Energy harvesting collects small amounts of power from the environment and stores it in a capacitor or rechargeable cell to run an ultra-low-power device. Indoor photovoltaic cells power wireless light switches and shelf labels; piezoelectric elements turn the press of a button into enough energy to send a radio frame; and RF harvesting feeds passive RFID tags. The goal is a device that never needs a battery change.
The constraint is that harvested power is tiny and intermittent, so the device must spend nearly all its time in deep sleep and send rarely and briefly. Designers budget energy per event rather than average current, and choose radios with very short transmissions. Where the ambient source is unreliable, a small battery or supercapacitor bridges the gaps, trading some of the maintenance-free promise for predictability.
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