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Energy Considerations of Anaerobic Digesters

Posted: September 3, 2004

Anaerobic digesters for manure are not new, although in recent years there is a better understanding of the proper operation and maintenance of digesters for higher levels of efficiency.

Using an anaerobic digester to turn manure into electricity is certainly the topic of the day. Anaerobic digesters for manure are not new, although in recent years there is a better understanding of the proper operation and maintenance of digesters for higher levels of efficiency.

Electricity can be produced from the methane that is liberated during the anaerobic digestion of the manure by using the methane to power a generator. The generation of electricity (regardless of what energy source is being used) is -- by its very nature -- an inefficient process. Therefore, dairy farmers will attain higher economic gain if the methane can be used directly on the farmstead as a heating source. It really is senseless to use methane to power an engine to drive a generator to produce electricity to heat water. Why not use the methane directly to heat water?

If you do use the methane to generate electricity, plan to use as much of the electricity on your farmstead as possible. Selling your electricity to your utility company sounds like a great idea, but you must understand the limitations and the restrictions involved. For more than the past two decades, federal regulations require utility companies to buy any electricity generated by their customers. The price they must pay in Pennsylvania is the wholesale market price for generated electricity. Since Pennsylvania is a net exporter of generated electricity, the wholesale price is only about 1.5 to 2 cents per kWh and it is predicted to get even lower in the future. This price is considerably below the cost for any dairy farmer to generate the electricity. There may be a market for your electricity at a considerably higher price, however, if the electricity can be sold as renewable or "green" electricity.

Selling electricity into the grid system involves expenses for the interconnection to ensure that the "two-way street" transfer of electricity is safe and reliable. Interconnection fees are very high depending on the utility companies involved. For this reason, you cannot justify selling the electricity into the grid unless you have a large supply of electricity to sell.

In the early 1980's, there was a brief period when utility companies in Pennsylvania were required to pay the retail price for electricity to any customer who generated it. That law is no longer in effect, but it has been "grandfathered" for any existing customer who has been generating electricity on a continuous basis since the early 80's.

Some states have passed net metering laws which require the utility companies in those states to pay the same rate for electricity that they charge for electricity. If and when Pennsylvania passes such a law, then there will be a whole new set of economic factors involved.

Bottom line recommendations. When doing the economic feasibly for an anaerobic digester, do not plan on any cash stream associated with selling electricity into the grid system. Use as much of the methane and/or generated electricity on the farmstead to reduce your expenditures for purchased energy. Talk to officials at your utility company about expenses involved for the interconnections. If you have excess electricity to sell, then that cash stream will be "icing on the cake."

Dennis Buffington
Agricultural and Biological Engineering