Piezo-mechanical harvesting
As mentioned earlier in this chapter, piezoelectric effects can be used as sensors, but they can also be used to generate power. Mechanical strains can be converted to energy through motion, vibration, and even sound. These harvesters could be used in smart roadways and infrastructures to harvest and change systems based on traffic movement, even when embedded in concrete. These devices produce currents on the order of milliwatts, and thus are suitable for very small systems with some form of energy collection and storage. This process can be performed using MEMS piezo-mechanical devices, electrostatic, and electromagnetic systems.
Electrostatic harvesting incorporates Faraday's law, which basically states that one can induce an electric current by changing the magnetic flux across a coil of wire. Here, the vibration is coupled either to the coil or a magnet. Unfortunately, this scheme in the IoT sensor area provides too little voltage for rectification.
Electrostatic systems use the change in distance between two capacitive plates held at a constant voltage or charge. As the vibration causes the distance to change between the plates, energy (E) can be harvested based on the following model:
Here, Q is the constant charge on the plates, V is the constant voltage, and C represents capacitance in the preceding equation. Capacitance can also be represented by the length of the plate Lw, the relative static permittivity as ε0, and the distance between plates d as shown:
An electrostatic conversion has the advantage of being scalable and cost-efficient to produce through micromachining and semiconductor fabrication.
The last method for mechanical-to-electrical conversion is piezo-mechanical, which was talked about earlier in this chapter when discussing sensor input. The same basic concept applies to energy generation. As the piezo-mechanical MEMS device attempts to dampen the mass attached to it, the oscillations will be converted into an electrical current.
Another consideration for the capture and conversion of vibrational or mechanical energy is the need for conditioning before the energy is used or stored. Normally, a passive rectifier is used for conditioning by incorporating a large filtering capacitor. Other forms of energy harvesting do not need such conditioning.