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Interviewees: Amanda Cuellar and Michael Webber, University of Texas at Austin
A dairy farm doubling as a power plant? It may surprise you, but farms full of livestock can be power plants. A dairy farm produces milk, of course, but it can also produce electricity, using that least-desirable by-product of cattle: manure.
Amanda Cuellar, an undergraduate engineering major at University of Texas at Austin, explains the benefits of this energy source: “We don’t have to come up with whole new technologies and novel methods of doing these sorts of things. We can produce energy and reduce our greenhouse gas emissions just by looking at resources we already have.”
Manure can be converted on site to a form of fuel called biogas, which is used for heat, cooking, or to generate electricity.
Cuellar says, “We found that biogas energy can produce about 2.5 percent of all the electricity consumed in the U.S., which is significant. And that we can displace about four percent of the greenhouse gas emissions from the electricity sector.”
From liability to windfall
Manure left to decompose in piles or “lagoons” on farms emits methane and nitrous oxide, two very potent greenhouse gases: 21 and 310 times the warming effect of carbon dioxide, respectively. Manure disposal also creates bad smells and contributes to air, soil and water pollution.
Webber says, “What we could do is take this manure which right now is an environmental liability, and turn it into a commodity. This is very appealing for agricultural operations. It’s feasible. It’s been done in other countries. We could do it here.”
In July, 2008, the Cal-Denier Dairy in Galt, California fired up the newest addition to their farm: a system that converts their cow manure into electricity right on site. Manure is washed into covered lagoons where microbes digest it, releasing biogas. The biogas is burned in small turbines to generate power that the farm sells back to the Sacramento Municipal Utility District (SMUD). SMUD contributed about 13 percent of the capital for the project. Additional funding came from the U.S. Department of Agriculture.
What makes the digestion process different from normal decomposition is the addition of specific anaerobic bacteria that break down manure and produce a mix of methane and carbon dioxide, called biogas. The process is called anaerobic digestion. There are a variety of different digester configurations depending on temperature and whether the waste is processed in batches or continuously.
The process is contained so that pollution can be controlled. Besides biogas, the solids which remain after processing, called “digestate”, make excellent fertilizer and have much less odor than manure.
Biogas can be burned on site for heat, cooking, or power generation, or it can be sold to the gas grid for distribution elsewhere. (To qualify for distribution in the gas grid, it must be “scrubbed” to remove carbon dioxide and small amounts of hydrogen sulfide.)
Webber says multiplying that effort across the one-billion tons of manure produced annually in America could have a huge effect.
“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.
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