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Iceland’s Blue Lagoonand Svartsengigeothermal power plant

21New Renewable EnergyAlternatives

Upon successfully completing thischapter, you will be able to:

� Outline the major sources of renewable energyand assess their potential for growth

� Describe solar energy and the ways it is harnessed,and evaluate its advantages and disadvantages

� Describe wind energy and the ways it is harnessed,and evaluate its advantages and disadvantages

� Describe geothermal energy and the ways it isharnessed, and evaluate its advantages and disadvantages

� Describe ocean energy sources and the ways theycan be harnessed, and evaluate their advantagesand disadvantages

� Explain hydrogen fuel cells and assess futureoptions for energy storage and transportation

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The Viking explorers who first set foot centuries

ago on the remote island of Iceland were

trailblazers. Today the citizens of the nation of

Iceland are blazing a bold new path, one they believe the

rest of the world may follow. Iceland aims to become the

first nation to leave fossil fuels behind and convert to an

economy based completely on renewable energy.

Iceland is essentially a hunk of lava the size of

Kentucky that has risen out of the North Atlantic

from the rift between tectonic plates known as the

Mid-Atlantic Ridge. Most of Iceland’s 290,000 people live

in the capital city of Reykjavik, leaving much of the

island an unpeopled and starkly beautiful landscape of

volcanoes, hot springs, and glaciers.

The magma that gave birth to the island heats its

groundwater in many places, giving Iceland some of the

world’s best sources of geothermal energy. Iceland has

also dammed some of its many rivers for hydropower.

Together these two renewable energy sources provide

73% of the country’s energy supply and virtually all its

electricity generation. Yet the nation, like most others,

also depends on fossil fuels. Oil powers its automobiles,

some of its factories, and its economically vital fishing

fleet—and together these have given Iceland one of the

highest per capita rates of greenhouse gas emission in

the world. Because it possesses no fossil fuel reserves, all

fossil fuels must be imported—a weak link in an

otherwise robust economy that has given its citizens one

of the highest per capita incomes in the world.

Enter Bragi Árnason, a University of Iceland professor

popularly known as “Dr. Hydrogen.” Árnason began

620

Iceland’s hydrogenbuses arrive inReykjavik

Central Case: Iceland Moves toward a Hydrogen Economy

“I believe that water willone day be employed asfuel, that hydrogen andoxygen which constitute it,used singly or together, willfurnish an inexhaustiblesource of heat and light . . . .Water will be the coal of thefuture.”—Jules Verne, in The

Mysterious Island, 1874

“Our long-term vision is ofa hydrogen economy.”—Robert Purcell, Jr.,

executive director,

General Motors, 2000

AtlanticOcean

Europe

Iceland

NorthAmerica

Africa

bre2ch21_619_645 12/16/05 4:09 PM Page 620

arguing in the 1970s that Iceland could achieve

independence from fossil fuel imports, boost its

economy, and serve as a model to the world by

converting from fossil fuels to a renewable energy

economy based on hydrogen. He suggested zapping

water with Iceland’s cheap and renewable electricity in

a chemical reaction called electrolysis, splitting the

hydrogen from the oxygen and then using the hydrogen

to power fuel cells that would produce and store

electricity. The process is clean; nothing is combusted,

and the only waste product is water.

In the late 1990s Árnason’s countrymen began to

listen. The nation’s leaders decided to embark on a grand

experiment to test the efficacy of switching to a

“hydrogen economy.” By setting an example for the rest

of the world to follow, and by getting a head start at

producing and exporting hydrogen fuel, these leaders

hoped Iceland could become a “Kuwait of the North” and

get rich by exporting hydrogen to an energy-hungry

world, as Kuwait has done by exporting oil.

The leaders planning the shift to a hydrogen

economy sketched a stepped transition in which fossil

fuels would be phased out over 30–50 years. Conversion

of the Reykjavik bus fleet to run on hydrogen fuel is the

first step. After that, Iceland’s 180,000 private cars would

be powered by fuel cells, and then the fishing fleet would

be converted to hydrogen. The final stage would be the

export of hydrogen fuel to mainland Europe.

