California dairy cows are giving the state's power grid a blast of renewable energy, but the real money maker in bovine biogas may lie elsewhere.Pacific Gas and Electric Company announced Tuesday it will receive up to three billion cubic feet of renewable natural gas a year from BioEnergy Solutions, a waste-to-energy firm. That's enough biomethane to meet the electricity needs of approximately 50,000 PG&E residential customers, the utility company says.

PG&E and other utilities are motivated to incorporate renewable energy sources, since California regulators have directed them to make renewable energy at least 20% of their electricity supplies by 2010. Qualifying renewable sources include solar, wind, biomass, geothermal, and small hydroelectric.

The technology used to convert grass and grain-fueled bovine emissions into electric energy is itself relatively simple. Manure is collected in a holding area (poo lagoon?), which is sealed in plastic to create an oxygen-free atmosphere that speeds bacterial growth. Methane gas is produced by the natural digestive process of the bacteria. The gas is then collected and "scrubbed" before being sent to the power plant.

The trick is not in extracting gas from cows (as anyone who has ever visited a farm can attest), but in converting the gas to methane and then getting it to the power plant. Herd size and proximity to an existing natural gas pipeline are two key factors in a biogas economy.

So how many cattle have to be enlisted to fulfill the BioEnergy Solutions contract with PG&E? If a 2,500-head dairy can provide enough waste to power more than 1,000 homes, as the company says, then it will take the efforts of 125,000 cows to light up 50,000 homes. California has about 2,000 dairy farms, with an average of 850 cows on each one.

BioEnergy Solutions contracts with dairy farmers to collect methane gas produced by cows. The company designs, installs, and maintains the equipment necessary to collect the methane. Then it splits the revenue from sale of the gas and carbon credits with the farmers.

Compared with other renewable sources of energy, the cost of electricity derived from cattle gases breaks down this way, according to the California Energy Commission:

Making energy from manure may seem fairly lucrative, but the real money in the biogas game is situated in other end of the cow -- up front. Professor Jan Bertilsson of the Swedish University for Agricultural Sciences says, "95% of the gas comes from the nose. Only 5% comes from the back of the cow."

Bertilsson recently received a half million dollar grant to study belching cows.

Ruminate on that.