Numerical Modeling of Kites for Power Generation Available to Purchase

A computational model of a massless kite that produces power in an airborne wind energy system (AWE) is presented. AWE systems use tethered kites at high altitudes to extract energy from the wind, and are being considered as an alternative to wind turbines since the kites can move in high-speed cross-wind motions over large swept areas to increase power production. In our model the kite completes successive power-retraction cycles where the kite angle of attack is altered as required to vary the resultant aerodynamic forces on the kite. The numerical simulation models the flow field in a two-dimensional domain near the flexible kite by solving the full Navier-Stokes equations. Eulerian grid points for the flow domain together with a Lagrangian representation of the kite are employed. The flow field is determined through a second-order finite difference projection method using a non-uniform mesh on a staggered grid. A corrector-predictor technique is employed to ensure the second-order accuracy in time of the numerical simulation. The two-dimensional kite shape is modeled as a slightly cambered immersed boundary that evolves with the flow. The flexible kite surface is modeled with a set of linear springs following Hooke’s law. The unstretched length of each elastic tether at a given time step is controlled using periodic triangular wave shapes to achieve the required power-retraction phases. A study was conducted in which the wave shape amplitude, frequency, and phase (between two tethers) was adjusted to achieve a suitably high net power output with very good agreement to predictions for Loyd’s simple kite in two-dimensional motion. Aerodynamic coefficients for the kite, tether tensions, tether reel-out and reel-in speeds, and vorticity flowfields in the kite wake are also determined.