To make this happen, Icelanders in 1999 teamed up

with corporate partners looking to develop technology for

the future. Auto company Daimler-Chrysler is producing

hydrogen buses, oil company Royal Dutch Shell is running

hydrogen filling stations to fuel the buses, and Norsk

Hydro is providing electrolysis technology.

In 2003, the world’s first commercial hydrogen filling

station opened in Reykjavik, and three hydrogen-fueled

buses began operation. The public-private consortium,

Icelandic New Energy (INE), is monitoring the technology’s

effectiveness and the costs of developing infrastructure.

Iceland’s citizens are behind the effort; a recent poll

showed 93% support among Icelanders.

Meanwhile, Daimler-Chrysler has introduced trios of

hydrogen-fueled buses to nine other European cities.

Hydrogen buses are also being developed by other

companies and run in cities in Europe and throughout

the world, from Tokyo to Chicago to Perth to Winnipeg.

Hydrogen refueling stations are being demonstrated in

Japan, Singapore, and the United States, and fuel-cell

passenger automobiles are being tested in Japan and

California. A global hydrogen economy could be closer

than we suspect.

“New” Renewable EnergySources

Iceland’s bold drive toward a hydrogen economy is onefacet of a global move toward renewable energy. Acrossthe world, nations are taking different approaches to fig-ure out how to move away from fossil fuels while ensuringa continued supply of energy for their economies.

In Chapter 20 we explored the two renewable energysources that are most developed and widely used: biomassenergy, the energy from combustion of wood and otherplant matter, and hydropower, the energy from runningwater. These “conventional” alternatives to fossil fuels arerenewable energy sources that can be depleted with overuseand that exert some undesirable environmental impacts.

In this chapter we explore a group of alternative energysources that are often called “new renewables.” These in-clude energy from the sun, from wind, from Earth’s geo-thermal heat, and from the movement of ocean water.These energy sources are not truly new. In fact, they are asold as our planet, and people have used them for millen-nia. They are commonly referred to as “new” because(1) they are not yet used on a wide scale in our modernindustrial society, as are fossil fuels and the conventionalrenewable alternatives; (2) they are harnessed using tech-nologies that are still in a rapid phase of development; and(3) it is widely believed that they will come to play a largerrole in our energy use in the future.

The new renewables currently providelittle of our power

The new renewable energy sources currently provide only0.5% of our global primary energy supply. Fossil fuelsprovide 80% of the world’s primary energy, nuclear en-ergy provides 6.5%, and renewable energy sourcesaccount for 13.5%, nearly all of which is provided bybiomass and hydropower (see Figure 20.1a, � p. 591). Thenew renewables make a similarly small contribution to ourglobal generation of electricity (see Figure 20.1b, � p. 591).Less than 18% of our electricity comes from renewable en-ergy, and of this amount, hydropower accounts for 90%.

Nations and regions vary in the renewable sourcesthey use. In the United States, most energy supplied by re-newable sources comes from hydropower and biomass, innearly equal proportions. As of 2004, geothermal energyaccounted for 5.6%, wind energy for 2.3%, and solar en-ergy for 1.0% (Figure 21.1a). Of electricity generated inthe United States from renewables, hydropower accounts

CHAPTER TWENTY-ONE New Renewable Energy Alternatives 621

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for nearly 75%, and biomass 14%. As of 2004, geothermalpower accounted for 8.3%, wind for 3.9%, and solar forjust 0.2% (Figure 21.1b).

