A Japanese university researcher developed a small-size vibration-driven generator that measures approximately 2 x 3 x 12mm and can output a power of 1.56mW with a vibration of 357Hz.
The energy density of the generator, which was developed by Toshiyuki Ueno, associate professor at Kanazawa University, is about 22mW/cm3. It is about 20 times higher than the energy density of normal small-size vibration-driven generators, which is about 1mW/cm3.
"The energy density is high enough to replace a button battery," Ueno said.
He is planning to apply the generator to tire pressure monitoring systems (TPMSes) for automobiles and mobile devices.
As a power generating element, Ueno used a magnetostrictive material called "Galfenol" (Fe81.4Ga18.6), which is an alloy of iron (Fe) and gallium (Ga) and has a large magnetostriction effect. Magnetostriction effect is a phenomenon in which the shape of an object changes due to magnetization. In the case of Galfenol, its magnetostriction effect is about 300ppm (a 1km-long bar would extend by about 300mm).
To generate electricity, Ueno used inverse magnetostriction effect, which is the converse phenomenon of magnetostriction effect. Inverse magnetostriction effect is a phenomenon in which the application of stress changes magnetization. In the case of Galfenol, its magnetic flux density decreases by up to more than 1T when compressional stress is applied. And the change in magnetic flux generates an induced voltage.
Galfenol was developed by the US Naval Research Laboratory (NRL) in 1998. But this is the first time that it has been used for a vibration-driven generator of small size, Ueno said.
Galfenol is a ductile material. Therefore, it can be easily processed by machines, and it does not easily break when warped. Piezoelectric elements, which are normally used for vibration induced power generation, are made of a ceramic, which is a brittle material.
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