Ambient Energy Harvesting-An Electrostatic Approach.

Abstract

With the rapid advancement of very-large-scale integration and miniaturization, wireless micro-sensors, wearable devices and biomedical implants have been implemented all over the world. Nowadays, batteries are considered as primary power sources for the majority of electronics. They need to be periodically charged or replaced due to their limited lifetime, which is inconvenient and may lead to increased costs. One promising solution is to harvest unused energy sources surrounding those electronic devices into electric energy, which can be considered as alternative or auxiliary power sources. There is a variety of ambient energy sources, such as solar, mechanical, thermal and chemical. Among the existing energy harvesting mechanisms, electrostatic energy harvesting draws our attention because of its various advantages, such as low-cost fabrication, high achievable energy harvesting efficiency and capability for large-scale integration and miniaturization. Furthermore, the electrostatic approach can be used to harvest a variety of ambient energy sources, such as mechanical, thermal and chemical. However, some challenges also exist, such as relatively small initial charge and capacitance, requirement for an external power supply, and complex power management control circuits to extract generated energy at the appropriate time. Two novel methods have been proposed in this dissertation to solve the current challenges for electrostatic energy harvesting. The first method utilizes an appropriate, repetitive reconfiguration process to create a positive feedback mechanism, thus, restoring the generated energy back to the system. Even a small disturbance on a system could significantly amplify electric outputs. Because of the exponentially growth rate of energy extraction, this method is particularly effective for distributed devices to scavenge energy from low-level ambient sources, thus enabling self-powered operation. As proof of concept, two rotary variable capacitors in addition with a fixed ceramic capacitor are used to establish a positive-feedback system. To achieve relatively high capacitance, liquid-contact variable capacitors are developed for the system. Because of contact electrification and electrostatic induction, the contact variable capacitors can provide relatively high extra charge in each cycle. In our experiments, the prototype using three mercury droplets can generate 10.2 J per cycle, corresponding to a harvesting efficiency of 12.2% and the values for the prototype with three water droplets will be 1.2 J per cycle and 7.9%. The efficiencies of the devices far exceed those of the existing droplet generators. Since the concept of exponential energy harvesting is not domain specific, it may lead to new research in directional energy transfer systems in various energy domains, such as salinity gradients and temperature differences. An additional proposed method utilizes water droplets that alternate contacts between CYTOP and PTFE thin films to provide high initial charge for vibration energy harvesting. Because CYTOP and PTFE develop significantly different surface charge densities during contact with water, they can be utilized to generate electricity effectively. More importantly, the proposed method utilizes the strong electrostatic induction in the water droplets due to the electrical double layer formed at the interface. A harvesting efficiency of 2.5% has been achieved in this study. It is more effective than existing methods that are based on the much weaker electrostatic induction in the substrate. Also, unlike existing methods, in which the charge of the drop is not delivered to external circuits, in our method, the water droplets possesses a dual function as both an electrode and a passive switch, leading to the direct harvesting of the peak electric potential energy. This method not only results in simple device architecture but also allows schemes based on variable capacitors to improve the performance. Using prototype devices, we demonstrate the effectiveness of this approach in scavenging energy from low-level and low-frequency ambient vibrations.