Propeller wind turbines are the most common way of using one of the most abundant energy sources on Earth to generate electricity. The tall three-bladed fans are the ubiquitous symbol of wind energy, but they aren't the only design on the market.
Vertical axis turbines, where the rotating axis stands upright, have been around as long as their horizontal brethren but have failed to catch on at large scales. "The simple story is that the companies that were commercializing [vertical turbines] were unable to make the inherent advantages exceed the inherent disadvantages," said Paul Veers, chief engineer at the National Renewable Energy Laboratory's Wind Technology Center.
The turbines come in two main varieties of rotors -- the part of the generator that catches the wind and spins -- and share many of the same advantages. The Darrieus rotor uses arced narrow blades anchored at both ends of a shaft, akin to a whisk or an eggbeater. The blades have a cross section similar to an airplane wing and generate force in similar fashion.
Savonius rotor turbines are another design, where air scoops are mounted on the shaft of the generator, forming a spinning cylinder. Where the Darrieus turbine spins by generating lift, the Savonius rotors spin from drag.
Since the turbines are vertical, the generators can be placed on or close to the ground, lowering maintenance costs. Their narrower vertical footprint allows them to be placed closer together, though spacing is limited by how much energy is extracted from passing breezes and by support structures like guy wires. Vertical turbines also tend not to be as tall and have a steady gravity load, meaning the effects of gravity don't change as the turbine spins, unlike in horizontal axis turbines. They can also accept wind from any direction with equal efficiency.
More mass, less power
"It's a technology that works, but it has some problems," said Veers. "The design is complicated. It's got very dynamic loading, and it's a very dynamic structure." Though the effects of gravity are constant, forces from wind change on various parts of the structure as it spins, said Veers. He noted that the frequency of the shifting forces can create a phenomenon known as resonance, where the effective force is amplified and the rotor ends up tearing itself apart. This limits the size of individual turbines.
In addition, the designs have an inherently higher mass-to-power ratio and an inherently lower efficiency, said Veers. Because the rotors tend to be shorter, vertical turbines only have access to lower wind speeds and are more vulnerable to turbulent air flows from buildings and trees. "There's a bit of an uphill climb to make them work," he said.
Mick Sagrillo, an independent consultant for small wind energy companies, agreed with this assessment. "The rotors themselves are actually less efficient," he said. "The successful technologies that are out there are horizontal. It's Darwinian economics. It would be nice if this weren't the case, because then we'd have other options, but unfortunately, that's where we're at."
Because of the lack of investment, there hasn't been much progress in improving vertical turbines as there has with horizontal windmills. "There are several decades of a growing knowledge base for horizontals," said Veers. "There's a lot of investing by knowledge, learning by failing."
Nonetheless, some engineers have tweaked the vertical turbine designs to improve their efficiency and cost-effectiveness. One method is to arrange the scoops or blades in a helix, thereby ensuring a constant level of force from wind, minimizing oscillations.
Fishing for better configurations
Another approach is to rearrange how the turbines are placed. John Dabiri, a professor of engineering and applied science at the California Institute of Technology, studied how schools of fish maneuver in water and extrapolated his findings to wind turbines. From computer models, he found that counter-rotating vertical turbines could be arranged in such a way that a vertical wind farm has a higher power output per unit of area than a horizontal farm, upward of 10 times the energy density.
The arrangement can even make use of erratic airflow. "The turbulence is a potential benefit, as it draws in kinetic energy from above the [vertical turbine] farm," said Dabiri in an email.
Dabiri said that for large-scale power generation, the hurdles at this point are technical. "We need to demonstrate a cost-effective, reliable vertical axis wind turbine. It's a straightforward engineering challenge, but it hasn't been done yet," he said.
However, on smaller scales, vertical turbines have found a niche among home installers, which mount them on poles or rooftops. Some companies advertise their models as being bird-friendly. Sagrillo is skeptical of this trend. "There's no fuel where you're siting them," he said, pointing out that there often isn't enough wind to make these installations cost-effective at these scales.
He also said that vertical turbines are no safer for wildlife than propeller turbines. "A spinning rotor is a spinning rotor; it doesn't matter how it's oriented," he said.
Still, there are some applications where vertical axis turbines would be ideal, according to Veers. "When people look at very, very large offshore systems, the advantages of vertical axis become interesting again," he said. The advantage of having a lower generator and consequently a lower center of gravity helps lower installation costs on floating platforms in the ocean, where wind speed is typically higher than on land, observed Veers.
In the end, both vertical and horizontal turbines have their trade-offs, though horizontals are a more mature technology. "It's all a matter of the details and how well you have designed the machine," said Veers. "Either one can be an effective machine. In any size range, either one could be effective if you did it right."
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500