A little while ago, I posted a message about charging batteries from the
voltage difference between a tree and the soil nearby. Since then, I have
found an online paper on the subject, and a company that has created a
commercial product, the Javelin lifetime, that can be charged with the
bioenergy harvester device.  It is a remote sensor/transmitter with a field
life of 15-20 years:

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0002963
http://www.voltreepower.com

Paul J.

-- 
Eastern Native Tree Society http://www.nativetreesociety.org 
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