This paper considers aerodynamic interactions among an array of tensioned ribbon energy harvesters capable of harvesting both wind and solar energy. Each harvester consists of a thin-film solar cell ribbon supported in tension by a pair of piezoelectric bimorph beams in an inverted-U configuration. These ribbons experience aeroelastic flutter when subjected to crossflow, and the energy from these vibrations can be harvested through the piezoelectric beams. The effect of wind speed on the interaction between two fluttering inverted U-shaped aeroelastic energy harvesters configured in a tandem array was investigated, as previous work suggests that synergistic wake interactions can occur between multiple fluttering energy harvesters. An experimental apparatus was constructed and two thin-film solar ribbons were placed in tandem at a fixed separation distance. Each ribbon was given an applied pre-tension, and wind tunnel testing was performed for a range of wind speeds between 7.5 m/s and 12.5 m/s for each ribbon when fluttering in isolation and when fluttering in tandem. Tandem array efficiency was calculated from the experimental data, and it was determined that there is a wind speed at which peak tandem array efficiency (significantly greater than unity) occurs. It was found that this peak corresponds to the wind speed at which constructive interference due to frequency lock between the two fluttering ribbons begins. Results also show tandem efficiency benefits in both the downstream and upstream harvester, as opposed to previous results that show benefits primarily in the downstream harvester. It is hypothesized that these upstream benefits are due to possible base excitations in the apparatus that have been transmitted by the downstream harvester.
Researchers have performed theoretical investigations of flow induced limit cycle oscillations (LCOs) of tensioned ribbons. Furthermore, attempts have been made to tap into the energy harvesting capability of such ribbons, owing to its structural simplicity, low weight and ease of fabrication. However, in order to tune the ribbon to perform optimally at a given location, a robust, reliable model of the ribbon is essential to predict the limit cycle behavior. The model needs validation across a broad spectrum of its operating envelope based on experimentally obtained results. This paper seeks to provide experimental data for a sample tensioned ribbon in cross flow to serve as basis for validation of an aeroelastic model. This paper experimentally characterizes a PTFE (polytetrafluoroethylene) ribbon of aspect ratio 18 across a range of applied axial preload tension and wind speeds.