Application of piezoelectric electrets to an energy-harvesting system

Abstract

We developed a prototype system with a simple structure for energy-harvesting as humans walk in their daily life, using piezoelectric electrets as piezoelectric-power-generating elements. We prepared a porous poly(tetrafluoroethylene) (p-PTFE) film with a thickness of 12 μm and an average pore size of 0.7 μm sandwiched by two films of tetrafluoroethylene–hexafluoropropylene copolymer (FEP) with a thickness of 6 μm (FEP/p-PTFE/FEP film) as an electret film used for the energy-harvesting device. A corona discharge system used to fabricate an FEP/p-PTFE/FEP film with an area of 20 × 20 cm2 that generates piezoelectricity (electret FEP/p-PTFE/FEP film). The electret FEP/p-PTFE/FEP film had a piezoelectric constant d33 of more than 100 pC/N. Then, we fabricated a multilayer film by stacking the metal foil and the electret FEP/p-PTFE/FEP film without forming wrinkles or streaks. The voltage, current, and power generated by the electret FEP/p-PTFE/FEP multilayer film during an exercise involving a research subject repeatedly stepping on the film placed in the floor were evaluated. The maximum instantaneous generated power was about 4500 μW each time the subject stamped up and down. The energy consumed in transmitting an 8-byte signal using a Bluetooth Low Energy (BLE) device is known to be about 600 μW. Considering the electricity consumption of BLE devices, the above result strongly indicates that the power generated by the electret FEP/p-PTFE/FEP multilayer film has great potential for use in BLE devices.