# Modeling a Wave Energy Harvester for Supplying Data Buoys
## 3 Results
### Energy Extraction in Presence of an Electronic Interface  
The capabilities of energy extraction of the considered harvester were investigated by considering two parallel tracks each equipped with eight pairs of coils. The two coils of each pair, placed in front, are connected in series. The terminals of each pair of coils are connected to diode bridge rectifiers, as shown in Figure 12. The outputs of the diode bridges are connected in parallel to a single capacitance at the input of a DC/DC converter. Figure 11: Measurements on the wave harvester prototype. (**a**,**c**,**e**) Time evolution of the cylinder position as captured by the video frames recorded by a high-speed camera. (**b**,**d**,**f**) Voltages at the output terminals of two series-connected pairs of coils placed at 85 mm from the track ends. Figure 10: Image acquired by a high-speed camera for measuring the time evolution of the cylinder position. The peak power and the energy per each oscillation cycle are reported in Figure 13a,b for different values of the dc voltage, \(V_{DC}\), on the capacitance. The data are reported for different values of the oscillation period \(T\), while the maximum inclination angle is set at \(\alpha_{M}=20^{\circ}\). Since the track length is \(L=200\) mm, according to Equation (21) and Figure 5, in all the considered test cases the bang-bang operation is ensured. Moreover, in Figure 13c,d, the power and energy data are reported for different values of the maximum inclination angle, while the oscillation period is set at \(T=2\) s. Also, in these test cases, the harvester operates in bang-bang mode, as predicted by Equation (21). It is interesting to observe that the values of period and angle, in which the maximum peak power is reached, are different from those of the energy per cycle because the peak power depends only on the speed at which the magnet passes near a single pair of coils, while the energy also depends on the time it takes to pass through all the coils. The plotted results also show that the DC/DC converter has to set, time by time, a different \(V_{DC}\) voltage in order to maximize the energy extraction in the considered operating conditions. To this end, a maximum power point tracking technique can be used. Figure 12: Schematic of the parallel connection of full-bridge rectifiers connected at the output terminals of the coil pairs. Figure 13: _Cont._  
Finally, the performances of the considered harvester were compared with those of the most similar wave energy harvester recently presented in the literature and based on a magnetic ball that can move inside an annular shell along which coils are placed [22]. However, for a fair comparison, instead of a magnetic ball, the same magnetic cylinder was considered, and it was assumed that the magnetic field generated by the magnet is always orthogonal to the coils. This assumption is obviously not verified in practice, but it can be assumed as the best limit condition of operation for the annular harvester. Moreover, the diameter of the annular shell was set to the same length of the linear tracks, i.e., \(L=200\) mm; the coils' characteristics are considered the same here, and the number of coils was set to sixteen (equal to those of two parallel linear tracks). The graphs of the peak power and the mean extracted energy per oscillation cycle are reported in Figures 13e and 13f, respectively. The data show that the behavior of the annular shell harvester is very irregular, since it does not have the same benefit of a bang-bang operation as the linear track harvester. The resulting lack of a clear energy maximum makes the use of maximum power point tracking techniques difficult.