Renewable energy is energy generated from natural resources such as water, sunlight, wind, rain, tides, geothermal sources and biomass sources as energy crops. Renewable energy sources are energy sources that are continually and naturally replenished in a short period of time. 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 world. 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.
On the other side 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.
The 7 renewable energy sources used most often are:
- Water – Hydropower
- Biomass — including wood and wood waste, municipal solid waste, landfill gas, and biogas, ethanol, and biodiesel
- Tidal Energy – using the ocean waves and currents to produce energy.
Most renewable energy comes either directly or indirectly from the sun. Sunlight, or solar energy, can be used directly for heating and lighting homes and other buildings, for generating electricity, and for hot water heating, solar cooling, and a variety of commercial and industrial uses.
In this context, “solar energy” refers to energy that is collected from sunlight. Solar energy can be applied in many ways, including to:
- Generate electricity using photovoltaic solar cells.
- Generate electricity using concentrating solar power.
- Generate electricity by heating trapped air which rotates turbines in a Solar updraft tower.
- Generate hydrogen using photoelectrochemical cells.
- Heat water or air for domestic hot water and space heating needs using solar-thermal panels.
- Heat buildings, directly, through passive solar building design.
- Heat foodstuffs, through solar ovens.
- Solar air conditioning
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
The parabolic troughs that make up this concentrating solar power system generate power from the sun on a large scale in California.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.
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.
The sun’s heat also drives the winds, whose energy is captured with wind turbines. Then, the winds and the sun’s heat cause water to evaporate. When this water vapor turns into rain or snow and flows downhill into rivers or streams, its energy can be captured using hydropower.
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.
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.
Along with the rain and snow, sunlight causes plants to grow. The organic matter that makes up those plants is known as biomass. Biomass can be used to produce electricity, transportation fuels, or chemicals. The use of biomass for any of these purposes is called biomass energy.
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.
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.
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.
Hydrogen also can be found in many organic compounds, as well as water. It’s the most abundant element on the Earth. But it doesn’t occur naturally as a gas. It’s always combined with other elements, such as with oxygen to make water. Once separated from another element, hydrogen can be burned as a fuel or converted into electricity.
Fill vehicle fuel tanks with it instead of gasoline. Pipe it to homes to generate electricity onsite, while providing heating and hot water, instead of sending electricity through transmission lines. And emit only water vapor where it is used.
Hydrogen offers great opportunities. Fuel cells that electrochemically combine hydrogen and oxygen to produce electricity and heat offer the promise of making hydrogen an ideal universal fuel. Make that an ideal energy carrier rather than a fuel, because while hydrogen does grow on trees and fall with the rain, it does not occur naturally by itself. It cannot be mined or harvested. But other energy sources can be used to make hydrogen, and then the hydrogen transported or stored for use where and when needed.
Most hydrogen production today is by steam reforming natural gas. But natural gas is already a good fuel and one that is rapidly becoming scarcer and more expensive. It is also a fossil fuel, so the carbon dioxide released in the reformation process adds to the greenhouse effect. Hydrogen has very high energy for its weight, but very low energy for its volume, so new technology is needed to store and transport it. And fuel cell technology is still in early development, needing improvements in efficiency and durability. The challenges NREL researchers are working on to help make a hydrogen economy a reality include:
- Fuel Cells — Improving fuel cell technology and materials needed for fuel cells.
- Production — Developing technology to efficiently and cost-effectively make hydrogen from renewable energy sources.
- Storage — Developing technology to efficiently and cost-effectively store and transport hydrogen.
For more basic information on hydrogen, see the National Renewable Energy Laboratory 2003 Research Review article “New Horizons for Hydrogen.” (PDF 1.1 MB).
Not all renewable energy resources come from the sun. Geothermal energy taps the Earth’s internal heat for a variety of uses, including electric power production, and the heating and cooling of buildings. And the energy of the ocean’s tides comes from the gravitational pull of the moon and the sun upon the Earth.
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 Estimated subterranean temperatures at a depth of 6 kilometers.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.
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.
There is also the potential to generate geothermal energy from hot dry rocks. Holes at least 3 km deep are drilled into the earth. Some of these holes pump water into the earth, while other holes pump hot water out. The heat resource consists of hot underground radiogenic granite rocks, which heat up when there is enough sediment between the rock and the earths surface. Several companies in Australia are exploring this technology.
The ocean can produce thermal energy from the sun’s heat and mechanical energy from the tides and waves. NREL does not conduct research in ocean thermal energy or ocean mechanical energy.
