 Electrostatic energy harvesting

Electrostatic energy harvesting uses mechanically varied capacitance to exchange mechanical and electrical energy. A capacitor must first carry charge or be electrically biased; changing an uncharged ideal capacitor does not by itself generate stored electrical energy.

For an isolated capacitor holding fixed electric charge $Q$, its stored energy is $U=Q^2/(2C)$. Pulling parallel plates apart reduces $C$ and increases $U$: the external mechanical input works against their electrical attraction. A switching circuit can then transfer part of this increased energy to a load and prepare another cycle.

The charge-preparation step, switching losses and leakage belong in the full energy balance. At fixed applied voltage the charge changes and the voltage source also exchanges energy, so the fixed-charge formula cannot alone describe the complete generator cycle. This distinction is useful when evaluating a vibrating variable-capacitance device.