Towers for wind turbines have done a good job hiding their high-tech origins. For instance, their sections, humbly called cans, are rolled from flat-sheet steel into precise, yet slightly tapered cones and then welded by CNC machines. More CNC machine tools drill holes and attach flanges that allow bolting cans together. But for all this sophistication, the enclosed steel tower may have reached its limit at 100m. Towers are trending toward taller versions, hybrid designs, and modularity.
Towers are getting attention because the tower-cost portion of a commissioned wind turbine is increasing from 10% toward 20% of system cost. Further attention is on cutting transportation costs and reducing weight. Higher steel prices were one cost driver over the last several years, although it is lessening now.
Most new towers in the U.S. are 80m, but the 100-m version will be the new norm, according to one manufacturer. While OEMs have focused on their blades and nacelle equipment, they have not given the same attention to towers. A length limit for transport by rail or truck is about 50m. A panelist at a supply-chain conference said their 12.5-ft width is about the limit for railroad tunnels and not much more for roadway bridges. To address the issues, third parties have offer alternate designs. Most are modular and have wider diameters at the foundation.
Diameters are likely to get larger to handle heights over 100m. Wider foundations will also allow less concrete-intensive designs, those shaped more like huge rings rather than thick discs. Modular towers made of curved and tapered panels could more easily ship on flatbed trucks and bolt together into cans or sections at the turbine site.
Another tower idea begins with a wide-diameter concrete base. The large diameter improves stability by spreading tower weight over a larger area, thereby avoiding the deep (2 to 3m in some cases) concrete foundations. An added benefit is more space available at the base for ground-mounted electrical and other turbine-support equipment. “The Concrete Tower Base has features that will optimize existing steel-tower technology and place the focus on height,” says Tindall Corp.’s Chris Palumbo. “For instance, the large-diameter, shallow-ring foundation can reduce material by 60 to 70%.
For the company’s concrete tower base, precast sections are positioned (leaned) against each other to form a cone-shaped section that provides stability necessary to extend hub heights above 100m. The 50-ton, 26-m long sections could be transported by truck or rail and even molded at the wind site, thereby avoiding the large transportation costs associated with huge and heavy components.
After assembling the tapered base, a circular concrete section fits like a collar atop it. All concrete sections are held together with post-tensioning tendons that loop through the collar sections and around an arched shelf of each taper section. Once the tendons are properly tensioned, the entire structure acts as a unit. A conventional steel tower then bolts to the top of the 31-m tall concrete base. Palumbo says such a base makes possible hub heights of 130m and higher.
Wind Tower Systems proposes a lattice-like tower that will allow reaching at least 100m, while lowering installation and transport costs, according to the company. WTS engineers have been working on the development of this space-frame tower for wind farms that require hub heights of 100-m or more. A durable fabric will cover the space frame to enclose it. WTS also has given thought to ways of transporting and installing its towers. They will use standard flatbed trucks and a jacking system to eliminate need for heavy-lift cranes. These developments help cost effectively extend tower height, which in turn lets the turbine produce more energy. GE liked the ideas so much, it bought the company in 2010.
“Taller towers are an important complement to longer blades,” says Victor Abate, VP of renewable energy for GE Power & Wind. Plans are underway to install a prototype of the GE’s space frame tower with commercial availability targeted for 2012. WPE
Filed Under: Towers