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Electronic Design

These Trees Are Voltage Sources, Not "Potato Batteries"

A scientific paper in an online journal resolves a number of questions about harvesting electrical energy from trees. A wireless mesh network designed to monitor forests for fires uses energy-harvesting hardware developed by Voltree Power that taps into to the trees themselves for energy (see “Tree-To-Earth pH Difference May Generate Harvestable Energy”). In our report, we left open the question of the electrochemistry behind the power. Here’s the rest of the story.

“After eliminating any extraneous capacitive or inductive couplings and ground-mediated electric current flows, we have measured sustained differences of 50 to 200 mV between the xylem region of a Faraday-caged, intact, potted Ficus benjamina tree and its soil, as well as between its cut branches and soils and ionic solutions standardized to various pH values,” wrote Christopher J. Love, chief technical officer of Voltree, in his paper, “Source of Sustained Voltage Difference between the Xylem of a Potted Ficus benjamina Tree and Its Soil.”

“Using identical platinum electrodes, no correlation between the voltage and time of day, illumination, sap flow, electrode elevation, or ionic composition of soil was found, suggesting no direct connection to simple dissimilar-metal redox reactions or transpirational activity,” Love continued in his paper. But where does the voltage come from?

According to Love, “the tree-root-soil system acts as a concentration pH cell, sometimes actively maintained by the tree’s homeostasis mechanisms.” Love and his co-authors further noted that the potential from such a concentration cell is the “Nernst potential, which depends only on a concentration gradient.” The rest of the paper addresses alternative hypotheses and their refutations. It also details specific experiments and their results.

The paper appeared in PLoS ONE, which is an open access, online scientific journal from the Public Library of Science. It covers primary research, and submissions are peer reviewed. Unlike print journals, however, the peer-review process proceeds at a brisk pace. Once articles are published, feedback is nearly in real time, with post-publication user discussion and rating features online.

Although PLoS ONE itself is easy to find, without Love’s help, the paper would still have been difficult to track down. The best search term would have been “Nernst.” (Check out the Wikipedia entry under “Nernst equation” for details.) But if you already knew that, you would also already be well up on electrochemistry and would probably never have been thinking about tree power in terms of potato cells in the first place.

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