


The GE Tehachapi facility’s enclosed-lattice, five-legged space frame prototype is 97 meters tall with a "brilliant" GE 1.7-megawatt, 100-meter rotor turbine on top. GE will introduce a 139-meter space frame for its 2.75-megawatt, 120-meter rotor turbine at the EWEA conference March 11.

A space frame is a three-dimensional structure built on struts locked together. These structures can accommodate very heavy weight with limited materials and supports.

Open lattice towers were used for early utility-scale wind turbines with unfortunate results. Bolts were rattled loose, leading to structural failures, and birds took up perches, leading to serious avian mortality.

In searching for a way to cut costs, GE engineers returned to the lattice. But the space frame eliminates danger to avian life by enclosing the lattice with a translucent, non-weight-bearing, UV-protected PVC-polyester fabric coating.

And it eliminates structural failure because of splined bolts, explained GE Wind Products General Manager Keith Longtin.

“The direction of 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 towers up and keep costs down. The space frame’s ten-meter diameter base will allow a 120-meter tower to use 20 percent to 30 percent less steel than a traditional 100-meter 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 its 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 parts to the 40 foot size of a standard shipping container so all 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 be maintenance free. The space frame’s splined bolts, once inserted, are essentially 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 economic wind speeds are higher up.

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 to penetrate the marketplace. Requests for information from Siemens about the bolted steel design’s commercialization were unanswered. Queries to wind industry professionals turned up no awareness of the Siemens design. Like the space frame, success for the Siemens concept probably awaits 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 economically assembled 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.
