Larger turbines have historically been considered suitable only for offshore applications, largely because of the constraints imposed by geography and transport logistics on moving and installing large towers, blades and nacelles. Other limitations relate to the physical and legal restrictions imposed on both hub and total installation heights.
However, new super-class onshore models indicate that this might be about to change.
Breaking the barrier
Germany's Enercon has been offering its E-126 turbines since 2007 exclusively for onshore use, initially in a 6MW form, later upgraded to 7.5MW. These giant turbines come with a standard prefabricated concrete tower, reaching 135 metres in hub height and a total installation height of 198.5 metres. By late October 2013, 52 Enercon E-126s were in operation, plus ten of their 4.5/6MW E-112 predecessors.
Following an operational wind farm in Belgium with 11 E-126 turbines, the largest E-126 project is currently under construction in the Netherlands, comprising 38 E-126 turbines.
The 3.6MW Siemens series, today's offshore workhorse, originates from 2004. The company recently announced the 2014 delivery of six SWT-3.6-120 turbines with an enlarged rotor diameter of 120 metres (initially 107 metres) to a coastal project in northern Germany.
Siemens's new SWT-4.0-130 turbine, with its 130-metre rotor diameter, could play a similar or growing role for high-wind onshore projects for which developers tend to prefer higher-rated units.
Gamesa installed the prototype of its G128-4.5MW onshore turbine series in 2009. This model features composite segmented blades, which, according to the company's technical documentation, provides a similar level of logistic complexity and construction requirement to Gamesa's smaller 2-2.5MW models. A FlexiFit add-on crane, for example, enables simplified installation and exchange of main components while the mass of individual main components and systems has been maximised.
Gamesa's scaled up G128-5.0MW successor turbine, currently being tested at a Spanish high-wind island location, is being marketed for onshore and offshore use with single-piece blades.
A new onshore version with an increased rotor diameter of 132 metres and single-piece 64.5-metre blades, indicates that views on road transport logistic capabilities may have evolved in recent years. Both 5MW onshore models come with hub heights up to 140 metres, which gives a total installation height of 204.5 metres for the G132 unit.
A total of 11 4.5MW Gamesa turbines were operational in 2013, with orders for nearly 200MW, concentrated in Finland. A recent major onshore framework agreement, again in Finland, has been added for 57 5MW turbines.
XEMC Darwind, a Dutch company with China's XEMC Windpower the majority shareholder, plans to enter Europe's main onshore markets with two new 4.5-5MW direct-drive turbines based on the 5MW XD115 offshore model.
During 2011 XEMC Darwind installed a XD115/5MW prototype designed for IEC wind class 1B in the Netherlands. Less than a year later a second prototype was installed near-shore in China.
The first XD115/4.5MW model version - with an unchanged 115-metre rotor diameter — is destined for noise-constrained IEC class 1A/1B high wind speed sites, enabled by slightly reduced-rated rotor speed simultaneously reducing rated power output.
For onshore locations without noise limitations, the XD115/5MW version will be supplied to maximise output. The 4.5MW turbine is technically very similar to the 5MW unit, but features a modified onshore-adapted generator cooling system that partly uses clean ambient air, enabled by less stringent completely sealed nacelle demands.
The company's second onshore model, the XD133/4.5MW, is for low and medium-wind IEC class IIIA/IIB conditions. It has a newly developed generator with increased length, and slender blades offering an enlarged 133-metre rotor diameter.
The XD133/4.5MW comes with tubular steel towers up to about 100-metre hub height, or with a concrete-steel hybrid that raises up to 140m with a total installation height of 206.5 metres. The ability to cross the 200-metre barrier is achieved with a combination of low head mass and the availability of cost-effective high-tower designs, together with growing acceptance of such landscape-dominant machinery, according to the company. The XD133/4.5MW prototype is planned for the first quarter of 2015 with commercial roll-out planned for the same year.
The first markets to be targeted for both 4.5MW models include northern and eastern parts of Germany, the Netherlands, the UK and Ireland. For Ireland, with it grid network constraints that limit the integration of more wind power, a planned interconnector cable with the UK might open up fresh opportunities.