The world’s ocean may eventually provide us with energy to power our homes and businesses. Right now, there are very few ocean energy power plants and most are fairly small. But how can we get energy from the ocean?
There are three basic ways to tap the ocean for its energy. We can use the ocean’s waves, we can use the ocean’s high and low tides, or we can use temperature differences in the water. Let’s take a look at each.
Kinetic energy (movement) exists in the moving waves of the ocean. That energy can be used to power a turbine. In this simple example, to the right, the wave rises into a chamber. The rising water forces the air out of the chamber. The moving air spins a turbine which can turn a generator.
When the wave goes down, air flows through the turbine and back into the chamber through doors that are normally closed.
This is only one type of wave-energy system. Others actually use the up and down motion of the wave to power a piston that moves up and down inside a cylinder. That piston can also turn a generator.
Most wave-energy systems are very small. But, they can be used to power a warning buoy or a small light house.
Another form of ocean energy is called tidal energy. When tides come into the shore, they can be trapped in reservoirs behind dams. Then when the tide drops, the water behind the dam can be let out just like in a regular hydroelectric power plant.
Tidal energy has been used since about the 11th Century, when small dams were built along ocean estuaries and small streams. the tidal water behind these dams was used to turn water wheels to mill grains.
In order for tidal energy to work well, you need large increases in tides. An increase of at least 16 feet between low tide and high tide is needed. There are only a few places where this tide change occurs around the earth. Some power plants are already operating using this idea. One plant in France makes enough energy from tides (240 megawatts) to power 240,000 homes.
This facility is called the La Rance Station in France. It began making electricity in 1966. It produces about one fifth of a regular nuclear or coal-fired power plant. It is more than 10 times the power of the next largest tidal station in the world, the 17 megawatt Canadian Annapolis station.
Ocean Thermal Energy Conversion (OTEC)
The idea is not new. Using the temperature of water to make energy actually dates back to 1881 when a French Engineer by the name of Jacques D’Arsonval first thought of OTEC. The final ocean energy idea uses temperature differences in the ocean. If you ever went swimming in the ocean and dove deep below the surface, you would have noticed that the water gets colder the deeper you go. It’s warmer on the surface because sunlight warms the water. But below the surface, the ocean gets very cold. That’s why scuba divers wear wet suits when they dive down deep. Their wet suits trapped their body heat to keep them warm.
Power plants can be built that use this difference in temperature to make energy. A difference of at least 38 degrees Fahrenheit is needed between the warmer surface water and the colder deep ocean water.
Using this type of energy source is called Ocean Thermal Energy Conversion or OTEC. It is being demonstrated in Hawaii. More info on OTEC can be found on the archive pages for the Natural Energy Laboratory of Hawaii at: www.hawaii.gov/dbedt/ert/otec-nelha/otec.html
Flowing water creates energy that can be captured and turned into electricity. This is called hydroelectric power or hydropower. NREL doesn’t perform any research in hydroelectric power technologies.
Hydropower is using water to power machinery or make electricity. Water constantly moves through a vast global cycle, evaporating from lakes and oceans, forming clouds, precipitating as rain or snow, and then flowing back down to the ocean. The energy of this water cycle, which is driven by the sun, can be tapped to produce electricity or for mechanical tasks like grinding grain. Hydropower uses a fuel—water—that is not reduced or used up in the process. Because the water cycle is an endless, constantly recharging system, hydropower is considered a renewable energy.
The Water (Hydrologic) Cycle
When flowing water is captured and turned into electricity, it is called hydroelectric power or hydropower. There are several types of hydroelectric facilities; they are all powered by the kinetic energy of flowing water as it moves downstream. Turbines and generators convert the energy into electricity, which is then fed into the electrical grid to be used in homes, businesses, and by industry.
Why Don’t We Use More Renewable Energy?
In the past, renewable energy has generally been more expensive to produce and use than fossil fuels. Renewable resources are often located in remote areas, and it is expensive to build power lines to the cities where the electricity they produce is needed. The use of renewable sources is also limited by the fact that they are not always available — cloudy days reduce solar power; calm days reduce wind power; and droughts reduce the water available for hydropower.
The production and use of renewable fuels has grown more quickly in recent years as a result of higher prices for oil and natural gas, and a number of State and Federal Government incentives, including the Energy Policy Acts of 2002 and 2005. The use of renewable fuels is expected to continue to grow over the next 30 years, although we will still rely on non-renewable fuels to meet most of our energy needs.