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Earth Battery

It seems that Wily gets these kind of calls every month or so.  This call came from an inventor who thought he had the solution to all of the energy problems of the world.  As the drawing below illustrates, the inventor pounds two metal rods into the ground.  One is made of copper; the second is made of aluminum.  When he measures the voltage across the two rods, he sees about one volt on his voltmeter.  His plan was to wire up 100s of these rods together to produce enough voltage to power his house. I asked him if he had measured the current this battery of his could produce.  Well, no, he h ADN’t.  Wily strongly suggested he make that measurement.  Wily also suggested that he use the simple volts times Amps equation to predict just how much power such electrodes in the ground could produce.  Wily told the caller to call back if the current was more than a few tens of milliamps.  The caller never did call back.

Wily had heard of these “earth batteries” before.  Some people pound metal nails in trees to produce the voltage.  Some dangle metal plates in seawater.  This guy pushed metal rods into the ground.  What people don’t understand is that these systems are just batteries. 

 

Their dissimilar metal electrodes in contact with an electrical conducting electrolyte will always produce a voltage.  However, the electrolytes in the ground are weak, so the power produced by those batteries is quite low.  Also, the metal surface area in contact with the soil is small.  It would take rather large metal plates to yield meaningful power.   

Wily wondered just how much power these earth batteries could produce. Willy went to his junk pile and grabbed an aluminum pipe and a copper pipe, both about ¾ of an inch in diameter.  He took them into his back yard and pounded them into the grass.  He only pushed them into the soil about 6 inches, so he could easily remove them.  He then measured the open voltage.  It was about 1.0v.  He then measured the short circuit current.  It measured 1ma.  Wily knew that by pushing the two metal pieces deeper into the soil it would most likely not increase the voltage but would increase the current, by bringing more metal in contact with the soil.  He figured

that if he could sharpen the two pipes and pound them all the way down to 6 feet, the available current might increase to 12ma.  But, if the current increase is proportional to the surface area, then he could also increase the current by using large metal plates instead of pipe.  Two plates, one of copper and one of aluminum, buried in the ground. would certainly have a larger contact area than pipes.  A ¾ inch pipe 6 inches long has a surface area of only 14 square inches and generated about 1ma of current. This yields a 0.07ma per square inch current factor.  If Wily used two large metal plates 24 inches by 24 inches, with both sides in contact with the soil, he would have a battery with 1152 square inches and should produce about 82ma of current.  Figuring the source impedance of such a power source is V/I or 12 ohms, then maximum power transfer would occur when the load was also 12 ohms.  So, if Wily buried two large plates of metal in the ground and put a 12 ohm load across them, the voltage would drop from 1.0v to 0.5v and the current would be 41ma.  This would yield a power of 20mw.  That is not much power.  In comparison, Wily knew that a quality single solar cell measuring just one square inch would produce about 100mw of power, which is 5 times more than the large earth battery.  So, those earth batteries are just not very useful for anything other than the novelty of producing some electricity out of the earth.


May 2010     Issue 9

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