Most wind energy systems, like wind turbines, extract power at low altitudes. Although extensive wind exploitation and remarkable growth happened thanks to conventional technologies, important benefits can be obtained by making the systems operate at high altitudes, where winds are stronger and less intermittent. In addition, the substitution of wind turbines by lightweight structures placed at a high altitude, such as kites, would reduce the costs and the visual impact. Airborne wind energy (AWE) systems are technologies that convert wind energy into electrical energy and are made of one or more aerial autonomous vehicles linked to the ground by at least one tether. These systems generate electricity by using the tether tension to move a generator on the ground (ground-generation) or using on-board wind turbines (fly-generation).

UC3M activities on AWE systems formally started in 2015 thanks to a Leonardo Grant funded by BBVA Foundation. From 2016 the team has performed research and development activities thanks to several projects granted by the Spanish Government: GreenKite (2016 – 2019), GreenKite-2 (2019 – 2023) and KITE2GRID (2023-2026). Additionally, UC3M AWE group has established a close collaboration with the Spanish company CT Ingenieros, through an Industrial PhD funded by Comunidad de Madrid, a Research Chair, and several joint research and development projects. In parallel, many Bachelor and Master Aerospace students developed their minor theses in our group, two PhD theses were defended since 2021 and two PhD theses are ongoing. UC3M research topics include the modelling, simulation and control of AWE systems, as well as the aerodynamic and aeroelastic characterization through numerical and experimental analysis. One of the most important results is the prototype of an AWE yo-yo machine.

Research capabilities

AWE Machine Prototype

Since 2015, UC3M has progressively acquired experimental capabilities on AWE systems. Starting from an experimental setup for the estimation of the aerodynamic parameters of AWE systems (DOIs: 10.2514/1.G003581 & 10.1002/we.2591), the group then developed an in-ground control system for power and rigid-framed kites and it is currently preparing a full prototype of a yo-yo AWE machine in collaboration with CT Ingenieros. Enabled by the multidisciplinary collaboration of the Aerospace Engineering Department and the Electrical Engineering Department of UC3M with CT Ingenieros, the prototype is designed to operate medium sized kites up to three lines, both connected to the grid and in islanded mode. The machine is a flexible and multi-purpose facility with application to the study of the dynamic and control of AWE systems, the mechanical-to-electrical power conversion, the optimization of on-ground and flying hardware and software elements and the aerodynamic characterization of AWE aircraft, among many others.

The AWE machine prototype is fully compatible with the experimental setup developed by UC3M to characterize the aerodynamic behavior of AWE systems. Fully compatible with 1, 2 and 3-line systems, the setup includes a mechanical control system with two linear actuators, a winch mechanism, inertial navigation unit with differential GNSS, a multihole pitot tube onboard the kite for the in-situ measurement of the full aerodynamic velocity vector, load cells at the tethers, and a weather station for measuring the wind velocity (modulus and direction). Dedicated estimation tools were also developed to fuse all measurements of the sensors and estimate the aerodynamic force and torque during in-flight testing campaigns and get the aerodynamic parameters by using estimation before modelling techniques.

The videos below show some of the in-flight tests that UC3M team carried out since 2015 with leading-edge inflatable and rigid-frame kites.

AWE simulator

UC3M has developed LAKSA, an open-source simulation package to study the dynamics and control of AWE systems (available for download). The software has 6 modules aimed at different machine architectures. The equations of motion for four of them were derived using a Lagrangian formulation with a minimal coordinate approach. This feature, along with the inelastic but flexible character of the tethers in the models, yields a non-stiff system of ordinary differential equations that is not coupled with algebraic constraints. As a result, the code is robust and efficient. LAKSA’s module, named KiteFlex, can be used to study both ground-generation and fly-generation systems. It considers the self-consistent dynamics of the aircraft (a rigid body), the on-board rotors, and a flexible tether. Its control vector, in the most sophisticated configuration, involves the length of the main tether for reel-in and reel-out operations, the lengths of the bridle lines, and the deflections of the ailerons, rudder, and elevator for steering the aircraft, as well as the torque of the motor controllers for the rotors. Thus, the code is prepared to analyze a wide variety of AWE machines, including those developed by companies such as Skysails, Kitepower, Mozaero, TwingTec, and Enerkite, among others. Recently, a new module called RAWE has been added, which can be used to study the dynamics of Rotary Airborne Wind Energy systems, such as the one developed by the company someAWE. All the simulators are linked to scientific articles that explain in detail the mathematical models and their structure (find links to them in the Publications section).

Computational Analysis of AWE Aerodynamics

An in-house three-dimensional unsteady panel method (DOI: 10.2514/6.2015-1185) and a high-fidelity open-source aerodynamic tool (SU2) have been applied to Rigid-Framed Delta (RFD) kites. This work opens the possibility to compare theoretical aerodynamic forces and moments to experimental ones (from flight-testing campaigns) for a deeper characterization and understanding of kites’ aerodynamics.

Contact

For further information please contact:

gonsanch@ing.uc3m.es