Researchers at the University of Washington (UW) have taken the term ‘green power’ literally by running an electric circuit from the power generated by trees. Sure, there isn’t much electrical power to harness, but the researchers say it should be enough to run wireless sensors that could be used to detect environmental conditions or forest fires and could also be used to gauge a tree’s health.

The UW team’s research follows on from a Massachusetts Institute of Technology (MIT) study last year that found plants generate a voltage of up to 200 millivolts (thousands of a volt) when one electrode is placed in a plant and the other in the surrounding soil. Those MIT researchers have since started a company developing forest sensors that exploit this new power source, but the UW team sought to further academic research in the field by building circuits to run off that energy. They successfully ran a circuit solely off tree power for the first time recently.

By hooking nails to trees and connecting a voltmeter, UW undergraduate student Carlton Himes found that big leaf maples generate a steady voltage of up to a few hundred millivolts. The team then developed a boost converter that takes a low incoming voltage and stores it to produce a greater output. The custom boost converter works for input voltages of as little as 20 millivolts and produces an output voltage of 1.1 volts, enough to run low-power sensors.

The circuit the UW team developed is built from parts measuring 130 nanometers and consumes on average just 10 nanowatts of power during operation. Normal electronics obviously aren’t going to run on these types of voltages and currents, so there’s no use packing that big screen TV for your next camping trip, but as new generations of technology come online they will continue to re-evaluate what is doable and not doable in terms of a tree power source.

Despite using special low-power devices, the boost converter and other electronics would spend most of their time in sleep mode in order to conserve energy. This created a complication because if everything were to go to sleep, the system would never wake up. To solve this problem, the team built a clock that runs continuously on 1 nanowatt, about a thousandth the power required to run a wristwatch, and when turned on operates at 350 millivolts, about a quarter the voltage in an AA battery. The low-power clock produces an electrical pulse once every few seconds, allowing a periodic wake-up of the system.

The researchers point out that the tree-power phenomenon is different from the popular high-school potato or lemon experiment, in which two different metals react with the food to create an electric potential difference that causes a current to flow. So, in order to distance the tree-power effect from the potato effect, they used the same metal for both electrodes.

The researchers say it hasn’t been established exactly where these voltages come from, but there seems to be some signaling in trees similar to what happens in the human body, but with lower speed. In this way they are hoping to apply the their results as a way of investigating what the tree is doing, much like measuring a pulse in a human.

The UW team’s findings appear in a study to be published in the journal, Institute of Electrical and Electronics Engineers' Transactions on Nanotechnology.

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