ENERGY
Renewable Energy: An Overview
DOE/GO-102001-1102 FS175 March 2001
EFFICIENCY AND
RENEWABLE ENERGY
What is Renewable Energy?
Renewable energy uses energy sources that are continually replenished by nature—the sun, the wind, water, the Earth’s heat, and plants. Renewable energy technologies turn these fuels into usable forms of energy—most often elec- tricity, but also heat, chemicals, or mechanical power.
Why Use Renewable Energy?
Today we primarily use fossil fuels to heat and power our homes and fuel our cars. It’s convenient to use coal, oil, and natural gas for meeting our energy needs, but we have a limited supply of these fuels on the Earth. We’re using them much more rapidly than they are being created. Even- tually, they will run out. And because of safety concerns and waste disposal prob- lems, the United States will retire much of its nuclear capacity by 2020. In the mean- time, the nation’s energy needs are expected to grow by 33 percent during the next 20 years. Renewable energy can help fill the gap.
Even if we had an unlimited supply of fos- sil fuels, using renewable energy is better for the environment. We often call renew- able energy technologies “clean” or “green” because they produce few if any pollutants. Burning fossil fuels, however, sends greenhouse gases into the atmos- phere, trapping the sun’s heat and con- tributing to global warming. Climate scientists generally agree that the Earth’s average temperature has risen in the past century. If this trend continues, sea levels will rise, and scientists predict that floods, heat waves, droughts, and other extreme weather conditions could occur more often.
Other pollutants are released into the air, soil, and water when fossil fuels are burned. These pollutants take a dramatic toll on the environment—and on humans. Air pollution contributes to diseases like asthma. Acid rain from sulfur dioxide and nitrogen oxides harms plants and fish. Nitrogen oxides also contribute to smog.
A PV-system at the Pinnacles National Monument in California eliminates a $20,000 annual fuel bill for a diesel generator that pro- duced each year 143 tons of carbon dioxide—a greenhouse gas.
This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory. The document was produced by the Information and Outreach Program at NREL for the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based on information known to EREC and NREL at the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC.
Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste
CLEARINGHOUSE
National Park Service, NREL/PIX04924
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Hydropower is our most mature and largest source of renewable power...
Renewable energy will also help us develop energy independence and secu- rity. The United States imports more than 50 percent of its oil, up from 34 percent in 1973. Replacing some of our petroleum with fuels made from plant matter, for example, could save money and strengthen our energy security.
Renewable energy is plentiful, and the technologies are improving all the time. There are many ways to use renewable energy. Most of us already use renewable energy in our daily lives.
Hydropower
Hydropower is our most mature and largest source of renewable power, pro- ducing about 10 percent of the nation’s electricity. Existing hydropower capacity is about 77,000 megawatts (MW). Hydro- power plants convert the energy in flowing water into electricity. The most common form of hydropower uses a dam on a river to retain a large reservoir of water. Water is released through turbines to generate power. “Run of the river” systems, how- ever, divert water from the river and direct it through a pipeline to a turbine.
Hydropower plants produce no air emis- sions but can affect water quality and wildlife habitats. Therefore, hydropower plants are now being designed and oper- ated to minimize impacts on the river. Some of them are diverting a portion of the flow around their dams to mimic the natural flow of the river. But while this improves the wildlife’s river habitat, it also reduces the power plant’s output. In addition, fish lad- ders and other approaches, such as improved tur- bines, are being used to assist fish with migration and lower the number of fish killed.
Bioenergy
Bioenergy is the energy derived from bio- mass (organic matter), such as plants. If you’ve ever burned wood in a fireplace or campfire, you’ve used bioenergy. But we don’t get all of our biomass resources directly from trees or other plants. Many industries, such as those involved in con- struction or the processing of agricultural products, can create large quantities of unused or residual biomass, which can serve as a bioenergy source.
