the installation of China’s first 3-MW (rotor diameter 90 metres) Sinovel offshore wind turbine. This first unit of the 100-MW Shanghai Donghai Bridge demonstration project undoubtedly marks the first of many future offshore wind farms to be built along Chinese shores.
Among several future Chinese wind industry players is former Dutch entrant DarWinD, which had just commenced building a 5-MW direct-drive prototype turbine before its June 2009 bankruptcy. Chinese firm Xiangtan Electric Manufacturing Group (XEMC) acquired DarWinD hardware and Intellectual Property (IP). Amongst other things XEMC manufactures generators, control equipment, large castings and forgings. The company already manufactures a 2-MW wind turbine in cooperation with Harokasan of Japan that has owned the direct drive technology from former Dutch group Zephyros since 2005.
DarWinD’s turbine concept also builds on Zephyros wind technology. Company sources say that the renamed XEMC DarWinD plans to erect a land prototype in the Netherlands and an offshore prototype in China.
US Offshore Wind
Recent reports from the USA refer to about 37 offshore wind projects that are ‘in the works’. A further prediction is that within the next five years the first U.S. offshore wind project will come on line.
Through its ScanWind purchase in September (see REW September–October 2009), GE can potentially become a major player in the emerging US offshore market, even though the initial focus is on the European sector, according to comments made by GE’s vice president of renewables, Victor Abate.
With regard to further ScanWind technology development, Abate explicitly referred to his company’s workhorse strategy, which is based upon producing large quantities of a limited range of products: ‘When we acquired Enron Wind in 2002, about 1000 1.5-MW turbines were operational. Today total numbers in operation worldwide exceed the 12,000 mark. The Arklow Bank offshore project served as a learning platform for us and we are excited about ScanWind direct drive technology in terms of proven reliability. At the moment availability of their turbines is already 96%–97%, achieved by operation under harsh Norwegian coastal conditions.’
According to the company, next year GE’s offshore turbine will be offered to the market, with first deliveries planned for 2012–2013. However, Abate did not make any specific mention of major product parameters such as the product power rating and rotor size.
Perhaps unsurprisingly, the long-delayed German offshore wind market kick-off – and many projects being in deep water locations far from shore – coincides with a steadily expanding market entry role for ‘super class’ 5–6 MW installations. The fact that Areva Multibrid and REpower will each erect six of their 5-MW turbine models in German waters this year can be regarded as another genuine milestone in offshore wind development.
This year a third German supplier, BARD Engineering, plans to begin installation of 80 Tripile foundations for its 400-MW BARD Offshore I North Sea project, in many respects a huge challenge. The Areva Multibrid and REpower 5-MW prototypes were erected in 2004, while the BARD prototype installation followed three years later.
Second Generation
REpower took 2–2.5 years to complete its second-generation 6.15-MW 6M, an up-scaling development of the 5M offshore turbine, now a serial product, with its initial capacity of 5 MW. The 126-metre rotor diameter remained unchanged and earlier this year the company erected three 6M onshore prototypes. Novelties associated with the design include a new 6.6 kV water-cooled generator (previously 950 kV) and a newly designed gearbox, with a 20% higher torque rating but a similar component mass when compared with the 5M.
Furthermore, the 6M’s frequency converter system features three modules, which as a redundancy measure enables continued operation at a reduced output of 67% in the event that one module fails. REpower says that the 6M yields 10% more energy compared to the 5M at sites with an average wind speed of 10 m/s (using LM 61.5 metre P blades), and will become available for offshore projects from 2012. In future the 6M will be delivered with in-house developed and manufactured RE 61.5 metre blades.
BARD Engineering focuses on high-wind offshore applications and has announced plans to erect two different prototype versions of its second-generation 6.5-MW BARD 6.5, with an unchanged 122-metre rotor diameter, in 2010. One version comprises a hydrodynamic WinDrive system from Voith that enables variable speed wind turbine operation combined with a directly grid-connected fixed speed generator, eliminating the need for a power converter. A second version will be fitted with a ‘conventional’ mechanical drive system (for more on this development see REW July–August 2009).
The most powerful turbine on the market currently is Enercon’s E-126 machine. This IEC Wind Class 1A direct drive turbine (with a rotor diameter of 127 metres) is now rated at 7.5 MW (previously 6 MW), but is not currently available for the near shore and offshore wind market.
