Researchers from the Massachusetts Institute of Technology (MIT) think it's possible to use the energy generated by trees to power a network of wireless sensors to prevent spreading forest fires. These sensors are equipped with off-the-shelf batteries that can be slowly recharged using electricity generated by the trees themselves. 'The system produces enough electricity to allow the temperature and humidity sensors to wirelessly transmit signals four times a day, or immediately if there's a fire.' Even if this would be a good application of wireless networking, the researchers also claim it opens the possibility of 'using trees as silent sentinels along the nation's borders to detect potential threats such as smuggled radioactive materials.' I suspect there is some exaggeration here, but read more...
You can see on the left how it is possible to capture energy from trees to power the wireless sensor networks envisioned by the MIT researchers. (Credit: MIT) You'll find more references about this illustration below.
This project has been led by Shuguang Zhang, the associate director of the MIT's Center for Biomedical Engineering (CBE). Zhang worked with Andreas Mershin, a postdoctoral associate at the CBE and Christopher Love, an MIT senior in chemistry.
Here are short quotes from the researchers. "A single tree doesn't generate a lot of power, but over time the 'trickle charge' adds up, just like a dripping faucet can fill a bucket over time,' said Zhang. Mershin adds that "it's really a fairly simple phenomenon: An imbalance in pH between a tree and the soil it grows in." Finally, Love said that the bioenergy harvester battery charger module and sensors are ready. "We expect that we'll need to instrument four trees per acre," he said, noting that the system is designed for easy installation by unskilled workers."
The wireless sensor network is developed by Voltree Power -- a company where Love and Mershin have financial interest. The company says on its website that its "bioenergy converter has been integrated into a power module that does not depend on wind, light, heat gradients or mechanical movement and is environmentally benign to produce and run. It parasitically harvests metabolic energy from any large plant without significantly harming it and the useful lifetime of the device is only limited by the lifetime of the host. It is weather resistant, completely quiet and has no heat or noise signatures making it also ideal for various security applications, currently under development."
This research work has been published in PLoS ONE on August 13, 2008 under the title "Source of Sustained Voltage Difference between the Xylem of a Potted Ficus benjamina Tree and Its Soil." Here is a link to this article, from which the above illustration has been extracted.
Here is the abstract. "It has long been known that there is a sustained electrical potential (voltage) difference between the xylem of many plants and their surrounding soil, but the mechanism behind this voltage has remained controversial. After eliminating any extraneous capacitive or inductive couplings and ground-mediated electric current flows, we have measured sustained differences of 50–200 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. 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. Instead, a clear relationship between the voltage polarity and magnitude and the pH difference between xylem and soil was observed. We attribute these sustained voltages to a biological concentration cell likely set up by the homeostatic mechanisms of the tree. Potential applications of this finding are briefly explored."
As you can see, there is a huge difference between the researchers' work and what the MIT News Office thinks about the future of this project.
Sources: Elizabeth Thomson, Massachusetts Institute of Technology News Office, September 22, 2008; and various websites
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