What is Renewable Energy?

Renewable energy uses energy sourcesthat are continually replenished bynaturethe sun, the wind, water, theEarths heat, and plants. Renewableenergy technologies turn these fuels intousable forms of energymost often elec-tricity, but also heat, chemicals, ormechanical power.

Why Use Renewable Energy?

Today we primarily use fossil fuels to heatand power our homes and fuel our cars.Its convenient to use coal, oil, and naturalgas for meeting our energy needs, but wehave a limited supply of these fuels on theEarth. Were using them much morerapidly 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 ofits nuclear capacity by 2020. In the mean-time, the nations energy needs areexpected to grow by 33 percent during thenext 20 years. Renewable energy can helpfill the gap.

Even if we had an unlimited supply of fos-sil fuels, using renewable energy is betterfor the environment. We often call renew-able energy technologies clean orgreen because they produce few if anypollutants. Burning fossil fuels, however,sends greenhouse gases into the atmos-phere, trapping the suns heat and con-tributing to global warming. Climatescientists generally agree that the Earthsaverage temperature has risen in the pastcentury. If this trend continues, sea levels

will rise, and scientistspredict that floods,heat waves, droughts,and other extremeweather conditionscould occur more often.

Other pollutants arereleased into the air,soil, and water whenfossil fuels are burned.These pollutants take adramatic toll on theenvironmentand onhumans. Air pollutioncontributes to diseaseslike asthma. Acid rainfrom sulfur dioxideand nitrogen oxidesharms plants and fish.Nitrogen oxides alsocontribute to smog.

Renewable Energy: An OverviewCLEARINGHOUSE

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This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory. Thedocument was produced by the Information and Outreach Program at NREL for the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiencyand Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based oninformation 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

DOE/GO-102001-1102FS175

March 2001

A PV-system at the Pinnacles National Monument in Californiaeliminates a $20,000 annual fuel bill for a diesel generator that pro-duced each year 143 tons of carbon dioxidea greenhouse gas.

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Renewable energy will also help usdevelop energy independence and secu-rity. The United States imports more than50 percent of its oil, up from 34 percent in1973. Replacing some of our petroleumwith fuels made from plant matter, forexample, could save money andstrengthen our energy security.

Renewable energy is plentiful, and thetechnologies are improving all the time.There are many ways to use renewableenergy. Most of us already use renewableenergy in our daily lives.

Hydropower

Hydropower is our most mature andlargest source of renewable power, pro-ducing about 10 percent of the nationselectricity. Existing hydropower capacityis about 77,000 megawatts (MW). Hydro-power plants convert the energy in flowingwater into electricity. The most commonform of hydropower uses a dam on a riverto retain a large reservoir of water. Water isreleased through turbines to generatepower. Run of the river systems, how-ever, divert water from the river anddirect it through a pipeline to a turbine.

Hydropower plants produce no air emis-sions but can affect water quality andwildlife habitats. Therefore, hydropowerplants are now being designed and oper-ated to minimize impacts on the river.Some of them are diverting a portion ofthe flow around their dams to mimic thenatural flow of the river. But while this

improves thewildlifes riverhabitat, it alsoreduces the powerplants output. Inaddition, fish lad-ders and otherapproaches, suchas improved tur-bines, are beingused to assist fishwith migrationand lower thenumber of fishkilled.

Bioenergy

Bioenergy is the energy derived from bio-mass (organic matter), such as plants. Ifyouve ever burned wood in a fireplace orcampfire, youve used bioenergy. But wedont get all of our biomass resourcesdirectly from trees or other plants. Manyindustries, such as those involved in con-struction or the processing of agriculturalproducts, can create large quantities ofunused or residual biomass, which canserve as a bioenergy source.

Biopower After hydropower, biomass is this coun-trys second-leading resource of renewableenergy, accounting for more than 7,000MW of installed capacity. Some utilitiesand power generating companies withcoal power plants have found that replac-ing some coal with biomass is a low-costoption to reduce undesirable emissions.As much as 15 percent of the coal may bereplaced with biomass. Biomass has lesssulfur than coal. Therefore, less sulfurdioxide, which contributes to acid rain, isreleased into the air. Additionally, usingbiomass in these boilers reduces nitrousoxide emissions.