Meanwhile, two novel offshore wind turbine development projects have come from the Netherlands. Emerging player in the sector 2-BEnergy is developing a 6-MW two-bladed turbine concept with a 130-metre rotor diameter and with prototype erection envisaged for early 2011.
The turbine will be fitted with a conventional geared drivetrain system, while downwind or upwind rotor configurations are both being evaluated as potential options, said company spokesperson Herbert Peels this September. A genuine offshore wind industry novelty is that the nacelle is mounted on a so-called ‘full truss’ support structure. This arrangement eliminates the traditional ‘tower plus foundation’ solution as, instead, an integrated single assembly extends from the seabed to the yaw bearing below the nacelle.
Denoted as the Delft Offshore Turbine (DOT), a second Dutch research and development project that commenced in 2008 is pursuing a radical step away from incremental offshore wind turbine development. Aimed at designing ‘the ultimate offshore turbine’, a key goal of the project is to drastically reduce the number of key components compared to ‘conventional’ state-of-the-art wind turbines currently applied offshore.
The DOT concept consists of a two-blade, fixed-pitch 5–10 MW wind turbine arrangement that directly drives a ‘closed loop’ water displacement pump located in the nacelle. High-pressure water flow generated by this pump drives a submerged hydraulic motor coupled to a second pump. That component in turn is part of an ‘open loop’ high-pressure seawater-based circuit. High-pressure seawater flow from individual wind farm turbines is then channelled to a central Offshore High Voltage Station (OHVS) where multiple hydro-generators convert the hydraulic power into electrical power of the required voltage level.
Delft researchers view power conversion into electricity at centralized level as a substantial system simplification and a major advantage compared to the current state-of-the-art solution in which electrical power conversion is executed at wind turbine level. In an approach analogous to ‘conventional’ offshore wind farms electrical power from the OHVS is finally fed to the onshore high-voltage network.
According to an ambitious programme, DOT technology will be ready for commercial applications as a package within the second half of the next decade.
Elsewhere, UK-based wind industry newcomer VertAx Wind Ltd revealed plans to develop a novel 10-MW three-blade vertical-axis offshore wind turbine it expects to enter the home wind market commercially by 2014. Its massive H-shaped stall-type Darrieus rotor is expected to reach 140 metres in diameter, while the blade length is 110 metres. The turbine will further be fitted with twin 5-MW direct driven generators made by Converteam, each located at a different elevation level. A major feature supporting operations and maintenance is a helicopter-landing platform integrated within the tower’s top section.
The company’s overall objective is to substantially reduce the cost of offshore wind power generation by offering installations boasting the fewest number of moving parts currently known in industry. The required total development budget, including a completed prototype, is estimated at about £35 million (€40 million).
In a more advanced product development phase than either DOT or VertAx is the 10-MW Clipper C-150 Britannia offshore turbine development, taking place in Blyth, in the UK. This giant will be fitted with in-house developed 71.5-metre rotor blades resulting in a record 150-metre rotor diameter, and the design is to be certified for a 30-year life.
As with the 2.5-MW Liberty turbine, this new machine also features a geared distributed drive system with four generators. And, according to Clipper Windpower Marine Limited managing director David Still, the total number of drive system components has not increased, despite a much larger total gear ratio (i.e. rotor speed 6.05–11.5 RPM). Still explained, ‘Another major benefit of the drive system choice is that the turbine can continue operations with one or more generators removed until replacement is performed.’
He further clarified confusion that has arisen over the C-150 power rating, saying that the land prototype will generate a maximum of 7.5 MW. The 10-MW offshore version will ‘simply’ spin a lot faster. Also remarkable is that the increase in the turbine’s Top Head Mass (THM) – nacelle + rotor – associated with the increased output (2.5 => 10 MW) will not be cubed but linear. Elaborating on product specifications Still said: ‘The C-150’s THM will be in line with the figure of state-of-the-art conventional 5–6 MW turbines, [Editors's note: state-of-the-art is about 350–460 tonnes] that in turn provides the right preconditions to our overall strategy of achieving a 50% reduction in installed costs per MW, and a further 30% reduction in foundation costs per MW.