The new renewables are growing fast

Although they comprise only a minuscule proportion ofour energy budget, the new renewable energy sources aregrowing at much faster rates than are conventional energysources. Over the past three decades, solar, wind, geother-mal, and ocean energy sources have grown far faster thanhas the overall primary energy supply (Figure 21.2). Among

renewable sources, the leader in growth is wind power,which has expanded by about 50% each year over the pastthree decades. Because these sources started from suchlow levels of use, however, it will take them some time tocatch up to conventional sources. For instance, the ab-solute amount of energy added by a 50% increase in windpower is still less than the amount added by just a 1%increase in oil, coal, or natural gas.

The rapid growth of the new renewables has been mo-tivated by concerns over diminishing fossil fuel suppliesand the environmental impacts of fossil fuel combustion(Chapter 19). As these concerns build, advances in tech-nology are making it easier and less expensive to harnessrenewable energy sources. The new renewables promiseseveral benefits over fossil fuels. As they replace fossil fuels,they help alleviate air pollution and the greenhouse gasemissions that drive global climate change (Chapter 18).Unlike fossil fuels, renewable sources are inexhaustible ontime scales relevant to human societies. Developing renew-ables can also help diversify an economy’s mix of energy,lowering price volatility and protecting against supply re-strictions such as those caused by the 1973 oil embargo orby Hurricane Katrina in 2005 (� p. 579). New energysources also can create new employment opportunities andsources of income and property tax for local communities,especially in rural areas passed over by other economic

622 PART TWO Environmental Issues and the Search for Solutions

Only 6% of the total primary energy consumed inthe United States each year comes from renewable sources. Of thisamount, most derives from hydropower and biomass energy.Geothermal energy accounts for 5.6% of this amount, wind for2.3%, and solar for 1.0% (a). Similarly, only 9% of electricitygenerated in the United States comes from renewable energysources, predominantly hydropower and biomass. Geothermalenergy accounts for 8.3%, wind for 3.9%, and solar for only 0.2%(b). Data from Energy Information Administration, U.S. Department of

Energy. 2005. Annual Energy Review, 2004.

FIGURE 21.1

(b) U.S. total electricity generation fromrenewable sources, 2004

Geothermal(5.6%)

Wind(2.3%)

Solar(1.0%)

Geothermal(8.3%)

Wind(3.9%)

Solar(0.2%)

Hydropower44.6%

Hydropower73.6%

Biomass46.5%

Biomass14.0%

(a) U.S. total primary energy fromrenewable sources, 2004

The “new renewable” energy sources are growingsubstantially faster than the total primary energy supply. Solarpower has grown by 32% each year since 1971, and wind powerhas grown by 52% each year. Because these sources began fromsuch low starting levels, however, their overall contribution to ourenergy supply is still small. Go to at www.aw-bc.com/withgott or on the student CD-ROM. Data from International Energy

Agency Statistics, 2002.

FIGURE 21.2

Total primary

energy supply

GeothermalSolar Wind Ocean

Energy source

10

0

20

40

30

60

50

Ann

ual g

row

th r

ate,

197

1–20

00 (%

)

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development. In many rural areas and developing coun-tries, locally based renewable sources may prove cheaper touse for electricity than would extending the electricity gridinfrastructure out from cities.

Rapid growth in renewable energy sectors seems likelyto continue as population and consumption grow, globalenergy demand expands, fossil fuel supplies decline, andcitizens demand cleaner environments. More govern-ments, utilities, corporations, and consumers are nowpromoting and using renewable energy sources, and, as aresult, the costs of renewables are falling.

The transition cannot be immediate, butit must be soon

If our civilization is to persist in the long term, it will needto shift to renewable energy sources. A key question iswhether we will be able to shift soon enough and smoothlyenough to avoid widespread war, social unrest, and fur-ther damage to the environment. The answer to this ques-tion will largely determine the quality of life for all of us inthe coming decades.