A process called gasificationthe conver-sion of biomass into gas, which is burnedin a gas turbineis another way to gener-ate electricity. The decay of biomass inlandfills also produces gas, mostlymethane, which can be burned in a boilerto produce steam for electricity generationor industrial processes. Biomass can alsobe heated in the absence of oxygen tochemically convert it into a type of fuel oil,called pyrolysis oil. Pyrolysis oil can beused for power generation and as a feed-stock for fuels and chemical production.

BiofuelsBiomass can be converted directly into liq-uid fuels, called biofuels. Because biofuelsare easy to transport and possess highenergy density, they are favored to fuelvehicles and sometimes stationary powergeneration. The most common biofuel isethanol, 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

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Hydropower is our

most mature and

largest source of

renewable power

A small-scale hydropower system in King Cove,Alaska, provides residents in this remote area with aless expensive source of electricity.

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3If youve ever burned

wood in a fireplace

or campfire, you've

used bioenergy.

ozone. Flex-fuelvehicles are alsonow on the market,which can use amixture of gasolineand ethanol, suchas E85a mixtureof 85 percentethanol and 15 per-cent gasoline.Another biofuel isbiodiesel, whichcan be made fromvegetable and ani-mal fats. Biodieselcan be used to fuela vehicle or as a

fuel additive to reduce emissions.

Corn ethanol and biodiesel provide about0.4 percent of the total liquid fuels market.To increase our available supply of biofu-els, researchers are testing crop residuessuch as cornstalks and leaveswoodchips, food waste, grass, and even trash aspotential biofuel sources.

Biobased ProductsBiomasscorn, wheat, soybeans, wood,and residuescan also be used to producechemicals and materials that we normallyobtain from petroleum. Industry hasalready begun to use cornstarch to pro-duce commodity plastics, such asshrinkwrap, plastic eating utensils, andeven car bumpers. Commercial develop-ment is underway to make thermosetplastics, like electrical switch plate covers,from wood residues.

Geothermal Energy

The Earths core, 4,000 miles below the sur-face, can reach temperatures of 9000 F.This heatgeothermal energyflows out-ward from the core, heating the surround-ing area, which can form undergroundreservoirs of hot water and steam. Thesereservoirs can be tapped for a variety ofuses, such as to generate electricity or heatbuildings. By using geothermal heatpumps (GHPs), we can even take advan-tage of the shallow grounds stable tem-perature for heating and cooling buildings.

The geothermal energy potential in theuppermost 6 miles of the Earths crust

amounts to 50,000 times the energy of alloil and gas resources in the world. In theUnited States, most geothermal reservoirsare located in the western states, Alaska,and Hawaii. GHPs, however, can be usedalmost anywhere.

Geothermal Electricity Production Geothermal power plants access the under-ground steam or hot water from wellsdrilled a mile or more into the earth. Thesteam or hot water is piped up from thewell to drive a conventional steam turbine,which powers an electric generator. Typi-cally, the water is then returned to theground to recharge the reservoir and com-plete the renewable energy cycle.

There are three types of geothermal powerplants: dry steam, flash steam, and binarycycle. Dry steam plants draw from reser-voirs of steam, while both flash steam andbinary cycle plants draw from reservoirsof hot water. Flash steam plants typicallyuse water at temperatures greater than360F. Unlike both steam and flash plants,binary-cycle plants transfer heat from thewater to whats called a working fluid.Therefore binary cycle plants can operateusing water at lower temperatures ofabout 225 to 360F.

All of the U.S. geothermal power plantsare in California, Nevada, Utah, andHawaii. Altogether about 2800 MW ofgeothermal electric capacity is producedannually in this country.

Geothermal Direct UseIf youve ever soaked in a natural hot spring,youre one of millions of people around theworld who has enjoyed the direct use of

This gasifier in Burlinton, Vermont, converts biomassinto a clean gas for electricity production

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The Steamboat Hills geothermal power plantin Steamboat Springs, Nevada has an electric-ity generation capacity of 13.5 MW.