With regard to component mass, individual 2.6-MW Britannia generators are, for instance, only about 20% heavier compared to their 0.66-MW Liberty equivalents.’ He added that this favorable figure can partly be attributed to a switch from 1320 V DC to a 3600 V AC, which requires less copper. A second major contributing factor to mass reduction is the fact that, according to Clipper technical specifications, Britannia generators turn about twice as fast when compared with the Liberty machine’s generators, rated at 2270 versus 1133 RPM, respectively. Testing the turbine’s subassemblies commenced in 2009 and will continue through 2010, with full drivetrain and blade testing scheduled in early 2011. Clipper plans to erect the onshore prototype by the end of 2011.
Offshore Workhorse
Siemens Energy released a second-generation 3.6-MW offshore wind turbine featuring a 120-metre rotor diameter. The new model’s power to swept area ratio (P/A) is only 0.32, a value that until recently was usually restricted to onshore turbines operating in low and medium wind speed conditions. Technologically the SWT-3.6-120 is based upon the proven SWT-3.6-107 (prototype erected in 2004) that has developed into the offshore wind industry workhorse over the last few years. For example, the world’s biggest offshore project so far, Greater Gabbard, will comprise 140 of the SWT-3.6-107 turbines when completed, planned for commissioning in May 2011.
‘We anticipate that our SWT-3.6-120 will generate roughly 10% more electricity at a typical offshore site compared to the SWT-3.6-107’, commented Siemens Wind Power CEO Andreas Nauen on the new product’s potential. Siemens has already installed 100 of its 3.6 MW turbines with another 700 units on order and earlier this year Dong Energy signed orders for over 450 SWT-3.6-120 machines.
Since 1995, Vestas Wind Systems of Denmark has installed over 400 offshore wind turbines with a total capacity exceeding 900 MW. In recent years the company has lost market share to Siemens, but aims to recapture it with a new V112-3.0 MW offshore model. According to the company, the new flagship turbine delivers optimal output in average wind speeds up to 9.5 m/s and will be individually IECS class certified for each separate offshore project.
V112-3.0 MW Offshore marketing will commence immediately. Vestas will install two V112-3.0 MW prototypes onshore in January and mostly likely April 2010 followed by serial delivery in 2011, a company spokesperson stated. Unlike the lightweight V90-3.0 MW turbine applied in several offshore wind farms, the new flagship features a conventional non-integrated geared drive system and, in a novelty for Vestas, a permanent magnet-type synchronous generator.
In addition, on 27 October Vestas partially lifted the curtain on a new 6-MW offshore turbine with a 130–140 metre rotor diameter that it is working on. Other technical specifications and envisaged market introduction time scale details have not been made available yet, but according to well-informed wind industry sources it will be a direct drive turbine fitted with a Vestas-designed annular generator.
Finally, Vestas has joined a research programme with Nowitech of Norway on the development of floating foundations suitable for water depths of more than 30 metres.
Downwind Floating Turbine Cooperation
Earlier this year French nuclear engineering giant Areva acquired German rotor blade manufacturer PN Rotor GmbH. The latter company was owned by Prokon Nord Energiesysteme GmbH and supplied all rotor blades for Multibrid M5000 turbines. Prokon is an Areva Multibrid minority shareholder with a 49% stake and also built and operates a foundry for M5000 main castings.
In a separate development, Areva and Norwegian renewable energy company SWAY AS announced a cooperation deal in August aimed at offering new solutions for exploiting offshore wind power in deep water. Areva Multibrid will adapt its 5-MW M5000 wind turbine, which was specifically designed for offshore applications, to enable downwind operation on SWAY’s new tower solution.
‘Our aim is to demonstrate that deep-water wind power is commercially attractive within the next four years’, said SWAY founder and CEO Eystein Borgen earlier this year.
SWAY has been granted a licence from the Norwegian Water Resources and Energy Directorate to build a floating offshore wind turbine approximately 7 km off Karmøy on the country’s west coast. Prototype construction is conditional on support from the recently established Norwegian financial support programme for marine renewable energy (Enova).
Finding a customer for this project is nonetheless essential and when that key hurdle is overcome the wind turbine may be up and running in 18-24 months, according to an optimistic Borgen, who said: ‘Our ambition is to demonstrate that such plants in a commercial phase shall be able to supply power at a price competitive with shallow water wind parks.’
