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

Abstract

The economic viability of future large-scale 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, as well as segment properties such as the turning radius of the circular manoeuvre, the wing lift coefficient, and the reeling speed. To analyse the scaling behaviour, we present a kite mass estimation model based on the wing area, aspect ratio, and maximum tether force. The computed results are compared with 6-degree-of-freedom simulation results of a system with a rated power of 150 kW. The results show the interdependencies between key environmental, system design, and operational parameters. Gravity penalises performance more at low wind speeds than at high wind speeds, and excluding gravity does not yield optimistic performance since it assists in the reel-in phase by reducing the required power. Thin tethers perform better at lower wind speeds but limit power extraction at higher wind speeds and vice versa for thick tethers. Upscaling results in a diminishing gain in performance with an increase in kite wing area. The proposed model is suitable for integration with cost models and is aimed at sensitivity and scaling studies to support design and innovation trade-offs in the conceptual design of systems.