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Technology 2012 - Industry innovations: Energy cost pressure stirs creative juices

Eize de Vries outlines the main trends and looks closer at the many developments in rotor blades, drive systems and towers.

Nordex says its N117/2400 machine can generate 3,500 full-load hours
Nordex says its N117/2400 machine can generate 3,500 full-load hours

Challenging market conditions have sparked heavy competition and spurred a continuous flow of innovative solutions, ranging from complete wind-turbine designs to main components and intelligent systems. Equally essential for the progress of the wind industry are the small-step evolutionary advancements, which over the years have delivered many successful optimised product designs.

The single most important factor driving new technologies is the need to reduce further lifecycle-based costs of energy (CoE), using clever designs to boost system performance with higher reliability, reduced loads, and lower investment and operating costs. A key objective for the offshore wind industry has become the need to substantially reduce current peak generating costs of about EUR170/MWh during a 20-25 year operational period to around EUR120/MWh or even lower by 2020. Onshore generating costs are generally much lower, but environmental conditions and turbine-specific variables have to be considered with the aim of matching, and in the future perhaps even beating, the costs of conventional and nuclear generation.

Large rotors

A technological development that is already having a huge impact is the fitting of large rotors to turbines of a given power rating, a move now valid onshore and offshore. Vestas initiated this trend in 2009 by introducing its V100-1.8MW onshore turbine with a 100-metre rotor diameter. Several competitors followed, including GE with a 1.6MW machine with 100-metre rotor diameter, and Nordex with its 2.4MW machine with a 117-metre rotor diameter. A more recent example is Gamesa's 2MW turbine with 114-metre blade diameter. A decade ago, 2MW and 80 metres was top of the bill; this advance represents a doubling in the rotor-swept area per megawatt.

Nordex says its N117/2400 machine at an inland site with 6-6.5 metres per second (m/s) average wind speed at hub height generates more than 3,500 full-load hours, corresponding to a 40% capacity factor. This is the ratio between all-year-round (8,760 hours) power generation at full capacity and the actual yield level, so, in the Nordex example 3,500/8,760 = 0.4 or 40%.

Many older-generation turbines, typically with much smaller rotors and short towers, under similar conditions, would only achieve capacity factors in the range of 20% or lower.

Taller towers

Another major parallel trend is the setting of onshore turbines on higher towers for better wind speeds. The more stable wind conditions found at greater heights can improve a turbine's operating life. As a rule of thumb, a given turbine operating with wind speeds of 6.7m/s at 70-metre hub height could generate 20-25% more if placed at 100 metres. The increment per extra metre added usually diminishes with height due to fewer obstacles that could slow down wind flow.

For offshore use, the number of new wind turbine models, especially in the 6-7MW class, keeps on growing. Rotor diameters of the 6MW turbines have jumped from a maximum of 126 metres to more than 150 metres today.

Size is still largely linked to nameplate capacity. The onshore wind market has gradually moved upward from about 1.5-2.5MW to 1.6-3MW-plus. Some experts predict 3MW-plus to become a future maximum in many wind markets, but with incremental rotor-diameter increases to optimise output per megawatt. One technology developer has indicated plans for a 3MW turbine with a 140-metre rotor diameter.

In countries with space constraints, 6-7.5MW-plus turbines could potentially develop into an additional (niche) market segment. Market potential in all cases depends on permitting restrictions, especially with regards to maximum installation height.

Offshore giants

Opinions are divided regarding future offshore turbine upscaling. Some believe that 6-7MW is only a transitional class, a stepping stone towards future 8-10MW-plus installations. Others argue that due to their increased sizes and mass, the latter will require huge investment in new factories, next-generation installation vessels and other necessary measures. There are also major manufacturing quality issues to consider with continued upscaling of components, and these are not always realised in full. Even bolder plans for 10-15MW turbines have been launched, while a leap into the 20MW-plus class was explored in the EU Upwind programme. Only time will tell whether these mega-turbine plans are realistic and feasible.

The hype that saw direct-drive generators using rare-earth-based permanent magnets as the ultimate drive system solution for future wind turbines had peaked by mid 2010. The price-hike of rare-earth elements in the first half of 2011 and perhaps increased fears of price volatility have at least coloured that perception and, some say, have fuelled a major technology rethink.