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geothermal energy. Direct-use applicationsrequire geothermal temperatures betweenabout 70 to 302Flower than thoserequired for electricity generation. TheUnited States already has about 1,300 geot-hermal direct-use systems in operation.

In a direct-use system, a well is drilledinto a geothermal reservoir, which pro-vides a steady stream of hot water. Somesystems use the water directly, but mostpump the water through whats called aheat exchanger. The heat exchanger keepsthe water separate from a working fluid(usually water or a mixture of water andantifreeze), which is heated by the geot-hermal water. The working fluid thenflows through piping, distributing theheat directly for its intended use.

The heated water or fluid can be used in abuilding to replace the traditional heatsourceoften natural gasof a boiler, fur-nace, and hot water heater. Some citiesand towns actually have large direct-useheating systemscalled district heatingthat provide many buildings with heat.Geothermal direct use is also used in agri-culturesuch as for fish farms and to heatgreenhousesand for industrial food pro-cessing (vegetable dehydration).

Geothermal Heat PumpsWhile air temperatures can vary widelythrough the seasons, the temperatures ofthe shallow ground only range from 50 to70F depending on latitude. GHPs drawon this relatively stable temperature as asource for heating buildings in the winterand keeping them cool in the summer.

Through underground piping, a GHP dis-charges heat from inside a building intothe ground in the summer, much like arefrigerator uses electricity to keep its inte-rior cool while releasing heat into yourkitchen. In the winter, this process isreversed; the GHP extracts heat from theground and releases it into a building.

Because GHPs actually move heat betweenhomes and the earth, instead of burningfuels, they operate very cleanly and effi-ciently. In fact, GHPs are at least three timesmore efficient than even the most energy-efficient furnaces on the market today.

Solar Energy

Solar technologies tap directly into theinfinite power of the sun and use thatenergy to produce heat, light, and power.

Passive Solar Lighting and Heating People have used the sun to heat and lighttheir homes for centuries. Ancient NativeAmericans built their dwellings directlyinto south-facing cliff walls because theyknew the sun travels low across the south-ern sky in the Northern Hemisphere dur-ing the winter. They also knew themassive rock of the cliff would absorbheat in winter and protect against windand snow. At the same time, the cliff-dwelling design blocked sunlight duringthe summer, when the sun is higher in thesky, keeping their dwellings cool.

The modern version of this sun-welcomingdesign is called passive solar because nopumps, fans, or other mechanical devicesare used. Its most basic features includelarge, south-facing windows that fill thehome with natural sunlight, and dark tileor brick floors that store the suns heat andrelease it back into the home at night. In thesummer, when the sun is higher in the sky,window overhangs block direct sunlight,which keeps the house cool. Tile and brickfloors also remain cool during the summer.

Passive solar design combined withenergy efficiency will go even further.Energy-efficient features such as energy-saving windows and appliances, alongwith good insulation and weatherstrip-ping, can make a huge difference inenergy and cost savings.

Solar Water Heating Solar energy can be used to heat water foryour home or your swimming pool. Mostsolar water-heating systems consist of asolar 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-transferfluid, such as a nontoxic antifreeze. Theheated water is then stored in a water tanksimilar to one used in a conventional gasor electric water-heating system.

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Altogether about

2800 MW of

geothermal electric

capacity is produced

annually in this

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This homeowner in Aurora,Colorado, uses a GHP to heatand cool his home.

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There are basicallythree types of solarcollectors for heat-ing water: flat-plate, evacuated-tube, and concen-trating. The mostcommon type, aflat-plate collector, isan insulated,weatherproof boxcontaining a darkabsorber plateunder a transpar-ent cover. Evacu-ated-tube collectorsare made up of

rows of parallel, transparent glass tubes.Each tube consists of a glass outer tubeand an inner tube, or absorber, coveredwith a coating that absorbs solar energybut inhibits heat loss. Concentrating collec-tors for residential applications are usuallyparabolic-shaped mirrors (like a trough)that concentrate the suns energy on anabsorber tube called a receiver that runsalong the axis of the mirrored trough andcontains a heat-transfer fluid.

