Vibration-based energy harvesting has long promised to provide perpetual power for small electronic components such as tiny sensors used in monitoring systems. If this potential can be realized, external energy sources such as batteries would no longer be needed to power these components.
Scientists at the Tokyo Institute of Technology and the University of Tokyo in Japan believe they have taken a step toward achieving self-powered components by developing a new type of micro-electromechanical system (MEMS) energy harvester. Their approach enables far more flexible designs than are currently possible—something, they say, that is crucial if such systems are to be used for the Internet of Things (IoT) and wireless sensor networks.
There are three basic ways to convert vibrations to electricity in a manner suitable to power miniature components: electromagnetic, electrostatic, and piezoelectric mechanisms. The Tokyo Tech scientists favor the electrostatic method because it provides a wider choice of frequencies at the low-frequency range, and because the output power density is relatively larger.
A MEMS electrostatic energy harvester uses an electret (the electrical equivalent of a permanent magnet with a permanently stored charge) and a MEMS variable capacitor. The capacitor employs a movable electrode attached to a spring, which moves with ambient vibration.
The capacitance of the electret circuit is fixed, whereas the variable capacitor changes according to the stretching of the spring. When the amount of variable capacitor charge is larger than the fixed charge, a movement of charges between the two electrodes is induced and the variable capacitor gains charge. Likewise, when the amount of charge in the fixed capacitor on the electret is larger, there is a movement of charges in the opposite direction and its electrode gains charge. It is this movement of charges that can be harvested as electricity, according to the principle of electrostatics.
"Until now, the design of MEMS electrostatic energy harvesters has been very constrained," says Daisuke Yamane, an assistant professor at Tokyo Tech, and one of the scientists who developed the new device.
The reason, he explains, is that conventional electrostatic harvesters place the electret together with the variable capacitor in the MEMS unit. "So it was difficult to optimize the design of each device element and to use the best materials for them because the electret is integrated into the MEMS structure," says Yamane.
The answer to this difficulty was to separate the electret from the MEMS structure by fabricating two separate chips. However, this required a complete redesign of the electret. While it was housed in the MEMS unit, one of its sides was attached to the fixed electrode, while the other side was open to the air to allow capacitance to take place.
In the newly fabricated chip, there is no air gap. So the scientists fabricated an additional electrode and a dielectric layer to sandwich the electret between two electrodes.
While the concept has been demonstrated to work, a number of challenges remain. For instance, parasitic capacitance needs to be properly estimated and further reduced. This can be achieved by creating a balance in the capacitance between the variable capacitor and the electret—a key step in ensuring the technology is successful. So the team is developing a simulation environment to aid them in this task.
They also need to reduce the size of both the MEMS unit and the electret, and they need to improve the efficiency of energy harvesting by optimizing both the design of the harvester and the materials used.
"We think conventional bonding techniques will help minimize the total size and also help improve harvesting performance," Yamane adds. He estimates these improvements will take place over the next several years, so commercialization is still some time away.
When the MEMS energy harvester is ready for practical use, the team expects it will be fit to power sensors in IoT and wireless networks. These include inertial sensors, pressure sensors, and temperature and humidity sensors. Such self-powered devices could monitor the environment and send information to various systems such as those monitoring traffic patterns, or natural disasters.
Yamane is the lead author of a paper about the new MEMS device that was presented at the IEEE International Conference on Micro Electro Mechanical Systems, which took place in Seoul, South Korea, from January 27 and 31.