Wind power has become a legitimate source of energy over the past few decades as larger, more efficient turbine designs have produced ever-increasing amounts of power. But even though the industry saw a record $99.5 billion global investment in 2014, turbine growth may be reaching its limits.
Transportation is increasingly challenging because of the size of the components: individual blades and tower sections often require specialized trucks and straight, wide roads. Today’s wind turbines are also incredibly top heavy. Generators and gearboxes sitting on support towers 100 meters off the ground can weigh more than 100 tons. As the weight and height of turbines increase, the materials costs of wider, stronger support towers, as well as the cost of maintaining components housed so far from the ground, are cutting into the efficiency benefits of larger turbines.
The alternative energy industry has repeatedly tried to solve these issues to no avail. But the latest entry promises a radically different type of wind turbine: a bladeless cylinder that oscillates or vibrates.
Spanish startup Vortex Bladeless has developed turbines that harness vorticity, the spinning motion of air or other fluids. When wind passes one of the cylindrical turbines, it shears off the downwind side of the cylinder in a spinning whirlpool or vortex. That vortex then exerts force on the cylinder, causing it to vibrate. The kinetic energy of the oscillating cylinder is converted to electricity through a linear generator similar to those used to harness wave energy.
David Yáñez, one of the company’s cofounders, first came across the concept as a student studying the collapse of the Tacoma Narrows Bridge in Washington. The bridge collapsed in 1940 due to excessive vibrations formed by the spinning motion of wind as it blew past the bridge and is a textbook engineering failure. Yáñez, however, learned a different lesson. “This is a very good way to transmit energy from a fluid to a structure,” he says.
Vortex’s lightweight cylinder design has no gears or bearings. Yáñez says it will generate electricity for 40 percent less than the cost of power from conventional wind turbines. The company has received $1 million in private capital and government funding in Spain and is seeking another $5 million in venture capital funding. Yáñez says the company plans to release a four-kilowatt system in 2016 and a much larger one-megawatt device around 2018.
The Vortex turbine sounds promising, but like any radical new alternative energy design, bladeless turbines have plenty of skeptics.
“If you have a common propeller-type wind turbine, you have a big area swept by the blades,” says Martin Hansen, a wind energy specialist at the Technical University of Denmark. “Here you just have a pole.”
In addition to capturing less energy, oscillating cylinders can’t convert as much of that energy into electricity, Hansen says. A conventional wind turbine typically converts 80 to 90 percent of the kinetic energy of its spinning rotor into electricity. Yáñez says his company’s custom-built linear generator will have a conversion efficiency of 70 percent.
Yáñez concedes that the oscillating turbine design will sweep a smaller area and have a lower conversion efficiency, but says significant reductions in manufacturing and maintenance costs will outweigh the losses.
As Vortex builds bigger devices that catch higher-speed winds further from the ground, it will also run up against other challenges inherent to the physics of fluid mechanics. Air or other fluids moving at low speeds past small-diameter cylinders flow in a smooth, constant motion. Increase the diameter of the cylinder and the speed at which the air flows across it, however, and the flow becomes turbulent, producing chaotic eddies or vortices. The turbulent flow causes the oscillating frequency of the cylinder to vary, making it difficult to optimize for energy production.
“With very thin cylinders and very slow velocities you get singing telephone lines, an absolutely pure frequency or tone,” says Sheila Widnall, an aeronautics and astronautics professor at MIT. “But when the cylinder gets very big and wind gets very high, you get a range of frequencies. You won’t be able to get as much energy out of it as you want to because the oscillation is fundamentally turbulent.”
Widnall also questions the company’s claim that its turbines will be silent. “The oscillating frequencies that shake the cylinder will make noise,” she says. “It will sound like a freight train coming through your wind farm.”
Oscillating cylinders are just one of several emerging technologies aimed at harvesting more of the wind for less. Makani Power is developing a tethered “energy kite” (see “Flying Windmills”). It flies in a large circle similar to the tip of a conventional turbine blade while harnessing wind power via smaller onboard turbines. Astro Teller, head of Google X, Google’s semi-secret research facility that acquired Makani in 2013, said in March that the company would soon begin tests of a full-scale, 600-kilowatt kite.
John Dabiri, an aeronautics and bioengineering professor at Caltech, is testing different configurations of vertical axis turbines, which are essentially windmills that spin like a merry-go-round rather than on a horizontal axis like a bicycle wheel. Typically wind turbines are placed far apart from each other to optimize energy production. Drawing on the same principles that fish use to conserve energy by schooling, Dabiri found that turbines placed close to each other could produce more energy than those that are far apart.
“You can coӧrdinate the operation of multiple wind turbines such that the whole is greater than the sum of its parts,” he says.
Dabiri says such synergistic effects could also apply to conventional, horizontal axis windmills or even oscillating turbines. The latter pose a greater challenge because the wake of such turbines is very chaotic but also a potential benefit because the wake packs a lot of energy, he says.
Much remains to be seen with Vortex’s oscillating turbine, Dabiri says, but he adds that he is excited by the company’s concept. “Anyone who says the three-bladed turbine is the best we can do is lacking in vision.”