Biopower
After hydropower, biomass is this coun- try’s second-leading resource of renewable energy, accounting for more than 7,000 MW of installed capacity. Some utilities and power generating companies with coal power plants have found that replac- ing some coal with biomass is a low-cost option to reduce undesirable emissions. As much as 15 percent of the coal may be replaced with biomass. Biomass has less sulfur than coal. Therefore, less sulfur dioxide, which contributes to acid rain, is released into the air. Additionally, using biomass in these boilers reduces nitrous oxide emissions.
A process called gasification—the conver- sion of biomass into gas, which is burned in a gas turbine—is another way to gener- ate electricity. The decay of biomass in landfills also produces gas, mostly methane, which can be burned in a boiler to produce steam for electricity generation or industrial processes. Biomass can also be heated in the absence of oxygen to chemically convert it into a type of fuel oil, called pyrolysis oil. Pyrolysis oil can be used for power generation and as a feed- stock for fuels and chemical production.
Biofuels
Biomass can be converted directly into liq- uid fuels, called biofuels. Because biofuels are easy to transport and possess high energy density, they are favored to fuel vehicles and sometimes stationary power generation. The most common biofuel is ethanol, an alcohol made from the fermen- tation of biomass high in carbohydrates. The current largest source of ethanol is corn. Some cities use ethanol as a gasoline addi- tive to help meet air quality standards for
A small-scale hydropower system in King Cove, Alaska, provides residents in this remote area with a less expensive source of electricity.
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Duane Hippe, NREL/PIX04410
This gasifier in Burlinton, Vermont, converts biomass into a clean gas for electricity production
If you’ve ever burned wood in a fireplace or campfire, you've used bioenergy. ozone. Flex-fuel vehicles are also now on the market, which can use a mixture of gasoline and ethanol, such as E85—a mixture of 85 percent ethanol and 15 per- cent gasoline. Another biofuel is biodiesel, which can be made from vegetable and ani- mal fats. Biodiesel can be used to fuel a vehicle or as a fuel additive to reduce emissions.
Corn ethanol and biodiesel provide about 0.4 percent of the total liquid fuels market. To increase our available supply of biofu- els, researchers are testing crop residues— such as cornstalks and leaves—wood chips, food waste, grass, and even trash as potential biofuel sources.
Biobased Products
Biomass—corn, wheat, soybeans, wood, and residues—can also be used to produce chemicals and materials that we normally obtain from petroleum. Industry has already begun to use cornstarch to pro- duce commodity plastics, such as shrinkwrap, plastic eating utensils, and even car bumpers. Commercial develop- ment is underway to make thermoset plastics, like electrical switch plate covers, from wood residues.
Geothermal Energy
The Earth’s core, 4,000 miles below the sur- face, can reach temperatures of 9000° F. This heat—geothermal energy—flows out- ward from the core, heating the surround- ing area, which can form underground reservoirs of hot water and steam. These reservoirs can be tapped for a variety of uses, such as to generate electricity or heat buildings. By using geothermal heat pumps (GHPs), we can even take advan- tage of the shallow ground’s stable tem- perature for heating and cooling buildings.
The geothermal energy potential in the uppermost 6 miles of the Earth’s crust amounts to 50,000 times the energy of all oil and gas resources in the world. In the United States, most geothermal reservoirs are located in the western states, Alaska, and Hawaii. GHPs, however, can be used almost anywhere.
Geothermal Electricity Production
Geothermal power plants access the under- ground steam or hot water from wells drilled a mile or more into the earth. The steam or hot water is piped up from the well to drive a conventional steam turbine, which powers an electric generator. Typi- cally, the water is then returned to the ground to recharge the reservoir and com- plete the renewable energy cycle.
There are three types of geothermal power plants: dry steam, flash steam, and binary cycle. Dry steam plants draw from reser- voirs of steam, while both flash steam and binary cycle plants draw from reservoirs of hot water. Flash steam plants typically use water at temperatures greater than 360°F. Unlike both steam and flash plants, binary-cycle plants transfer heat from the water to what’s called a working fluid. Therefore binary cycle plants can operate using water at lower temperatures of about 225° to 360°F.
All of the U.S. geothermal power plants are in California, Nevada, Utah, and Hawaii. Altogether about 2800 MW of geothermal electric capacity is produced annually in this country.
