WATERTOWN, S.D. – The blades of the giant propellers sprouting from cornfields look like they're almost dragging on the ground.
The Dakota Range 1 & 2 wind farm under construction near here will sport the largest turbines of any of Xcel Energy's many wind projects — and even bigger turbines are on tap for southwestern Minnesota.
The yet-to-be built Plum Creek project will feature turbines 655 feet tall when their blades are fully extended, about 160 feet higher than Xcel's new South Dakota project and roughly the height of a 60-story building.
"They could easily have a position on the Minneapolis skyline," John Tuma, a member of the Minnesota Public Utilities Commission (PUC), said during a review of Plum Creek.
And they are the future. Bigger blades capture more wind energy. Bigger turbines require less overall space because fewer are needed for the same amount of energy.
Plus, bigger turbines can generate more power at lower wind speeds, turning some uneconomic projects into profitable ones.
"This really opens up possibilities," said Kim Randolph, Minneapolis-based Xcel's vice president of energy supply projects. "The high-wind areas are filling up with wind farms."
Still, there are potential drawbacks to the big new turbines if you live close to them. Bigger blades can increase "shadow flicker," or shadows cast on nearby properties, already a complaint about wind farms.
And while Plum Creek's towering heights sparked no significant opposition from its soon-to-be neighbors, the visibility of larger turbines could become an issue, industry observers say.
"They are certainly more visible from far away," Mark Bolinger, a renewable energy research scientist at Lawrence Berkeley National Laboratory in California.
Bigger turbines will help reach goals
Minnesota and the Dakotas — already big wind-power producers — will likely be hosting a lot more turbines if long-term clean energy goals are to be met.
Minnesota in 2020 got 21% of its electricity from wind, a figure that does not include power produced in the Dakotas that is consumed here.
"There is still a good 8 to 10 percent left for wind to contribute to our power sector," said Aditya Ranade, the Minnesota Department of Commerce's deputy commissioner for energy resources.
Bigger turbines will help reach that goal more efficiently, sweeping up more wind with each turn of their blades.
"We expect to see the cost reduction driven by the size," Ranade said. "You can get more energy on the same footprint."
Wind farm levelized costs fell much more than experts anticipated between 2014 and 2019 — 28% to 36% — and the adoption of larger turbines was a key factor, according to a study by several authors published recently in Nature Energy, a scientific journal.
Regardless of any federal tax credits, costs will continue falling substantially as individual wind turbines' production capacity increases from an average of 2.5 megawatts in 2019 to a forecast of 5.5-megawatts by 2035, the study said.
At Dakota Range, most turbines will be 4.2 megawatts and boast a 446-foot rotor diameter — the cross section of the circle swept by its blades. They will produce about 70% more energy than a 2-megawatt turbine with a 393-foot wingspan.
Even with the bigger blades, the towers holding the turbines at Dakota Range are about the size of those at current large Xcel wind farms. But at Plum Creek, the blades are longer than those at Dakota Range so the wind towers themselves must be taller.
"At some point, you have to increase tower size, or you won't have adequate space on the blade's downswing," Bolinger said.
Turbine visibility already an issue
In one way, bigger turbines can help countryside aesthetics because developers need fewer of them (if a project's total size is held constant). But because they are taller, they tend to stick out more.
"Which do people prefer — more or bigger? Probably either if you are a [wind] supporter, and neither if you are detractor of building on the landscape," said Beth Soholt, head of Clean Grid Alliance, a St. Paul group that represents renewable energy developers and advocates.
Turbine visibility is a sticky issue for the proposed Big Bend Wind project in southwestern Minnesota. Its proximity to the Jeffers Petroglyphs site near Comfrey has alarmed the Lower Sioux Indian Community and the Minnesota State Historical Society.
The rock carvings date back 7,000 years and are considered sacred to several tribes. The tribes and historical society argued Big Bend's turbines would be too close to the petroglyphs, marring the landscape and thus diminishing the historic site.
Big Bend's developer, Apex Clean Energy, responded by reducing the number of planned turbines from 64 to 56 and moving some farther out. But to maintain the wind farm's total production capacity, Apex swapped in bigger turbines. At a height of 655 feet, they will be 85 feet taller than what was in the original plans.
Now, both the proximity and the height of the turbines is a contested issue.
Big Bend would be one of the biggest wind farms in Minnesota, with the capacity to generate 308 megawatts of electricity; Plum Creek, with individual turbines of up to 6.2 megawatts, would be by far the largest at 414 megawatts.
Plum Creek, which would sit between Westbrook and Walnut Grove, was approved unanimously by the PUC earlier this month.
However, Tuma, the PUC commissioner, struck one wary note on the new breed of wind turbines at developments like Plum Creek.
"I love them. I think we should build them," Tuma said. "But the one thing that is going to be a problem for these communities that was never a problem with [the turbines] we built before is that the shadow flicker is greater."
The expected shadow flicker at several sites in Plum Creek's footprint could well exceed the 30 hours per year threshold that's considered an industry best practice. Those sites are all owned by "participants" — property owners who are leasing their land to the wind farm's developer, National Grid.
The PUC required Plum Creek to come up with a shadow flicker plan, or get agreements or consent waivers from participating landowners so they know how much flicker they might experience.
Size makes construction trickier
The bigger the turbine, the longer the blades and the heavier the components.
Foundations for the largest new turbines contain up to 1,000 cubic yards of concrete, double the amount needed for more conventionally sized machines, said Tim Maag, vice president, general manager for wind energy at Golden Valley-based Mortenson, one of the largest U.S. wind farm builders.
Mortenson is constructing Xcel's Dakota Range. On a recent day, about 160 union workers — crane operators, iron workers, millwrights and laborers — were raising turbines on the slightly rolling fields north of Watertown.
For a smaller turbine, they would have assembled its "nacelle" — the tower-top housing for all of the generating components — on the ground. But because the bigger turbine's drivetrain and generator weigh considerably more, Mortenson's crew needed to assemble the nacelles "in the air," a trickier task for the workers.
Each component was lifted individually by a 400-ton crane and set atop the tower, instead of lifting the nacelle in one piece. The crane made eight or nine "picks," as opposed to four or five, before assembly was completed.
But the biggest challenge for building larger turbines is transportation, from stuffing giant blades onto ships to hauling them to remote sites like Dakota Range by truck. Most blades at Dakota Range are 73 yards long, leaving a truck driver with a harsh turning radius.
"Eventually the blades will become so large they will come in two pieces and be assembled on site," Maag said.