High efficient kite for harvesting high altitude wind power: Investigation of new concepts and materials for the structure

Abstract

The Center for Synergetic Structures at EMPA is currently developing high efficient ultra-light plane-like structures, known as Twing, for generating wind energy at high altitudes. This work is part of a larger collaborative project called Swiss Kite Power in which other universities are involved. The elec-tricity is produced by the tension of a rope which connects the ground generator with the flying Twing. The structure needs to be lightweight enough to be airborne and it is required to hold correctly the aerodynamic loads. The current Twing fulfills perfectly the weight requirement but it is not stiff enough to carry properly the loads. Thus, the aerodynamic performance is not optimal and the generated en-ergy per cycle is much lower than desired. Object of this work is to improve the structural concept of the Twing by increasing its stiffness but not increasing its weight. This master thesis aims to study the structural behavior of the new Twing ribs made out of composite sandwich with numerical models and experiments. In the first part of the thesis, the numerical model of the Twing is implemented based on the finite el-ement method. In order to investigate the influence on the spacing between ribs, the structure is modeled with several rib distributions. A rib distribution is decided concerning the weight and the de-formation of the structure. Afterwards, the Twing is modeled with different concepts for the ribs in or-der to evaluate their effect on the deformations of the structure. The analysis regards the thickness of the sandwich core, the type of reinforcing composite material and the introduction of holes into the rib structure. Finally, a definitive stiffer design for the ribs which fulfills the weight requirement is pro-posed. In the second part of the thesis, the loads onto the ribs are obtained numerically with the finite ele-ment method and analytically with the corresponding assumptions. In general, the numerical solutions show a strong agreement with the analytical estimates. In the last part of the thesis, rib samples similar to the modeled in the numerical study are manufac-tured by hand through distinct fabrication techniques. The prototypes are then experimentally and numerically tested. The ribs of the old Twing design are as well manufactured and tested. Despite the experimental results and the numerical results are slightly different, their magnitude is coherent. The results prove that the stiffness of the ribs is dramatically increased from the old ribs to the new sand-wich ribs.