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Finding the optimum low-wind design combination

WORLDWIDE: Power from wind depends upon two main variables: the wind speed cubed at hub height and rotor-swept area.

Direct drive… A prototype of Enercon's 2.5MW E-115 is planned for this year
Direct drive… A prototype of Enercon's 2.5MW E-115 is planned for this year

As fewer favourable high-wind speed sites are available, interest in effectively exploiting regions with lower wind speeds is growing worldwide. But in order to optimise results at these locations, wind technology is having to evolve, and equipment is being developed to target this market.

Developers have three hardware-configuring options to achieve economically viable yields at inland sites with low and medium wind speeds: increase rotor-swept area for a given power rating; put the turbine on a higher tower; or use a combination of the two, which in many circumstances offers the best package in terms of energy production potential, equipment durability and appearance.

A new market is born

Early this century, the first-generation 2MW turbines were typically fitted with 70-80 metre rotor diameters. They represented the top of the market with specific power ratings of 520-398 Watts per square metre (W/m2). These turbines were generally used on high-, mediumand low-wind sites, accepting inherent and substantial yield variations.

Vestas was the first manufacturer to develop a turbine specifically for low and medium wind conditions in 2009. Its V100 1.8MW turbine was an expansion of the V80-2.0MW/V90-2.0MW product platform and featured a combination of enlarged 100-metre rotor diameter and reduced power rating. Together, these features gave it a much-reduced 223W/m2 specific power rating.

However, it is US firm GE that is widely credited for kick-starting today's major low-wind turbine trend in 2010 when it introduced a 1.6MW turbine model with 100-metre rotor diameter. Based on the proven 1.5MW series, the extreme combination of such a low power rating for a 100-metre rotor swept diameter was initially met with scepticism, with many believing that a turbine configuration with 204W/m2 specific power rating for the 1.6-100 was too extreme.

But GE turned the product into a huge commercial success. The extreme configuration turned out to be a successful marketing ploy and despite all the scepticism the model apparently sold out in a few months, prompting a range of other suppliers to introduce their own low and medium wind speed models. Nordex, another low-wind pioneer, successfully began series production of a 2.4MW N117/2400 onshore model last year, offering 223 W/m2.

Slow turn

With an enlarged rotor, low and medium wind turbines must turn more slowly so that the rotor blade tip speed range is maintained to curb aerodynamic noise. As a result, the gearbox ratio of geared turbines must be increased to retain the generator operating-speed range. With direct-drive turbines, the generator and rotor speed are identical. If the rotor speed drops, but the power output remains the same, the generator torque increases, which requires generator modifications to prevent overheating.

As well as drive-system configuration, structural modifications might be necessary, such as a strengthened hub with stronger pitch bearings, a thicker main shaft, larger diameter main bearings and/or extra yaw motors.

Low-wind turbines placed on to high towers offer significant benefits in forested inland regions because the wind profile could start building upward from the treetops rather than ground level as is normally the case.

Natural obstacles such as hills and forests, or man-made structures like buildings, all act as wind-flow disturbances, slowing down wind speed while simultaneously raising turbulence level. Enercon has been pioneering wind-power generation at German inland sites for years, developing in-house concrete and concrete-steel towers, which now have hub heights up to 149 metres. These enable the whole rotor to be raised well above tree tops or other obstacles into relatively undisturbed, stable, strong winds.

As a rule of thumb, annual energy production increases by about 0.75-1% for each extra metre of hub height but site-specific conditions will affect this. Also, as wind flow is more consistent at higher locations, as there are fewer obstacles, the height-related increments typically drop with increased elevations. However, reduced turbulence also means the operating life of a turbine could be longer.

Latest models

The 2.7MW Alstom ECO 122 turbine builds on the proven 3MW ECO 100 and ECO 110 turbines. With its 2.7MW power rating and 122-metre rotor diameter, the ECO 122 is one of the more powerful low-wind turbines offering 231W/m2.

Earlier this year, Alstom signed a memorandum of understanding (MoU) with German construction specialist Max Bogl to develop a 139-metre concrete-steel hybrid tower for ECO 122 turbines aimed at Germany's inland wind market.

The MoU also covers a technology partnership for developing an innovative tower, nacelle and rotor installation method for sites with space constraints, such as forested areas, which involves self-climbing tower cranes.

A prototype is planned for the second half of the year and series production should start in early 2014. Even before prototype installation, Alstom has secured orders from Brazil for at least 200 units.

The 2.5MW Enercon E-115 direct drive wind turbine is a low-wind de-rated sister model of the 3MW E-101 turbine, but with an enlarged 115-metre rotor diameter. Designed for inland sites, the E-115 features a liquid-cooled ring generator and will be fitted with new-generation, slender, segmented rotor blades for easier and cheaper road-transport logistics. The inner composite blade section measures about 12 metres and the composite outer sections around 44 metres - they are joined on site. A specific power rating of 241W/m2 is somewhat higher than some competing products, but Enercon turbine blades feature a yield-boosting spoiler integrated into the inner section as standard. The E-115 is available with hub heights of 92-149 metres, with the latter configuration offering a total installation height of 206.5 metres. Introduced at the 2012 Husum trade fair, a prototype is planned for this year with series production to start in 2014.

Gamesa's 2MW geared turbine platform dates back to 2002. The initial 80-metre rotor size has been expanded with 87 metre, 90 metre, and 97-metre options. The latest platform extension is the Gamesa G114-2MW, an IEC class IIIA turbine with trendsetting 114-metre rotor diameter offering a wind industry record-low 196W/m2 specific power rating. A prototype is planned for September or October this year.

The recently introduced GE 1.7-100 is the second turbine to carry GE's 'Brilliant' branding and is an optimised version of its 1.6MW 100-metre rotor diameter 1.6-100 model. The company claims that this model has benefits over its current technology, including electrical-system upgrades and industrial internet capabilities such as data handling and networking.

The 1.7-100 specific power rating is somewhat higher compared with the 1.6-100 - 217W/m2 versus 204W/m2 - but claims to generate 6% more energy when both are measured at average wind speeds of 7.5m/s.

Simultaneously, a 6% nameplate power rating increase from 1.6 to 1.7MW without changing the rotor size further reduces the 1.7-100 capacity factor from 54% to 53%. GE announced it is supplying 59 1.7-100 turbines with a hub height of 80 metres to a project in Michigan. Fine-tuning the 1.6-100 into the latest 1.7-100 clearly highlights that wind-turbine product development, testing and measuring is a continuous learning process.

How to choose a turbine

As always, the most important factor for buyers of low-wind technology to bear in mind is the cost of the energy generated over the 20-year lifetime of a turbine. Potential buyers need to work out how much it would cost to invest in and operate a machine compared to how much energy it will generate. This lifecycle cost of energy is what really counts.

Therefore, when choosing a low-wind product, turbine configurations, in terms of hub heights and blade length, power specifications and operational performance figures are all key and require careful consideration both as individual factors and in cross comparisons.

Above all, focusing on the lowest W/m2 figures or maximised capacity factor alone could miss the point that the only thing that really counts is how much the energy produced over 20 years is going to cost.

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