By Tim Miser, Associate Editor
Siemens’ concrete wind turbine tower technology, shown here at a wind farm in Iowa, is designed to capture stronger winds at higher altitudes.
Photo courtesy: Siemens
You won’t be surprised to read that Iowa is big farm country. If you’ve ever driven through the state, you doubtless noticed the vast expanses of land given over to agriculture. By some estimates, about 90 percent of Iowa is dedicated to growing crops of some kind. The flat, open country makes it the perfect location for large-scale agricultural operations, and the business of growing commodities has a long and proud tradition in the state.
These days though, Iowans aren’t just farming crops on their land. Turns out those same wind-swept fields which are so perfect for raising corn and soybeans are also very good for farming wind energy. It’s a newer brand of undertaking, one which is perhaps less storied for its relative novelty, but it may prove to be a key component in Iowa’s evolving economy.
The business of harvesting wind energy makes for a wild ride. The economic climate surrounding the industry continues to evolve rapidly, and investors hoping to profit from new wind infrastructure would do well to breathe deep and hang on tight. Between the fits and starts of various tax credits and environmental regulations that impact the industry, throwing hundreds of millions of dollars into wind turbines is not for the faint of heart. You’d be forgiven for thinking that wind investors must be adrenaline junkies.
Even so, across the country the grand sweeping towers continue to rise from the horizon in still greater numbers, and R&D investments into wind technologies continue to yield exciting developments that promise good things for the future.
Last year MidAmerican Energy announced it was building the nation’s tallest onshore wind turbine in Iowa. At 115 meters from ground to hub, the tower going up at the Adams County wind farm in the southern part of the state will stand more than a hundred feet above other turbines at the site. Add in the sweep of the uppermost blade and the tower will collect wind at nearly 170 meters above its foundation.
Reaching such heights is no easy task. To maintain stability at these altitudes, basic physics requires the diameter of the base of such a tower to be very wide, and the logistical constraints associated with transporting such components are enough to keep trucking companies awake at night. Just because they’re making wind towers taller these days, doesn’t mean they’re also making highway overpasses taller to accommodate them.
So what do you do when there is no longer any room to increase the diameter of a tower’s base? Traditionally, wind turbine towers have been made of steel. One way to make a tower taller without expanding its waist line is to construct components using thicker steel. This does work, but it creates much heavier tower modules, which in turn require foundations to be substantially reinforced. Ultimately, this chain of events amounts to projects that are not financially viable.
To combat this problem, Siemens has introduced concrete towers to the American market. Using concrete to construct large infrastructure like bridges is nothing new. For that matter, concrete has been used to build towers before. So far though, concrete towers have been little utilized in the wind industry, but they have an advantage over their steel counterparts. Concrete tower components can be cast in the field at the construction site, leveraging local labor and materials, and avoiding the transportation costs associated with towers fashioned from steel in remote factories.
When in early 2015 MidAmerican called on Siemens to build their record-breaking project at the Adams site, a concrete tower seemed like the obvious solution.
“Siemens is a technology company,” says Michael McManus, head of business development strategy at Siemens Wind Power Americas. “We focus on improving the technologies of components to boost performance at an acceptable cost. We’re excited to introduce our innovative concrete tower to the Americas.”
Siemens builds their concrete towers in much the same way that concrete bridges are constructed. Segmental concrete bridges are assembled using modules. These modules are made stable using compressive forces supplied by cables called tendons, which are post-tensioned as part of the erection process, essentially holding everything together. Like concrete bridges, concrete wind turbine towers are assembled one piece at a time, stacked together like enormous children’s blocks, and finally made strong using steel tendons. “That’s because wind turbine towers are enormous,” says McManus, “so they must be built using smaller, more manageable parts. The concrete cylinders we use are about 3.5 to 5 meters tall, and 7 meters in diameter at ground level. We stack them on top of one another and tension them using nine tendons, which provide for a very stable structure.”