Geothermal Direct Use
If you’ve ever soaked in a natural hot spring, you’re one of millions of people around the world who has enjoyed the direct use of
The Steamboat Hills geothermal power plant in Steamboat Springs, Nevada has an electric- ity generation capacity of 13.5 MW.
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Joel Renner, INEEL, NREL/PIX07658
Warren Gretz, NREL/PIX04744
Altogether about 2800 MW of geothermal electric capacity is produced annually in this country. geothermal energy. Direct-use applications require geothermal temperatures between about 70° to 302°F—lower than those required for electricity generation. The United States already has about 1,300 geot- hermal direct-use systems in operation.
In a direct-use system, a well is drilled into a geothermal reservoir, which pro- vides a steady stream of hot water. Some systems use the water directly, but most pump the water through what’s called a heat exchanger. The heat exchanger keeps the water separate from a working fluid (usually water or a mixture of water and antifreeze), which is heated by the geot- hermal water. The working fluid then flows through piping, distributing the heat directly for its intended use.
The heated water or fluid can be used in a building to replace the traditional heat source—often natural gas—of a boiler, fur- nace, and hot water heater. Some cities and towns actually have large direct-use heating systems—called district heating— that provide many buildings with heat. Geothermal direct use is also used in agri- culture—such as for fish farms and to heat greenhouses—and for industrial food pro- cessing (vegetable dehydration).
Geothermal Heat Pumps
While air temperatures can vary widely through the seasons, the temperatures of the shallow ground only range from 50° to 70°F depending on latitude. GHPs draw on this relatively stable temperature as a source for heating buildings in the winter and keeping them cool in the summer.
Through underground piping, a GHP dis- charges heat from inside a building into the ground in the summer, much like a refrigerator uses electricity to keep its inte- rior cool while releasing heat into your kitchen. In the winter, this process is reversed; the GHP extracts heat from the ground and releases it into a building.
Because GHPs actually move heat between homes and the earth, instead of burning fuels, they operate very cleanly and effi- ciently. In fact, GHPs are at least three times more efficient than even the most energy- efficient furnaces on the market today.
Solar Energy
Solar technologies tap directly into the infinite power of the sun and use that energy to produce heat, light, and power.
Passive Solar Lighting and Heating
People have used the sun to heat and light their homes for centuries. Ancient Native Americans built their dwellings directly into south-facing cliff walls because they knew the sun travels low across the south- ern sky in the Northern Hemisphere dur- ing the winter. They also knew the massive rock of the cliff would absorb heat in winter and protect against wind and snow. At the same time, the cliff- dwelling design blocked sunlight during the summer, when the sun is higher in the sky, keeping their dwellings cool.
The modern version of this sun-welcoming design is called passive solar because no pumps, fans, or other mechanical devices are used. Its most basic features include large, south-facing windows that fill the home with natural sunlight, and dark tile or brick floors that store the sun’s heat and release it back into the home at night. In the summer, when the sun is higher in the sky, window overhangs block direct sunlight, which keeps the house cool. Tile and brick floors also remain cool during the summer.
Passive solar design combined with energy efficiency will go even further. Energy-efficient features such as energy- saving windows and appliances, along with good insulation and weatherstrip- ping, can make a huge difference in energy and cost savings.
Solar Water Heating
Solar energy can be used to heat water for your home or your swimming pool. Most solar water-heating systems consist of a solar collector and a water storage tank.
Solar water-heating systems use collectors, generally mounted on a south-facing roof, to heat either water or a heat-transfer fluid, such as a nontoxic antifreeze. The heated water is then stored in a water tank similar to one used in a conventional gas or electric water-heating system.
This homeowner in Aurora, Colorado, uses a GHP to heat and cool his home.
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Warren Gretz, NREL/PIX06537
The Four Times Square Building in New York City uses thin-film PV panels to reduce the building’s power load from the utility grid.
Passive solar building techniques turn homes into huge solar collectors.