All three types of collectors heat water bycirculating household water or a heat-trans-fer fluid such as a nontoxic antifreeze fromthe collector to the water storage tanks. Col-lectors do this either passively or actively.

Passive solar water-heating systems usenatural convection or household waterpressure to circulate water through a solarcollector to a storage tank. They have noelectric components that could break, afeature that generally makes them morereliable, easier to maintain, and possiblylonger lasting than active systems.

An active system uses an electric pump tocirculate water or nontoxic antifreezethrough the system. Active systems areusually more expensive than passive sys-tems, but they are also more efficient.Active systems also can be easier to retrofitthan passive systems because their storagetanks do not need to be installed above orclose to the collectors. Also, the movingwater in the system will not freeze in coldclimates. But because these systems useelectricity, they will not function in apower outage. Thats why many active

systems are now combined with a smallsolar-electric panel to power the pump.

The amount of hot water a solar waterheater produces depends on the type andsize of the system, the amount of sunavailable at the site, proper installation,and the tilt angle and orientation of thecollectors. But if youre currently using anelectric water heater, solar water heating isa cost-effective alternative. If you own aswimming pool, heating the water withsolar collectors can also save you money.

Solar ElectricitySolar electricity or photovoltaic (PV) tech-nology converts sunlight directly into elec-tricity. Solar electricity has been a primesource of power for space vehicles sincethe inception of the space program. It hasalso been used to power small electronicsand rural and agricultural applications forthree decades. During the last decade, astrong solar electric market has emergedfor powering urban grid-connected homesand buildings as a result of advances insolar technology along with globalchanges in electric industry restructuring.

Although many types of solar electric sys-tems are available today, they all consist ofbasically three main items: modules thatconvert sunlight into electricity; invertersthat convert that electricity into alternat-ing current so it can be used by mosthousehold appliances; and possibly orsometimes batteries that store excess elec-tricity produced by the system. Theremainder of the system comprises equip-ment such as wiring, circuit breakers, andsupport structures.

Todays modules can be built into glassskylights and walls. Some modules resem-ble traditional roof shingles, but they gen-erate electricity, and some come withbuilt-in inverters. The solar modules avail-able today are more efficient and versatilethan ever before.

In over 30 states, any additional powerproduced by a PV system, which is notbeing used by a home or building, can befed back to the electric grid through aprocess known as net metering. Net meter-ing allows electricity customers to payonly for their net electricity, or the

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Passive solar building

techniques turn

homes into huge

solar collectors.

The Four Times Square Building in New York City usesthin-film PV panels to reduce the buildings power loadfrom the utility grid.

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amount of power consumed from theirutility minus the power generated by theirPV system. This metering arrangementallows consumers to realize full retailvalue for 100 percent of the PV energyproduced by their systems.

Grid-connected PV systems do not requirebatteries. However, some grid connectedsystems use them for emergency backuppower. And of course in remote areas, solarelectricity is often a economic alternative toexpensive distribution line extensionsincurred by a customer first connecting tothe utility grid. Electricity produced bysolar electric systems in remote locations isstored in batteries. Batteries will usuallystore electricity produced by a solar-electric system for up to three days.

What type of system to purchase willdepend on the energy-efficiency of yourhome, your homes location, and yourbudget. Before you size your system, tryreducing energy demand through energy-efficient measures. Purchasing energy-sav-ing appliances and lights, for example, willreduce your electrical demand and allowyou to purchase a smaller solar-electricsystem to meet your energy needs or getmore value from a larger system. Energyefficiency allows you to start small andthen add on as your energy needs increase.

Solar Thermal Electricity Unlike solar-electric systems that convertsunlight into electricity, solar thermal elec-tric systems convert the suns heat intoelectricity. This technology is used primar-ily in large-scale power plants for power-ing cities and communities, especially inthe Southwest where consistent hours ofsunlight are greater than other parts of theUnited States.

Concentrating solar power (CSP) tech-nologies convert solar energy into electric-ity by using mirrors to focus sunlight ontoa component called a receiver. Thereceiver transfers the heat to a conven-tional engine-generatorsuch as a steamturbinethat generates electricity.

