Almost all renewable energy resources originate in the sun. Non-hydropower renewable energy currently accounts for only four percent of U.S. energy and two percent of the electricity supply. Hydropower provides an additional 10 percent of production and seven percent of electricity. In the last decade, the growth in U.S. renewable energy production outpaced all sources except for nuclear energy.
Barriers to renewable energy development include high up-front costs and higher power costs. For example, electricity produced from natural gas currently costs three cents per kilowatt-hour, compared to about six cents for solar energy. But the generating costs for renewable energy are shrinking, and surveys show that Americans are increasingly supportive of non-polluting power.
Hydropower
Hydropower is produced by channeling the flow of rivers or by storing water in reservoirs behind dams and directing it through turbines. There is no pollution of the types associated with the burning of fossil fuels. Hydropower is clean, renewable, and domestically produced, yet it supplies less than 20 percent of the world's electricity. Advocates of hydropower cite the recreational benefits derived from reservoirs and the provision of water for irrigation.
On the other hand, dams disrupt river ecosystems, causing upstream flooding and downstream flow depletion. Water redistribution adversely affects many habitats and can make it impossible for anadromous fish such as salmon to travel upstream to spawn. Though fish ladders have helped mitigate this problem, there is still growing public opposition to dams.
Solar Energy
This most basic source of energy is produced in the sun's core by nuclear fusion. The slight mass lost in this process is emitted as radiant energy; though less than one percent of it reaches the Earth, in 30 minutes it can provide a year's worth of human energy needs. The amount of solar energy a specific place receives depends on such factors as the season and proximity to the equator.
Humans have long used sunlight to cook food and heat water and homes. Today, solar energy is still used for those purposes and to provide hot water for industries such as laundries.
Photovoltaic cells, made of semiconducting materials, are used to collect solar energy and generate electricity. Solar electrical plants are not suited to locations with scarce or unreliable sunlight. Large solar plants can also involve clearing of land for infrastructural components.
Wind Energy
Wind is moving air produced by uneven solar heating of the Earth's surface. Wind power has long been used for grinding grain and pumping groundwater.
Windmills' modern equivalent, tall wind turbines, use wind energy to generate electricity. Turbines catch the wind with blades mounted around a shaft to form a rotor. On the downwind side of the blade, blowing wind forms a low-pressure pocket, which pulls the blade, turning the rotor to spin an electrical generator.
Wind power is now the fastest-growing energy source worldwide. However, land clearing for vast "wind farms" may produce environmental concerns. Many predict that wind energy will provide more U.S. electrical production as new turbine designs enhance economic and environmental viability.
Geothermal Energy
Geothermal energy comes from intense heat within the Earth, which also produces hot springs, geysers, and volcanoes.
Geothermal resources are found where the Earth's crust is relatively thin. The only widely used type of geothermal energy is hydrothermal, produced when subsurface water contacts hot rock and turns to steam, which is piped to the surface. In some cases, water or steam is used directly to heat homes or provide process heat for businesses. In a typical geothermal electric plant, steam is piped to a turbine to power an electrical generator.
Geothermal development has disadvantages, particularly the hydrogen sulfide gas emitted during extraction. Many of the same environmental concerns surrounding exploitation of oil and gas may also impact the development of geothermal resources, which must be similarly drilled and piped to the point of use. The advantage to geothermal energy, however, is that it does not produce pollution when used.
Biomass and Biofuels
Biomass is any modern organic matter used as an energy source. The most common examples are wood, bioenergy crops, and organic wastes such as agricultural residues. Unlike other renewable energy sources, biomass can be burned or converted directly into liquid biofuels.
All biomass is solar energy transformed through photosynthesis. Biomass energy is usually released by burning, and less often by bacterial decay and fermentation. If vegetation is regrown as biomass is used, the net release of carbon dioxide due to the burning of biomass is zero.
Today, wood stoves are used world-wide for heating and cooking, making biomass one of the most common energy resources. Biopower is the burning of biomass to generate electricity. Waste-to- energy biopower plants use organic garbage as a feedstock, which reduces the amount of waste entering landfills.
Toxic substances may enter the atmosphere when municipal waste is incinerated, so contaminants should be removed for treatment before waste incineration. As landfill sites become harder to find, waste-to-energy plants may be an increasingly attractive option.
Biofuels
Alternative fuels offer another application for biomass technology. Crops can be fermented to produce liquid biofuels, the most common of which are ethanol and methanol. Today these alcohols are relatively high-cost, and oil prices would have to double to make them a cost-effective alternative. But gasohol, a mixture of just 10 percent ethanol and 90 percent gasoline, is highly cost-competitive and can be used in a traditional gasoline engine. It also has higher octane than gasoline and is far cleaner-burning. The air pollution savings from the increased use of ethanol and/or gasohol could be significant.
Scientists debate the consequences and benefits of genetically modified crops and forests that are managed for biomass resources.
Biogas
Biogas is methane produced from animal waste and by the decay of organic garbage. Because of current natural gas prices, biogas is usually flared as waste. More productive uses include onsite burning of biogas for heating of livestock barns and greenhouses.
Most experts agree that with some additional guidelines and new technologies, biomass can be part of a "greener" U.S. power portfolio.
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