"Overall you get a net effect of saving over 100 million metric tons of greenhouse gases per year — if we're able to get all that manure and put it into action as an energy source," he says.
That billion tons of manure is renewable. "It's a resource we have lying around," says Cuellar. And, she adds that it's not as controversial as corn ethanol. "Ethanol production or even biodiesel production uses a food source, and this has caused problems worldwide, particularly for the food shortages we're seeing this year."
But what about the capital costs?
Cuellar and Webber are not actually proposing a specific policy at this point. They said they did the analysis to show the potential reduction of greenhouse gas emissions.
"What we will consider next," says Webber, "is how feasible this might be. But it looks very feasible for these concentrated animal feeding operations where you have a lot of manure in one place anyway."
"It does cost some money, however. You have to buy the equipment… But in an era of high electricity prices and high gas prices, this becomes more economical than ever before," he says.
Depending on the size of the operation and what the biogas is used for, anaerobic digesters can pay for themselves in as little as 5 years.
And with the prospect of a carbon tax on coal-fired power generation, the investment in biogas plants look even more attractive.
If it's such a good idea, why isn't someone doing it?
Webber says, "People often ask, 'Is this new technology?' or 'Is it expensive?' It's not new technology, it's been demonstrated for decades in countries like Germany. It's an age-old process, people have known about it for a long time."
Anaerobic digesters were first used in the 19th century. India today has over 2 million biogas plants, mostly small units used to generate cooking fuel. Biogas-powered bus fleets are springing up all over Europe, and Germany is using biogas generators as significant contributors to their national electrical and natural gas grids.
Webber says, "We're trying to find ways to reduce our greenhouse gas emissions, and we often think of difficult, long term, multi-decade, high-technology solutions. But it turns out the solution might be laying on the ground around us."
Now that's putting the "moo" into the green energy movement.
This research was published in the July 2008 edition of Environmental Research Letters, Institute of Physics, and funded by The Center for International Energy and Environmental Policy at University of Texas at Austin.