The effect of kite control unit mass and drag on the flight behavior of an airborne wind energy system kite

Abstract

Airborne wind energy (AWE) is an emerging field that aims to revolutionize the wind energy sector.This work focuses on the aspects of modeling AWE kite flight behavior. Research on such modelsis important since new or improved models can potentially reduce development time and costs andprovide insight in the role of key system parameters.This research investigates the effects of the mass and drag of the kite control unit (KCU) on the kite’sflight behavior since it is hypothesized that the mass of the KCU causes an outward swing of the KCUduring turns and that KCU drag reduces the angle of attack during the retraction phase.For this investigation, the simulation of a kite and tether system is required. Existing simulation environ-ments could not be used due to legacy code, limited validity or unsuitable model complexity. Therefore,a suitable kite and tether model need to be selected for implementation in a simulation environment(SE). Two kite models have been found suitable: Fechner’s and Ruppert’s model.In the early stages of this research the aim was to use a model that has been fitted to experimental flightdata. Due to the unavailability of experimental flight data, the parameters of both models are identifiedusing simulated flight data. This approach provides Kitepower with a method to identify the modelparameters with experimental flight data in the future. Kitepower is an AWE company that facilitatedon this research.The groundstation controller and flight controller signals of Kitepower have been integrated in the SE.Two experiments have been performed in the SE. For the first experiment, flights are performed withdifferent and increasing values for the drag coefficient of the KCU. In the second experiment the massof the KCU and kite have been varied independently. Both experiments have been performed in a lowand high wind condition.The high wind condition experiment showed that and increasing drag results in a reduction of 7% ofthe average cycle power. The KCU drag force can reach up to to 5% of the kite drag force for a KCUdrag coefficient of 1.5.Increasing the combined mass of the kite and KCU results in an increase of >150% in average cyclepower output for the low wind conditions and an increase of >40% for the high wind condition. Theincrease in power is caused by a reduction in elevation angle in combination with the assumption of aflat wind profile. Additionally an outward swing of the KCU of 4.5 ∘ has been observed.It is recommended to compare Fechner’s and Ruppert’s rigid body kite models through using experi-mental flight data in order to investigate which model reproduces actual kite flight the best. Furthermoreit is recommended to investigate the optimal altitude or elevation angle at which to operate the AWEsystem for maximal power output considering a non-flat wind profile, tether drag and kite and KCUdynamics. It is suggested that these findings should then be incorporated in the flight controller ofKitepower in order to increase the system power output.