Category Advances in Energy Harvesting Methods

Summary

In this chapter, an energy harvesting platform based on high-permeability material was theoretically studied and tested. Two generations of devices were designed. The first generation consisted of a high-p cantilever vibration energy harvester incorporated a vibrating high-permeability cantilever, a solenoid, and a bias magnet pair. Interaction between the high-permeability magnetic beam and the bias magnets leads to complete flux reversal of the high-permeability beam, which generates a maximum power of 74 mW and a high power density of 1.07 mW/cm3 at an ambient vibration frequency of 54 Hz and at an acceleration of 0.57 x g...

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Discussion

Compared to the previous vibration energy harvester design based on a vibrating high-p beam and a stationary bias hard magnet pair, this new generation device utilizes a vibrating hard magnet pair and a stationary solenoid pair with thick multilayer high-p core materials. The multilayer high-permeability solenoids core leads to significantly increased flux change in the solenoid within one oscillation period without increasing the total volume of the device. In addition, the solenoid at both sides of the vibrating magnets makes full use of the spatially inhomogeneous bias magnetic fields on both sides of the magnets, leading to a doubled power output, and a dramatically enhanced power density by approximately 20 times over the previous energy harvester design with high-p materials.

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Results and Analysis

Figure 18.14 shows the measured open circuit voltage of the energy harvester with different springs and resonant frequencies. For spring #1, with resonant frequency of 27 Hz, the peak voltage is 1.18 V for an acceleration amplitude of 2 x g; for spring #2, with resonant frequency of 33 Hz, the generated maximum voltage is 1.64 V for an acceleration of 3 x g; spring #3, increased the peak voltage to

Fig. 18.14 Measured results of the open circuit voltage for the energy harvesting device with three different springs at respective resonant frequencies: spring #1 at 27 Hz; spring #2 at 33 Hz, and spring #3 at 42 Hz

Fig. 18.15 Measured maximum output power of the harvester with three different springs and their associated resonant frequencies

2...

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Theoretical Model

The mass of the hard magnet pair, the stiffness of the supporting spring, and the magnetostatic coupling between the solenoids and the hard magnet pair determine the resonant vibration frequency and the output voltage of the energy harvester. The equivalent spring-mass system becomes a nonlinear oscillation system due to the magnetostatic coupling between the solenoids and the hard magnet pair. This nonlinear effect can be explained from the potential energy point of view, as shown in the previous section. The magnetostatic potential energy has two identical minimum values due to the coupling between the magnets and solenoids. These minimums occur when the magnets move a short distance up or down from the equilibrium position in the middle of the hard magnet pair. As a result, the
superposi...

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Second Generation High Output Power Vibration Energy Harvester with High-Permeability Material

18.3.1 Prototype and Testing Platform

The schematic design of the high output power energy harvester is shown in Fig. 18.12. Two identical solenoids with high-permeability/insulator multilayer cores were placed on two sides of a vibrating hard magnet pair with antiparallel magnetization. The key components of this energy harvester are the two identical solenoids with a high-permeability (high-ц,) MuShield core inside, which are placed at two sides of the magnet pair and fixed on a vibrating stage. The magnets have antiparallel magnetization and are supported by a regular circular cross­section spring with its bottom fixed on the surface. When the magnet pair moves up and down with respect to the vibrating stage, the magnetic field inside each solenoid periodically changes its direction...

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Discussion

Table 18.1 shows the figures of merit for vibrating energy harvesters with dif­ferent types of working mechanisms and materials, including magnetoelectric, electrostatic, piezoelectric, magnetoelectric sensor based, magnetostrictive, and high-permeability material-based devices. Among all these different mechanisms, the wide bandwidth energy harvester based on the magnetic coupling between the high-|x material and the bias field of the hard magnets, generates a relatively high power density and a wide working bandwidth. The metallic high-permeability single-layer beam has advantages from the material point of view as well. First of all, it is mechanically more robust compared with most of the piezoelectric materials or glue bonded multilayer materials...

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Nonlinear Effect

From Fig. 18.10, it can be seen that the working bandwidth is about 10 Hz, 18.5% of the central frequency, compared with 2.1 Hz (Ferro Solution VEH360) or 3.5% of the central frequency, for a typical linear oscillator harvester. The major reason for the large bandwidth is that the magnetic coupling is not linear to the displacement of the oscillator, so that the nonlinear effect provides the system with a wider working bandwidth [16]. As shown in Fig. 18.11, compared with the elastic potential energy, the magnetic potential energy curve has two potential wells distributed right next to each other, resulting in a wider total potential well...

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Results and Analysis

Figure 18.8 shows calculated and measured results of the open circuit voltage in two cases. When the magnet pair is set to have antiparallel magnetization, the energy harvester shows a high open circuit voltage with a peak value of 544 mV at a vibration frequency of 54 Hz and an acceleration amplitude of 0.57 x g. As expected, for the case when the two bias magnets are arranged with parallel magnetizations, the output voltage has a significantly lowered peak value of 8 mV at double the driving frequency (i. e. at 108 Hz). It is interesting to note that the mechanical vibration source is a sine wave signal, while the output voltage is not, but with narrow peaks with a full width at half maximum of 1 ms...

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