Poised halfway between the complexity of Makani and the simplicity of SkySails, the TwingTec TwingKite (or simply Twing, short for tethered wing) uses an advanced lightweight construction to generate energy from the wind. It has been awarded 10,000 Swiss Francs by the Venture Kick start-up funding program.
TwingTec's Twing (or tethered wing) prototype
As anyone who has flown a kite or a paraglider, or landed under a parachute can tell you, the wind is stronger the higher off the ground you are. The wind slows with the drag of the earth and obstacles sticking up from it, and as this drag is reduced with altitude, the air's velocity increases. This has been a substantial factor behind the growth in generation capacity of traditional three-blade, horizontal axis wind turbines, the iconic white towers that dot the countryside in so many parts of the world. As they have been built taller and taller, the blades have been exposed to more and more wind, so they fly faster and in turn generate more electricity.
But many people have asked if there isn't a better way, perhaps one that does away with the tall white tower entirely.
TwingTec believes it has the answer: simply fly a rigid, very lightweight kite more than 100 m (328 ft) higher than the highest point a modern wind turbine blade reaches, let the strong wind generate power as it strips cable from a drum on the ground, then fly it out of the power zone and pull it back upwind with little effort. Repeat every two minutes or so, and a fair amount of electricity can be generated without a single white tower in sight.
Its system is evolving. In the past they had been using simple airfoil traction kites in a manner very similar to the SkySails Power System. However, the company knew that more power could be generated with a rigid wing kite. Its current model has an airfoil, a tail and an aileron that allows it to be controlled from the ground. As such, it has put a more complexity and potential fragility into the air, moving in the direction of Makani's rigid wing approach, recently acquired by Google.
Its secret sauce is Tensairity, a Swiss-invented, ultra-lightweight construction technology that has been around for a decade or so . It uses an overpressurized air filled cylinder with a lightweight compression spine and cables for tension. When filled correctly, it creates a very stiff beam at a fraction of the weight of conventional structures. This construction technique has been looking for a suitable application and has only been used in a handful of demonstration builds to date.
The leading edge of the Twing is a Tensairity beam, providing the basis for a rigid airfoil that is very lightweight yet also strong and crash resistant. The prototype flies much faster than the fabric airfoils previously used and creates more lift which can be used to generate more electricity.
It is unclear whether this advantage will outweigh the additional complexity of the rigid wing, added aileron and an inflatable chamber that likely needs sensors and an onboard pump to maintain the overpressure needed for rigidity. The power for the pump will have to be provided too, either with a battery requiring changing or by turning one of the load-bearing cables into a conductor or by mounting a small generator on the wing itself. The complexities will multiply.
It's also clear that the automated flight control systems that both Makani and SkySails have built over the years are more of a gleam in TwingTec's eye at this point. All flying is done by a ground-based pilot with joysticks controlling the winches. Automated kiting when the wing is in the air isn't hard; stunt kiters can easily fly figure eights when blindfolded. But launching and landing when the wind isn't blowing even up in the sky is much more sophisticated, as is dealing with strongly variable winds.
Finally, the company seems yet to address that its kilometer-long cables and rigid wing introduce obstacles for land-based generation. Both Makani and SkySails are primarily focused on offshore wind farms as a primary model at present.
But it is early days. TwingTec has received 10,000 Swiss Francs from the Venture Kick start-up funding program, the first funding in a pipeline that could reach 130,000 Swiss Francs (about $140,000 USD) if they hit their marks. As their competition includes both Google and a firm which has successfully towed cargo ships with parafoil fabric kites, they have their work cut out for them.
By Le My