Harvesting Energy from Ambient Vibrations

Abstract

In vibratory energy harvesting, the energy flow generally goes through three stages: the external vibration energy is firstly coupled into the device as kinetic energy, which is partially converted to electricity through electromechanical conversion unit (such as electromagnetic, piezoelectric, and electrostatic), then the generated electricity is applied to the electrical load circuit. The process of such energy flow indicates that the energy coupled in the first stage determines the maximum available energy converted to electricity, while the electricity delivered to the load depends on the characteristics of electrical load circuits. Note that the first stage for coupling energy is achieved by device dynamics. To best understand vibratory energy harvesting, the effects of device dynamics and electrical load circuits on energy harvesting performance are investigated in this dissertation. For the effects of device dynamics, we do the study from four areas: parametric oscillator/device, global resonance, the roles of excitation, and dynamics outside the potential well. At first, we investigate the potential of using a nonlinear parametric oscillator/device to harvest energy. In such device, the excitation appears as a parameter of the dynamical system. Such parameterization of the excitation provides a cross-frequency energy transfer in the excitation, resulting in modifying the frequency content of the excitation, i.e. modulation of the excitation, which enables the device into the orbits of higher-order subharmonic oscillations more easily. A device with a pendulum-type architecture is proposed and used as an illustrative example. The further investigation in device dynamics has proved that for a nonlinear device, there exists a generalized, global resonance condition which requires matching of all of the frequencies between the device and the excitation. Under global resonance, the device performance is optimized with the maximum energy harvesting efficiency, but its corresponding displacement is not the largest because the amplitude of global resonance response is strongly correlated with the fundamental frequency supported by a nonlinear potential well (e.g. potential function). Such results suggest that traditionally relying only on increasing the device response in nonlinear systems can be misleading. The global resonance condition also shows that damping of the device and modulation of the excitation play critical roles in facilitating the frequency match required for resonance. According to the global resonance condition, it is revealed that the potential of nonlinear device in harvesting energy from multi-frequency vibrations benefits from multiple frequency match, not from the wider bandwidth obtained from single-frequency response. To harvest energy from multi-frequency vibration using non- linear devices, a device-design concept based on global resonance is thus proposed. When the global resonance condition is satisfied, the instantaneous power of the excitation is always non-negative, resulting in the maximum device performance. Conversely, when the condition is not satisfied, the excitation does negative work for a duration per cycle, leading to the reduction of the energy harvested. During such duration, the excitation actually takes energy back from the device, acting as a sink. The extent to which the excitation behaves as a sink determines the energy harvesting performance. We find that instantaneously changing device response to ensure the velocity in phase with the excitation can reduce the behavior of the excitation as a sink, resulting in dramatic increase of the energy harvested. Based on our findings, it has shown that an active method based on manipulating the roles of excitation would be more promising in bringing vibration energy harvesting to fruition. Although the responses of a device are usually constrained by its potential well, it is possible for the dynamics of a pendulum-type device to escape from the potential well. Here, we also investigated the possibility of utilizing the dynamics outside the potential well of a device for harvesting energy from vibrations. A pendulum-type device is used as an example. Results show that when the device dynamics is outside the potential well and stays in stable orbits of period-one rotations, the harvested energy is proportional to the energy level of the orbit, neither depending on the natural frequency of the device nor on the intensity of the excitation. For the effects of electrical load circuits, we consider three types of non-resistive loads, such as a resistive load with a rectifier, a resistive load with a rectifier and a regulating capacitor, and a simple charging circuit consisting of a rectifier and a storing capacitor. Numerical results suggest that when the harvested energy is to be stored in capacitors, the ultimate voltages across capacitors are the same as the open- circuit voltage of the device minus the rectifier drop. For charging loads, therefore, the amount of stored energy is determined by the capacitance and the device performance under open circuit. Moreover, a larger capacitor is beneficial for an electromagnetic harvester, but not for a piezoelectrical harvester.