There are three types of CSP systems:power towers (central receivers), parabolictroughs, and dish/engine systems. Apower tower system uses a large field of

mirrors to concentrate sunlight onto thetop of a tower, where a receiver sits.Molten salt flowing through the receiver isheated by the concentrated sunlight. Thesalts heat is turned into electricity by aconventional steam generator. Parabolic-trough systems concentrate the sunsenergy through long, parabolic-shapedmirrors. Sunlight is focused on a pipefilled with oil that runs down the axis ofthe trough. When the oil gets hot, it isused to boil water in a conventional steamgenerator to produce electricity. Adish/engine system uses a mirrored dish(similar in size to a large satellite dish).The dish-shaped surface focuses and con-centrates the suns heat onto a receiver atthe focal point of the dish (above and cen-ter of the collectors). The receiver absorbsthe suns heat and transfers it to a fluidwithin an engine, where the heat causesthe fluid to expand against a piston to pro-duce mechanical power. The mechanicalpower is then used to run a generator oralternator to produce electricity.

Concentrating solar technologies can beused to generate electricity for a variety ofapplications, ranging from remote powersystems as small as a few kilowatts (kW) upto grid-connected applications of 200 MWor more. A 354-MW power plant in South-ern California, which consists of nine troughpower plants, meets the energy needs ofmore than 350,000 people and is theworlds largest solar energy power plant.

Wind Energy

For hundreds of years, people have usedwindmills to harness the winds energy.Todays wind turbines, which operate dif-ferently from windmills, are a much moreefficient technology.

Wind turbine technology may look simple:the wind spins turbine blades around acentral 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 windsenergy by means of new blade designs orairfoils. Modern, mechanical drive systems,combined with advanced generators, con-vert that energy into electricity.

Wind turbines that provide electricity tothe utility grid range in size from 50 kW to

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Wind energy has been

the fastest growing

source of energy since

1990

This dish/Stirling solar power system in Arizona iscapable of producing 25 kW of electricity.

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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 turbinesspread over manyacres of land.Small turbines,below 50 kW, areused to charge bat-teries, electrifyhomes, pumpwater for farmsand ranches, andpower remotetelecommunica-tions equipment.Wind turbines canalso be placed inthe 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 growingsource of energy in the world since 1990,increasing at an average rate of over 25percent per year. Its a trend driven largelyby dramatic improvements in wind tech-nology. Currently, wind energy capacityamounts to about 2500 MW in the UnitedStates. Good wind areas, which cover 6percent of the contiguous U.S. land area,could supply more than one and a halftimes the 1993 electricity consumption ofthe entire country.

California now has the largest number ofinstalled turbines. Many turbines are alsobeing installed across the Great Plains,reaching from Montana east to Minnesotaand south through Texas, to take advantageof its vast wind resource. North Dakotaalone has enough wind to supply 36 percentof the total 1990 electricity consumption ofthe lower 48 states. Hawaii, Iowa, Min-nesota, Oregon, Texas, Washington, Wiscon-sin, and Wyoming are among states wherewind energy use is rapidly increasing.

Hydrogen

Hydrogen is high in energy, yet its use as afuel produces water as the only emission.Hydrogen is the universes most abundantelement and also its simplest. A hydrogenatom consists of only one proton and one electron. Despite its abundance and

simplicity, it doesnt occur naturally as agas on the Earth.

Today, industry produces more than 4 tril-lion cubic feet of hydrogen annually. Mostof this hydrogen is produced through aprocess called reforming, which involvesthe application of heat to separate hydro-gen from carbon. Researchers are develop-ing highly efficient, advanced reformers toproduce hydrogen from natural gas forwhats called Proton Exchange Membranefuel cells.

You can think of fuel cells as batteries thatnever lose their charge. Today, hydrogenfuel cells offer tremendous potential toproduce electrical power for distributedenergy systems and vehicles. In the future,hydrogen could join electricity as animportant energy carrier: storing, mov-ing, and delivering energy in a usable formto consumers. Renewable energy sources,like the sun, cant produce energy all thetime. But hydrogen can store the renew-able energy produced until its needed.

