Making Electricity From Dirt

By harnessing power from soil microbes, one engineer is trying to charge cell phones across rural Africa.
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By harnessing power from soil microbes, one engineer is trying to charge cell phones across rural Africa.

In the United States, renewable power is mostly about being bigger. Take BrightSource’s plans for a solar thermal power plant in the California desert: hundreds of mirrors will focus the sun’s rays on a central “boiler,” creating enough power for 140,000 homes. Billions of dollars thousands of acres of land; 392 megawatts of production—more than a standard coal fired plant.

But the development of renewable energy also has another path, particularly outside the U.S. That path is very tiny. Miniscule, in some cases. It strives to supply exactly the right number of electrons at the right time in the right place. In sub-Saharan Africa 500 million people have no access to electricity, but nearly a quarter of them have cellphones. Charging these phones is tough: some people walk miles to commercial charging stands that cost 50 cents or a dollar to top up the battery, a significant part of a family’s income. Solar panels for the purpose can cost $20 or more and they have a local reputation for being hard to repair.

What these people need is a very cheap, very simple, very tiny trickle charger to keep their phones going. And that is how Aviva Presser Aiden, a Ph.D. engineer who is now a student at Harvard Medical School, became interested in generating electricity from dirt.

A few years ago Presser was working with some students from Africa to figure out how to get more electric light in remote parts of the continent. Lots of people are working in solar, but Presser and friends were interested in a phenomena where soil microbes produce power. “There’s a lot of soil in Africa,” says Presser, and what they wanted was to design a charger that costs less than $5, hopefully more like $1 or even free. If the very idea sounds absurd—electricity doesn’t grow on trees, never mind in dirt—it attracted a very big backer. With a $100,000 Grand Challenges in Global Health grant from the Gates Foundation, Presser and her colleagues have done extensive lab tests on a prototype and are preparing to test it in Uganda later this summer.

How do you make electricity with dirt? First you need some kind of jar, with a piece of graphite or some other non-corrosive metal, at the bottom. Then put in dirt with very little oxygen, and another piece of graphite. Soil microbes are constantly making electrons, but if there’s oxygen about they’ll put the electrons into the oxygen. If there isn’t any oxygen, they’ll dump the electrons on pieces of metal—i.e. the graphite.

There are a lot of variables, says Presser: microbes “like” some metals more than others, and some microbes produce more electrons than others. In the early days, experiments produced a microwatt of power from a bucket of soil.

As they kept tinkering, the team learned a few things. One was that while bigger buckets produced a little more power, it wasn’t a linear relationship. “Bigger is better, but 'n' bigger is not 'n' better,” says Presser. Ultimately, they decided to use smallish plastic containers kitted out with graphite that can be wired together. Now their cells are producing 75 to 400 microwatts of power&mdash:enough to charge a small cell phone in about a week. (This may sound impractical but if the phone is plugged in most of the time it will rarely require a full charge.) The problem is that charging a cell phone requires a minimum of 4 volts of power, and these cells make between 4 and 8 tenths of a volt. So Presser’s team needed to incorporate a small computer chip to boost the voltage. The chip costs between $2 (in large quantities) and $7 (on eBay) and, in addition to increasing the cost of the system, it drags down the efficiency somewhat.

Right now Presser is studying furiously for med school and getting her dirt-kits ready to travel to Uganda for field tests next month. She hopes that feedback from farmers will help the team refine their design to be more practical and efficient. Ultimately, she hopes that increasing refinements will reduce the price to $5 or less for those who build the devices out of scraps. “It would be a lot less expensive than the phone or a solar panel,” she says, “and maybe we can make the voltage booster available for $2.” Presser is encouraged by farmers who’ve been “very receptive.” They know a lot about the earth and ground, she speculates, and are comfortable with this sort of technology.

Presser sees the design as part of a wave of appropriate lower-cost high-tech for
Africa that can help people live better. The idea also changed her life: after thinking about how designing better and cheaper energy sources for African medicine could improve people’s lives, she decided to study medicine.

“This shaped my career,” she says, “I hope to integrate engineering with medicine. But my life has gotten a lot crazier.”

Will this “small” revolution in greentech get bigger? Presser's team's dramatic improvement in performance and the possibility of reducing prices to almost nothing is promising. In developed economies like the U.S., small tech may offer a new paradigm for energy security. (Brooklyn's Madagascar Institute, for example, offers classes in building innovative gadgets from industrial scrap to trickle-charge phones. It's both a neat idea and a sort of “we-can-survive” Mad Max Manifesto.)

But in un-wired parts of the world, the technology has to deliver real performance rather than a DIY philosophy. Rural African consumers are tough customers: If even a few dirt batteries fail, other villagers will not waste their money on them.