Advances in Airborne Wind Energy and Wind Drones

Among novel technologies for producing electricity from renewable resources, a new class of wind energy converters has been conceived under the name of Airborne Wind Energy Systems (AWESs) or Wind Drones (WDs). This new generation of systems employs flying tethered wings or aircraft in order to reach winds blowing at atmosphere layers that are inaccessible by traditional wind turbines.

The economics of AWESs is very promising for mainly two reasons. First, winds high above ground level are steadier and typically much more powerful, persistent and globally available than those closer to the ground, and second, the structure of AWESs is expected to be orders of magnitude lighter than conventional wind turbines.These plants are therefore interesting for their potential high power density, i.e. ratio between nominal power and weight of required constructions, that makes it possible to forecast a low Levelized Cost of Energy (LCOE) for the produced electricity.

Despite this interesting potential, two important issues might be a major limitation to AWESs development. First, the large requirement in terms of airspace and the related safety issues, and second, the power dissipation through the aerodynamic cable drag. In a scenario where AWESs beat conventional wind turbines in terms of LCOE, the large airspace requirement is a logistic constraint that might slow down the economic development of AWESs, but the relatively small size/weight of AWESs foundations might be a key factor that enables the development of inexpensive floating offshore platforms thus solving the airspace limitations thanks to practically unlimited sea area. A significant part of this thesis investigates the performance of floating offshore AWESs by means of dynamic models, first with a single Degree of Freedom (D.o.F.) of the floating platform, then with a more complex multi D.o.F. model.

The second issue, the aerodynamic cable drag, is a physical constraint that is already limiting the potential of AWESs. In short, in order to reach higher altitudes, current AWESs must increase the cable length, but the power that would be dissipated by sweeping a longer cable through the air exceeds the power that would be gained from stronger winds at higher altitudes. This prevents current AWESs from working at very high altitudes where the jet streams carry up to 15.5 kW/m2 of wind power density.

A possible solution to this second problem is represented by a dual Wind Drone architecture in which two aircrafts, with on-board generators, are connected to the ground with a ‘Y’ shaped tethering; a concept that was first envisioned in 1976. At that time it was hard to envision a real operation of this system but recent studies are starting to investigate this concept in more detail. The last two chapters contain an attempt to estimate the power output of a large scale dual Wind Drone system and a proposal for a novel take-off method that might enable a first implementation of the dual Wind Drone system. It is first introduced a power model that captures the most significant real-world issues such as the effect of the weight of airborne components, the limits of structural/electrical elements, and considers take-off constraints. A numerical case study is analyzed considering a large scale system in Saudi Arabia. For such a device, the power curve is computed and, using real wind data, a nominal power output of approximately 15 MW with 30% capacity factor is estimated. Finally an experimental campaign that was carried out in TU Delft is described. In that campaign a take-off system for dual drones inspired to the so called ‘control line flight’ was investigated. The passive flight stability of a single wind drone in axi-symmetric configuration is a positive experimental evidence that encourages further research in dual drone systems.