All processes that involve energy conversion are, to some degree, inefficient. Motors get hot, as do power transistors, automobile engines, and light bulbs; in each case energy is wasted as heat. Radio stations put out megawatts of RF but their signals reach antennas as microwatts. Energy harvesting devices capture some of this wasted energy, convert it to electricity, and put it to work.
The best known energy harvesting collectors are large solar panels and wind generators, which have become major alternative energy sources for the power grid. But small embedded devices must rely on energy scavenging systems that can capture milliwatts of energy from light, vibration, thermal, or biological sources.
The most widely used energy harvesting devices rely on solar, thermal, RF, and piezoelectric sources of energy.
Photovoltaic (PV) or solar cells convert light energy into electricity. Photovoltaic cells have the highest power density and highest power output of the various energy harvesting devices.
Small rechargeable Li-Ion batteries are popular in space-limited portable applications. The Texas Instruments bq24210 Li-Ion Battery Solar Charger incorporates an input voltage regulation loop with a programmable input voltage regulation threshold, making it suitable for charging from alternative power sources such as a solar panel or an inductive charging pad. TI’s bq25505 is an ultra-low-power boost converter with battery management for energy harvesting applications.
For micropower devices, thin-film batteries are an attractive storage option. The Cymbet CBC3150 EnerChip integrates a 3.3V/50 µA thin-film battery with a charge pump, power manager, and temperature-compensated charge control. The Cymbet CBC-EVAL-10 Energy Harvesting Evaluation Kit includes a small solar panel.