There are basically three types of solar collectors for heat- ing water: flat- plate, evacuated- tube, and concen- trating. The most common type, a flat-plate collector, is an insulated, weatherproof box containing a dark absorber plate under a transpar- ent cover. Evacu- ated-tube collectors are made up of rows of parallel, transparent glass tubes. Each tube consists of a glass outer tube and an inner tube, or absorber, covered with a coating that absorbs solar energy but inhibits heat loss. Concentrating collec- tors for residential applications are usually parabolic-shaped mirrors (like a trough) that concentrate the sun’s energy on an absorber tube called a receiver that runs along the axis of the mirrored trough and contains a heat-transfer fluid.
All three types of collectors heat water by circulating household water or a heat-trans- fer fluid such as a nontoxic antifreeze from the collector to the water storage tanks. Col- lectors do this either passively or actively.
Passive solar water-heating systems use natural convection or household water pressure to circulate water through a solar collector to a storage tank. They have no electric components that could break, a feature that generally makes them more reliable, easier to maintain, and possibly longer lasting than active systems.
An active system uses an electric pump to circulate water or nontoxic antifreeze through the system. Active systems are usually more expensive than passive sys- tems, but they are also more efficient. Active systems also can be easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. Also, the moving water in the system will not freeze in cold climates. But because these systems use electricity, they will not function in a power outage. That’s why many active systems are now combined with a small solar-electric panel to power the pump.
The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, proper installation, and the tilt angle and orientation of the collectors. But if you’re currently using an electric water heater, solar water heating is a cost-effective alternative. If you own a swimming pool, heating the water with solar collectors can also save you money.
Solar Electricity
Solar electricity or photovoltaic (PV) tech- nology converts sunlight directly into elec- tricity. Solar electricity has been a prime source of power for space vehicles since the inception of the space program. It has also been used to power small electronics and rural and agricultural applications for three decades. During the last decade, a strong solar electric market has emerged for powering urban grid-connected homes and buildings as a result of advances in solar technology along with global changes in electric industry restructuring.
Although many types of solar electric sys- tems are available today, they all consist of basically three main items: modules that convert sunlight into electricity; inverters that convert that electricity into alternat- ing current so it can be used by most household appliances; and possibly or sometimes batteries that store excess elec- tricity produced by the system. The remainder of the system comprises equip- ment such as wiring, circuit breakers, and support structures.
Today’s modules can be built into glass skylights and walls. Some modules resem- ble traditional roof shingles, but they gen- erate electricity, and some come with built-in inverters. The solar modules avail- able today are more efficient and versatile than ever before.
In over 30 states, any additional power produced by a PV system, which is not being used by a home or building, can be fed back to the electric grid through a process known as net metering. Net meter- ing allows electricity customers to pay only for their “net” electricity, or the
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Andrew Gordon and Fox & Fowle Architects, NREL/PIX09052
This dish/Stirling solar power system in Arizona is capable of producing 25 kW of electricity.
Wind energy has been the fastest growing source of energy since 1990... amount of power consumed from their utility minus the power generated by their PV system. This metering arrangement allows consumers to realize full retail value for 100 percent of the PV energy produced by their systems.
Grid-connected PV systems do not require batteries. However, some grid connected systems use them for emergency backup power. And of course in remote areas, solar electricity is often a economic alternative to expensive distribution line extensions incurred by a customer first connecting to the utility grid. Electricity produced by solar electric systems in remote locations is stored in batteries. Batteries will usually store electricity produced by a solar- electric system for up to three days.
What type of system to purchase will depend on the energy-efficiency of your home, your home’s location, and your budget. Before you size your system, try reducing energy demand through energy- efficient measures. Purchasing energy-sav- ing appliances and lights, for example, will reduce your electrical demand and allow you to purchase a smaller solar-electric system to meet your energy needs or get more value from a larger system. Energy efficiency allows you to start small and then add on as your energy needs increase.
Solar Thermal Electricity
Unlike solar-electric systems that convert sunlight into electricity, solar thermal elec- tric systems convert the sun’s heat into electricity. This technology is used primar- ily in large-scale power plants for power- ing cities and communities, especially in the Southwest where consistent hours of sunlight are greater than other parts of the United States.
