Wind speed increases with altitude and wind power increases with the cube of velocity, so it makes sense to build wind turbines on the tallest towers possible. But there are practical limits to tower height. First, tubular steel towers need to be transported from the manufacturing facility to the wind farm. The US Department of Energy estimates that a 100 meter tower, which must be moved in four sections, could cost up to $180 per mile to transport. According to the American Wind Energy Association (AWEA), there are fewer than 20 manufacturers of large (utility-scale) wind towers in the United States, so it’s likely that a tower will have quite a long trip from the factory to the wind farm. Erecting such a tower requires an enormous crane, which also much be transported to and from the site. And of course, all that size means that it’s only economical to build wind farms near major roadways.
Currently, most wind farms in the US have towers that reach up to 80 meters. Increasing that height to 100 meters could result in a 16% increase in available wind power. If the cost of building, transporting, and erecting the tower can be contained, it would make economic sense to build towers that reach or exceed 100 meter hub heights.
At the moment there are two possible designs that could achieve the goal. General Electric recently introduced their Space Frame tower, a lattice tower that looks like a traditional tubular design tower from the outside. The lattice is made of steel beams that can be transported in standard 12m shipping containers and assembled on site. The steel frame is wrapped in a translucent fabric, making it appear solid from the outside but letting ambient light into the interior. These towers can reach a staggering height of 139 meters, which allows the turbines to capture as much as 40% more wind power than the same turbine at 80 meters. GE plans to begin mass production of the Space Frame towers in 2015.
Image courtesy of General Electric
Nearly half of the world’s steel is produced in Asia, so US companies often import steel to make turbine towers. Concrete, on the other hand, is produced domestically at many locations. That’s why a team of structural engineers at Iowa State University is studying towers made of prefabricated high strength concrete panels. They’ve tested 100 meter concrete towers and found no signs of distress at operational lateral loads of 100,000 pounds. Unlike GE, this team is still in the R&D stage; no word on when the concept could make it to production. The university was recently awarded a $1M grant from the US Department of Energy to conduct further studies.
Image courtesy of Iowa State University
The engineers at Iowa State claim that ultra high performance concrete will outlive steel, giving the concrete towers a longer lifespan. I tried to verify that claim but I’ve found no concrete evidence either way; the construction industry itself continues to debate the merits of one vs the other. It’s possible that one will emerge as the clearly superior material for wind turbine towers, or that both will be feasible, with local conditions dictating a preference of one or the other. Either way, it’s good to have options.