Renewable Energy Sources

What is Renewable Energy?
Renewable energy resources are virtually inexhaustible in duration but limited in the amount of energy that is available per unit of time. Renewable energy resources include biomass, hydropower, geothermal, solar, wind, ocean thermal, wave action, and tidal action. (The last three are not discussed in this brochure because there were no commercial operations using these resources in the United States in 2004.) In 1850, about 90% of the energy consumed in the United States was from renewable energy resources. Now the United States is heavily reliant on the non-renewable fossil fuels: coal, natural gas, and oil. In 2004, about 6% of all energy consumed and about 9% of total electricity production was from renewable energy resources.

How Is Renewable Energy Used?
Renewable energy is used for electricity generation, heat in industrial processes, heating and cooling buildings, and transportation fuels. In 2004, electricity generation accounted for about 70% of total renewable energy consumption. The total amount of electricity generated from renewable energy was about 359 billion kilowatthours (kWh), about 9% of total U.S. electricity generation. Industrial process heat and building space heating accounted for 25% of renewable energy use and the remainder was used as vehicle fuels.



What Are The Different Types of Renewable Energy?

Biomass Energy
Biomass energy is produced from non-fossilized materials derived from plants. Wood and wood waste are the largest sources of biomass energy followed by energy from municipal solid waste (MSW) and alcohol fuels. In 2004, biomass accounted for 47% of renewable energy consumption, with about 50% of this used for heating, 40% for electrical power production, and the rest as transportation fuel.

1.Wood – Wood biomass includes wood chips from forestry operations, residues from lumber, pulp/paper, and furniture mills, and fuel wood for space heating. The largest single source of wood energy is “black liquor,” a residue of pulp, paper, and paperboard production. It supplies over 50% of these industries’ energy requirements. Lumber mills and furniture manufacturers use chips, sawdust and bark for nearly 60% of their energy requirements. A small but growing amount of wood is co-fired with coal in utility power plants. Cordwood, wood chips, and pellets made from sawdust are used for space and water heating in buildings, including in over two million households as primary or supplemental heating fuels.

2.Municipal Solid Waste and Biogas – Waste-to-energy facilities burned 29 million tons of MSW in 2004 to produce heat and electricity. There are also about 380 landfills that recover methane, which forms as waste decomposes in low-oxygen (anaerobic) conditions. The methane is burned to produce electricity and heat. Methane is also produced in anaerobic “digesters” for heat and electricity generation at municipal sewage treatment facilities, concentrated livestock operations, and dairy farms.

3.Biofuels – Biofuels include alcohol fuels, such as ethanol, and “biodiesel,” a fuel made from grain oils and animal fats. Most biofuel used in the United States is fuel ethanol produced from corn. Nearly all fuel ethanol is consumed as a gasoline oxygenate in oxygenated or reformulated gasoline or as a gasoline enhancer in fuels such as “gasohol” (also known as E-10, a 10% ethanol- 90% gasoline fuel). In 2004, total consumption of fuel ethanol was about 3.5 billion gallons, the equivalent of about 2.3 billion gallons of gasoline on an energy content basis, or the amount of gasoline consumed in 4.3 million passenger cars. Biodiesel use is small but its use in diesel engines and for heating is increasing.

Type of Renewable Energy Consumption by Sector, 2004 (Percent)

Hydropower
Hydropower is electricity produced from flowing water. As a result, hydropower output varies widely according to rainfall. Most hydropower is produced at large facilities built by the Federal Government, such as Grand Coulee Dam on the Columbia River in Washington State - the largest single electric power facility in the United States. Most of the largest dams are located on rivers in the western United States, but there are numerous smaller facilities operating around the country. Hydropower production varies from year to year, depending on precipitation. In 2004, hydropower dams produced 270 billion kWh, which was about 7% of total U.S. electricity production and accounted for about 45% of total renewable energy consumption. The newest “wave” in hydropower technologies is being developed to harness the energy in ocean tides, waves, and currents.


Geothermal Energy
Geothermal energy is energy from the hot interior of the earth. Fissures in the earth’s crust allow water heated by geothermal energy to rise naturally to the surface at hot springs and geysers. Wells drilled into the earth allow heated steam or water to escape to the surface in a controlled manner to operate steam turbines and electricity generators. In a different type of application, the temperature of the earth or ground water relatively near the earth’s surface is used as a heat source and sink for “geothermal” heat pumps that heat and cool buildings. Geothermal energy accounted for about 6% of total renewable energy consumption in 2004, with about 89% used to produce electricity. Geothermal power plants in California, Hawaii, Nevada, and Utah generated approximately 14.4 billion kWh of electricity in 2004. Hot springs have long been used at resorts and spas, and for heat in buildings, greenhouses and aquaculture facilities, and in industrial processes. There are about 500,000 geothermal heat pumps in the United States, and the number is increasing by around 37,000 per year.

Wind Energy
Water pumping windmills and small wind electric generators were once used throughout the United States. Rural electrification programs of the 1930’s and 1940’s largely replaced the need for these systems. Starting in the early 1980’s, Federal and State Government policies and incentives led to a revival in wind power generation.

Solar Energy
Solar energy systems use solar radiation to produce heat and electricity. In 2004, 90% of solar energy consumed was used for heat. The other 10% was used to produce about 580 million kWh of electricity. The three basic categories of solar systems are discussed below.

1.Solar Thermal Systems for Heating Buildings and Water – Solar thermal systems use solar collectors to absorb solar radiation to heat water or air for space and water heating. Between 1975 and 1985, sales of solar thermal energy collectors grew dramatically due to Federal and State income tax credits for the installation of solar energy equipment. When the Federal tax credits ended in 1985, sales dropped. In the past few years, however, there has been an increase in sales of relatively low-cost collectors for heating swimming pools.

2.Solar Thermal-Electric Power Plants – Solar thermal-electric power plants use concentrating solar collectors to focus the sun’s rays to heat fluid to a high temperature. This working fluid can then be used to generate steam to operate a turbine, which is then used to produce electricity in a generator. The three types of solar-thermal power systems deployed or developed in the United States are parabolic trough, solar dish, and solar power towers. The parabolic trough is used in the largest solar power facility in the world located in the Mojave Desert at Kramer Junction, California. This facility has operated since the 1980’s and accounted for the majority of solar electricity produced by the electric power sector in 2004. DOE and industry partners built and successfully operated a demonstration solar power tower near Barstow, California, during the 1980’s and 1990’s. Solar dish technologies have been developed but are still not fully commercialized.

3.Photovoltaic Systems – Photovoltaic (PV) systems are based on solar electric cells, which convert solar radiation directly into electricity. Individual PV cells are configured into modules of varying electricity producing capacities. PV applications range from single solar cells for powering watches to large installations with hundreds of modules for electric power production. Until a few years ago, most PV systems were installed where utility power line extensions or the use of fossil fuel generators was technically or financially infeasible. Financial incentives in several States have led to the installation of these systems on houses and buildings that are connected to electric utility power lines. These “grid-connected” systems are now a major application of PV in the United States.