It's not so much green energy as brown power: a dairy farm in California said yesterday that it had found a new way to generate electricity for households — using a vat of liquid cow manure, 33ft deep and big enough to cover five football fields.

“When most people see a pile of manure, they see a pile of manure. We saw it as an opportunity for farmers, for utilities, and for California,” said David Albers, a partner in the Vintage Dairy, near Fresno, which has 5,000 cows and calls its new facility the Vintage Dairy Biogas project.

As cow manure decomposes it produces methane, a greenhouse gas more damaging than carbon dioxide. Scientists say that controlling methane emissions from animals such as cows will be hugely important in preventing climate change.

Methane can also be captured and treated to produce renewable gas, which can be used instead of coal to run electricity-generating plants — the excretion of a single cow can produce about 100 watts of power.

Although other farms in California already generate natural gas from cow dung, this marks the first time that it has actually been supplied via a pipeline to a utility company, PG&E Corp.

The pipeline will allow PG&E to generate power for about 1,200 homes a day in California's agricultural heartland.

The energy is certainly renewable — as long as no count is made of the energy consumed during the farming of the grain that is fed to the cows — but no one could call it clean.

In addition to being a partner in the Vintage Dairy, Mr Albers is also president of BioEnergy Solutions, the company that funded and built the so-called digester, which turns the cow faeces into gas and saves farmers the cost of disposing of the waste.

BioEnergy Solutions now intends to build digesters at other farms, ultimately generating enough gas to supply 50,000 homes.

The digester works by mixing the manure with microbes. This breaks down the faeces and the resulting gases are then captured.

At the Vintage farm, the digester prevents about 1,500 tons of methane gas from escaping into the atmosphere every year. It also helps to prevent groundwater pollution, a common side-effect of manure storage.

Mr Albers described PG&E as a customer and declined to give details of their agreement.

California's regulators — encouraged by the Governor, Arnold Schwarzenegger, a recent green convert — have ordered PG&E and other utilities to make renewable energy at least 20 per cent of their electricity supplies by 2010.

PG&E expects to reach 14 per cent this year, thanks in small part to Mr Albers's vat of dung.

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