Redesigning the traditional wind turbine -- large three-bladed rotors with controllable pitch -- is sort of like reinventing the wheel. Decades of trial, error and testing go into it. Bob Thresher, a research fellow at the National Renewable Energy Lab in Golden, Colorado, would know. He’s spent more than 40 years working in the wind energy field and helped start the National Wind Technology Center.
“I’ve spent my entire working life developing kinetic art that’s useful,” he said.
But conventional wind turbines remain expensive and require tons of material. The ongoing quest for cheaper, more efficient wind energy has produced wacky wind turbine ideas. Thresher offers his grounded take on some of the most promising ones.
Airborne wind turbines resembling kites, planes, balloons, flying figure eights and nearly everything in between are actively being developed. Joby Energy’s multi-wing system and KiteGen’s kite-like turbines have made headlines. In 2009, the Airborne Wind Energy Industry Association formed to promote the stakeholders in this nascent sector.
The idea of tapping winds higher in the atmosphere makes sense, Thresher said. But the challenges for flying or light air devices also increase exponentially. Airspace is tight. Plus, a bad day for a wind turbine on the ground is when the rotor smashes the tower. “If you’ve got a bad day in the air you might cut your cable and then where are you going to land? You’ve got some liability issues,” Thresher said. Going offshore or a place with a large expanse could help minimize the risks, he added.
Some engineers have looked to building structures, redesigning them to tap wind energy. Last year a company called Clean Wind Energy Tower Inc. got approval to develop two http://news.discovery.com/tech/downdraft-wind-tower-120606.htm Downdraft Towers in Arizona. The concept works like this: Water is pumped to the top of the 2,000-foot-tall cylindrical tower and sprayed into a mist. That cools the air, causing it to sink rapidly, at speeds up to 50 miles per hour. The falling air rushing through the bottom, where there are turbines that convert the motion into electricity.
Thresher compared the tower to an ocean thermal energy conversion or OTEC, where a device in the ocean picks up cold water from the bottom and warm water from the surface to run a heat engine on the temperature difference. The trouble is that to produce significant amounts of energy, the device has to be huge, he said. Then the structural cost is also high due to the large size. And, forget about planning offices or homes for wind energy buildings. In general, Thresher said, people tend to avoid living in windy places. “Look at Wyoming: great wind, no people.”
Vertical-axis wind turbines are among the less wacky on the list. Operational ones are already up and running. Their power curve is similar to conventional horizontal axis turbines. In these designs, the main rotor shaft is arranged vertically, often with the blades swirling around in a helix formation. The Swedish company Ehmberg Solutions AB took this a step further, creating a flywheel design called SeaTwirl intended to be anchored on the seafloor.
The big challenge here is making such a system cost-effective, Thresher said, but with enough development it could be a horse race with traditional turbines. “We proposed looking back at vertical axis turbines for offshore,” he added. “We have some energy around the idea that it might be doable.”
Funnel-like wind devices can be traced back to the 1970s, when the first diffuser was proposed. Thresher said the most high-tech one today is an aerodynamic jet-engine style structure from Massachusetts-based FloDesign Wind Turbine Corporation. Their shrouded turbine works by creating a rapid-mixing vortex when wind hits it, and has fins that allow it to turn into the wind for maximum energy. The tricky part is that a device with a shroud needs to be able to survive hurricane-force winds.
“You can’t feather it,” Thresher said. “You can’t minimize the drag -- you’ve got this big shroud out there.” Another challenge, as usual, is cost. The larger the foundation and structure, the higher the cost. To mitigate the added weight and cost, some designers are considering structural fabric or polymer skin for the parts. Thresher added that he thinks a shroud does improve performance but the cost is still unproven.
Several years ago Yuji Oyha, a professor at Kyushu University, captured imaginations with his novel design for what he called a “Wind Lens.” While a giant rim-like rotating wheel for offshore energy has yet to be fully realized, several smaller experimental versions have been installed high above a park in Fukuoka, Japan -- part of a major green energy expansion in the country.
Thresher called the Wind Lens a type of wind augmenter that has some structural advantages in a smaller size. In the past he’s seen a similar device with a rim drive so all the equipment could be on the ground. The problem, he said, is that you can’t hide in high winds. Any lens-like structure has to be built to withstand them. “It might work for small devices,” he said. “Particularly if your turbine is in a low wind-speed place where the augmentation might be important.”
We’ve crossed a few odd designs off the list as too improbable to feature. Bob shoots down to a satellite-dish shaped turbine he says will wobble like a sign on the highway. But he perks up about a bladeless turbine called an Electrostatic Wind Energy Convertor or EWICON for short created by researchers in the Netherlands at Delft Technical University. A steel frame holds insulated tubes that spraying positively charged water particles into the air. As the breezes blow, the particle are driven through an oppositely charged collector array, generating a current.
“I do have doubts as to whether it can be made cost effective, and then there is the added requirement for a pure clean water supply,” he cautioned, but called the approach innovative. “This is truly a unique wind device.”