From Surf-kites and Sailplanes to Twings

45 Rolf H. Luchsinger Empa Center for Synergetic Structures Überlandstrasse 129 CH-8600 Dübendorf Switzerland rolf.luchsinger@empa.ch http://www.empa.ch/css

From Surf-kites and Sailplanes to Twings

Empa – Center for Synergetic Structures

The kite is arguably the most critical piece of hardware of a kite power system. Its performance dictates the efficiency and the capacity factor of the system and thus to a large ex-tend the levelized costs of energy (LCOE), the ultimate bench-mark for airborne wind energy. The emergence of power-ful surf-kites from the sports industry in the last decade has fueled the devel-opment of kite power. No surprise that most of the kitepower groups start out with surf-kites, either from the shelf or slightly modified. Surf-kites are easy to get, rel-atively cheap, rather simple to control and to a large extend crash resistant. On the other hand a few groups are pursuing concepts with a rigid wing. These plane-like configurations are “borrowed” from the avia-tion industry and are in most re-spects very different from surf-kites. So what is really crucial for the kite? For a pumping cycle kitepower system, the four most important wing properties with respect to the efficien-cy of the system are the lift to drag ratio, the wing loading,

the mass per wing area and the ability to fly depowered dur-ing the retraction phase. While the outstanddur-ing feature of the surf-kite is its very low weight, it fails short with respect to the three other criteria. On the other hand, a rigid, sailplane like structure can have a high lift to drag ratio, good depow-er and high wing loading. How-evdepow-er, the structural weight is a major challenge.

We at the Center for Synergetic Structures have put our focus on the development of the wing for pumping cycle kitepow-er. In the framework of the SwissKitePower R&D project we have performed theoretical studies, numerical simulations and numerous field tests. Based on these results we have developed the twing (tethered wing), which can be seen as a synergetic combination of the sailplane and the surf-kite. The presentation will de-scribe our quest for the optimal wing for kitepower and give insights into our twings.