Energy harvesting devices based on the inverted flag configuration have recently attracted attention as a viable option to power small electronics and remote wireless sensors from wind excitation. Despite showing high potential, relatively little research has considered how the dynamics and power generation performance of these devices vary with planform geometry. This study considers composite inverted flags constructed using flexible polyvinylidene difluoride strips as active elements sandwiching a stainless-steel shim as elastic structural support, a configuration allowing for enhanced ability to tailor mechanical and geometrical properties (and hence responses) of the flag. We explore the effect of three key parameters (planform aspect ratio, mass ratio, second moment of area) on flag dynamics (amplitude, flapping frequency) and operational characteristics (power, velocity range). Twelve flags have been manufactured and tested under controlled wind excitation, selected to cover aspect ratios in the range of 0.9-7 and mass ratios in the range of 7-14. Our results expose a number of useful insights into how these devices perform. First, we show that for a given mass ratio, flapping amplitude, frequency, power, and power density are all inversely proportional to aspect ratio and proportional to second moment of planform area. Second, we show that second moment of planform area is a better indicator parameter to assess the flags performance, as compared to the aspect ratio. Third, we show that as mass ratio increases higher power densities and wider operational velocity ranges are allowed; however, this is on the expense of the flag being more sensitive to variations in operating conditions and the possibility of experiencing some unfavourable motion behaviour such as the 'one-sided' low-amplitude high-frequency low-power flapping mode.
This paper presents results from a practical assessment of the endurance of an inverted flag energy harvester, tested over multiple days in a wind tunnel to provide first insights into flapping fatigue and failure. The inverted flag is a composite bimorph, composed of PVDF (polyvinylidene difluoride) strips combined with a passive metallic core to provide sufficient stiffness. The flag, derived from an earlier, more extensive study, flaps with a typical amplitude of ~120 degrees and a frequency of ~2 Hz, generating a constant power of ~0.09 mW in a wind velocity of 6 m/s. The flag was observed to complete ~5×105 cycles before failure, corresponding to ~70 h of operation. The energy generated over this lifespan is estimated to be sufficient to power a standard low-power temperature sensor for several months at a sampling rate of one sample/minute, which would be adequate for applications such as wildfire detection, environmental monitoring, and agriculture management. This study indicates that structural fatigue may present a practical obstacle to the wider development of this technology, particularly in the context of their usual justification as a ‘deploy and forget’ alternative to battery power. Further work is required to improve the fatigue resistance of the flag material.
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