Philip Small, formerly of Transect Points, seems to have completed his migration to his new weblog at the National Society of Consulting Soil Scientists site.

His most recent post links this fascinating paper: Source of Sustained Voltage Difference between the Xylem of a Potted Ficus benjamina Tree and Its Soil which investigates the sustained electrical potential difference between the xylem of many plants, such as trees, and their surrounding soil. The phenomenon was known, but the mechanisms were poorly understood. They conclude from their experiments that "a biological concentration cell likely set up by the homeostatic mechanisms of the tree" are responsible.

Consider the implications:

We found it difficult to resist speculating that there may be possible practical applications of these findings beyond monitoring pH changes such as a wide variety of trickle chargers for niche, low-power, pulsed, off-grid distributed systems–including forest fire detectors; environmental sensors; and “smart dust” or mesh-networked devices drastically decreasing the need for in-the-field battery changes. Interestingly, ionic flows through microfluidic circuits have already been investigated as viable sources of microwatt level electrical power [18] via the streaming potential effect (in the case of trees and other plants we can expect between 1–10 mV from sap flow alone as discussed above). If a method for easily inserting low-impedance microelectrodes in high-flow areas was developed the sap flow could be converted into electrical power. In addition, the voltage generated by the mismatch in pH between xylem and soil can also be harvested by such circuits that would act as a low-impact “parasite” on the tree drawing on its metabolism, assuming the homeostatic mechanisms would continue supplying the puncture site with redox-mediating molecules.

Such devices must necessarily draw some current, deplete redox mediators at the positive-electrode site, and lead to some negative-electrode degeneration over long times. However, it is possible, by correctly choosing a bio-friendly positive-electrode plating material (e.g. graphite, platinum, gold etc.), to harness large plants' metabolic power and drive tiny load resistors.

As devices continue to grow smaller, and use less power, such small but constant sources of power become sufficient to do useful work. I once said that the world sizzles with energy, it's all around us, we are bathed in energy from ambient sources - solar, vibration, pressure and temperature gradients - and now voltage differences between plants and soil.

Research into nanogenerators is relevant:

The idea is to create a thin-film, solid-state heat pump that turns temperature variance directly into electricity—taking advantage of a phenomenon called the Seebeck effect. As with much of this research, the scientists behind it saw an application in implantable biomedical devices, but they also offered an innovative potential use as a layer on top of computer chips. As the chip heats up, it could produce enough power to run a fan on the heat sink that would cool it down. Since heat from computer chips is entirely the product of energy waste, the solution seemed particularly elegant.

Research into nanoantennas is relevant:

The nanoantennas' ability to absorb infrared radiation makes them promising cooling devices. Since objects give off heat as infrared rays, the nanoantennas could collect those rays and re-emit the energy at harmless wavelengths. Such a system could cool down buildings and computers without the external power source required by air-conditioners and fans. . .

If these technical hurdles can be overcome, nanoantennas have the potential to be a cheaper, more efficient alternative to solar cells. Traditional solar cells rely on a chemical reaction that only works for up to 20 percent of the visible light they collect. Scientists have developed more complex solar cells with higher efficiency, but these models are too expensive for widespread use.

Nanoantennas, on the other hand, can be tweaked to pick up specific wavelengths depending on their shape and size. This flexibility would make it possible to create double-sided nanoantenna sheets that harvest energy from different parts of the sun's spectrum, Novack says. The team's stamp-and-repeat process could also be extended to large-scale roll-to-roll manufacturing techniques that could print the arrays at a rate of several yards per minute. The sheets could potentially cover building roofs or form the "skin" of consumer gadgets like cell phones and iPods, providing a continuous and inexpensive source of renewable energy.

Heat is everywhere, even in space. Energy scavenging antennas that can receive such frequencies and convert them to electricity could be very useful. The ability to collect infrared radiation and re-emit it as visible light, for example, might be an interesting illumination hack.