Piezoelectric vibration energy harvesting device based on water sloshing-inspired extensions Open Access

With the development of IoTs (Internet of things) and other applications, various wireless sensors and wireless sensor networks are widely demanded.¹ How to power the wireless sensors has been receiving increased attention.^2–4 Batteries are widely used as conventional energy sources; however, they are limited due to their low lifespan and energy storage capacity, which are commonly needed to be regularly recharged or replaced. In some situations, such as outdoors and physically inaccessible environments, battery maintenance is costly and inconvenient. Thus, the energy acquired automatically from the ambient sources (wind, fluid flow, solar, mechanical, etc.) to recharge or replace the batteries is thought to have a good application foreground, which does not need maintenance.^5–7 Energy harvesting devices converting dynamic mechanical vibrations to electric energy based on piezoelectric materials have intrigued many researchers, which are typically called piezoelectric energy harvesting devices.^8–11 

Piezoelectric energy harvesting devices based on ambient vibration will exhibit good output power, while they are excited at frequencies close to the resonant frequency. However, the ambient vibrations distribute in a wide frequency range in majority of applications. The narrow response bandwidth of the devices limits the use of the energy harvesting techniques due to their weak performance when the excitation frequencies are away from the resonant frequencies. Several approaches have been applied in order to change the resonant frequency or increase the response bandwidth of the harvester. Passive methods, based on regulating the dynamics of the system, are applied to tune the resonant frequency of the devices. The method applying prestressed beams realizes to regulate the resonant frequency but does not increase the bandwidth of the system.^12,13 Thus, two PVDF bimorphs with different resonant frequencies building a bi-resonant structure harvester have been investigated to widen the frequency bandwidth, which improves the output power compared to the two distinct beams.¹⁴ Active methods are also adopted to alter the resonant frequency of the harvester by actuating the structure.^15,16 However, the power consumption of the added actuating system is more than the system benefit. In order to obtain better performance of the harvester, nonlinear energy harvesting devices have been proposed in recent years. Harvesters using multi-degrees-of-freedom have been presented to increase the frequency bandwidth, which commonly use piezoelectric–magnetic structures with one or more magnets fixed on the frame.^17–19 Nevertheless, implementation of the magnetic attached to the beam is cumbersome.²⁰ How to increase the frequency bandwidth or adjust the resonant frequency is still one of the most-researched subjects on the piezoelectric energy harvesting devices.

In order to tune the performance of the piezoelectric energy harvesters, piezoelectric energy harvesting devices based on the inverted cantilever beam with extensions have been proposed.^21,22 Inspired by these studies, we use the sloshing water as the extensions to build the nonlinear energy harvesting device. In this paper, a novel type of vibration-based piezoelectric energy harvesting device is proposed, which is composed of a piezoelectric beam and a tank with water filling for forming a cantilever beam. On account of the sloshing of water and other factors, the resonant frequency and frequency bandwidth of the proposed harvester can be adjusted by filling water at different heights in the tank. Thus, a series of experiments have been designed to investigate the influences on the performance of the harvester under different operating conditions (different water volumes, different vibration frequencies, different load resistances, etc.). The remainder of this article offers the following:

1. The structure of the piezoelectric energy harvesting device is presented to illustrate how electric power is produced. Then, the details of the device structure are given.

2. The prototype of the proposed piezoelectric energy harvesting device is designed and tested by a series of experiments. Then, the experimental data are provided and analyzed.

Motivated by shallow-water sloshing in a moving tank, we develop the configuration of the piezoelectric energy harvesting device. The piezoelectric cantilever beam configuration is widely applied in vibration-based energy harvesting, which is simple in structure and commonly includes a bimorph beam with an inertial mass added to the tip of the beam for improving the mechanical response. Thus, we proposed a vibration-based energy harvesting device based on a piezoelectric bimorph beam with a tank added to the tip, as shown in Fig. 1. The bimorph beam is fixed at one end to form a cantilever beam, with another end connected to the tank, which is used for tuning the resonant frequency of the harvester by regulating the filling water height. In the presence of the ambient vibrations (wind, fluid flow, and sea waves), acting like the base excitation to the system, the cantilever beam bends according to the vibration direction. Accordingly, water sloshes in the tank and affects the mechanical response of the harvester. Due to the nonlinear phenomenon of the slashing water, the frequency response of the proposed piezoelectric energy harvesting device can be regulated by adjusting the height of water in the tank.

The way of changing the water height in the tank attached to the piezoelectric beam is of interest to us. Altering the water height changes the inertial mass added at the tip of the beam, leading to the varying mechanical response. Sloshing of water is a nonlinear and complex phenomenon, which is influenced by many external variables, such as excitation frequency, excitation amplitude, and water height. Thus, we use the tank fixed at the tip of the beam and adjust the resonant frequency and frequency bandwidth of the harvester by changing the water height for acquiring better performance of the vibration-based harvester. In addition, the internal resistance of the piezoelectric bimorph beams changes according to the different operating conditions. Thus, the open-circuit output responses are not enough for investigating the performance of the energy harvesting device. Different external resistances are connected to the system in our study for testing in order to seek an optimal load resistance for optimizing the system. In this paper, the effects of the different operating conditions (i.e., different water heights, different excitation frequencies, and different load resistances) on the performance of the inverted cantilever piezoelectric bimorph beam are studied and analyzed.

