By Tracie McMillan
“The Plate,” National Geographic • June 21, 2016
Summer is high season for composting food waste—and, at large scale operations, for generating power by burning the biogas it generates. But scientists around the globe are figuring out new ways to turn decomposing food into power beyond the trash heap, and they’re finding that some foods are better-suited to the job than others.
That matters because figuring out which foods turn into fuel efficiently makes it easier to reuse waste where it starts: in the fields and supermarkets. Every year, more than half the fruits and vegetables produced in North America and Ocenania end up in the garbage heap, and a full 20 percent of produce grown fails to even make it off the farm.
Take, for instance, the beloved tomato. In south Florida, where much of the American east coast’s winter tomato crop is grown, tomato fields generate 396,000 tons of food waste, owing to fruit that’s not suitable for sale. The resulting surplus of rotting tomatoes either creates methane in landfills or wastewater treatment problems if it gets dumped in water.
Yet all those tomatoes are rich in lycopene, a powerful antioxidant—and, as it turns out, a powerful mediator for electrical charges. Researchers have found that tomato-based microbial fuel cells, which function by harnessing the electrons produced when bacteria break down organic matter, hold promise for generating power. Early research shows that 10 mg of tomato waste can generate 0.3 watts of power.
Both of those are tiny increments, but it still wouldn’t take much to run a light off tomato waste. A 60-watt light bulb, for instance, would require 2,000 mg of tomato waste—still less than one ounce—to run for an hour. And best of all, say researchers, rotting tomatoes are better at transmitting changes than edible ones, which means we don’t have to sacrifice our sandwiches.
There are, of course, difficulties to taking it to scale says Namita Shrestha, one of the researchers on the project. “When we try to scale it up, you might encounter challenges, she says. For example, “electrodes in a bigger size might not be as efficient as at the lab size.” But the benefits could be significant, too. The annual tomato waste in south Florida, says Shrestha, could power Disney World for three months.
But even as high-tech solutions like fuel cells gain ground, simpler food-based power holds promise, too.
Researchers looking for a way to broadly provide simple, cheap power in the world’s developing countries, for instance, have revisited an elementary school favorite: Potato batteries. When researchers tested samples of the “Desiree” potato, the world’s most popular red-skinned and yellow-fleshed potato, they found boiling it boosted power transmission by up to 10 times.
That’s particularly useful in the developing world, where rural electrical power grids can be rare, but agriculture is not. And as more and more people rely on battery-powered devices of all kinds, potatoes offer a cheap power source. Power from potato batteries ends up costing about $9 U.S. for every kilowatt hour. That makes spud-based batteries far cheaper than traditional batteries, which can run anywhere from $68 per kilowatt hour for a C battery to $1,200 per kilowatt hour for a six-volt lithium battery. Researchers didn’t specifically explore whether rotting potatoes held much promise, but even basic field excess in the U.S.—about 1.3 million tons in 2015, according to U.S. Department of Agriculture statistics—suggests there’s plenty of power in those taters.