Airborne Wind Energy Systems

Flight dynamics and control of airborne wind energy systems:

Airborne wind energy systems allow for the harnessing of winds at high altitudes through the replacement of a conventional wind turbine tower with tethers and a lifting body such as a wing, kite, or lighter-than-air shell. These systems either use their lifting body to elevate a turbine (or turbines) to high altitudes and transmit electricity down a conductive tether or execute periodic motions that result in net energy production on the ground. Either way, airborne wind energy systems represent a rich playground for the meaningful application of advanced dynamic modeling and control techniques. Because the optimal airborne shape for power production differs significantly from traditional flight vehicles or aerostats, their flight modes typically do not align with traditional aircraft, and their dynamics demand unique attention. And since tethered systems are not constrained to a particular altitude or required to remain stationary, their altitude and motion can be optimized for power production.

My work in the area of tethered wind energy systems involves two key research thrusts:

  • Characterization and optimization of closed-loop flight performance for airborne wind energy systems;
  • Optimization of altitude and motion to maximize energy harvesting, subject to constraints.

Characterization and optimization of closed-loop flight performance for airborne wind energy systems:

In the first area, I initially collaborated with Altaeros Energies and the University of Michigan Department of Aerospace Engineering to develop the first lab-scale, water channel-based platform for characterizing the flight dynamics of tethered wind energy systems at fractions of the cost of full-scale prototypes.  This approach involves 3d printing of roughly 1/100-scale models that can be ballasted to achieve different buoyancy and inertial properties, followed by dynamic characterization of the full tethered system under a variety of flow speeds. Full-scale flight testing conducted by Altaeros in the fall of 2013 revealed good correlation between water channel tests and full-scale results for the configurations flown.

Since working on the above passive flight characterization platform, my research group has instrumented UNC-Charlotte’s world class, 1m x 1m water channel with a high speed camera system, rapid control prototyping system, and DC motors, which will enable higher fidelity dynamic characterization of tethered wind energy system, along with experimental validation of control system designs. The UNC-Charlotte system represents the first-in-world system for lab-scale experimental characterization and optimization of the combined plant and controller for tethered wind energy systems. Our present research in this area focuses on a novel approach to combined plant and controller optimization, shown below, wherein 1/100-scale experiments are combined with numerical optimization in order to efficiently optimize the airborne wind energy system’s lifting body and controller.

Experimentally infused optimization framework

Simultaneous wind shear mapping and altitude optimization:

In the second area of research, our group is conducting research on algorithms that optimize the operating altitude of airborne wind energy systems while simultaneously mapping the wind shear profile. These necessarily competing optimization and mapping objectives lead to interesting control design formulations that can involve variants on extremum-seeking control and information maximization approaches.

Crosswind Flight for Power Augmentation:

A high lift-to-drag wing is capable of executing crosswind motions at far greater speeds than the true wind speed. Consequently, flying an AWE system in a crosswind pattern has the potential to substantially increase the amount of power generated from a given swept area. This has led numerous researchers to study the design of control algorithms to harness the vast potential promised by crosswind flight.

The CORE Lab is the first to have demonstrated crosswind flight in a lab-scale environment, using the UNC Charlotte water channel. Initial demonstrations were conducted in March 2016, and ongoing research within the lab focuses on validating crosswind flight control algorithms over a wide variety of flow speeds and optimizing the periodic crosswind flight trajectories.

Industrial/start-up collaboration opportunities:

My research group maintains an industrial collaboration with Altaeros, inc., which enables selected students to participate in full-scale flight tests and internships at Altaeros’ test site and headquarters (in Boston, MA), respectively.

Flight test at Boston airport

Current Funding:

1) CAREER: Efficient Experimental Optimization for High-Performance Airborne Wind Energy Systems, NSF Award Number 1453912.

2) Collaborative Research: Multi-Scale, Multi-Rate Spatiotemporal Optimal Control with Application to Airborne Wind Energy Systems, NSF Award Number 1711579

3) Collaborative Research: An Economic Iterative Learning Control Framework with Application to Airborne Wind Energy Harvesting, NSF Award Number 1727779

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