The German manufacturer's announcement of a 3.4MW turbine with a record rotor diameter of 140 metres was one of the four low-wind turbines launched at the 2015 Husum WindEnergy fair last September.
The new model builds and expands on the company's 2MW and 3MW designs, but its real novelty is its 68.5-metre single-piece blade, the longest yet announced for an onshore turbine. A prototype of the 3.4M140 is expected in 2017, with series production due to start the following year.
The 3.4M140 is aimed at future low-wind market conditions in Germany, says Senvion product manager Stephan Kirchhoff, who highlights the factors that are important to this sector. "First, the future wind market will be characterised by direct marketing of electricity," he says.
"Secondly, the main volume turbine market segments are increasingly shifting to inland regions with average wind speeds in the 6-6.5m/s range. Turbines that are capable of generating substantial amounts of power at modest wind speeds will provide the maximum revenue to owners and operators." Kirchhoff adds that older-generation turbines, typically fitted with relatively modest rotor diameters, are still often found at sites with lower wind speeds, even though their characteristics are better suited for higher-wind conditions. And even recent power rating and rotor diameter combinations for IEC III turbines, once thought of as state-of-the-art and "future proof", already now seem outdated for meeting today's low-wind demands.
Maximising yield
Compared with Senvion's current IEC IIIA flagship - the 3.0M122 with a rotor diameter of 122 metres and a specific power rating of 257W/m2 (generator size in watt divided by swept area in square metres) — the 3.4M140 offers 400kW more in capacity, and a 32% increase in rotor-swept area. These technical specifications translate into an additional yield of up to 20%, Senvion says.
Equally important, the new turbine's 221W/m2 specific power rating is fully in line with the values offered by some of the latest low-wind designs, such as the 223W/m2 of the Nordex N131/3000. In general, site-optimised specific power ratings with matching hub heights are the key contributing factors for maximising yield potential at the lowest possible levelised cost of energy (LCOE) at low-wind sites.
Achieving a favourable sound power level of only 104db(A) was another key design driver for the Senvion team. Such a figure could only be attained by combining aero-acoustic optimising of the blade aerofoils with limiting the rated blade tip speeds to a modest 70m/s (252km/h).
This is becoming a hot issue in the wind industry, with some manufacturers opting for tip speeds in the 80-85m/s range to avoid sacrificing performance and yield potential, but at the expense of aerodynamic noise levels.
"Being capable of offering competitive low-noise turbines has already become a major selling factor in several large onshore markets, including Germany, Australia, France, Canada and the UK," says Kirchhoff. "Additional benefits linked to reducing rotor blade tip speeds are the potential to curb blade-bearing loads, and the minimising of overall turbine structural loading."
Carbon free
The new in-house 68.5-metre blade is still in development, but Kirchhoff confirms that it will not contain carbon fibre.
"Carbon is expensive and increases the risk of introducing faults during handling and manufacturing states," he says. "Carbon further attracts lightning and is an excellent conductor of electric currents, a combination that puts many demands on the design of an effective lightning protection system. "We therefore chose an epoxy-based blade design with fibreglass only, but which is pre-coned to compensate blade deflection impact under load."
Fierce debates have raged on the maximum size of blade that can be transported cost effectively by road in one piece ever since the introduction of 32-37-metre blade lengths in the second half of the 1990s. Remarkably, the development of transportation equipment has kept pace with the increasing length of blades, and blades of more than 60 metres can now be transported in one piece.
Most wind turbine suppliers - with Enercon and Gamesa notable exceptions - have chosen not to switch to segmented blades, largely on the grounds of their compexity, cost and fears over the fatigue life performance of the joints. A major challenge for Senvion's design team has therefore been to find whether a single-piece blade of nearly 70 metres in length could still be transported cost effectively.
Kirchhoff defined the blade length, chord length (the widest part of the unit), pre-bending, and blade root diameter as the main impacting variables. "Our R&D staff travelled across Europe, mapping main routes and all possible situations to be encountered when bringing such huge blades to various destinations," he says. "Their final conclusions were positive."
The 3.4M140 is equipped with a novel pitch-control system, the Eco Blade Control, or EBC. This high-performing pitch communication and advanced sensor concept can reduce extreme loads on the turbine and thus allow for a cost-efficient design. In addition, the design lifetime of the 3.4M140 was extended to 25 years.
Growing sector
For many years the 3-3.5MW onshore class was a minority segment in the industry, but it is now becoming a main volume sector. Vestas took the lead by presenting the V90-3.0MW in 2003, with Senvion, then known as Repower, the first to follow suit with the installation of a 3.4MW 3.XM series prototype in 2008. Founded only seven years earlier, the Germany company rapidly developed a strong reputation for quality and technological competence.
The 3.XM with its 104-metre rotor diameter and in-house developed RE-type blades bridged the gap between Senvion's 2MW MM series, introduced in 2002, and its 5-6MW offshore turbines (from 2004). The principles applied in the 3.XM prototype were largely similar to the 2MW MM series and its 1.5MW MD predecessor, all featuring a non-integrated high-speed geared drivetrain with three-point gearbox support and doubly fed induction generator (DFIG). This proven drivetrain solution is popular with other leading suppliers.
The 3.XM prototype evolved into two serial models in 2010: the 3.4M104 and 3.2M114. All later variants in the 3-3.4MW range continued to build on these technology principles.
Gradual switch from DFIG
The future of DFIG is another topic of hot industry debate. At stake is whether DFIGs will be capable of meeting ever more stringent future demands for connecting turbines to grid networks. Opinions in the industry are, to say the least, mixed.
Senvion recently started a gradual switch from DFIG with partial power converter, to induction generator with full converter in its 3.XM platform, called the Next Electrical System (NES). This was introduced in a 3.4M114 NES prototype during 2015, and will be followed by a 3.2M122 NES prototype this year, says Kirchhoff.
"The 3.4M140 will be standard-fitted with NES. This electrical system technology switch is specifically aimed at being capable of fully meeting new binding Entso-E network codes for connecting wind turbines to electricity networks, which will be introduced from 2017 in all EU member states."
Kirchhoff adds that a full power converter gives superior grid behaviour and fully isolates a turbine from the grid. The induction generator provides higher efficiency during partial load at lower wind speeds, and the elimination of slip rings allows for an uncomplicated generator design.
Like other 3.XM variants, the 3.4M140 uses a high-speed three-stage gearbox. The combination of a large rotor diameter and low tip speed increases rotor torque, which, as a countermeasure, demands gearbox strengthening as well as adapting the total gear ratio to match these new specifications and load conditions.
The 3.4M140 will use a cast main chassis with a steel fabricated welded generator frame. Initital hub heights available will be 110 metres (for asteel tower) and 130 metres (for a hybrid tower), but some competitors have already announced various concrete-steel hybrid and other (new) tower concepts that can offer maximum hub heights of up to 160 metres in the future.
"The 3.4M140 is the first turbine model of our new onshore product platform," says Kirchhoff. "It will be supplemented by new variants for IEC II and IEC I in the next few years."
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