Siemens towers are unique for another innovation as well. McManus says: “There are many examples of post-tension construction in the world, but most require the use of some form of joinery mechanism like grout during construction in the field.” McManus explains that the mating surfaces of the various components of a concrete tower are not perfect. Such imperfections result in imperfect load transfer, which in turn results in instability as cylinders are stacked on top of each other. To solve this problem, some type of grout is required to bridge two pieces together, allowing for even load transfer and creating a stable unit. Alternatively, joints can be ground smooth to create even load transfer. Both methods add time and cost to a project.
Siemens’ tower segments are different because of a patented manufacturing process called match casting. This system eliminates much of the expense and complexity of tower construction because no grout work or grinding is required at all. Instead, individual tower components are cast against one another in such a way that the surface of one cylinder is used as the casting mold to form the segment of the tower that will be stacked on top of it. “This means that any contour at the joint of one tower segment will have a corresponding contour in the adjoining segment, creating a matched set that will mate together perfectly,” says McManus.
This “dry joint” process entirely eliminates the need for joinery, which saves time and money. It also allows jobs to be completed in the field using local laborers that do not need advanced training. “The process is brilliant in its simplicity,” McManus says. “Tower segments are held together relying on nothing but gravity and post-tensioned steel tendons.”
Because of these innovations, Siemens is able construct taller and taller towers while adhering to more linear cost curves. By contrast, steel towers experience cost curves that increase exponentially as towers are pushed toward greater heights. “Due to the modularity of our form system, now Siemens is pursuing even taller towers and different turbine models,” says McManus. “This is being considered where sufficiently strong wind resources can only be accessed at greater altitudes.”
Michael Groggin, senior director of research for the American Wind Energy Association, echoes this. “We haven’t seen much increase in the height of wind turbine towers in the interior of the U.S.,” he says. “This is because there’s no need to go higher in these areas because there are already very good wind resources at 80 meters. However, parts of the East Coast are beginning to install towers that exceed 100 meters, since there are greater wind resources at higher altitudes in those regions.”
Towers aren’t the only components of wind turbines currently enjoying an impressive evolution; blades too a being improved at an exciting pace.
“One of the biggest evolutions we’ve seen in wind energy in recent years is longer rotor blades,” says Groggin. This helps to account for some of the cost reductions seen in the industry over the last few years, he explains. In five years rotor diameters have grown by about 20 meters. “That doesn’t sound like much,” Groggin says, “but it represents about a 55 percent increase in the swept area of a wind field. That captures a lot more energy.”
Siemens also understands this. “We’re always pursuing longer rotor diameters because they increase the annual production of a turbine,” says McManus. “It’s a tool by which we can increase the value of our machines. To keep the costs of electricity down, you always want the largest rotor you can get.”
Siemens currently has an offshore rotor with a diameter of 154 meters. “There are logistical constraints that govern the sizes of blades,” McManus adds. “We are capable of putting a blade into service that exceeds 75 meters, but the cost of getting such a blade to an onshore facility can create challenges that interfere with our commitment to low-cost electricity.” In contrast, McManus says, offshore turbines provide better opportunities for larger blades because rotors can be built at or near a port and shipped to their final destination.
Presently, most blades are constructed in a single piece from fiberglass, but other materials are being experimented with across the industry.
“To this point, the industry hasn’t made large use of carbon fiber,” says Groggin. “But as the cost of carbon fiber continues to come down through economy of scale and technology improvements, the use of carbon fiber will increase. This will greatly increase blade size, in addition to reducing costs.”
Groggin explains there has also been some interest in moving to modular blades. “As blade length grows, we begin to approach the limits of what can be transported efficiently over land,” he says. “In Europe, some manufacturers have begun to use modular blades that come in two pieces or even more.” These blades can be assembled on site, he says, which helps alleviate transportation problems. “That hasn’t really caught on in the U.S.,” Groggin adds, “because we have wider open spaces and less logistical constraints. We don’t have to worry about moving large objects through medieval roads, as they do in Europe.”
