Cow power refers to the generation of relatively clean, renewable electricity from the anaerobic decomposition of livestock manure. In a typical system, manure is fed into an anaerobic digester, basically a covered tank where bacteria break down organic materials in the absence of oxygen and produce biogas, which can then be burned in a modified natural gas engine that spins an electric generator. One cow's manure—about 30 gallons (114 l) a day—can produce enough electricity to light two 100-watt light bulbs 24 hours a day. Biogas can also be produced from the anaerobic decomposition of sewage sludge, landfill waste, or other organic materials.
Biogas is typically 50–70% methane (CH4), a high-energy fuel, and 30–50% carbon dioxide (CO2). Methane is about 21 times more effective at heating Earth's atmosphere than CO2 and may account for 23% of human-caused global warming. About 8% of methane emissions generated from human activities comes from the anaerobic
decomposition of livestock manure. Cow power and other biogas collecting systems keep methane out of the atmosphere by burning the gas, which converts it into CO2 and so reduces its impact on global climate.
Historical Background and Scientific Foundations
Biogas from anaerobic digestion was first used to heat bathwater in Assyria in the tenth century BC and in Persia in the sixteenth century. Through the 1700s, scientists determined that the decomposition of organic material could produce flammable gas, and in the early 1800s isolated methane—the primary component of natural gas, a fossil fuel often found in association with petroleum—as one of the gases produced by the anaerobic digestion of cattle manure. The first modern biodigester was built at a leper colony in Bombay, India, in 1859, and biogas from a sewage plant was used to light the streets of Exeter, England, after 1895.
Fuel shortages during World War II (1939–1945) and the energy crisis of the 1970s greatly increased interest in biogas energy in the United States and Europe, but early efforts in the United States to operate digesters were often plagued by design and construction problems. Over the 1990s and 2000s, increasing recognition of global climate change, the emergence of carbon regulations and carbon markets, climbing fossil fuel prices, and a growing worldwide push for renewable power sources have reinvigorated broad interest in harnessing biogas for energy.
The countries of the European Union were producing 32,122 gigawatt hours' worth of biogas by 2002, a 6% increase over 2001. In the United States, there were 125 operational anaerobic digesters by 2006. The U.S. Environmental Protection Agency (EPA) estimates that these systems produced 275 gigawatt hours of electricity and kept approximately 88,134 tons (80,000 metric tons) of methane out of the atmosphere in 2007 alone.
Biogas is also popular for cooking, heating, and producing electricity in developing countries, where small-scale systems better serve dispersed, rural populations than do centralized gas and power plants. In China, 7.5 million household biogas systems and 750 industrial-scale plants had been installed as of 2002. About 3.4 million family-sized biogas plants had been installed in India as of 2002.
Impacts and Issues
In addition to climate benefits, manure-based biogas-to-electricity systems can increase the income and long-term viability of agricultural operations like dairies, where commodity prices are often low. Such systems also control odors and help clean pathogens and weed seeds from manure by essentially cooking it with the concentrated heat from decay. The solid byproduct of the process can be used as animal bedding, saving farmers money, or sold as compost, generating cash income. Closed biodigesters may also help farmers better comply with regulations protecting water from manure runoff.
Large systems can be prohibitively expensive, however, with traditional digesters costing as much as $3 million. Farms generally must be large, with a large number of animals contained in a small area, to collect enough manure for electricity sales to offset the infrastructure costs of generating power. To overcome this hurdle, some governments and utilities offer grants and cost-sharing programs to smaller farmers. Green pricing programs, where utilities purchase power from farmers at a premium and sell it at elevated cost to customers who choose to pay more because of its added environmental value, also assist deployment of biogas-to-electricity systems. The sale of carbon credits, which represent a biogas system's climatic benefits, may also help offset costs.
The Blue Spruce Farm in Bridport, Vermont, is an example of a success story. In 2005, the 2,000 cow dairy—the first to participate in Central Vermont Public Service (CVPS) Company's “Cow Power” program— produces enough electricity to power 300 to 400 average homes. CVPS provided incentives to offset the initial cost of system installation, and now purchases and sells the electricity at a premium. Overall, the Blue Spruce Farm has invested about $1.3 million in its system, but expects to recoup the cost in electricity sales over seven years and eventually to realize a profit.
WORDS TO KNOW
ANAEROBIC: Lacking free molecular oxygen (O2). Anaerobic environments lack O2; anaerobic bacteria digest organic matter such as dead plants in anaerobic environments such as deep water and the digestive systems of cattle. Anaerobic digestion releases methane, a greenhouse gas.
BIOGAS: Methane produced by rotting excrement or other biological sources. It can be burned as a fuel.
CARBON CREDIT: A unit of permission or value, similar to a monetary unit (e.g., dollar, euro, yen) that entitles its owner to emit one metric ton of carbon dioxide into the atmosphere.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
METHANE: A compound of one hydrogen atom combined with four hydrogen atoms, formula CH4. It is the simplest hydrocarbon compound. Methane is a burnable gas that is found as a fossil fuel (in natural gas) and is given off by rotting excrement.
RENEWABLE ENERGY: Energy obtained from sources that are renewed at once, or fairly rapidly, by natural or managed processes that can be expected to continue indefinitely. Wind, sun, wood, crops, and waves can all be sources of renewable energy.
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