Greentech Media got an early look at GE's new space frame wind turbine tower in advance of the technology's official debut at next week’s European wind industry conference.
The space frame advances the potential of GE to deliver taller towers capable of more power production at a lower cost.
GE's enclosed-lattice, five-legged space frame prototype, sited at the company's Tehachapi, California facility, is 97 meters tall with a "brilliant" GE 1.7-megawatt, 100-meter rotor turbine on top. GE will introduce a 139-meter-tall space frame for its 2.75-megawatt, 120-meter rotor turbine on March 11 at the European Wind Energy Association conference.
A space frame is a three-dimensional structure built on struts that are locked together. These structures can accommodate very heavy weights with limited materials and supports.
Open-lattice towers were used for early utility-scale wind turbines, often with poor results. Bolts frequently rattled loose, leading to structural failures, and birds took up perches in the structures, leading to significant rates of avian mortality.
In searching for a way to cut costs, GE engineers returned to the lattice concept. But the space frame eliminates danger to avian species by enclosing the lattice with a translucent, non-weight-bearing, UV-protected PVC-polyester fabric coating.
The new design also uses splined bolts, which eliminates the risk of structural failure, according to GE's general manager of wind products, Keith Longtin.
The overarching trend in "the wind industry is higher hub heights,” Longtin said. “But tube towers scale poorly, because increasing load and material requirements do not pay off in increased output.”
GE’s R&D goal is to scale up towers and keep costs down. The space frame’s 10-meter-diameter base will allow a 120-meter-tall tower to use 20 percent to 30 percent less steel than a traditional 100-meter-tall tube tower, because the broader base means less support is needed from the tower walls.
Another benefit of the larger tower base is that advanced power electronics and battery storage capability can be housed inside and protected from weather and vandalism.
Because the space frame narrows at the top, it can interface with any nacelle without structural alterations.
One of the two key design parameters of the space frame, Longtin said, was limiting all of the parts to the 40-foot size of a standard shipping container so all of the pieces of a tower can be delivered by long-haul trailers. That should have a significant impact on transportation logistics.
Turbine manufacturers now deliver tube towers in three 30-meter-long, 60-ton sections. It requires special vehicles, elaborate planning and permitting and, often, police escorts.
The space frame will arrive in shipping containers. On-site assembly will replace complex transport logistics. It took about 30 days to assemble the Tehachapi prototype, but Longtin believes the average assembly time can be four days.
The other key design parameter was that the fastening system needed to be maintenance-free. The space frame’s splined bolts, once inserted, essentially function like rivets. They have long been the standard, maintenance-free fasteners used in bridges, aircraft carriers, and skyscrapers. Accelerated testing by GE engineers and third-party labs validated the maintenance-free durability of the GE splined bolts.
The cost savings from the space frame will be site-specific, Longtin explained. Savings from reductions in materials, shipping time and costs may be offset by increased on-site labor and time.
Because the space frame’s economic advantages will be greater for taller towers, Longtin expects the balance to come out strongly in GE’s favor in heavily forested places like Sweden, where the rotor needs to be above the treetops, and in places like northern Germany and the U.S. Southeast, where economically viable wind speeds are found higher in the sky.
An alternate strategy for cutting costs is substituting concrete for steel at the tower’s base. That can be cost-effective if a project is near a concrete source, Longtin acknowledged. But many are not.
Siemens, one of GE’s biggest competitors, introduced a bolted steel design aimed at reduced costs for towers as tall as 140 meters in 2011. It offered many of the space frame’s advances. Bent steel plate shells and other parts can be delivered to the project site by standard trucking for on-site assembly with maintenance-free bolts. A broader base provides increased stability.
The concept, developed with Denmark’s Andresen Towers, has apparently failed thus far to penetrate the marketplace to a significant degree. Requests for information from Siemens about the bolted steel design’s commercialization went unanswered. Queries to wind industry professionals turned up no awareness of the Siemens design. As with the space frame, success for the Siemens concept is probably likely to occur when there is greater demand for taller towers.
GE is presently working with ARPA-E on a truss-structured, fabric-covered turbine blade that can be shipped in containers and assembled economically onsite -- even as they continue to get bigger. These advances show that even in a maturing industry like wind, there are still plenty of logistics and costs to attack.
Editor's note: That's intrepid reporter and tower climber, Herman Trabish, once again risking life, limb and syntax to bring you the high-altitude clean energy news.