A new perspective on the aerodynamic performance and power limit of crosswind kite systems

Abstract

In this paper, a new perspective on the aerodynamic performance modelling of crosswind kite power systems (CKPSs) is provided, where the effects of the induction factor or flow retardation by the kite are taken into account. For simplicity, only CKPSs in straight downwind configuration are considered, where the kites sweep an area perpendicular to the wind direction. Moreover, the in-plane or tangential induction factor is neglected. It is argued that the concept of the swept-area-normalized power coefficient, which is commonly used for conventional wind turbines, is not practically important for CKPSs. Instead, a kite-area-normalized power coefficient concept is adopted and is shown to be a more appropriate metric of performance of kite systems. The kite-area-normalized useful and wasted power coefficients for both a lift and a drag mode CKPS are plotted at different values of solidity factor and aerodynamic efficiency. Moreover, it is shown that the two modes of power generation, i.e. lift and drag modes, yield the same amount of useful power when the kite system solidity factor is essentially zero – this is in agreement with the underlying assumption in the crosswind kite power principle. For non-zero solidity factors, thus non-zero induction factors, however, our results suggest that the drag mode has higher power generation potential than the lift mode.