New and diverse technologies are harnessing sunlight, wind, water and biomass, converting their raw power into usable energy. From on-site systems that run homes, schools and businesses, to personal power products that give people the flexibility to unplug indefinitely, renewable energy is transforming how we live and work in coexistence with the environment.
There are 5 main types of modern sources of renewable energy:
SOLAR ENERGY :: FUN WITH THE SUN
Solar cells, also called photovoltaics (PV) convert sunlight directly into electricity. Solar cells are often used to power calculators and watches. They are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic (PV) effect.
Traditionally, solar cells are typically combined into modules that hold about 40 cells; about 10 of these modules are mounted in PV arrays that can measure up to several meters on a side. These flat-plate PV arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day. About 10 to 20 PV arrays can provide enough power for a household; for large electric utility or industrial applications, hundreds of arrays can be interconnected to form a single, large PV system.
Thin film solar cells use layers of semiconductor materials only a few micrometers thick. Thin film technology has made it possible for solar cells to now double as rooftop shingles, roof tiles, building facades, or the glazing for skylights or atria, or products such as the Juice Bags or Powerpockets.
Some solar cells are designed to operate with concentrated sunlight. These cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible.
The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain energies will work efficiently to create electricity, and much of it is reflected or absorbed by the material that make up the cell. Because of this, a typical commercial solar cell has an efficiency of 15%—about one-sixth of the sunlight striking the cell generates electricity. The first solar cells, built in the 1950s, had efficiencies of less than 4%.
Here is a little more on Thin Film Solar
To learn more about solar power, renewable energy and energy efficiency, visit the U.S. Department of Energy’s website at www.eere.doe.gov.
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WIND ENERGY :: YOU SPIN ME ‘ROUND
Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetation. Humans use this wind flow, or motion energy, for many purposes: sailing, flying a kite, and even generating electricity.
The terms wind energy or wind power describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity.
So how do wind turbines make electricity? Simply stated, a wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity.
The wind turbine technology that followed the oil embargoes of the 1970s refined old ideas and introduced new ways of converting wind energy into useful power. Many of these approaches have been demonstrated in "wind farms" or wind power plants — groups of turbines that feed electricity into the utility grid — in the United States and Europe.
Today, the lessons learned from more than a decade of operating wind power plants have made wind-generated electricity very close in cost to the power from conventional utility generation in some locations. Wind energy is the world's fastest-growing energy source and will power industry, businesses and homes with clean, renewable electricity for many years to come.
To learn more about wind power, renewable energy and energy efficiency, visit the U.S. Department of Energy’s website at www.eere.doe.gov.
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OCEAN ENERGY :: MINE THE SEAS
The world's ocean may eventually provide us with energy to power our homes and businesses. 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.
Wave Energy
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.
Tidal Energy
Another form of ocean energy is called tidal energy. When tides comes 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 to 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.
To learn more about ocean energy, visit the Ocean Renewable Energy Coalition at www.oceanrenewable.com
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BIOMASS ENERGY :: POWERFUL PLANTS
Biomass is plant matter such as trees, grasses, agricultural crops or other biological material. It can be used as a solid fuel, or converted into liquid or gaseous forms, for the production of electric power, heat, chemicals, or fuels. By integrating a variety of biomass conversion processes, all of these products can be made in one facility, called a biorefinery. You might have heard of Ethanol or Biodiesel, fuels that are gaining in popularity because of their ability to increase our country’s energy independence while being better for the environment.
Ethanol and biodiesel, made from plant matter instead of petroleum, can be blended with or directly substitute for gasoline and diesel, respectively. Use of biofuels reduces toxic air emissions, greenhouse gas buildup, and dependence on imported oil, while supporting agriculture and rural economies. When fossil fuels such as petroleum are burned, they also release carbon dioxide that was captured by plants billions of years ago. This release contributes to the buildup of greenhouse gases that contributes to climate change. On the other hand, carbon dioxide released from burning biofuels is balanced by the carbon dioxide capture by the recent growth of the plant materials from which they are made. Depending on how much fossil energy is used to grow and process the biomass feedstock, this results in substantially reduced net greenhouse gas emissions. Biobased products that provide equivalents or alternatives to those made from petroleum and natural gas also contribute to oil import and greenhouse gas reduction, while enhancing biorefinery economics.
To learn more about biomass energy, renewable energy and energy efficiency, visit the U.S. Department of Energy’s website at www.eere.doe.gov.
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GEOTHERMAL ENERGY :: THE POWER BENEATH YOUR FEET
Geothermal ("earth heat") energy has tremendous potential for producing electricity. About 8,000 megawatts (MW) of geothermal electricity are currently produced around the world, including about 2,800 MW of capacity in the United States. Today's technology produces electricity from hydrothermal (hot water/steam) resources. In the future, we may be able to use the heat of the deep, hot, dry rock formations of Earth's crust, and possibly the even deeper, almost unlimited energy in Earth's magma.
Two basic types of geothermal power plants are used today: Steam and Binary.
Steam plants use very hot (more than 300° F) steam and hot water resources (as found at The Geysers plants in northern California—the largest geothermal electricity producer in the world). The steam either comes directly from the resource, or the very hot, high-pressure water is depressurized ("flashed") to produce steam. The steam then turns turbines, which drive generators that generate electricity. The only significant emission from these plants is steam (water vapor). Minute amounts of carbon dioxide, nitric oxide, and sulfur are emitted, but almost 50 times less than at traditional, fossil-fuel power plants. Energy produced this way currently costs about 4-6 cents per kWh.
Binary plants use lower-temperature, but much more common, hot water resources (100° F – 300° F). The hot water is passed through a heat exchanger in conjunction with a secondary (hence, "binary plant") fluid with a lower boiling point (usually a hydrocarbon such as isobutane or isopentane). The secondary fluid vaporizes, which turns the turbines, which drive the generators. The remaining secondary fluid is simply recycled through the heat exchanger. The geothermal fluid is condensed and returned to the reservoir. Because binary plants use a self-contained cycle, nothing is emitted. Energy produced by binary plants currently costs about 5 to 8 cents per kWh. Because these lower-temperature reservoirs are far more common, binary plants are the more prevalent.
Although geothermal power plants have many features in common with more traditional power plants, they also pose special challenges: non-condensable gases and minerals in the geothermal fluid, need for a greater amount of heat rejection, use of hydrocarbon fluids, and lack of cool water to cause condensation.
To learn more about geothermal energy, renewable energy and energy efficiency, visit the U.S. Department of Energy’s website at www.eere.doe.gov.
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