But Christy maintains that her team's cow-powered batteries are a greener, more efficient alternative.
"No methane is involved," she said. "It is a direct conversion into electricity. Therefore no greenhouse gases are released, no combustion inefficiencies are encountered, and capital costs are reduced."
While Christy says her team's microbial fuel cells are the first to use the body fluids of animals, other researchers have developed MFCs that run on human sewage.
At Pennsylvania State University, scientists are working to develop this technology to enable sewage treatment plants to power themselves.
And last month, National Geographic News reported the development of a tiny battery which runs on human urine. The biodegradable batteries are designed as a disposable power source for medical test kits.
Meanwhile scientists are looking to microbial fuel cells as an energy supply for autonomous robots. These robots could be programmed to find the raw fuel for these cells on their own, becoming completely self-sufficient.
The British-based EcoBot Project has created a robot that runs on flies, which are fed to microbes taken from sewage sludge.
Chris Melhuish, director of the Intelligent Autonomous Systems Laboratory at the University of the West of England in Bristol, leads the project.
"We've shown that just from using the energy from dead flies, EcoBot has created enough power to be able to sense its environment using a temperature probe," he said. The robot can then "process this information, actuate wheels to move slowly towards light, and transmit the information back to a base station over a radio linkall powered by eight dead flies."
"We can also use things like shellfish, such as the carapace [shell] of a prawn or plant material," Melhuish added.
The scientists says there are many possible applications for a robot that can generate energy from its surrounding environmentfrom monitoring crops on a large farm to gathering data on the ocean floor.
"There are lots of variations on these themes. But whatever it's doing, it's doing it without external power," he added.
Experts say the big question is whether microbial fuel cell power as a form of clean, renewable energy can be scaled up to a degree that would reduce our reliance on fossil fuels.
"I think the prospects are very promising," said Christy of Ohio State University. "But the amounts of electricity from our laboratory-based systems are still small. More research on scale-up is definitely needed."
Melhuish agrees. "I'm not going to put my hand on my heart and say this is definitely going to be the technology for the future," he said. "It might be. But unless we experiment and invest money in finding out how good or bad this thing is, we'll never know. It's got to be worth a [try]."
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