Renewable Source Overview

Renewable energy sources are energy sources that are continually replenished. These include energy from water, wind, the sun, geothermal sources, and biomass sources such as energy crops. In contrast, fuels such as coal, oil, and natural gas are non-renewable. Once a deposit of these fuels is depleted it cannot be replenished – a replacement deposit must be found instead. Both renewable and non-renewable energy sources are used to generate electricity, power vehicles, and provide heating, cooling, and light.

Renewable sources of energy vary widely in their cost-effectiveness and in their availability across the United States. Although water, wind, and other renewables may appear free, their cost comes in collecting, harnessing, and transporting the energy so that it can do useful work. For example, to utilize energy from water, a dam must be built along with electric generators and transmission lines.

Renewables themselves are non-polluting, while the structures built to harness them can have positive or negative environmental impacts. For example, dams may affect fish migration but may also create wildlife habitat.

An impoundment hydropower plant dams water in a reservoir.

Hydropower refers to using water to generate electricity. Water is the most common renewable source of energy in the United States today.

Many hydroelectric power plants use a dam on a river to store water. Water released from behind the dam flows through a turbine, spinning it, which then turns a generator to produce electricity. Electricity generated this way is known as hydroelectricity, and it accounts for about 7% of the electricity used by the nation. Hydroelectric power doesn't necessarily require a large dam – some hydroelectric power plants just use a small canal to channel the river water through a turbine. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, or ranch.

The Tazimina project in Alaska is an example of a diversion hydropower plant. No dam was required.

Dam sites for hydropower plants are limited both by available rivers and by competing uses for those rivers, such as recreation, tourism, industry, and human settlements. Because of such limitations, water power could never generate all the electricity used in the United States. In addition, environmental impacts are considered when locating dams.

While all hydroelectric dams have some environmental impact, the impacts vary widely, and current regulations and policies attempt to address environmental concerns. A dam may either create a reservoir or may be a run-of-river project that does not store large amounts of water but simply takes advantage of a river's natural flow.

Fish ladder.

A dam that creates a reservoir may flood a large area upstream, and can change flow patterns and impact flooding downstream with resulting environmental consequences, either positive or negative. Fish migration, which has long been a concern associated with dams, is often addressed with fish ladders and other structures to ensure the successful movement of fish both upstream and downstream.

In addition to power, dams often provide other benefits such as recreation opportunities on upstream reservoirs, habitat for a wide variety of aquatic and terrestrial species, diversion of water for irrigation, and control of destructive flooding and environmental damage downstream.

Hydropower is one of the least expensive sources of electricity and areas with good sources of hydropower tend to attract industries with large needs for electricity. Major hydroelectric dams in the United States are found in the Northwest, the Tennessee Valley, and on the Colorado River.

Click image for large view.

Existing hydroelectric plants (yellow) and potential high head/low power energy sites (orange) in the conterminous United States. Purple represents areas excluded from hydropower development due to Federal statutes and policies.

Turbines at Martinsdale Hutterite Colony.

For hundreds of years, humans have used wind to pump water or grind grain, usually with small windmills. Large, modern wind turbines are used to generate electricity, either for individual use or for contribution to a utility power grid. Wind turbines usually have two or three blades and, because winds above the ground tend to be faster and less turbulent than those near the surface, the turbines are mounted on tall towers to capture the most energy. As the blades turn, the central shaft spins a generator to make electricity.

In recent years, wind has become an increasingly attractive source of renewable energy – wind energy is the world's fastest-growing energy technology. Wind turbines placed at sites with strong, steady winds can economically generate electricity without producing pollutants. The power in wind increases rapidly with its speed, which means that locating windmills in areas of strong winds is critical. The strongest winds in the United States tend to be in Alaska, the western United States, and the Appalachians. Wind power currently supplies about 1% of United States electricity needs, but capacity is expanding rapidly. Although wind will contribute more to the United States electric supply in the future, like hydropower it cannot be expected to supply all of our electric needs.

United States wind resource map.

