Great River Energy receives a total of 5 megawatts of methane derived energy from anaerobic digesters. |
Anaerobic digesters: power from cows Great River Energy purchases the energy output from anaerobic digesters located on dairy farms in Princeton and Morris, Minnesota. The digesters capture methane gas and use it to run generator units that in turn produce renewable electricity. The anaerobic digesters are large, cement tanks outside a barn, mostly underground, which capture the cows’ waste. Capturing the waste greatly reduces odor from the dairy operation. |
When manure breaks down, it creates a bio-gas, which is primarily methane. The methane rises to the top of the tank and is captured. Methane is a greenhouse gas which, when released into the atmosphere, is 23 times more potent than carbon dioxide.
Once captured, the methane is typically destroyed through “flaring,” during which it is burned through a flame that flares out the end of a pipe. The business earns carbon offsets or carbon reduction credits based on the amount of methane destroyed in the process, which is measured by a third party. Some dairy operations go one step further and destroy the methane by burning it in internal combustion engines, rather than simply flaring it. The combustion engines turn a generator which produces electricity.
The electricity generated in this process is eligible under Minnesota’s Renewable Energy Standard, which requires Great River Energy to produce 25 percent of its electricity using renewable energy sources by the year 2025. The systems will be registered and their energy production tracked in the Midwest Renewable Energy Tracking System (M-RETS). M-RETS is one of several regional tracking systems in the U.S. used to verify renewable energy production and utilities’ compliance with renewable energy production goals.
According to the Environmental Protection Agency Climate Leaders white paper published in 2006, of 92,000 dairy operations in the United States, only 1.7 percent had more than 500 head of cows and only 62 made use of anaerobic digester systems.
A large dairy digester system will generate approximately 14,000 megawatt-hours of electricity each year, which is enough to meet the annual energy needs of about 1,400 homes. Liquids left over from the biological breakdown of the manure have less odor and can be used as fertilizer for crops. Solids left over have no bacteria and can be used as bedding. Waste heat from the generators is captured and used to aid the biological process in the digester.
