Dairy farms turn manure into electricity
Dairy farmers are increasingly turning their manure piles and lagoons into a cash crop, thanks to technology that allows them to turn that waste into electricity. Anaerobic digestion is the process by which organic waste is broken down in an oxygen-free environment, producing biogas. That gas can be used in the same way as natural gas to fuel generators that produce electricity.
On-farm anaerobic digesters allow farmers to redirect their animals’ manure into a sealed tank, where the digestion occurs. The tank is temperature-controlled at 104 degrees Fahrenheit, the optimal climate for the naturally occurring bacteria to do their work. As methane and carbon dioxide rise out of the decomposing waste, they are captured in tanks or bags, or piped directly to an on-farm power plant.
The size of the power plant is based on the amount of manure produced on the farm. It runs 24 hours a day, burning the gas and generating electricity. Most produce far more than can be used on-farm, so they’re tied in with the power grid to sell that electricity to the local utility.
Along with the gas, the tanks yield what’s left over from the decomposition process, a mixture of liquid manure and dry matter. The two products are separated during extraction from the tank, with the liquid stored for application on fields as fertilizer and the dry matter being heat-dried further and then used for animal bedding or compost.
While there are fewer than 200 of these plants currently in operation on farms in the U.S., interest is rising, and several states have launched initiatives to incentivize their use. In January, the New York State Energy Research & Development Authority announced that it would make $20 million in grants available to farms to offset the costs of installing digesters. Many of Pennsylvania’s more than 30 on-farm digesters were funded in part by resources from the state’s Commonwealth Financing Authority and Department of Environmental Protection.
David Dunn, of Green Mountain Power’s Energy Innovation Center, helped lead Vermont’s effort to create legal, legislative and regulatory systems that would ease the process and cost of installing and using on-farm digesters. The state’s first unit went online in 2003, and today there are 15 in use. Together they have produced more than 77,000 megawatt-hours of electricity.
“It’s not difficult for farms to earn a reasonable price for energy they produce from manure,” says Dunn – but utility companies have to cooperate. In Vermont, that meant setting a 20-year fixed rate for the electricity the utility would buy from each farmer and adding a bonus of 4 cents per kilowatt-hour for the environmental and renewable benefits created by the digesters. Funding for that bonus comes from a voluntary tariff on customers’ bills.
Green Mountain Power also helps farmers with the initial costs of the digesters, which can easily top $2 million, by helping them find financing from a variety of sources. Federal grants from the USDA’s Rural Energy for America Program can fund the generator, for instance, and the Natural Resources Conservation Service’s Environmental Quality Incentives Program often has funding for the manure management side of the project. The state of Vermont’s Clean Energy Development Fund offers grants of as much as $250,000 for these projects as well. Green Mountain Power itself created a fund to help cover the costs of electrical system upgrades and equipment needed to tie the on-farm power plants to the grid.
Altogether, Dunn says the grants can cover as much as 60 percent of the project’s cost, and traditional farm lenders have put together loans to help farmers finance the remainder of the cost for a seven-year term, usually with monthly payments that can be covered by the amount the farm is earning from the electricity it generates.
Maxwell Farm, a third-generation dairy farm in Coventry, Vt., was looking for a steady second income stream when milk prices bottomed out in 2007. After researching a number of possible projects, they built a digester that went online in December 2008.
“The $1.5 million price tag made us hesitate, and it wasn’t a completely proven technology,” Matt Maxwell says. The grant and loan packages that Green Mountain Power helped them put together made it possible, and the project has been a success. “We’re making back enough money to pay the loan off,” he notes, adding that they will earn money from the generator when the loan is cleared by mid-2015. The plant generates 180 to 200 kilowatt-hours of electricity, enough to provide power to 150 average U.S. households.
At first the manure from the farm’s 800 Holsteins wasn’t quite enough to generate the power their plant was designed for, says Maxwell. By bringing in organic substrate to add to the digester along with the manure, the plant began running at 30 to 35 percent efficiency, far better than the efficiency rate of a typical coal-burning power plant.
Farmers find that the power-generating capacity of their manure changes with the season and what they feed their cows, so sometimes what they put into the digester has to be tweaked in order to keep it running at optimal capacity. Adding additional materials, such as waste oils and fats, whey, or even crops grown specifically for the digester, is common and can help improve efficiency, as well as keep significant amounts of organic materials out of the waste stream.
Having a power plant on-farm does mean more chores, says Maxwell. It takes a little more than an hour each day to check all of the systems, add oil, adjust valves and do other basic maintenance. Occasional large maintenance jobs may require the help of an outside mechanic and shutting the generator down for a short period of time. Every two to three weeks, the oil needs to be changed, and many farms use the 30 to 40 gallons of waste oil per change as heating fuel. There is also regular maintenance on the digester itself, which has pumps, paddles to stir the waste matter, and a screw press that separates the dry matter from the liquids.
The dry matter that comes out of the digester has turned out to be a boon for the farm. Where the Maxwells were buying a tractor-trailer load of sawdust every week in the past ($2,200 per load), now they buy just one load a month to use for calves, heifers and sick cows, and the rest of their animals have the dried manure for bedding.
As an added bonus, Maxwell notes that their animals’ somatic cell counts and mastitis rates have dropped significantly since they started using the new bedding material. A recent study by the University of Vermont found that this experience is common. The farm is producing more of the dried bedding than they can use, so some is sold to neighboring farms and landscaping companies for additional income.
Maxwell Farm further increases the power plant’s efficiency by capturing excess heat from the generator’s water jacket and exhaust and using it to heat their shop and garage, the digester tank itself, water used for cleaning the milk house, a drying floor where they dry the bedding material from the digester, and a new 36-by-72-foot greenhouse where they grow greens and spinach through the winter to sell to local restaurants. Maxwell estimates that the farm saves more than $4,000 a year on oil just for the heated water used in the milk room.
The reduction in flies and odor is another nice benefit, Maxwell says. Dunn points out that the odor reduction is an indication of the environmental benefits of the digesters. Since methane is captured and burned in the power plant, less of it is being released into the atmosphere. He says that the 15 digesters in Vermont have reduced methane emissions by 219,000 metric tons of carbon dioxide equivalent (MTCDE) and offset an additional 35,000 MTCDE with the power they have produced.
Running manure through the digester doesn’t mean losing the nutritive value for fertilizer, says Dunn. In fact, the liquid manure byproduct of the digestion process actually has better nutrient availability, which can reduce the need for commercial fertilizer.
These systems are not for all farms, of course. Dairies that predominantly graze their animals, for instance, don’t have an easy way to capture and move manure, so they likely wouldn’t have enough to keep a digester running consistently and efficiently, nor would dairies that milk seasonally. The cost of on-farm digesters usually makes them economically feasible only for large dairies with 1,000 cows or more – larger if the project is to be built without grant support – but smaller digesters are in development that are intended for use on dairies with fewer animals.
Maxwell notes that there’s another side effect of having an on-farm digester: having to offer a lot more tours. The farm gets regular requests from student groups, from grade school to college, as well as other farmers and environmental groups, all interested in seeing how manure can be turned into electricity and help sustain dairy farms.