At least one supplier has redesigned a permanent-magnet-based-direct drive generator into a "classic" design with electrically excited field magnets. And the leading direct-drive supplier, China's Goldwind, has announced that it has developed a turbine with a medium-speed, two-stage gearbox and permanent-magnet generator (PMG).

System design trends

Two-stage systems are claimed to offer superior CoE performance compared with direct-drive and single-stage, low-speed systems, all PMG-based. Some other technology developers and suppliers are reported to be working on lowand medium-speed systems, with either conventional synchronous or asynchronous generators.

Nearly all turbine configurations - with or without gearbox - operate with variable rotor speed and therefore require either a partial or full power electronic converter to convert generator power with variable frequency into grid-compliant 50Hz/60Hz power.

Doubly fed induction generators, or DFIGs, need only a partial converter rated at 20-34% of nominal power rating. A smaller rated converter is cheaper and reduces internal losses, making the combination with DFIG an economical and popular choice for high-speed turbines up to 6.15MW. PMGs still occupy a minority share in the high-speed segment but seem gradually to be gaining ground. Vestas switched from DFIG to PMG in its 1.8MW/2MW GridStreamer series, based on considerations relating to lower mass, being more compact and with higher partial-load efficiency. PMGs are also claimed to be superior in meeting future grid requirements compared with DFIG, including in high wind power penetration markets.

However, one leading turbine supplier is reported to have carried out a reverse product redesign from high-speed PMG to DFIG, a strategy it said was aimed primarily at lowering the turbine cost price.

Several suppliers are in the process of developing, or have already developed, alternative drive solutions, ranging from mechanical-hydraulic to full hydraulic or full mechanical systems. Most of these enable an operational combination of variable rotor speed and fixed generator speed, eliminating the need for a power electronic converter.

Two-blade turbines

Horizontal-axis, three-bladed turbines continue to dominate the global wind market, but at least four different wind-industry players are working on two-bladed equivalents aimed especially at offshore use. Envision's 3.6MW E128 is referred to as a modular turbine aimed at "proof of concept" demonstration rather than a prototype.

The test programme focuses on two main product innovations: a rigid, two-bladed upwind rotor with partial-pitch control, where the outer blade section can be turned. UK-based Condor Wind Energy is developing a 5MW upwind turbine with yaw control, a technology that turns the rotor plane gradually out of the wind at high-wind conditions. SCD of Germany has developed a downwind 6.5MW medium-speed offshore turbine, as well as a smaller 3MW medium-speed upwind model, both licensed to Ming Yang of China. The smaller version is already operational. Finally, 2-B Energy of the Netherlands is working on a 6MW downwind turbine.

If wind turbines with high capacity factors become accepted, they offer a huge potential for driving down the costs of energy from wind power. Onshore and offshore turbines offering more megawatt hours per megawatt at the lowest possible lifecycle cost will ultimately prove the real winners regardless of their key technology preferences and principles.

Drive system innovations

Earlier this year Peter Tavner, professor of new and renewable energy at Durham University in the UK, presented his novel brushless DFIG design. This avoids the need for expensive magnets and is claimed to offer higher reliability and hence a lower cost of energy than conventional brush-based DFIGs. Also of UK origin is a modular axial-flux generator for direct drive and geared applications, developed by NGenTec. Each generator module functions electrically as a separate generator, providing inherent system redundancy. This enables continued operation at reduced capacity if one module develops a failure - a benefit especially offshore, where access can be complex at certain times of the year.

Emerson's French subsidiary Leroy Somer has introduced a copper-only classic synchronous generator in combination with an optimised power electronic converter, aimed at matching PMG efficiency without the disadvantages of rare earth, such as serviceability challenges and over-speed related issues.

Winergy of Germany will soon ship its first medium-speed HybridDrive for fitting in a new 3MW Fuhrlander model. This will be a compact drive system comprising a gearbox and generator that is flanged on, resulting in a combined gearbox-generator unit that is shorter than non-integrated drive trains. Winergy claims HybridDrive only needs 20% of the magnets of a direct-drive generator of similar rating. The current maximum range is 6-8MW, but larger capacities are being researched.

Finnish companies Moventas & The Switch have developed a functionally comparable medium-speed drive system called FusionDrive. A prototype will be fitted in a new 3MW DeWind turbine, while an upscaled 7MW version is in the pipeline. Meanwhile Vestas, Gamesa and DSME of Korea have each announced the development of 7MW medium-speed offshore turbines.