The demonstration plant features a 188-metre tower structure, of which 104 metres is submerged. The ballasted tower bottom is anchored to the seabed with the aid of tension leg technology, including a suction anchor. A genuine novelty is a subsea yaw mechanism located between the tension leg and the tower plus wind turbine assembly. The downwind rotor allows the floating tower to tilt (6-8 degrees) due to wind pressure (thrust), and is claimed to be acceptable in terms of yield loss. Transformers and power electronics will be located in the tower, and a subsea cable will connect the plant to the onshore electrical grid.
All-Weather Access
One of the many specific challenges linked to offshore wind power is all-weather installation access. A combination of turbine failure and being unable to access the machine during adverse weather conditions can easily result in extended downtime and subsequent loss of revenue. A common transfer access method to offshore wind turbines on fixed foundations is with the aid of a ‘people mover’ or other service vessel. The rubber-lined bow section is steered firmly against a ‘matching’ steel structure that is often an integral add-on part of the substructure/foundation. Engaging full throttle on the vessel introduces increased friction between it and the corresponding steel structure and slows down wave movements to such an extent that it affords relative safe passage to service personnel.
Among alternative, novel, access technologies is the Ampelmann, developed by Dutch researchers at the TU Delft. The vessel-based self-stabilizing platform actively compensates for all ship motion. Ampelmann systems are already employed in the North Sea and elsewhere and they are claimed to reduce the necessity for jack-ups, semi-subs and helicopters, potentially turning it into a cost effective system, even under marginal weather conditions.
In comparison, according to an expert delegate, floating offshore wind turbine access from a vessel represents a much bigger challenge as waves and wind cause both objects to move continuously. Another challenge may prove to be conducting servicing activities, including the exchange of large and small components (like circuit boards) alike while the entire structure continuously moves in all directions.
The Future’s Out There
During 1991 former technology pioneer Bonus of Denmark (now owned by Siemens) erected the world’s first offshore wind farm comprising of 11 of its 450-kW fixed-speed stall regulated turbines in shallow water at Vindeby. Since then wind turbine size and complexity, average offshore wind farm capacity, distance to shore and water depth have all grown dramatically. Despite the challenges such changes have inevitably wrought, a steadily increasing number of international players continues to reinforce the rapidly expanding wind industry base.
Meanwhile, highly experienced offshore wind market entrants complement established parties with new spirit, fresh ideas and, perhaps most importantly, a strong belief in what they individually and as partners can accomplish. Accelerated availability of reliable high-performance wind technology, fast vessels and clever installation methods, all-weather access technologies, well-trained specialists, and sufficient finance are all essential preconditions for meeting Europe’s ambitious offshore wind objectives.
Equally important for streamlining and speeding up projects and processes is a facilitating role at national and EU political level, with committed leaders taking the responsibility to act. The idea that a winning combination of ambitious renewables (including wind power) and environmental targets can also create many well paid long-term green careers is also fortunately gaining ground. In this respect Swedish deputy prime minister & minister of Enterprise and Energy, Maud Olofsson, deserves much praise. During the conference opening session she inspired the audience by voicing these much needed views and visions.
Eize de Vries is a wind technology correspondent for Renewable Energy World Magazine.
Sidebar: New Products
Offshore wind farms could not function without subsea cables transporting the electrical power to shore for feeding into the high-voltage network. Currently four large cable suppliers dominate the world market, including ABB, Prysmian (formerly Pirelli), Nexus, and Draka. The latter has manufactured electricity transport cables for 100 years, including 30 years of subsea cables. The company’s commercial director of subsea cables, Martin Dale, explained: ‘In past years Draka made a large investment in our Norway plant capable aimed at producing a new generation (up to) XLPE 36 kV AC medium voltage cables for the offshore wind market. [Editor's note: Such cables typically extend from the offshore high voltage station (OHVS) – where the cumulative wind farm power output is brought together and transformed to the required voltage level – to shore.]
Draka has now expanded 36-kV AC cable production and in addition offers a complete range of accessories and installation services along with circuit testing or commissioning. We in fact offer a complete solution from design through installation that helps make project management and field installation for our customers more efficient than trying to coordinate several vendors and installers.’
Commenting on dedicated product features Dale says that a lot of effort was put into making the XLPE series more flexible when compared with state-of-the-art equivalents. A second built-in product feature he mentioned explicitly is an extra protective layer aimed at enhancing long-term marine performance.
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