How Plant-microbial Fuel Cells Work

PMFCs: All Wet, or Outstanding in Their Field?

As of 2012, PMFCs don't produce much energy and work only in aquatic environments, with plants like reed mannagrass (Glyceria maxima), rice, common cordgrass (Spartina anglica) and giant reed (Arundo donax) [sources: Deng, Chen and Zhao; PlantPower]. If you ran across a field of PMFCs, like the rooftop patch at the Netherlands Institute of Ecology in Wageningen, you'd never know it was anything more than a collection of plants, except for the colorful wiring trailing out from the soil [source: Williams].

Still, their potential applications in addressing other global sustainability problems, including the strain placed by biofuels on an already overburdened global food supply system, continues to inspire researchers and at least one exploratory venture, the 5.23-million-euro project PlantPower [sources: Deng, Chen and Zhao; PlantPower; Tenenbaum].

Because PMFCs already work on aquatic plants, farmers and villages need not dump their water-based rice crops in order to implement them. On a larger scale, communities could set up PMFCs in wetlands or areas of poor soil quality, avoiding land competition between energy and food production [source: Strik et al.]. Manufactured settings like greenhouses could produce energy throughout the year, but farmland electricity production would depend on the growth season [source: PlantPower].

Producing more energy locally could lower carbon emissions by reducing the demand for fuel shipping -- itself a major greenhouse gas contributor. But there's a catch, and it's a pretty significant one: Even if PMFCs become as efficient as possible, they still face a bottleneck -- the photosynthetic efficiency and waste production of the plant itself.

Plants are surprisingly inefficient at transforming solar energy into biomass. This conversion limit springs partly from quantum factors affecting photosynthesis and partly from the fact that chloroplasts only absorb light in the 400-700 nanometer band, which accounts for about 45 percent of incoming solar radiation [source: Miyamoto].

The two most prevalent types of photosynthesizing plants on Earth are known as C3 and C4, so named because of the number of carbon atoms in the first molecules they form during CO2 breakdown [sources: Seegren, Cowcer and Romeo; SERC]. The theoretical conversion limit for C3 plants, which make up 95 percent of plants on Earth, including trees, tops out at a mere 4.6 percent, while C4 plants like sugar cane and corn climb nearer to 6 percent. In practice, however, each of these plant types generally achieve only 70 percent of these values [sources: Deng, Chen and Zhao; Miyamoto; SERC].

With PMFCs, as with any machine, some energy is lost in running the works -- or, in this case, in growing the plant. Of the biomass built by photosynthesis, only 20 percent reaches the rhizosphere, and only 30 percent of that becomes available to microbes as food [source: Deng, Chen and Zhao].

PMFCs recover around 9 percent of the energy from the resulting microbial metabolism as electricity. Altogether, that amounts to a PMFC solar-to-electrical conversion rate approaching 0.017 percent for C3 plants ((70 percent of the 4.6 percent conversion rate) x 20 percent x 30 percent x 9 percent) and 0.022 percent for C4 plants (0.70 x 6.0 x 0.20 x 0.30 x 0.09) [sources: Deng, Chen and Zhao; Miyamoto; SERC].

In fact, some researchers think those assumptions may underestimate the potential of PMFCs, which can only be good news for consumers.

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