We cannot switch completely to renewable energysources overnight, because there are technological andeconomic barriers. Currently, most renewables lack ade-quate technological development and lack infrastructureto transfer power on the required scale. However, dra-matic improvements in technology and infrastructure inrecent years suggest that most of the remaining barriersare political. Renewable energy sources have received farless in subsidies, tax breaks, and other incentives fromgovernments than have conventional sources. By one es-timate, of the $150 billion in subsidies the U.S. govern-ment provided to nuclear, solar, and wind power in thepast half century, the nuclear industry received 96%, thesolar industry received 3%, and the wind industry lessthan 1%. For decades, research and development of re-newable sources have suffered from the continuingavailability of fossil fuels made inexpensive in part bygovernment policy.

Many corporations in the fossil fuel and automobileindustries understand that they will eventually need toswitch to renewable sources. They also know that when thetime comes, they will need to act fast to stay ahead of theircompetitors. However, in light of continuing short-termprofits and unclear policy signals, companies have notbeen eager to invest in the transition. Under these circum-stances, our best hope may be for a gradual transition fromfossil fuels to renewable energy sources, one driven largelyby economic supply and demand. However, if the transi-tion proceeds too slowly—if we wait solely for economicsto do its work, without government encouragement—

diminishing fossil fuel supplies could outpace our ability todevelop new sources, and we could find our economiesgreatly disrupted, and our environment greatly degraded.Thus, encouraging the speedy development of renewableenergy alternatives holds promise for bringing us a vigor-ous and sustainable energy economy without the environ-mental impacts of fossil fuels.

Solar Energy

The sun provides energy for almost all biological activityon Earth (� pp. 105–106) by converting hydrogen to he-lium in nuclear fusion (� pp. 595–596). Each square meterof Earth’s surface receives about 1 kilowatt of solar energy—17 times the energy of a lightbulb. As a result, an average-sized house whose roof is covered in panels that harnesssolar energy has enough roof area to generate all its powerneeds. The sun’s raw energy is so strong that if only 0.1% ofEarth’s surface—roughly the combined area of New Mexicoand South Dakota—were covered with solar panels, wewould have enough solar energy to power all the world’selectrical plants. The amount of energy Earth receivesfrom the sun each day, if it could be collected in full for ouruse, would be enough to power human consumption for27 years.

Clearly, the potential for using sunlight to meet ourenergy needs is tremendous. However, all this “free” en-ergy from the sun cannot be harnessed just yet. We are stillin the process of developing solar technologies and learn-ing the most effective and cost-efficient ways to put thesun’s energy to use.

The most commonly used way to harness solar energyis through passive solar energy collection. In this approach,buildings are designed and building materials are chosen tomaximize direct absorption of sunlight in winter, even asthey keep the interior cool in the heat of summer. This ap-proach contrasts with active solar energy collection, whichmakes use of technological devices to focus, move, or storesolar energy.

We tend to think of using solar power as a novel phe-nomenon, but people have chosen and designed their livingsites with passive solar collection in mind for millennia.Moreover, solar energy was first harnessed with active solartechnology in 1767, when Swiss scientist Horace de Saussurebuilt a thermal solar collector to heat water and cook food.In 1891, U.S. inventor Clarence Kemp claimed the firstcommercial patent for a solar-powered water heater. TwoCalifornia entrepreneurs bought the patent rights and out-fitted one-third of the homes in Pasadena, California, withsolar water heaters by 1897.


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Passive solar heating is simple and effective

One passive solar design technique involves installing low,south-facing windows to maximize sunlight capture in thewinter (in the Northern Hemisphere; north-facing win-dows are used in the Southern Hemisphere). Overhangsblock light from above, shading these windows in the sum-mer, when the sun is high in the sky and when cooling, notheating, is desired. Passive solar techniques also include theuse of heat-absorbing construction materials. Often calledthermal mass, these materials absorb heat, store it, and re-lease it later. Thermal mass (of straw, brick, concrete, orother materials) most often makes up floors, roofs, andwalls, but also can comprise portable blocks.

