"The process involves plants growing in modules—two-square-foot plastic containers connected to other modules—where they undergo the process of photosynthesis and convert sunlight, air, and water into sugars. The plants use some of the sugars to grow, but they also discharge a lot of it back into the soil as waste. As the waste breaks down, it releases protons and electrons. Plant-e conducts electricity by placing electrodes into the soil."

http://www.yesmagazine.org/planet/dutch-company-powers-streetlights-with-living-plants-will-your-cellphone-be-next

New life for formerly illegal hydroponic plant rooms?