Clever ideas float more than a turbine

Offshore looks like the next big expansion area for the wind industry. Conventional ideas extended to offshore tasks will mount turbines on towers anchored to the ocean floor in not much more than 30-m depths. Floating turbines, however, would allow placing them farther from shore but will probably be expensive. Inventor Doug Selsam suggests an answer to the shortcomings with some out-of-the-box thinking in his Superturbine (superturbine.net), a floating design that omits the tower altogether and sports several other advantages.

For instance, a direct-drive generator would mount in a floating pod tethered to the ocean floor so water depth becomes less of an issue. A long carbon-composite tube would hold rotors positioned along its length while sea breezes provide lift to keep the string of rotors aloft. Selsam says the design omits all components that do not contribute to power generation, resulting in a low cost turbine. This stripped-down design would be more efficient, cheaper, and easier to produce, he says, than what he calls lumbering windmills. They’re also more versatile. For instance, they could be attached atop skyscrapers.

Selsam has been nurturing the multi-rotor turbine for several years in prototypes that have produced more power than similarly sized single-rotor units. For instance, prototypes of his initial idea put several 7-ft, two-blade rotors on a single shaft that runs through a generator. With one more rotor downstream than upstream, the unit points into the wind without a rudder or yaw motors. In 32.5 mph winds, this setup generates 6 kW, about six times more power, he reports, than a 7-ft single-rotor turbine would produce in the same wind.

“The rotors act like gyroscopes stabilizing the driveshaft where they attach,” he says. “Because smaller diameter rotors turn at higher speeds than larger rotors on conventional units, a small and light direct-drive generator allows omitting a gearbox.”

Such a design, he says, is easily deployed on land or at sea. The effectiveness of offshore versions is amplified by attaching a balloon to lift the free end. When necessary, this design can lay itself down on the sea, or submerge completely using flooding chambers. Selsam says it would pose no risk to passing vessels, because it is relatively lightweight.

More advantages come from minimal spacing requirements. For instance, conventional wind farms separate rotors by several diameters to reduce downstream wake effects. But in Selsam’s design, the vertical offset allows closer placement because positioning the rotor array relative to the wind direction is predetermined. The top of one wake transits to the lower edge of the succeeding rotor, so energy normally lost to wake vorticity is recaptured. “Each rotor adds lift which helps support the driveshaft against gravity and downwind thrust forces. And the power reaching all rotors tends to be equal,” he says. WPE

Scratch the tower. Let the turbines float

Norway’s StatoilHydro and its new division Hywind have towed and anchored a floating 2.3-MW wind turbine to a spot about 10 km off the southwest coast of Norway. A 100-m buoyancy section of the tower keeps the rest of the unit floating upright. It’s anchored to the seabed with cables. The turbine can be placed at depths from 120 to 700 m. The Hywind, built by Siemens, combines technologies from the wind farming industry and oil and gas sectors, and will serve as a two-year test bed.

“This should help move offshore wind farms out of sight and away from where they cause disruption,” says Statoil spokesperson Alexandra Beck Gjorv. This would benefit military radar operations, the shipping industry, fisheries, bird life, and tourism.

“Taking wind turbines to sea presents new opportunities,” adds Gjorv. “Wind is stronger and more consistent and the areas are large.” Floating wind farms will be connected to mainland grids by cables on the seabed.

Offshore wind farms cost considerably more than wind farms on land, and initially floating ones will be more expensive than static offshore installations. But over time, says Gjorv, the floating designs should not cost more than fixed ones. Statoil plans to target markets where there is an ability to pay along with a growing demand for energy. Floating wind farms could be established off the coasts of North America, the Iberian peninsula, Norway, and the U.K. she said. Floating wind farms could also provide an additional source of energy for countries that have run out of space for their onshore wind farms, or where there is not enough wind on land.

“The global market for such turbines is potentially enormous, depending on how low we can press costs,” she said, though she was not able to quantify them or to outline a timescale for when floating wind farms would become commercially available.

StatoilHydro is investing around NOK 400 million in the construction and development of the pilot. The public corporation Enova SF, whose aim is to promote the transition to environmentally friendly energy use and energy production in Norway, has granted NOK 59 million in support.

A few specs for the floating turbine

Turbine works 1,000 ft up without a tower

Instead of waiting for fickle winds to come to MARS (Magenn’s Air Rotor System) it goes to the wind. It is a 50 by 120-ft lighter-than-air device from Magenn Power LLC, Washington, D.C., that floats 600 to 1,000 feet up to catch wind currents present almost everywhere. MARS rotates to generate up to 100 kW/h, and feeds it down a tether to a grid or battery array. “Traditional fixed turbines work in 15% of the world. We’re the solution for the other 85%,” says Magenn CEO Mac Brown.

MARS is a lighter-than-air tethered wind turbine that rotates about a horizontal axis in response to wind. Helium sustains MARS and lets it ascend to a higher altitude than traditional wind turbines. MARS captures the energy available in the 600 to 1,000-foot level that exists most everywhere. Its rotation also generates an effect that provides additional lift, stabilizing the turbine and positioning it in a controlled and restricted location to adhere to FAA and Transport Canada guidelines. “SolidWorks CAD software helps us experiment with different turbine configurations, compare their power outputs, and save thousands that we once spent on outsourced simulations,” says Brown.

The design removes all placement limitations. Coast-line and off-shore locations are unnecessary to capture higher speed winds. Reaching winds at 1,000-feet up allows installing MARS closer to the grid. The mobile turbine can be rapidly deployed, deflated, and redeployed without need for towers or heavy cranes. What’s more, MARS is said to be bird and bat friendly, generates little noise, and operates in a wider range of wind speeds, from 4 mph to greater than 60 mph.

Brown sees applications in developing nations where infrastructure is limited or non-existent and off-grid combined wind and diesel for island nations, farms, and remote areas. Rapid deployment diesel and wind could be air dropped to disaster areas for power to emergency and medical equipment.

Capacity factors, MARS versus conventional designs