Thermal mass may be strategically located to capturesunlight in cold weather and radiate heat in the interior ofthe building. In warm weather, the mass should be locatedaway from sunlight so that it absorbs warmed air in theinterior to cool the building. Passive solar design can alsoinvolve planting vegetation in particular locations arounda building. By heating buildings in cold weather and cool-ing them in warm weather, passive solar methods helpconserve energy and reduce energy costs.

Active solar energy collection can heat airand water in buildings

One active method for harnessing solar energy involvesusing solar panels or flat-plate solar collectors, most ofteninstalled on rooftops. These panels generally consist ofdark-colored, heat-absorbing metal plates mounted inflat boxes covered with glass panes. Water, air, or

antifreeze solutions are run through tubes that passthrough the collectors, transferring heat throughout abuilding. Heated water can be pumped to tanks to storethe heat for later use and through pipes designed to re-lease the heat into the building. Such systems have provenespecially effective for heating water for residences.

Active solar energy is being used for heating, cooling,and water purification in Gaviotas, a remote town in thehigh plains of Colombia far from any electrical grid. Engi-neers have developed inexpensive solar panels that har-vest enough solar energy, even under cloudy skies, to boildrinking water for a family of four. They also have de-signed, built, and installed a unique solar refrigerator in arural hospital, along with a large-scale solar collector toboil and sterilize water. Their innovations show that solarpower does not need to be expensive or confined to re-gions that are always sunny.

Concentrating solar rays magnifies theenergy received

The strength of solar energy can be magnified by gather-ing sunlight from a wide area and focusing it on a singlepoint. This is the principle behind solar cookers, simpleportable ovens that use reflectors to focus sunlight ontofood and cook it (see Figure 3.13, � p. 71). Such cookersare becoming widespread and proving extremely useful inparts of the developing world.

The principle of concentrating the sun’s rays has alsobeen put to work by utilities in large-scale, high-techapproaches to generating electricity. In one approach,mirrors concentrate sunlight onto a receiver atop a tall“power-tower” (Figure 21.3). From the receiver, heat is


At the Solar Two facilityoperated by Southern California Edison inthe desert of southern California, mirrorsare spread across wide expanses of land toconcentrate sunlight onto a receiver atop a“power-tower.” Heat is then transportedthrough fluid-filled pipes to a steam-driven generator that produces electricity.

FIGURE 21.3

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transported by fluids (often molten salts) piped to a steam-driven generator to create electricity. These solar powerplants can harness light from large mirrors spread acrossmany hectares of land. The world’s largest such plant sofar—a collaboration among government, industry, andutility companies in the California desert—producespower for 10,000 households. Another approach is the useof solar-trough collection systems, which consist of mir-rors that gather sunlight and focus it on oil in troughs. Thesuperheated oil creates steam that drives turbines, as inconventional power plants.

Photovoltaic cells generate electricitydirectly from sunlight

A more direct approach to producing electricity fromsunlight involves photovoltaic (PV) systems. Photovoltaic

(PV) cells collect sunlight and convert it to electrical energydirectly by making use of the photovoltaic, or photoelectric

effect, first proposed in 1839 by French physicist EdmundBecquerel. This effect occurs when light strikes one of apair of metal plates in a PV cell, causing the release of elec-trons, which are attracted by electrostatic forces to the op-

posing plate. The flow of electrons from one plate to theother creates an electrical current, which can be convertedinto alternating current (AC) and used for residential andcommercial electrical power (Figure 21.4).

The plates of a PV cell are made primarily of silicon,enriched on one side with phosphorus and on the otherwith boron. Silicon is a semiconductor, so it conducts andcontrols the flow of electricity. Because of the chemicalproperties of boron and phosphorus, the phosphorus-enriched side has excess electrons, and the boron-enrichedside has fewer electrons. When sunlight strikes the cellsurface, it transfers energy and causes electrons to move.When wires connect the two sides, electricity is created aselectrons flow from the phosphorus-enriched side to theboron-enriched side. Photovoltaic cells can be connectedto batteries that store the accumulated charge until it isneeded.