Mitsubishi is developing a 7MW offshore turbine called SeaAngel with hydraulic drive system and two high-speed medium-voltage generators, enabling a combination of variable rotor speed and fixed synchronous generator speed. Norwegian company Chapdrive has developed a comparable hydraulic drive system, but with a single hydraulic motor and medium-voltage synchronous generator. Chapdrive technology has been field-tested in 225kW and 900kW turbines and, in late 2009, the company secured EUR6 million in an industrial research and development agreement with Statoil and Innovation Norway to develop a 5MW hydraulic transmission system.

A mechanical-hydraulic, semi-integrated drive system for future 9MW offshore turbines is in development with HydrauTrans of the Netherlands. Nestor Management Consultants, which early this century led the Zephyros direct-drive turbine development, is co-inventor and initiator. Nestor owner Bart van Neerbos explains that core HydrauTrans technologies include a double-sided, crown-type gearwheel, fitted with six patented "floating-cup" low-speed pumps, a high-speed floating-cup-type motor and a high-speed generator.

He adds: "Artisan-made crown-wheels of wood are known from the mechanical drive systems of classic Dutch windmills. A key advantage compared to bevel gear systems (coning gearwheels) is that the matching pump's pinion wheels can be fitted and removed effortlessly without additional adjustments. The floating cup pumps and motors operate without any metal-to-metal friction, offering long service life and superior efficiency."

The distributed drive system design is aimed at combining system robustness with high efficiency and favourable head mass.

TOWER DEVELOPMENTS HYBRIDS TRENDING

Concrete-tubular steel hybrid structures seem to be gaining ground for hub heights above 100 metres. A total installation height of 200 metres is currently regarded as the legal maximum. Within this hybrid segment Enercon's E-101, with a record 149-metre hub height, reaches 199.5-metre installation height.

Fuhrlander plans to offer a Ventur hybrid tower with hub heights up to 140 metres for its new FL 3000 turbine with a 120-metre rotor diameter. The concrete bottom part comprises multiple pre-stressed, prefabricated elements, mostly up to 3 metres wide and 10 metres long, positioned in a polygon-shaped base frame. In 2006, a 2.5MW Fuhrlander turbine with 90-metre rotor diameter was installed on a lattice-type tower with 160-metre hub height, developed by SeeBA.

Siemens and Danish steel structure supplier IB Andresen have jointly developed an alternative bolted steel shell tower with polygonal shape (pictured below). It consists of multiple, easily transportable, pre-manufactured, installation-ready coning steel sections designed for onsite assembly. Because the tower is built of standardised individual sections, foot-size restrictions found with tubular steel equivalents could be eliminated. A prototype with a 2.3MW turbine atop was installed in late 2011. A planned SWT-2.3-113 direct-drive turbine with a maximum 142.5-metre hub height has a 199-metre tip height and measures 8 metres between the tower base's parallel sides.

BLADE INNOVATIONS LONGER AND LIGHTER

LM Wind Power and Siemens have each developed a long and slender 6MW class offshore blade of 73.5 and 75 metres respectively. Both single-piece components offer favourable mass figures in the 25-26 tonne range by avoiding the incorporation of carbon fibres.

Nordex has introduced a slender blade of nearly 58 metres with carbon fibres incorporated in the load-carrying main girders. The NR58.5 weighs 7% less than its N50 predecessor, at less than 11 tonnes.

California-based Modular Wind Energy has developed a novel segmented rotor blade series, enabling production and transportation in modular sections of around 15 metres. Its innovative assembly technology is claimed to avoid weight penalties linked to 'traditional' bolted segmented blades. The company also claims the blade has a mass up to 20% lower compared to conventional blades, enabling greater blade lengths and a lower cost of energy.

US-based Blade Dynamics has developed segmented blades that allow manufacture in small, individual components and blade transportation in two sections. This patent-pending technology is also claimed to greatly enhance quality and performance, as only uncomplicated production and assembly moulds are required.

The significant Blade Dynamics innovations include the lightweight root attachment and a modular central spar-joining technology, enabling larger rotors 6-12% higher annual energy yield and significantly higher financial returns.

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