Sandra Upson, in "IEEE Spectrum", June 2008
http://www.spectrum.ieee.org/jun08/6254
This article discusses works-in-progress to allow smart sensors to harvest their energy from radio-frequency energy.
"The concept was once dismissed as unfeasible because of the rapid dissipation of electromagnetic waves as they travel from their source. But even microwatts, (...) can be enough to power some sensors (...)."
"Typically, wireless sensors are designed to observe environments in a more flexible way than wired ones can. (...)"
"(...) A sensor's power supply is the most confounding problem. Each option has its limitations: a battery alone has a short lifetime, and solar cells (...) can't soak up photons from inside an airplane's wing."
"The technology for harvesting wireless power is essentially based on (...) RFID. A transmitter can both recharge and query RFID-based sensors."
According to the article, a key development is a steady growth in tag communication distances (now up to 100 meters and increasing). One essential problem remains: a reliable source of RF energy is not always available. The solution might be hybrid power supplies, where a sensor can make good use of different tentative power sources, like temperature changes or mechanical movements.
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