Nothing new about energy harvesting

-March 14, 2013

Energy harvesting (I like to think of it as “scavenging”) is a hot topic, and with good reason. The combination of transducers, energy-capture circuitry, power management ICs, energy-storage elements (battery or supercapacitor), and wireless links, all operating with minuscule power, means it is an effective way to implement self-powered, long-life data acquisition, logging, and reporting systems. We may think of it as a new development, and today’s sophisticated designs certainly are so. But the basic harvesting idea is not new, not at all.

I realized this very clearly when we had an AC power failure, yet my gas-fired water heater kept on working just fine. The unit is controlled by a simple bimetallic thermostat switch closure - no fancy Internet-enabled Nest Leaf thermostat unit here. It has an electrically operated solenoid valve which controls the gas flow, based on that switch closure. But what powers the solenoid valve, since there is no electrical power connection to the heater?

The answer is simple: while we normally think of a thermocouple as a temperature-sensing transducer, it can also be an energy source. There is a thermocouple positioned in the always-on pilot light of the heater, and the modest amount of power it generates is enough for the valve. It’s a good fail-safe mechanism, too: if the pilot light goes out or the thermocouple fails, the solenoid cannot be energized and the valve stays shut, so there is no gas flow. (Note: a similar energy-production technique is used to power some spacecraft, with radioactive decay as the heat source captured by a surrounding array of thermocouples.)

There’s another energy-harvesting design that electrical engineers have used for about 100 years: the legendary crystal radio. Comprised of just an antenna, tuning coil, diode, capacitor, and headphones, this radio can capture and demodulate AM broadcasts, with no batteries needed. The received signal itself provides the power to drive the headphones (which must be high-impedance, sensitive units). A crystal radio is a great project for any youngster with a hands-on, build-it, “wow” inclination - and you can find plenty of schematics for them online.

Are there any other harvesting applications of which you are aware, and which require no advanced ICs or perhaps even pre-date our IC era?

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