Table 1.

The power densities of different energy sources [14].

Energy Source	Types	Energy Harvesting Method	Power Density
Radiant	Solar	Solar cells	15 mW/cm${}^2$
	Wind Flow	Electromechanical conversion	16.2 $\mathsf{\mu }$W/cm${}^3$
Mechanical	Acoustic Noise	Piezoelectric	960 nW/cm${}^3$
	Motion	Piezoelectric	330 $\mathsf{\mu }$W/cm${}^3$
Thermal	Body heat	Thermoelectric	40 $\mathsf{\mu }$W/cm${}^2$