도로용 압전 에너지 하베스팅 시스템의 모듈 및 내부 회로 설계 연구

Abstract

Energy harvesting is a technology to convert energy gathered from the environment to electrical energy. Ambient energies such as potential energy, gravity, the sun, human body, thermal, and vibration energy are being widely used. I studied piezoelectric energy harvesting technology using vibration energy. Piezoelectric effect is understood to have reversible process called direct piezoelectric effect and reverse piezoelectric effect. Direct piezoelectric effect is when mechanical force is applied to a piezoelectric substance thus changing the surface charge density causing electricity generation, and is used in places such as piezoelectric harvester and sensor. In contrast, reverse piezoelectric effect is when electric field is applied to a piezoelectric substance causing mechanical deformation, and is used in places such as piezoelectric speaker, motor, and vibration exciter. In this thesis, research on piezoelectric energy harvesting technology using direct piezoelectric effect is discussed. First of all, I studied an optimization of array methods of piezoelectric device, which is one of the most essential parts of a piezoelectric energy harvesting system. A vibration energy harvesting using piezoelectric devices favors a cantilever structure. It is characterized by concentrated stress near its fixed end. The device also shows low efficiency due to its localized power generation. Therefore, I conducted a research to distribute the concentrated stress evenly to produce higher electrical energy. In the cantilever structure of a piezoelectric device, the radius of curvature decreases from the free end to the fixed end. Thus, the substrate of the piezoelectric cantilever beam was changed to have a thick body having a large second moment of inertia near the fixed end, where the radius of curvature is the smallest, and becomes thinner with a smaller second moment of inertia towards the free end to reduce the standard deviation among the various radii of curvature. I could confirm that the radii of curvature of each point of the substrates were lowered than those of the conventional ones, and that smaller thickness made the radius of curvature also smaller. The actual experimental results showed that the output power was higher when the radius of curvature was smaller. Secondly, I studied the way to transfer the output power of a piezoelectric device to a load as much as possible. In the previous research on vibration energy harvesting, efforts to increase output power had focused on using heavier tip mass creating larger deformation. But, it also made the resonance frequency smaller and the matching resistance larger, only to make it unsuitable to utilize its high output power. It is because a resonance frequency decreases as the tip mass increases. I adopted permanent magnets, rather than tip mass, to the piezoelectric device to increase output power. By increasing the magnetic force, both the deformation and the resonance frequency increased and the matching impedance could be kept small. Therefore, higher output power was realized. Thirdly, I studied the cases for an impact energy harvesting. It was done by increasing impact frequency, or reducing impact period, to increase a resonance frequency of a piezoelectric device. When the resonance frequency was increased, the resistance fit for impedance matching decreased and the output power became higher than before. Fourthly, I manufactured a footpad energy harvester with saw-tooth wheels to increase the impact frequency for a piezoelectric device. A ZigBee RF wireless communication module was powered with the harvester. When the harvester operated five times, the enough power was generated to run the ZigBee module to measure indoor temperature. Finally, I made an impact-type piezoelectric energy harvester using a magnet, which was equipped with twenty piezoelectric devices in parallel. It was designed to harvest energy from weight of vehicles on a road. When it was placed under a speedbump, it did not fully work since the vehicles could only run slowly. Thus, it turned out that the road energy harvester was better to be buried under a road pavement generating higher output power, since the vehicles could run over it freely. With a piezoelectric energy harvester for roadways, an energy-independent roadway that supplies energy to its surrounding environment can be realized.