While wind power helps the environment by producing electricity without producing pollution, there can be negative environmental impacts of wind power generation, including wildlife deaths. However, recent studies suggest that the number of birds and bats killed by collision with wind turbines is far lower than the number killed by collisions with other tall structures such as buildings. Appropriate siting of wind farms and individual turbines can reduce the impact on wildlife. Noise, which was a problem with older turbine designs, has mostly been eliminated through improved engineering.

United States map showing annual average daily solar radiation per month

Annual average daily solar radiation per month, using a flat-plate collector facing south at a fixed tilt equal to the latitude of the site. Capturing the maximum amount of solar radiation throughout the year can be achieved using a tilt angle approximately equal to the site's latitude.

Solar technologies use the sun's energy to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and industry. Despite sunlight's significant potential for supplying energy, solar power provides less than 1% of U.S. energy needs. This percentage is expected to increase with the development of new and more efficient solar technologies.

Different types of solar collectors are used to meet different energy needs. Passive solar building designs capture the sun's heat to provide space heating and light. Photovoltaic cells convert sunlight directly to electricity. Concentrating solar power systems focus sunlight with mirrors to create a high-intensity heat source, which then produces steam or mechanical power to run a generator that creates electricity. Flat-plate collectors absorb the sun's heat directly into water or other fluids to provide hot water or space heating. And solar process heating and cooling systems use specialized solar collectors and chemical processes to meet large-scale hot water and heating and cooling needs.

Solar technologies produce few negative environmental impacts during collector operation. However, there are environmental concerns associated with the production of collectors and storage devices. In addition, cost is a great drawback to solar power. Although sunlight is free, solar cells and the equipment needed to convert their direct-current output to alternating current for use in a house is expensive. Electricity generated by solar cells is still more than twice as expensive as electricity from fossil fuels. Part of the problem with cost is that solar cells can only operate during daylight hours. In contrast, a coal or natural gas plant can run around the clock, which means the cost for building the plant can be spread over many more hours of use.

The parabolic troughs that make up this concentrating solar power system generate power from the sun on a large scale in California.

The parabolic troughs that make up this concentrating solar power system generate power from the sun on a large scale in California.

Around the United States, available sunlight varies considerably as a result of differences in cloud cover and latitude, and also varies with the seasons. In the summer, longer daylight hours and a higher sun angle provide more solar power, compared to the winter when the sun is up for fewer hours and at a lower position in the sky. These variations must be taken into consideration when planning solar collection facilities.

Geothermal power plant at The Geysers, California.

Geothermal power uses the natural sources of heat inside the Earth to produce heat or electricity. Currently, most geothermal power is generated using steam or hot water from underground. Geothermal power generation produces few emissions and the power source is continuously available.

There are three geothermal technologies currently in use in the United States: direct-use systems, use of deep reservoirs to generate electricity, and geothermal heat pumps.

In direct-use geothermal systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. The water is brought up through the well, and a mechanical system—piping, a heat exchanger, and controls—delivers the heat directly for its intended use. A disposal system then either injects the cooled water underground or disposes of it in a surface storage pond. Geothermal hot water is used for heating buildings, raising plants in greenhouses, drying crops, heating water for fish farms, or for industrial processes, at hundreds of sites around the country. Geothermal reservoirs appropriate for direct-use systems are widespread throughout the western United States.

Geothermal power plants convert hydrothermal fluids (hot water or steam) to electricity. The oldest type of geothermal power plant uses steam, accessed through deep wells, to directly drive a turbine to produce electricity. Flash steam plants are the most common type of geothermal power plants in operation today. They use extremely hot water (above 300 degrees F (149 degrees C)), which is pumped under high pressure to the generation equipment at the surface. The hot water is vaporized and the vapor in turn drives turbines to generate electricity. Binary-cycle geothermal power plants use moderate-temperature water (100-300 degrees F (38-149 degrees C)). The water is used to vaporize a second fluid that has a much lower boiling point than water. The vapor from this second fluid is then used to drive the turbines to produce electricity. California, Hawaii, Nevada, and Utah currently have operating geothermal power plants.

Estimated subterranean temperatures at a depth of 6 kilometers.

