Generation and storage of electricity from power harvesting devices

The concept of capturing the normally lost energy surrounding a system and converting it into electrical energy that can be used to extend the lifetime of that system's power supply or possibly provide an endless supply of energy to an electronic device has captivated many researchers and has brought forth a growing amount of attention to power harvesting. One of the most common methods of obtaining the energy surrounding a system is to use piezoelectric materials. Piezoelectric materials have a crystalline structure that provides a unique ability to convert an applied electrical potential into a mechanical strain or vice versa, or convert an applied strain into an electrical current. The latter of these two properties allows the material to function as a power harvesting medium. In most cases the piezoelectric material is strained through the ambient vibration around the structure, thus allowing a frequently unused energy source to be utilized for the purpose of powering small electronic systems. However, the amount of energy generated by these piezoelectric materials is far smaller than that needed by most electronic devices. For this reason, the methods of accumulating and storing the energy generated, until sufficient power has been captured, is the key to developing completely self-powered systems. This article quantifies the amount of energy generated by a piezoelectric plate and investigates two methods of accumulating the energy thus produced. The first method uses a capacitor, which in early research has been the most common method of storing the energy generated and the second utilizes rechargeable nickel metal hydride batteries. The advantages of each method are discussed and the rechargeable battery is found to have more desirable qualities for power harvesting than the capacitor. Additionally, this manuscript represents, for the first time, the fact that the power output by a piezoelectric material is capable of recharging a discharged battery. Through the excitation of a piezoelectric plate, it is demonstrated that a 40 mAh battery can be charged in less than half an hour at resonance and in only a few hours with a random signal similar to that of a typical vibrating piece of machinery.