Performance modelling and scaling of fixed-wing ground-generation airborne wind energy systems

Abstract. The economic viability of large-scale future airborne wind energy systems critically hinges on the achievable power output in a given wind environment and the system costs. This work presents a fast model for estimating the net power output of fixed-wing ground-generation airborne wind energy systems in the conceptual design phase. In this quasi-steady approach, the kite is represented as a point mass and operated in circular flight manoeuvres while reeling out the tether. This phase is subdivided into several segments. Each segment is assigned a single flight state resulting from an equilibrium of the forces acting on the kite. The model accounts for the effects of flight pattern elevation, gravity, vertical wind shear, hardware limitations, and drivetrain losses. The simulated system is defined by the kite, tether and drivetrain properties, such as the kite wing area, aspect ratio, aerodynamic properties, tether dimensions and material properties, generator rating, maximum allowable drum speed, etc. For defined system and environmental conditions, the cycle power is maximised by optimising the operational parameters for each phase segment. The operational parameters include cycle properties such as the stroke length (reeling distance over the cycle), the flight pattern average elevation angle, and the pattern cone angle, and include segment properties such as the turning radius of the circular manoeuvre, the wing lift coefficient, and the reeling speed. To analyze the scaling behaviour, we present a kite mass estimation model based on the wing area and the maximum tether force. The model mainly aims at sensitivity and scaling studies to support design and innovation trade-offs. It is also suitable for integrating cost models and systems engineering tools that assist in the conceptual design of systems. The computed results are compared with six-degree-of-freedom simulation results of a system with a rated power of 150 kW. The interdependencies between key environmental, system design, and operational parameters are presented. The model's capability to capture scaling effects is shown through an example of varying kite wing area and tether diameter.