Insights from an experimental study on the crosswind flight of a lab-scale buoyant air turbine

This paper presents an experimental investigation into the crosswind motion of a lab-scale buoyant air turbine under a variety of flow conditions. A buoyant air turbine consists of a horizontal-axis turbine placed inside a helium-filled shroud and connected to the ground by tethers. Because of this setup, the system can reach higher altitudes where winds are typically stronger. Furthermore, there is an opportunity to improve power production by executing crosswind flight. This paper builds directly on (i) previous work that uses a water channel to develop a lab-scale setup that is dynamically equivalent to a full-scale system and (ii) previous work that uses this setup to execute crosswind motion induced by a square wave roll set-point trajectory. The water channel setup allows for many tests to be run at a relatively low cost. In this paper, we use the water channel to run crosswind experiments at a variety of flow speeds and with a variety of control parameters. These experimental conditions are selected using the G-optimal Design of Experiments. To the best of the authors' knowledge, this is the first time that such a variety of crosswind flight conditions have been tested on a lab-scale buoyant air turbine. Results show that (i) crosswind motion can be achieved at a variety of flow speeds, (ii) roll set-points with periods between 4 and 6 seconds are most effective out of the conditions tested, and (iii) at low flow speeds, the system can oscillate, even when unprompted by a periodic roll set-point trajectory.