Last year, MIT researchers found that plants generate a voltage of up to 200 millivolts when one electrode is placed in a plant and the other in the surrounding soil. A University of Washington team followed up on this research, and has run a custom circuit entirely off tree power.
By hooking nails to trees and connecting a voltmeter, Carlton Himes, a UW undergraduate student, found that bigleaf maples generate a steady voltage of up to a few hundred millivolts.
The UW researchers next built a device that could run on this available power. Brian Otis, assistant professor of Electrical Engineering, led the development of a boost converter - a device that takes a low incoming voltage and stores it to produce a greater output.
The team's custom boost converter works for input voltages of as little as 20 millivolts, an input voltage lower than any existing such device. It produces an output voltage of 1.1 volts, which is enough to run low-power sensors.
The UW circuit is built from parts measuring 130 nanometers and it consumes on average just 10 nanowatts of power during operation.
"Normal electronics are not going to run on the types of voltages and currents that we get out of a tree. But the nanoscale is not just in size, but also in the energy and power consumption," said Babak Parviz, associate professor of Electrical Engineering.
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, creating a complication. "If everything goes to sleep, the system will never wake up," Otis explained.
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 wakeup of the system.
The tree power phenomenon is different from the well-known 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. To avoid confusion, the UW researchers used the same metal for both electrodes.
Parviz admits that tree power is unlikely to replace solar power for most applications. But the system could provide a low-cost option for powering tree sensors that might be used to detect environmental conditions or forest fires. The electronic output could also be used to gauge a tree's health.