The gas from Jensen's 875 cows produces about 775 kilowatts of electricity, enough for about 600 homes, according to EPC.

The machines heat the manure, which also creates useful leftovers once the gas is removed. Jensen uses some of the byproduct as pathogen-free bedding for his cows.

The machines reduce the size of manure lagoons, making them less likely to leach into local water supplies. The process creates seed-free fertilizer to boot. And there's another benefit. "The odor reduction is probably the biggest thing that everybody likes," Jensen said in a phone interview.

The first methane digester was built in 1859, at a leper colony in India. Use of the machines is a small, but established practice in Europe.

The trend is narrow in the United States, but with high energy prices and tight electricity generation capacity, it could catch on. "We're likely to see a number of cottage industry-type solutions to the energy problems and although they may only be a drop in the bucket, every drop helps," said Jason Schenker, an economist with Wachovia Securities in North Carolina.

EPC estimates 1,000 dairy farms across the United States could be tapped, as well as pig farms, and at pork and beef production factories. The potential annual market for manure gas is about 150 trillion British thermal units, or more than 250 million barrels of oil equivalent, it says.

If it capitalizes on 10 percent of the US market potential, and if natural gas prices average US$7 per mmBtu, EPC says it could realize US$120 million in annual earnings from manure alone.

The technology has advantages over solar and wind power because it doesn't need subsidies. And since cows will always make waste, there is less risk than drilling for gas. "We know a well (from the farm) is not going to come up dry," Albert Morales, EPC executive vice president, said in an interview.

Since methane is a greenhouse gas 21 times stronger than the main greenhouse gas carbon dioxide, some farmers have been able to capitalize on selling credits for burning the gas in voluntary greenhouse gas markets.

Minnesota dairy farmer Dennis Haubenschild, for instance, sells credits for capturing methane on the Chicago Climate Exchange. Last year Haubenschild, who financed his own digester, got about US$10,000 from CCX. He sold his credits to members of the exchange who were not able to reduce their emissions of greenhouse gases under levels to which they had agreed.

EPC is in final permitting stages with Joseph Gallo Farms in California, where it expects two digester tanks for manure from 3,000 cows will produce 130 billion British thermal units of energy per year, or enough for 2,200 homes.