Three standard approaches are piezoelectric, electromagnetic and electrostatic energy harvesting.
- A piezoelectric microcantilever or nanowire develops electric polarization when strained. An inertial mass, bending substrate or vibration bends the element; electrodes and a rectifier collect charge. PZT, ZnO and PVDF illustrate different material choices. This approach needs no external bias for transduction and can give appreciable voltage in a compact element. Its capacitance gives high source impedance and relatively small electric current; brittle materials, fatigue, depolarization and a narrow mechanical resonance can limit performance.
- Electromagnetic harvesting uses relative motion of a magnet and coil to change magnetic flux, producing . A micromachined coil and movable magnet can implement it at the microscale. It can offer low source impedance and useful electric current, but the small flux and coil area make voltage and power increasingly difficult to maintain on miniaturization; coil resistance and fabrication of magnets also matter. A practical microscale device does not imply efficient scaling all the way to individual nanowires.
- Electrostatic harvesting uses a variable capacitor, for example MEMS comb fingers or moving plates. Mechanical motion changes capacitance; an initial charge, electrical bias or electret allows this change to do electrical work. Small gaps favor strong coupling and compatibility with microfabrication. Limitations include the need for charge or bias, parasitic capacitance, leakage, pull-in, stiction and dielectric breakdown.
In all three cases the output is constrained by the available mechanical input and damping. Nanoscale dimensions alone do not supply an energy advantage.
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