Blades also continue to get smarter.
Siemens wind turbines incorporate areoelastic taylor blades (ATB). These blades have a pre-bend and a twist, which help them deflect load and maintain structural integrity. “In a perfect lab environment, wind comes from a single direction and is very uniform,” McManus explains. “In reality though, gusts come from many different directions and affect the loading of blades.”
If on a given day the wind changes direction entirely, McManus says, yaw and pitch systems on a turbine can account for these variables. But smaller secondary gusts will always load blades in divergent ways, fatiguing them in the process. “When gusts come out of direction, it creates a load on the blade as it bends,” he explains. Because of their design and flexibility, Siemens’ ATB blades twist at the tip when they experience out-of-direction winds, relieving a portion of this load. “They actually repitch themselves,” McManus adds. “This process is different than simply pitching a wind turbine, because it is not controlled by a dispatch system. Rather, the blade’s response is passive in nature, reacting autonomously to small variations in wind direction and speed.”
This is important because smart blades are more efficient. “Responsive blades can be built lighter while still withstanding loads,” McManus says. “Without ATB technology, we could still build blades that could endure the elements, but we would have to fortify them much more rigorously. In the end, these heavier, beefier blades would not be as efficient. It’s a bit like the difference between an oak tree and a blade of grass.”
At 115 meters from ground to hub, MidAmerican Energy’s first concrete wind tower stands more than 30 meters taller than its neighbors at the Adams wind farm in Adams County, Iowa. The concrete tower is a prototype that will allow MidAmerican to evaluate the amount of additional energy that can be generated from wind at higher altitudes.
Photo courtesy: MidAmerican Energy Company
Innovations have also been borrowed from the aeronautics industry. “Vortex generators are now being applied to rotor blades which increase their aerodynamic efficiency,” Groggin says. “These create greater air flow separation, resulting in better lift and improved efficiency.” The industry’s understanding of aerodynamics and atmospheric science has improved markedly, Groggin explains. “This is partly due to the increased use of remote-sensing technologies like radar. This makes it possible to take measurements at the full height of the uppermost blade of a turbine, not just at the height of the tower hub itself.” Prior to this, it has been unusual to achieve measurements from such heights, which has impacted the industry’s understanding of turbulence and wind speed at such elevations. “This is important, because these variables can have real impacts on output performance, in addition to loads and wear issues,” Groggin says.
The Wind Farm as Super Organism
Wind farming is a deceptively complex operation. So many disciplines of science converge on the industry that it becomes necessary to adopt a syncretic and inclusive approach to technology. Any industry in which basic physics collides with aerodynamics and atmospheric science will not fare well amidst a climate of compartmentalized knowledge. Engineering problems can appear intermittent or unpredictable, but only because they are the product of the complex interactions of multiple and subtle variables and co-variables. Mastering any wind operation means maintaining a broad view of a project.
Toward this end, the industrial internet is now being used to leverage big data in an attempt to better understand wind farming. It’s no longer sufficient to simply understand the workings of individual wind turbines in isolation, since a single turbine inevitably affects the other turbines on a farm.
Groggin explains: “We now have a better understanding of the interactions of individual wind turbines within the context of the larger farm. A wind turbine upwind can impact the performance of a wind turbine downwind by creating turbulence and other undesirable circumstances. We now know that by changing the direction of output of a turbine in the front row of a farm, you can improve the productivity of turbines multiple rows to the rear.”
Because of this, it’s possible to produce higher cumulative output by managing these interactions, even if it means deliberately compromising the performance of certain turbines, Groggin explains. “Even if we’re not utilizing wind turbines on the front row to their fullest,” he says, “we might be creating a chain of events that helps turbines in the secondary and tertiary rows perform more optimally, and this impacts the success of the wind farm as a whole.”