What are renewable sources of energy?

Renewable sources of energy can be replenished or renewed on a relatively short time scale. For example, we can get energy from burning wood, and we can plant trees to supply new wood within a few years. Non-renewable sources of energy can't be renewed on a human timescale. For example, coal takes thousands or millions of years to form. Once it's gone, it's gone!

The five most used forms of renewable energy are:

Biomass — organic material from plants and animals, such as wood and ethanol — can be burned to release energy in the form of heat.

Hydropower — moving water — can be harnessed to give mechanical energy or electricity.

Geothermal energy — heat from within the earth — can be used to heat buildings or generate electricity.

Wind can be harnessed to give mechanical energy or electricity.

Solar energy — radiation from the Sun — can be used directly for heating, or it can be converted to electricity.

Get accessible information about renewable energy: Visit the US Department of Energy to read quick overviews of renewable energy in its many forms. These provide the right level of detail for CSET Science I. Click on the links, read the material, and return for additional details and illustrations, an overview of renewable energy sources, and some recent statistics.

Advantages and disadvantages of renewable energy

Renewable energy has some big advantages over non-renewable energy. The use of renewable sources produces little or no waste products such as carbon dioxide, a greenhouse gas. And by definition, the source of energy can be renewed, so we don’t have to worry about its running out.

On the other hand, the use of renewable energy may have some disadvantages compared to the use of nonrenewable sources such as fossil fuels.

Many forms of renewable energy, such as solar energy and wind energy, depend on the weather. Windmills only turn and generate electricity when the wind blows at a speed of at least 13 miles per hour; solar cells function best under clear skies.
Renewable sources tend to produce energy at lower rates than fossil fuels.
Finally, the main reason renewable sources are not used as much as nonrenewable sources are at this time is that fossil fuels and nuclear energy are still relatively inexpensive. The technology to harness energy from the sun and other renewable sources is still new and expensive.

The five different types of renewable energy listed above also have their individual adverse environmental impacts. For example, wind farms (arrays of windmills harnessing the wind) can pose a problem to migrating birds, and hydroelectric dams can impede the migration of salmon.

Sustainability

The topic of renewable energy often comes up in the context of sustainability. Sustainability is meeting our present needs while preserving resources for future generations. To grow food sustainably, for example, might mean to grow enough for the present time without depleting the soil, polluting water that runs off the farm, overusing groundwater, harming pollinators through pesticide use, or exposing workers and consumers to dangerous chemicals.

To determine if an action, product, or service is sustainable, we consider its effects in relation to three components: the needs of people, the protection of the environment, and the health of the economy.

Renewable energy is generally sustainable because it does not deplete finite resources such as fossil fuels and contributes little to the emission of greenhouse gasses.

Energy efficiency is generally considered to be an aspect of the sustainable use of energy because it prolongs the use of a resource and reduces waste.

Renewable energy in California

In 2007, 11.8% of all California's energy came from renewable sources, including wind, solar, geothermal, biomass, and small hydroelectric plants. This figure does not include large hydroelectric plants, which are sometimes classified separately from renewable resources because of their environmental impacts (e.g., on salmon that cannot spawn because their upstream trips are blocked by dams). Large hydroelectric plants contributed 11.7% of California's energy that year.

Hydropower

Hydroelectric power is one of the most widely used forms of renewable energy worldwide. About 20% of global electricity production comes from hydroelectric power, mostly from large dams. Some famous dams and associated hydroelectric power projects you should have heard about include the following:

The Three Gorges Dam in China is partially operational now, and expected to be fully operational in 2011. It will control the Yangtze River and supply 22,500 MW, more than any other in the world. (The Grand Coulee Dam in Washington, the largest in the United States, provides 6,500 MW.)

Three Gorges Dam on the Yangtze River Source: Wikipedia

The Itaipu Dam, on the border of Brazil and Paraguay, has a total capacity of 12,600 MW (enough to power most of California all by itself).

The Itaipu Dam on the Parana River. Source: Itaipu Binacional

The Grand Coulee Dam in Washington is the biggest dam in the United States and the third largest hydroelectric facility in the world.

The Grand Coulee Dam on the Columbia River. Source: Photo courtesy of Bureau of Reclamation

Hoover Dam was, at the time of its construction in 1936, by far the largest dam in the world and the most challenging to build. It is still one of the top five hydroelectric power plants in the United States.