Experiments are set up to investigate the influences of the different factors (i.e., water height, excitation frequency, and load resistance) on the performance of the proposed piezoelectric energy harvesting device, as shown in Fig. 2. It consists of a fixed base, an electromagnetic shaker, some clamping mechanism, the energy harvesting prototype, and an accelerometer. The harvester is placed on the electromagnetic shaker (LDS v406) to apply the harmonic base excitation. The excitation signal is generated using a vibration controller (Spider 81B), which is amplified using a power amplifier (LDS PA100E). The accelerometer (PCB 352C33) is installed on the base, used to measure the base excitation. In the experiment, variable load resistances are connected to electrodes covering the piezoelectric elements by two electrical wires, and the output voltage of the load is recorded using a digital oscilloscope.

The detailed dimensions of the tank and piezoelectric bimorph beam are shown in Fig. 3. The piezoelectric bimorph beam is composed of two piezoelectric PZT layers with one aluminum plate in between. The piezoelectric is lead zirconate titanate [PZT-5H; Young’s modulus of the piezoelectric element (Y_P), 71 GPa; mass density of the base structure (ρ), 7.45 g/cm³; relative dielectric constant (ε₃₃^T), 4500; and piezoelectric constant (d₃₁), 186*10⁻¹² C/N]. A cylindrical tank is chosen in this study, providing a total capacity of 4.2 ml. Four kinds of water height configurations are applied in this study: A mode, B mode, C mode, and D mode, as shown in Fig. 4. The empty tank (E mode) is also tested for comparison. The load resistance range is chosen from 100 KΩ to 1 MΩ, which is close to the internal resistance of the piezoelectric bimorph beam. A sinusoidal excitation with an acceleration of 0.1 g (g = 9.8 m s⁻¹) is used as the base excitation with frequency varying from 5 to 30 Hz. In our work, the base excitation frequency leading to the maximum open-circuit output voltage is considered as the resonant frequency of the harvester. We record the amplitude of the output voltage for four different water heights according to the various load resistances at different excitation frequencies, as well as the output power being calculated. The frequency span leading to the output voltage greater than 1/$2$ of its amplitude is considered as the frequency bandwidth.

First, the output voltage amplitude at the load resistance of 1 MΩ is recorded among different water heights (A mode to E mode) according to different base excitation frequencies. From the experimental results shown in Fig. 5, a large difference is observed. The maximum output voltage for the piezoelectric bimorph beam with full water (D mode) is close to 50 V in magnitude, while for the beam with the 1/4 water height (A mode), the maximum output voltage is only around 10 V in magnitude. However, the maximum output voltage of the empty tank (E mode) is around 15 V in magnitude. In addition, it can be seen that the E mode has a larger output voltage than A and B modes, which means that sloshing of water has the influence on the performance of the energy harvesting device. The secondary oscillation of the water is more obvious with the lower height of the water filling in the tank, which will weaken the vibration energy from the base excitation. The resonant frequency of the A mode is around 18 Hz, while that of the D mode is around 13 Hz. As the height of the water increases, it is found that the resonant frequency of the harvester decreases, but the bandwidth of the harvester is broadened. Based on the experimental results, conclusion can be made that the frequency response of the harvester can be adjusted by regulating the filling water height of the tank, which can enhance the energy harvesting performance of the device at different ambient vibration frequencies.

Then, load resistances from 100 KΩ to 1 MΩ are tested for these different water heights under different base excitations, and the results are plotted in Fig. 6. At most excitation frequencies, the output voltage of the harvester increases with the water height. However, at some excitation frequencies, such as 14 Hz, the output voltage of the harvester is almost the same between the C mode and the D mode. In some cases, the output voltage of the harvester with a low water height is larger than that with a high water height, such as the output voltage of the A mode and B mode at an excitation frequency of 18 Hz. It means that the water filling in the tank is not the higher the best for the energy harvesting performance of the device. Comparing the produced voltage of the empty tank with that of the tank with water filling, it is found that the energy harvesting performance of the empty tank is better than those of A and B modes but worse than those of C and D modes. In addition, the output power of the device is computed accordingly, as shown in Fig. 7. The maximum harvesting power of the device is around 1 mW. The load resistances leading to the maximum output power of the piezoelectric energy harvesting device for A, B, C, D, and E modes are around 200, 200–300, 300, 500, and 200 KΩ, respectively. It means that the internal resistance of the piezoelectric bimorph beam increases with the height of the water filling in the tank.

In this paper, a vibration-based piezoelectric energy harvesting device is proposed, which is composed of a piezoelectric beam and a tank with water filling for forming a cantilever beam. The operating principle and structure of the system are proposed. Sloshing of water in the tank influences the mechanical response of the harvester. Thus, the method for modulating the height of the water filling in the tank is studied to investigate the effect of the water height on the resonant frequency and bandwidth of the harvester. Experiments are carried out to investigate the actual device performance by changing the base excitation frequency and load resistance for different heights of water filling in the tank. The results show that with the increase in the water height, the resonant frequency of the device increases, while the bandwidth of the device decreases. The present research, though at its incipient stage, provides some clues to improve the performance of the vibration-based piezoelectric energy harvesting devices in the future.

This work was supported by the National Natural Science Foundation of China (Grant No. 51905486) and the Zhejiang Provincial Natural Science Foundation (Grant No. LGG19E050003).

The data that support the findings of this study are available from the corresponding author upon reasonable request.