Although tall towers have become increasingly commonplace, recent product announcements from Vestas, GE and Alstom feature amenities that resolve previous design and structural constraints. As a result, taller towers are now able to harness wind resources up to 150 meters in some locations, meaning less windy sites – once thought unworkable – have become viable options.
“Low-wind sites are a huge segment of the global market,” notes Matthew Keith Whitby, spokesperson at turbine manufacturer Vestas, which introduced a Large Diameter Steel Tower (LDST) that raises tower heights on its 3 MW machines to 140 meters. “Higher towers have become increasingly more important for maximizing output.”
Just the same, going taller typically means more force, which requires the manufacturer to use thicker, steel plates. Therefore, costs increase. However, the LDST offers the same protection against force while requiring little extra steel, Whitby notes. The LDST increases the diameter of the tower’s bottom section to 6.3 meters, as opposed to a 4.2-meter base used in Vestas’ conventional steel towers. Increasing the diameter of the bottom section of the tower results in significantly more strength, Whitby notes, adding that the LDST allows the tower to withstand the “bending moment” – a phenomenon caused by force exerted on the tower by the higher wind speeds.
On a typical site with a mean wind speed of 6.5 m/s, Vestas claims the LDST will boost annual energy production by up to 8% at a hub height of 137 meters for the V126-3.3 MW, compared to a hub height of 117 meters for the conventional steel tower.
In addition, turbine maker GE recently introduced a five-legged space frame lattice tower for its 2.75-120 machine.
GE claims the product – in full operation at the company’s prototype site in Tehachapi, Calif. – provides a more logistics-friendly option for developers, with an aesthetic and maintenance profile comparable to conventional tubular towers. Keith Longtin, GE’s general manager of global wind products and services, says the space frame design enables towers up to 139 meters to be built in locations that were once difficult to access.
“While the trend in the wind industry means higher hub heights, the tube towers scale poorly, because increasing load and material requirements do not pay off in increased output,” says Longtin.
Notably, GE designers went to school on improving upon early lattice wind tower design flaws from the 1980s.
For example, Longtin explains GE’s tower has a “maintenance free” bolting system, an upgrade from early maintenance-intensive lattice towers that required technicians to constantly re-tighten tower bolts.
Longtin says that the space frame tower’s structural fasteners maintain their tension throughout the life of a turbine. The design and number of fasteners in each joint provide sufficient clamping force so that all the load is carried in friction, meaning that there is no cycling that would cause the joint to loosen.
By contrast, Longtin says, flange bolts used in tube towers are subject to what he describes as “axial loading” – an occurrence that causes a continual stretching and relaxing that can cause flange bolts to loosen over time. As a result, flange bolts need to be sampled periodically. If one on a flange is found to have insufficient torque, Longtin says, all are tightened.
The lattice tower frame is wrapped with a non-weight-bearing PVC material – the same material used to cover domed stadiums. Wrapping the tower with a material covering gives the appearance of a solid structure and, more importantly, prevents raptors and other avian species from using the lattice tower as a nesting perch the way birds did with the first-generation lattice towers in the early to mid 1980s.
GE plans to initially offer the lattice tower in Northern Europe. Still, GE is mulling over the possibility of offering a 97-meter version of the space frame tower for the U.S. market. Longtin claims developers will benefit from a transportation and logistics perspective, as a 97-meter tower can be shipped via rail using 11 40-foot containers. As an added bonus, rail transport alleviates the need to obtain a special permit for transporting oversize loads via a flatbed, thus allowing customers faster delivery.
Not to be outdone, turbine maker Alstom will rely on partnerships to provide steel or concrete-steel hybrid tall towers at less windy sites, according to Laurent Carme, vice president of the company’s onshore platform.
In March, Alstom signed a partnership with French provider Freyssinet to develop a 119-meter concrete tower specifically designed for its ECO122 wind turbine. This new tower will be made of 11 concrete sections, the lowest measuring 7.2 meters in diameter, for the base of the structure.
Using concrete for tall towers allows for increased local content and reduced transport costs, notes Alstom. Additionally, Freyssinet has developed a unique installation method called “Eolift” that is based on heavy lifting techniques used in civil engineering. The method facilitates the nacelle and tower assembly at heights exceeding 120 meters while reducing contingencies relating to stringent conditions, such as high-wind conditions during installation.
The company has also partnered with German provider Max Bogl to develop a 139-meter tower for the ECO122 turbine model. Max Bogl’s website touts that its hybrid towers – able to reach 150 meters – provide “high efficiency and energy yield even in less windy regions.”
Alstom says it will offer its customers both high-tower options.
“By providing a comprehensive range of towers made of concrete, steel or both,” Carme explains, “it is now possible to consider installing a wind farm within more complex environments, at competitive costs and within competitive completion times.” w
Marketplace: Tower Technology
OEMs Building Bigger, Better Mousetraps
By Mark Del Franco
Several recent product introductions alleviate some of the previous constraints to harnessing wind resources at taller tower heights.
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