Fifteen years ago, lattice towers dominated wind-farm landscapes. When hub height hit the 60m mark, developers turned to steel tubular designs that have not changed much – until recently. For this blog, we contacted several tower manufacturers to see what’s new and what might push towers beyond 100m.
Towers that reach over 100m are a great idea because they let a turbine work in stronger and steadier winds, which give them higher capacity factors. But does the extra height pay off in saleable wind power?
NREL says 100m towers (versus 80m) would produce about 12% more power per installed turbine, but it won’t happen without addressing these key height issues:
• Weight scalability with height: Greater overall forces act on a 100m tall tower compared to an 80m tower. Conventional tubular-steel towers combat these forces by adding lots of steel mass to increase stiffness. As a result, the cost of steel towers grows exponentially with tower height, making taller towers economically impractical.
• Transportation limitations: A high cost comes with transporting large, tubular-steel wind towers. They are often built in four sections with the heaviest weighing over 50 tons. Highly specialized $500,000 trailers are needed to move each section. Tower transport costs up to $180 per mile.
• Construction challenges: Due to the immense weight of steel-tube towers, large crawler cranes are required to install the towers and turbines. The 600 to 1,000 ton cranes needed to erect turbines above 100m are few and expensive. The high mobilization and demobilization cost for the largest cranes is a key inhibitor of smaller, community wind projects, and a recurring problem for wind turbine, heavy-maintenance operations. Finally, excessive 32-ft wide roads required by large crawlers ferment community opposition in many areas due to the amount of woodland clear-cutting required.
The cost of offshore wind is leading people to seek out taller towers. Wind advocates in Europe are among those asking questions about costs. Developing offshore wind farms is “the wrong road to go down, economically and technologically speaking,” concludes one analysis by the consumer rights group the Federation of German Consumer Organizations. With “encouragement” like that it’s no wonder taller towers are getting more attention in onshore Europe.
We know towers made only of steel that exceed 100m approach a cost-prohibitive limit. But a 40m concrete based section is less expensive and has several plusses. For one, it can be cast and assembled on site thereby avoiding transportation costs. (For reasons not entirely clear, the modular steel tower has not yet caught on.)
The good news is there is no shortage of ideas for pushing towers up. In a nutshell, these include hybrid towers of a concrete base and conventional steel top, which are making appearances in Europe and soon here. The lattice tower covered in fabric might make a comeback. And self-erecting towers and turbines, ones that need no 120m crane, would be a holy grail.
Hybrid towers in Germany
Wind-turbine manufacturer, REpower Systems SE (www.repower.de) has recently commissioned its tallest wind turbine. The 3.2-MW turbine carries a 114-m diameter rotor and sits on a 143-m concrete and steel hybrid tower. The company says the turbine and tower are best suited for locations with low-wind speeds, for which there is much in Europe.
“We can gain an additional energy yield of up to 1% with the hub height above 100m,” says Matthias Schubert, CTO at the company. By raising the hub height from 93m to 143m, the company expects an increase in yield of up to a whopping 50% in low-wind locations.
The company says its hybrid tower is a combination of pre-manufactured steel-concrete segments and a standard steel tower. “The segmented construction supports an economic and nearly weather-independent installation, and guarantees a relatively short construction time,” says Schubert. The increase in hub height also brings new challenges with transport and installation.
An important part of the technical design is a mobile tower crane that “climbs up” along with the progress of construction, thus permitting a cost and space-saving installation of the turbine even in forested areas and on complex terrain. (Sorry, no pictures.) The company says the concept significantly reduces the time often required for onsite installation.
The first variant of the low wind speed turbine with a hub height of up to 139m is for sale now. The 3.2M114 turbine will be available in three hub heights – 93m, 123m, and 143m – in the corresponding IEC II wind class by mid-2013.
Henkel and TimberTower GmbH
As an alternative to steel, Henkel (www.henkel.com) and TimberTower GmbH (www.timbertower.de) suggest wood for turbine towers. This sustainable raw material can reach hub heights of 200m, they say, and weigh significantly less than traditional steel towers. According to Henkel, using wood for a 100m tower can save about 300 tons of sheet steel. The first TimberTower was inaugurated last December in Germany.
The tower is manufactured as an assembly of laminated and glued timber panels, and surface components. No dowels, nails, screws, or bolts are used to hold this timber construction together. Workers assemble the tower on-site into a closed, hollow body with a hexagonal, octagonal, or dodecagonal cross section. The base measures 7 x 7m and the top, 2.9 x 2.9m. A waterproofing membrane placed on the wood protects it from the elements. Segments are transported in 40ft containers. The fasteners are integrated in the tower’s individual components, along with a ladder and lift system. Round, steel adapters connect the nacelle to the tower. The foundation has a mass and diameter similar to that used under tubular steel towers. The wooden tower weighs about 90 metric tons, with the top-mounted nacelle and rotor adding another 100 tons. The design firm ensures a lifecycle of 20 years.
“This tower offers nothing but advantages,” says TimberTower’s general manager, Holger Giebel. “It has greater stress resistance, can be built significantly higher, and is 20 to 30% cheaper to make.”
Tindall’s hybrid tower
The Atlas Concrete Tower Base (Atlas CTB), designed by Tindall Corp. (www.atlasctb.com), can raise hub heights to 110m and higher. The GL-Certified precast tower base is the first of its kind in North America, says the company. Benefits include a 50% or more reduction in foundation concrete, reinforcing steel, and installation costs.
Twenty concrete pieces form the CTB and are held securely together with about 13 miles of post-tensioned cable. The base uses no bolts. “After three years of development, the innovative design provides a reliable, efficient solution that eliminates height barriers,” says Tindall Vice Chairman William Lowndes IV.
The tower base sits 40-m high and can support 1.5 to 3+MW turbines on steel mono-towers of 70 to 100m. Components can be factory manufactured or site cast. What’s more, it lets contractors reduce excavation costs, labor costs, and shorten the project schedule. The company says thermal cracking concerns are eliminated because there are no large concrete pours. Lowndes says it will be strong and stable fifty years from now.
“The 16-m diameter conical base supports a concrete transition and multiple circular sections to form a platform that supports conventional steel monotowers. A clever ring foundation, with a larger than conventional diameter, provides maximum resistance to bending and overturning,” adds Lowndes. This allows for minimal foundation thickness. The design of the base takes full advantage of gravity, effectively resisting overturning loads and preventing uplift. Lastly, it helps reduce life cycle costs for blades, turbines, and the upper tower.
Towers from ATS
Another tower that breaks the perceived 100-meter barrier is the Advanced Tower Systems (ATS) (www.advancedtowers.com) towers of the Gau-Bickelheim wind farm in Germany. They reach hub heights of 145m. Hybrid towers made from concrete and steel make these heights possible.
The advantage of a hybrid system is that it can come in pieces, says Felix Wächter, a spokesman for Germany-based ATS. He says the lower section of the tower can be replaced by precast reinforced concrete segments. Reinforced concrete is available everywhere and is relatively inexpensive, he adds. The design is built in easily transported segments. In contrast, transporting a complete steel tower needs a special truck due to large diameters in the lower sections that often exceed 8m. This dimension also calls for special routes and escort vehicles.
Cranes are also limited in terms of traction capacity and towing heights. Because of the steel tubes in the upper levels of a hybrid tower, the crane makes fewer lifts and with less weight.
The greatest advantage of the ATS tower, says Wächter, is the relatively simple production of its reinforced concrete sections. It is produced with few molds and locally in every country that has a concrete industry. That makes the system a good fit to emerging and established wind markets.
– Nic Sharpley, Steven Bushong, and Paul Dvorak