Two coincidental items this week about getting "free" energy via RF energy harvesting made me stop and think about the topic. The items are "Managing the power in wireless sensor networks powered by energy-harvesting circuitry" and "Sensors Expo: Powercast demos prelude to mobile network RF energy harvester".
RF-sourced energy harvesting is not a new idea, not at all. I'm not referring to Nicolai Tesla's frustrating attempts to transmit power wirelessly from large towers, for use in illumination, motors, and other higher-power uses. But in the earliest days of radio, at the beginning of the 20th century, the receiver was entirely RF powered: it was called a crystal radio. The circuit was simple and the BOM was short: a long-wire antenna, a tuner coil using wire wound around a cardboard tube or container, a crystal, a capacitor, and headphones, period. It was all assembled on a real breadboard—a wooden board previously used for slicing bread and rolls. Batteries were not needed, since all operating power was derived from the received RF signal.
You can still build such a radio, using a packaged diode instead of a raw hunk of crystal (usually galena) which radio pioneers had to use; it’s a great starter project for an aspiring engineer, and it even has a little touch of "magic". (If you Google "crystal radio plans" you'll get thousands of hits; I leave it to you choose among them. Or, you can get a beginner's electricity and electronics book from the 1940s and earlier, still available at many libraries, and you'll see some plans.) The challenge in building such a radio today is getting the headphones, which need to have high-impedance of 1000 to 2000 Ω, while most headphones today are in the 8 to 32-Ω range. (Note: If you don’t know why the headphones need to be high-impedance units, you need to brush up on your "analog" basics, please!)
So RF energy harvesting is not new, but there is a major difference between the harvesting done by the crystal radio "back in the day" and the harvesting in today's systems. No, it is not that our circuits today are more complicated, or we use batteries or supercapacitors to store some ofthat energy. The real difference is that the crystal radio used harvested energy to receive, while most of today's energy harvest systems use it to transmit (usually but not exclusively sensor data, and often in a networked scheme). That represents a fundamental shift in the use of the harvested energy, and also allows us store and use it later for sporadic bursts, rather than the continuous operation which a crystal-radio receiver needs to be useful.
One more point: after I built several crystal radios and read up on them, I understood the concept of "envelope detection" and AM demodulation using the diode and capacitor of the circuit, or so I thought. But it wasn't until years later, when I came to understood circuits better, that I finally realized that another way of looking at the envelope-detection function is this: it's a peak-detector circuit, with rise/fall time constant such that it captures the signal peaks–the carrier "envelope"–while being oblivious to the much-higher carrier frequency itself. At last, I had achieved true insight! ♦