Fig 1.
Properties of model cottonwood leaf in fan-generated wind: (A) Power dissipated across 10 MΩ load (circles), and frequency of major band (triangles) as function of wind speed; (B) Raw voltage data at two wind speeds; (C) Power spectra at four wind speeds, showing shift of major band to higher frequencies with increase in wind speed. Leaf was angled 60 degrees downward from horizontal.
Fig 2.
Effect of excitation frequency and load resistance on power output from PVDF strip exposed to repetitive pulses of N2 gas at 34 PSI.
Optimal RL corresponds to capacitive reactance of piezo. Sweet spot appears to exist at ~ 4 Hz, perhaps due to mechanical resonance.
Fig 3.
Effect of load resistance on energy conversion by a triplex stack of PVDF elements excited by repetitive pulses of N2 gas at 4 Hz: (A) Primary data show progressive drop in voltage with RL. (B) Measured (circles) and theoretical (crosses) RMS voltage drops across load resistance; (C) Effect of RL on power (V2/RL), with optimal harvesting near 1 MΩ.
Fig 4.
Power generation by a stack of 10 PVDF elements attached to base of 48 cm cattail-inspired model, flexed at 1.3 Hz by a gated air stream (11 knots).
Individual elements connected in parallel to increase C step-wise. Approximate ten-fold increase in power with ten-fold increase in C. For RL = 1 MΩ, Fc = 16.7, 2.3 and 1.7 Hz at 1, 7 and 10 elements connected in parallel.
Fig 5.
Cottonwood-shaped plastic leaves mounted on aluminum trellis.
Output from each Kynar-based petiole was rectified before summation over leaf population.
Fig 6.
Power output by faux cottonwood trellis as function of wind speed.
(A) Average wind speed estimated from mean of highest and lowest speed during the 25 s recording period. At ten knots, power was ~ ten-fold that of single cottonwood mimic indoors. (B) Power spectrum of single leaf during 500 s, in wind fluctuating from 7.5 to 9 knots. (C) Primary voltage data show variation due to fluctuating wind speed and direction.
Fig 7.
Directional sensitivity of power output from vertical flapping stalk and cottonwood leaf models.
Power was maximal for both systems when PVDF petiole was oriented edge-on into wind (8.9 vs. 7.5 knots for flapper vs. cottonwood) and decayed with increase in azimuthal angle. Max power was greater but decayed more steeply for vertical flapping stalk. RL = 10 MΩ both systems.