Everything for Electronics
Posted in: Developing Perspectives (February 2009)

Energy Harvesting

By Bryan Bergeron

The poster child for mechanical energy harvesting is the perpetual, self-wound wristwatch. Walking and other normal body movement is typically sufficient to keep a self-wound watch going. Furthermore, unless the wearer has a sedentary job and lifestyle, there is no need to consciously shake the watch. Reclaiming ambient energy is almost like getting energy for nothing. In this regard, energy harvesting is different from manually operating self-charging devices such as flashlights that must be shaken vigorously for a minute or more, emergency radios with built-in generators that must be cranked..

There have been numerous demonstrations of energy harvesting using piezoelectrics, ranging from sidewalks and backpacks to combat boots. Piezoelectrics harvest energy by converting an applied stress to an electrical charge. In the case of sidewalks, the stress generated by the movement of pedestrians is converted to an electric charge. Similarly, piezoelectric backpack straps and piezoelectric elements in the heels of combat boots convert the bouncing and pounding to useful energy by charging a supercap or battery..

The high cost of installation and maintenance and the relatively small return on investment have kept energy harvesting either in the lab or in highly publicized (but short-lived) demonstration projects. What's been lacking is an affordable, general-purpose energy harvesting technology that can be applied to a variety of application areas with a high return on investment. For example, while energy-generating sidewalks may — at best — illuminate a store sign, personal energy harvesting systems could save lives. Think backup power for a cardiac pacemaker or a telemetry system for a soldier stranded for weeks without any other means of charging the batteries in his communications gear..

I had a chance to evaluate a general-purpose energy harvesting device about the size of a 9V battery. The piezo-based device is called Joule-Thief, from AdaptivEnergy ([url=http://www.rlpenergy.com]http://www.rlpenergy.com[/url]). The electrical equivalent of a self-wound watch, the Joule-Thief converts movement into electrical energy. That is, the Joule-Thief transforms the movement of a piezoelectric sensor to a 3.6V output. The movement could be from walking, running, the vibration of an engine, or the rocking motion of a boat moored to a dock. Regardless of the input source, the output is on the order of 1 mW /second — enough to power the transmitter that accompanies the demonstration kit. .

While AdaptivEnergy plans to offer their energy harvester at about $10 per unit, the full evaluation kit is currently priced at about $500 in single unit quantities. If you don't want to wait until this summer to experiment with a more affordable Joule-Thief, then consider experimenting with an ordinary piezoelectric transducer, diode, and supercap. My favorite source for transducers is Parallax, which sells a Piezo Film sensor manufactured by Measurement Specialties ([url=http://www.msisa.com]http://www.msisa.com[/url]) for about $2. You can find Piezo sensors at Digi-Key and Mouser, as well. You probably won't be able to harvest enough energy to power a transmitter with a single sensor, but you should be able to track energy capture with a voltmeter or micro-ammeter. .

If you want to learn more about how the Joule-Thief operates, see the AdaptivEnergy website where you'll find several informative PDFs available for free download. Similarly, information is available on the piezo Film Vibra Tab vibration sensor at the Parallax website ([url=http://www.parallax.com]http://www.parallax.com[/url])..

I don't want to give the impression that piezo is synonymous with energy harvesting. For example, there is promise in using the temperature differential between the body and the environment as a source of energy. .

If you have a working energy harvesting circuit or device — regardless of the technology — please let me hear from you. I'd like to share your experiences with our other readers. NV