Geothermal heat pumps are used for space heating and cooling as well as water heating, for residential and commercial applications. The technology relies on the fact that beneath the surface, the Earth remains at a relatively constant temperature throughout the year, warmer than the air above it during the winter and cooler in the summer. A geothermal heat pump takes advantage of this by transferring heat, stored in the ground, into a building during the winter, and transferring it out of the building and back into the ground during the summer. The heat pump consists of a series of pipes, buried in the ground near a building to be conditioned or where water is to be heated. Fluid is circulated through the pipes to either absorb heat from the ground or distribute heat to the ground. Geothermal heat pumps can be used in most areas of the United States.

While geothermal energy use is efficient, reliable, and environmentally friendly, it currently meets less than 1% of U.S. power needs.

McNeil Generating Station, Burlington, Vermont, the country's only utility-owned and operated wood-fired power plant.

Biomass power is power obtained from the energy in plants and plant-derived materials, such as food crops, grassy and woody plants, residues from agriculture or forestry, and the organic component of municipal and industrial wastes. Biomass power provides two valuable services: it is the second most important source of renewable energy in the United States and it is an important part of our waste management infrastructure. In the future, farms cultivating high-yielding energy crops (such as trees and grasses) will significantly expand our supply of biomass. These energy crops, coupled with high-efficiency conversion technologies, can supplement our consumption of fossil fuels and help us respond to global climate change concerns.

Wood has been used for energy longer than any other biomass source and today is still the largest biomass energy resource. The largest source of energy from wood is pulping liquor or "black liquor," a waste product from processes of the pulp, paper, and paperboard industry. Biomass energy can also be derived from waste and from alcohol fuels. Waste energy is the second-largest source of biomass energy. The main contributors of waste energy are municipal solid waste, manufacturing waste, and landfill gas.

Biomass can be used for direct heating (such as burning wood in a fireplace or wood stove), for generating electricity, or can be converted directly into liquid fuels to meet transportation energy needs.

Truck unloading wood chips that will fuel the Tracy Biomas Plant, Tracy, California.

Electricity generated from biomass is also called biopower. Biopower facilities use many different technologies; the most common is burning of wood or other biomass feedstocks to produce steam which then is used to drive turbines and produce electricity. Some generators use a mix of biomass and fossil fuels to generate electricity, while others burn methane, a product of the natural decay of organic materials. In the United States, the pulp and paper industries are major producers of biopower, using residues from paper production to produce electricity for industrial plant use.

Biomass power is close to a carbon-neutral electric power generation option — biomass absorbs carbon dioxide from the atmosphere during its growth and then emits an equal amount of carbon dioxide when it is processed to generate electricity. Thus, biomass fuels "recycle" atmospheric carbon, and may reduce global warming impacts. Biopower facilities produce fewer other pollutants than equivalent fossil fuel power facilities.

Biofuels are liquid fuels produced from plants. The two most common types of biofuels are ethanol and biodiesel. Ethanol is an alcohol, the same as in beer and wine. It is made by fermenting any biomass high in carbohydrates through a process similar to beer brewing. The majority of ethanol produced in the United States is made from corn. Current research is exploring ways to efficiently convert cellulose (agricultural waste, forest residue, municipal solid waste, and energy crops) to ethanol. Ethanol is mostly used as a fuel additive for vehicles to increase octane and cut down carbon monoxide and other smog-causing emissions. Biodiesel is made by processing vegetable oil, animal fat, or recycled cooking grease with alcohol or other chemicals. It can be used as an additive (typically 20%) to reduce vehicle emissions or in its pure form as a renewable alternative fuel for diesel engines.

Biomass and biofuels resource potential in the conterminous United States.

Because biomass power is produced from plant sources, it can potentially be produced almost anywhere in the United States.

While biomass is a renewable energy resource, it can have both negative and positive environmental impacts. It may reduce emissions and pollutants, but factory farming of biomass crops can reduce biodiversity and negatively impact wildlife habitat. Municipal solid waste may contain toxins which could cause pollution if it is used as a biomass feedstock. As with other renewable resources, use of appropriate technology will promote the most positive environmental impacts.

Source: U.S. National Atlas