Hoover Dam on the Colorado River. Source: Photo courtesy of Bureau of Reclamation

The Aswan High Dam was built to control flooding on the Nile. The dam (which is actually the second and more modern dam, upstream from an older one) was funded by the Soviet Union, which also provided design engineers. Construction began in 1960 and ended in 1970. At the time of construction, the dam provided about half of Egypt’s electricity.

Photograph of the Aswan High Dam taken by astronauts aboard the International Space Station. Source: NA

A newer form of hydroelectric energy is from small-scale projects, supplying less than 10-30 MW (depending on local definitions of "small"). Small-scale projects usually do not completely dam a river. They can often avoid the environmental problems of large-scale projects, such as interfering with salmon runs, and are also less expensive to build and maintain.

Learn more:

Hydroelectric power in California

California gets about 16% of its electricity from hydroelectric power plants. That’s more than the national average of about 10%.

For more on hydroelectricity in California, see California Energy Commission.

Oroville Dam on the Feather River, Butte County. Under California laws, only small hydroelectric projects, producing less than 30 MW of electricity, are classified as renewable. Source: California Department of Water Resources

Geothermal energy

For a quick review of geothermal energy, read this first.

California has the largest geothermal energy generating capacity in the United States. The generating plants are mostly in Napa and Sonoma counties, in Northern Caliifornia.

An area called "the Geysers," north of San Francisco, has been generating electricity since the 1960s. The city of San Bernardino heats at least three dozen buildings, including city hall, directly from geothermal sources. (Who knew San Bernardino could be so interesting!)

Learn more about geothermal energy in California.

Hot Creek in east-central California, one of the state's many natural hot springs. Source: USGS

Solar energy

For a quick review of solar energy, read this.

California's legislature has encouraged homeowners and businesses to take advantage of small-scale solar heating and energy generation through tax incentives, and has contributed to the construction of large-scale pilot solar energy projects to demonstrate the commercial potential of new designs and technology.

Learn more about the large-scale demonstration projects, including Solar One/Solar Two, here,
here, and here.
Fun fact: Starting in 2008, home builders were required to offer solar panels as options for new homes in California. This applies to subdivisions of 25 homes or more.

Parabolic solar collector in California's Mojave Desert. Source: Wikipedia

Wind energy

For a quick review of wind energy, read this first.

You can also read an illustrated history of wind power development here.

California was the first state to develop large wind farms, and is a world leader in the development of wind energy. In the mid-1980s, California wind farms produced about 90% of all the wind-generated electricity worldwide. The wind farms that are operational today are located on mountain passes, which act like natural wind tunnels, channeling the movement of air.
For more on wind energy in California, go here, and here.

An aerial view of some of the wind turbines in the San Gorgonio Pass. This wind farm, a familiar sight from the 10 freeway, has more than 4,000 individual windmills and provides enough electricity for Palm Springs and the Coachella Valley. The photo shows the turbines and the access roads to service them. Source: NASA

Energy from the Ocean

For a quick review of energy from the ocean, read this first.

Some countries are just beginning to make use of energy from ocean waves, one of the forms of ocean-related energy generation. A Scottish company built the world's first commercial "wave farm" a few miles off the coast of Portugal. The farm officially became operational in late September 2008. Read more about the AguÃ§adoura Wave Park.

California's long coastline and surfer-pleasing waves offer good potential for the development of ocean wave energy conversion facilities. Estimates are that waves could produce 17 MW per miles of coastline. Some economic feasibility studies have been performed, and in December, 2008, a California-based company demonstrated its technology in Monterey Bay. Learn more.

Read more about ocean energy in California.

Ethanol

For a quick review of ethanol, read this first.

As of October 2008, California has five operating ethanol plants. Most of the corn to supply the plants arrives by train from the Midwest, although some California-grown corn may be used as well.

Gasoline in California currently contains 6% ethanol, but that percentage is supposed to rise to 10% in 2010.

Read more about ethanol.

Biomass energy

For a quick review of biomass energy, read this first.

You may be surprised to learn that there are at least 36 biomass-fueled power plants in California. Fuels burned include wood, lumber mill waste, agricultural waste, and urban wood waste. An industry group, the California Biomass Energy Alliance, says its member companies have 700 MW of generating capacity. Learn more.

Biodiesel fuel

For a quick review of biodiesel fuel, read this first.

The California Energy Commission says that more than 4 million gallons of diesel fuel are used each year in California, in several national parks, in the city of Berkeley, and at military installations. The US Navy is the largest consumer of biodiesel in the world!

To buy biodiesel, you need to be a member of a biodiesel user's group.
If you're curious, you can see where to buy biodiesel.