One of our contributing editors, Elizabeth Montalbono, wrote a blog for another UBM site, Design News, about harvesting energy from the air. A major discussion ensued. I had initially hoped this had to do with the electrostatic gradient in the air or the Earth's magnetic field. But it did not.
Instead, the post had to do with a proposal by Dennis Siegel, a student at the University of Arts in Bremen, Germany, to tap the energy available in the E-M fields around high-tension power lines, radio broadcast towers, cellphones, and electric appliances. The intent was to draw just a small amount of energy to charge a small battery, for example, that could be used to power a cellphone, flashlight, or radio.
With regard to using small appliances as the power source, the assumption is made that the appliance has a motor, or that it has an internal power supply that uses a transformer. Either way, the stray magnetic field that leaks out would be intercepted with an open frame inductor. Of course, the coupling is rather loose, so the energy transfer level would be pretty low, and the charge time would be pretty long. Siegel notes this but says it is still a workable idea.
My first impression was that, if you could increase the coupling coefficient, that would help. Then, in an a-ha moment, I thought, “If you're that close to the appliance, couldn't you just plug a wall wart into the power mains?” That would increase the coupling coefficient substantially (like to unity). If you're just trickle-charging a small battery, the cost of the electricity used would be practically nil.
But that's not what really got me concerned about this method of getting power. Apparently, Siegel thinks harvesting energy from utility power lines and radio transmitters is a perfectly OK idea with no consequences. And a significant number of the people who commented on the blog also think it's perfectly OK. The assumption is that the field is there whether we snag it or not, and that you can couple to the power line inductively, capacitively, or not at all — there's no effect on the utility's power distribution. This was clearly thought up by someone who had not paid attention during an AC Circuits 201 class.
As for harvesting energy from a nearby radio station, I guess that same someone was not paying attention during RF Circuits 301. We could conduct an experiment: Buy a house in the northern suburbs of Cincinnati, and build a nice vertical antenna tuned to 700kHz. Add a center loading coil, so the antenna doesn't have to be quite so tall. From its base feed point, couple it into a suitable transformer matching network, rectifier diodes, and filter capacitors. Et voila — free electricity.
Just let me know how long it takes before the chief engineer and an attorney from WLW knock on your door.