Concentrating solar power (CSP) tech- nologies convert solar energy into electric- ity by using mirrors to focus sunlight onto a component called a receiver. The receiver transfers the heat to a conven- tional engine-generator—such as a steam turbine—that generates electricity.
There are three types of CSP systems: power towers (central receivers), parabolic troughs, and dish/engine systems. A power tower system uses a large field of mirrors to concentrate sunlight onto the top of a tower, where a receiver sits. Molten salt flowing through the receiver is heated by the concentrated sunlight. The salt’s heat is turned into electricity by a conventional steam generator. Parabolic- trough systems concentrate the sun’s energy through long, parabolic-shaped mirrors. Sunlight is focused on a pipe filled with oil that runs down the axis of the trough. When the oil gets hot, it is used to boil water in a conventional steam generator to produce electricity. A dish/engine system uses a mirrored dish (similar in size to a large satellite dish). The dish-shaped surface focuses and con- centrates the sun’s heat onto a receiver at the focal point of the dish (above and cen- ter of the collectors). The receiver absorbs the sun’s heat and transfers it to a fluid within an engine, where the heat causes the fluid to expand against a piston to pro- duce mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity.
Concentrating solar technologies can be used to generate electricity for a variety of applications, ranging from remote power systems as small as a few kilowatts (kW) up to grid-connected applications of 200 MW or more. A 354-MW power plant in South- ern California, which consists of nine trough power plants, meets the energy needs of more than 350,000 people and is the world’s largest solar energy power plant.
Wind Energy
For hundreds of years, people have used windmills to harness the wind’s energy. Today’s wind turbines, which operate dif- ferently from windmills, are a much more efficient technology.
Wind turbine technology may look simple: the wind spins turbine blades around a central hub; the hub is connected to a shaft, which powers a generator to make electric- ity. However, turbines are highly sophisti- cated power systems that capture the wind’s energy by means of new blade designs or airfoils. Modern, mechanical drive systems, combined with advanced generators, con- vert that energy into electricity.
Wind turbines that provide electricity to the utility grid range in size from 50 kW to
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Bill Timmerman, NREL/PIX08982
The 6-MW Green Mountain power plant in Searsburg, Vermont, consists of eleven 550-kW wind turbines.
Hydrogen is high in energy, yet its use as a fuel produces water as the only emission.
1 or 2 MW. Large, utility-scale pro- jects can have hun- dreds of turbines spread over many acres of land. Small turbines, below 50 kW, are used to charge bat- teries, electrify homes, pump water for farms and ranches, and power remote telecommunica- tions equipment. Wind turbines can also be placed in the shallow water near a coastline if open land is limited, such as in Europe, and/or to take advan- tage of strong, offshore winds.
Wind energy has been the fastest growing source of energy in the world since 1990, increasing at an average rate of over 25 percent per year. It’s a trend driven largely by dramatic improvements in wind tech- nology. Currently, wind energy capacity amounts to about 2500 MW in the United States. Good wind areas, which cover 6 percent of the contiguous U.S. land area, could supply more than one and a half times the 1993 electricity consumption of the entire country.
California now has the largest number of installed turbines. Many turbines are also being installed across the Great Plains, reaching from Montana east to Minnesota and south through Texas, to take advantage of its vast wind resource. North Dakota alone has enough wind to supply 36 percent of the total 1990 electricity consumption of the lower 48 states. Hawaii, Iowa, Min- nesota, Oregon, Texas, Washington, Wiscon- sin, and Wyoming are among states where wind energy use is rapidly increasing.
Hydrogen
Hydrogen is high in energy, yet its use as a fuel produces water as the only emission. Hydrogen is the universe’s most abundant element and also its simplest. A hydrogen atom consists of only one proton and one electron. Despite its abundance and simplicity, it doesn’t occur naturally as a gas on the Earth.