Eventually, researchers would like todirectly produce hydrogen from waterusing solar, wind, and biomass and bio-logical technologies.

Ocean Energy

The ocean can produce two types of energy:thermal energy from the suns heat, andmechanical energy from the tides and waves.

Ocean thermal energy can be used formany applications, including electricitygeneration. Electricity conversion systemsuse either the warm surface water or boilthe seawater to turn a turbine, which acti-vates a generator.

The electricity conversion of both tidaland wave energy usually involvesmechanical devices. A dam is typicallyused to convert tidal energy into electric-ity by forcing the water through turbines,activating a generator. Meanwhile, waveenergy uses mechanical power to directlyactivate a generator, or to transfer to aworking fluid, water, or air, which thendrives a turbine/generator.

Most of the research and development inocean energy is happening in Europe.

The 6-MW Green Mountain power plant in Searsburg, Vermont, consists of eleven 550-kW wind turbines.

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NASA uses liquid hydrogento launch its space shuttlesand hydrogen fuel cells toprovide them with electricity

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8Resources The following are sources of additional information onrenewable energy. The list is not exhaustive, nor does themention of any resource constitute a recommendation orendorsement.

Ask an Energy ExpertDOEs Energy Efficiency and Renewable Energy Clearinghouse (EREC) P.O. Box 3048Merrifield, VA 22116Phone: 1-800-DOE-EREC (363-3732)TDD: 1-800-273-2957Fax: (703) 893-0400E-mail: doe.erec@nciinc.comOnline submittal form: www.eren.doe.gov/menus/energyex.htmlConsumer Energy Information Web site: www.eren.doe.gov/consumerinfo/Energy experts at EREC provide free general and technicalinformation to the public on many topics and technologies per-taining to energy efficiency and renewable energy.

DOEs Energy Efficiency and Renewable Energy Network (EREN) Web site: www.eren.doe.govYour comprehensive online resource for DOEs energy effi-ciency and renewable energy information.

OrganizationsCenter for Energy Efficiency and Renewable Energy(CEERT)1100 Eleventh St., Suite 311Sacramento, 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 technologiesand resources.

National Renewable Energy Laboratory (NREL)1617 Cole Blvd.Golden, CO 80401Web site: www.nrel.govDOE-lab devoted to researching and developing renewableenergy and energy efficiency technologies.

Renewable Energy Policy Project (REPP)1612 K St. NW, Suite 202Washington, DC 20006 Phone: (202) 293-2898; Fax: (202) 293-5857Web site: www.repp.orgWorks to advance renewable energy technologies.

Web Sites CADDET Renewable Energy Web site: www.caddet-re.orgProvides technical information on renewable energy projectsand technologies from around the world.

Clean Energy BasicsNRELWeb 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.comFeatures information and news on renewable energy technol-ogy developments in Europe and around the world.

Planet EnergyThe Renewable Energy Trail United Kingdom Department of Trade and Industry Web site: www.dti.gov.uk/renewable/ed_pack/index.htmlSpecifically gears its information for students and teachers,from grade school through high school.

Solstice Center for Renewable Energy and Sustainable Technology (CREST) Web site: http://solstice.crest.orgProvides an online source of information on renewable energyand technology development.

Further ReadingAchieving Energy IndependenceOne Step at a Time, J.Yago, Dunimis Technology, 1999, 190 pp.

Charging Ahead: The Business of Renewable Energy andWhat It Means for America, J. Berger and L. Thurow, Uni-versity of California Press, 1998, 416 pp.

Clean Energy Choices: Tips on Buying and Using RenewableEnergy at Home, DOE Office of Energy Efficiency andRenewable Energy, 2000, 48 pp. Print copy availablefrom EREC (see Ask an Energy Expert above), and aPDF is available at www.nrel.gov/docs/fy00osti/27684.pdf.

The Real Goods Solar Living Sourcebook: The Complete Guideto Renewable Energy Technologies and Sustainable Living, D.Pratt ed., Real Goods, 1999, 562 pp.