Energy harvesting means scavenging energy already present in a device's surroundings — photons, vibrations, temperature differences, radio waves — and converting it into usable electricity on the spot. No fuel delivered, no battery swapped. The crucial word is small: harvesting operates at microwatts to milliwatts, powering sensors and IoT devices rather than homes.
The oldest ambient source is light. A photovoltaic cell under full sunlight delivers roughly 20 milliwatts per square centimetre; under office lighting, the same cell yields 10–50 microwatts. Piezoelectric harvesting exploits the Curie effect: crystals like PZT and the polymer PVDF separate charge when squeezed or bent. Fingernail-sized cantilevers on vibrating machinery generate tens to hundreds of microwatts — enough for wireless condition-monitoring sensors that run for years without a battery change.
Thermoelectric generators use the Seebeck effect to convert temperature gradients into voltage. On a human wrist, a small TEG yields about 20 microwatts. RF harvesting — the crystal radio's descendant — rectifies ambient radio waves into DC, powering billions of passive RFID tags worldwide.
This comprehensive reference does honest math on viral claims (spoiler: 12 million footsteps per day to power one house) and defines the real niche where harvesting already ships: battery-free switches, self-winding watches, vibration-powered rail monitors, and indoor-light sensor nodes. At the frontier, neutrinovoltaic research by the Neutrino Energy Group investigates whether multilayer graphene-silicon nanostructures can tap the one ambient flux that never depends on weather, daylight, or motion.
