Figure 5: HBS piezoelectric device developed by TIMA
3. Electrostatic devices
Electrostatic converters are capacitive structures made of two plates separated by air, vacuum or any dielectric materials. In these devices, a relative movement between the two plates generates a capacitance variation and then electric charges .
As explained in [5
], electrostatic devices are well-suited for size reduction and enable to decouple the mechanical system (mass-spring) from the mechanical-to-electrical converter. An electret-based converter has been chosen to enable a direct mechanical-to-electrical conversion. A schema of the device is introduced in figure 6a
; patterned electrets are presented in figure 6b
. Once again, a mass is added in order to increase the output power.
Figure 6: (a) Electret-based energy harvester developed by CEA-LETI and (b) patterned electrets
A first MEMS electrostatic prototype has been manufactured using cleanroom processes and is presented in figure 7a
. The energy harvester has a total volume of 1cm³ and an output power of 10µW will be available as soon as the device is implanted in the heart. A schema of the future complete autonomous pacemaker, with its vibration energy harvester, is represented in figure 7b
Figure 7: (a) MEMS electret-based vibration energy harvester (b) schematic of the future device
Piezoelectric or electrostatic devices deliver an AC output voltage that cannot be used as is to supply electronic devices: a power management circuit is required.
1. For more information on electrostatic vibration energy harvesters: Electrostatic conversion for vibration energy harvesting, S. Boisseau, G. Despesse, B. Ahmed Seddik, Small-scale Energy Harvesting, Intech, 2012