# Comparison Among Different Rainfall Energy Harvesting Structures
## 3 Experimental Study  
Piezoelectric transducers were exposed to artificial rain from different heights: 0.5, 1.0, 1.5 and 2.0 m.  
In order to test the harvesting structures, different raindrops were dropped, and an average waveform was identified. Figure 7 shows the minor differences in the behaviors of the cantilever structures with raindrops falling from a 0.5 m height. The output was recorded using a Teledyne LeCroy WaveRunner 6 Zi scope, used for the real-time monitoring and acquisition of the related waveforms. Internal impedance was set at 1 M\(\Omega\), in order to have an open circuit condition. Moreover, Figure 8 shows the different behaviors of the cantilever structure raindrops falling from a 2.0 m height. The comparison between Figures 7 and 8 shows that the behavior of the waveform is quite the same, it is a form of damped oscillation, for which the time constant of the damping is equal to twice the period of oscillation. The structure of the cantilever given by the piezoelectric film, encapsulated into two mylar layers is defined in [26]. The LDT1-028K series is a flexible component comprising  
Figure 8: Voltage obtained at the ends of the structures due to a raindrop falling from 2.0 m and hitting the free edge. Figure 7: Voltage obtained at the ends of the structures due to a raindrop falling from 0.5 m and hitting the free edge. a 28 \(\upmu\)m thick piezoelectric PVDF (polyvinylidene fluoride) polymer film with screenprinted silver ink electrodes, laminated onto a 0.125 mm polyester substrate, and fitted with two crimped contacts. The active part has the following dimensions: length 30 mm, width 12.19 mm, thickness 0.028 mm; it is encapsulated on a shield of length 41.40 mm, width 16.26 mm, with two mylar layers of 0.125 mm thickness. [PERSON] and [PERSON] [27] described an equivalent electro-mechanical model for a piezoelectric cantilever; the stress given by the source (in the present case the raindrop) is correlated with voltage output by taking into account the piezoelectric strain coefficient, the electric field, the displacement field, the permittivity, the mass of the cantilever and inertia moment. The scheme presented in [27] was made for bimorph models, so a three-layer system was developed in [11]. The characterization was performed for a one-edge fixed cantilever, the transfer function was developed, and the traced Bode diagram showed an amplitude maximum located at 260 rad/s, with a corresponding period of 25 milliseconds [11]. The average values for the peaks are about 12 V and 17 V, for 0.5 m and 2.0 m heights, respectively. The exact damping ratio can be evaluated by logarithmic decrement method [28], and is about 0.25-0.27. As expected, the frequency of oscillation remains the same, as it is related to the length of the cantilever. Figure 9 shows the performance of the bridge cantilever. In order to present a clean figure, only two drops were considered, but more were acquired to confirm the average behavior. The first part of the waveform is similar to the previous case; the second part shows a very low-frequency oscillation above zero value for the blue curve and below zero for the orange one. This behavior, in which there is a strong initial impulse, followed by lower oscillations, is not appreciable by a rectifying system, since the voltage drops in the diodes could cancel the waveform of voltage. Figure 10 shows the voltage waveform for the floating circle of Figures 3 and 6. This behavior is closer to the behavior of the bridge harvester than the cantilever one, since again there is a great initial pulse and sub-sequential periodic oscillations due the floating system. Figure 9: Voltage obtained for the bridge structure. Two raindrops were considered falling from 0.5 m, hitting the central point of the bridge. Different tests were conducted, also considering the 2 m height, and in such cases the behavior shown in Figures 9 and 10 was replicated with twice the voltage.