Today, industry produces more than 4 tril- lion cubic feet of hydrogen annually. Most of this hydrogen is produced through a process called reforming, which involves the application of heat to separate hydro- gen from carbon. Researchers are develop- ing highly efficient, advanced reformers to produce hydrogen from natural gas for what’s called Proton Exchange Membrane fuel cells.
You can think of fuel cells as batteries that never lose their charge. Today, hydrogen fuel cells offer tremendous potential to produce electrical power for distributed energy systems and vehicles. In the future, hydrogen could join electricity as an important “energy carrier”: storing, mov- ing, and delivering energy in a usable form to consumers. Renewable energy sources, like the sun, can’t produce energy all the time. But hydrogen can store the renew- able energy produced until it’s needed.
Eventually, researchers would like to directly produce hydrogen from water using solar, wind, and biomass and bio- logical technologies.
Ocean Energy
The ocean can produce two types of energy: thermal energy from the sun’s heat, and mechanical energy from the tides and waves.
Ocean thermal energy can be used for many applications, including electricity generation. Electricity conversion systems use either the warm surface water or boil the seawater to turn a turbine, which acti- vates a generator.
The electricity conversion of both tidal and wave energy usually involves mechanical devices. A dam is typically used to convert tidal energy into electric- ity by forcing the water through turbines, activating a generator. Meanwhile, wave energy uses mechanical power to directly activate a generator, or to transfer to a working fluid, water, or air, which then drives a turbine/generator.
Most of the research and development in ocean energy is happening in Europe.
NASA uses liquid hydrogen to launch its space shuttles and hydrogen fuel cells to provide them with electricity
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NASA, NREL/PIX03814
Green Mountain Power Corporation, NREL/PIX05768
Resources
The following are sources of additional information on renewable energy. The list is not exhaustive, nor does the mention of any resource constitute a recommendation or endorsement.
Ask an Energy Expert
DOE’s Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
Phone: 1-800-DOE-EREC (363-3732)
TDD: 1-800-273-2957
Fax: (703) 893-0400
E-mail: doe.erec@nciinc.com
Online submittal form: www.eren.doe.gov/menus/ energyex.html
Consumer Energy Information Web site: www.eren.doe. gov/consumerinfo/
Energy experts at EREC provide free general and technical information to the public on many topics and technologies per- taining to energy efficiency and renewable energy.
DOE’s Energy Efficiency and Renewable Energy Network (EREN)
Web site: www.eren.doe.gov
Your comprehensive online resource for DOE’s energy effi- ciency and renewable energy information.
Organizations
Center for Energy Efficiency and Renewable Energy (CEERT)
1100 Eleventh St., Suite 311
Sacramento, CA 95814
Phone (916) 442-7785; Fax (916) 447-2940 E-mail: info@ceert.org
Web site: www.cleanpower.org
Promotes the development of renewable energy technologies and resources.
National Renewable Energy Laboratory (NREL)
1617 Cole Blvd. Golden, CO 80401
Web site: www.nrel.gov
DOE-lab devoted to researching and developing renewable energy and energy efficiency technologies.
Renewable Energy Policy Project (REPP)
1612 K St. NW, Suite 202
Washington, DC 20006
Phone: (202) 293-2898; Fax: (202) 293-5857 Web site: www.repp.org
Works to advance renewable energy technologies.
Web Sites
CADDET Renewable Energy
Web site: www.caddet-re.org
Provides technical information on renewable energy projects and technologies from around the world.
Clean Energy Basics
NREL
Web site: www.nrel.gov/clean_energy/
Provides basic information on renewable energy technologies, including specific links for homeowners, small business own- ers, students, and teachers.
European Renewable Energy Exchange (EuroREX)
Web site: www.eurorex.com
Features information and news on renewable energy technol- ogy developments in Europe and around the world.
Planet Energy—The Renewable Energy Trail
United Kingdom Department of Trade and Industry Web site: www.dti.gov.uk/renewable/ed_pack/ index.html
Specifically gears its information for students and teachers, from grade school through high school.
Solstice
Center for Renewable Energy and Sustainable Technology (CREST)