You may be familiar with small PV cells that poweryour watch or your calculator. Atop the roofs of homesand other buildings, multiple PV cells are arranged inmodules. These modules can comprise panels, which canbe gathered together in flat arrays. Increasingly, PV roof-ing tiles are being used instead of these arrays. PV roofing

A photovoltaic (PV) cell converts sunlight to electrical energy. When sunlight hits alayer of silicon infused with phosphorus, electrons are released and travel toward the layer of siliconlaced with boron. This movement of electrons induces an electric current, producing electricity. PVcells are grouped in modules, which can comprise panels, which can be erected in arrays.

FIGURE 21.4

Boron-enrichedsilicon

Junction

Phosphorus-enrichedsilicon

Electricitygenerated(direct current)

Photovoltaiccell

Module

Panel

Light

Array


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tiles look like normal roofing shingles but generate elec-tricity by the photovoltaic effect. In some remote areas,such as Xcalak, Mexico, PV systems are being used incombination with wind turbines (� pp. 627–628) and adiesel generator to power entire villages.

Solar power is little used but fast growing

Although active solar technology dates from the 18th cen-tury, it was pushed to the sidelines as fossil fuels gained astronger foothold in our energy economy. Even as solartechnology was being refined for applications rangingfrom handheld calculators to spacecraft, it was not beingdeveloped for the roles that fossil fuels have filled. In re-cent U.S. history, funding for research and developmentof solar technology has been erratic. After the 1973 oilembargo, the U.S. Department of Energy funded the in-stallation and testing of over 3,000 PV systems, providinga boost to companies in the solar industry. As oil pricesdeclined, however, so did government support for solarpower, and funding for solar has remained far below thatfor fossil fuels.

Largely because of the lack of investment, solar powercurrently contributes only a minuscule portion of our en-ergy production. In 2004, solar accounted for only 0.06%—less than 6 parts in 10,000—of the U.S. primary energysupply, and only 0.02% of U.S. electricity generation. How-ever, use of solar energy has grown by nearly one-third an-nually worldwide since 1971, a growth rate second only tothat of wind power. Solar power is proving especially at-tractive in developing countries, many of which are rich insun but poor in power infrastructure, and where hundredsof millions of people are still without electricity. Somemultinational corporations that built themselves on fossilfuels are now investing in alternative energy as well. BP So-lar, British Petroleum’s solar energy wing, recently com-pleted $30 million projects in the Philippines andIndonesia, and is working on a $48 million project tosupply electricity to 400,000 people in 150 villages.

Sales of PV cells are growing fast—by 25% per year inthe United States, for instance, and by 63% annually inJapan, which uses PV roofing tiles widely. Use of solartechnology is widely expected to continue increasing asprices fall, technologies improve, and economic incentivesare enacted. However, the very small amount of energycurrently produced by solar power means that its marketshare will likely remain small for years or decades tocome—unless governments, businesses, and consumersbecome more motivated by the benefits that solar energycan provide.

Solar power offers many benefits

The fact that the sun will continue burning for another4–5 billion years makes it practically inexhaustible as anenergy source for human civilization. Moreover, theamount of solar energy reaching Earth’s surface should beenough to power our civilization once solar technology isadequately developed. Although these overarching bene-fits of solar energy are clear, the technologies themselvesalso provide benefits. PV cells and other solar technolo-gies use no fuel, are quiet and safe, contain no movingparts, require little maintenance, and do not even requirea turbine or generator to create electricity. An average unitcan produce energy for 20–30 years.

Another advantage of solar systems is that they allowfor local, decentralized control over power. Homes, busi-nesses, and isolated communities can use solar power toproduce their own electricity and may not need to be neara power plant or connected to the grid of a city.

In developing nations, solar cookers enable families tocook food without gathering fuelwood; as a result, theylessen people’s daily workload and help reduce deforesta-tion. In locations such as refugee camps, solar cookers arehelping relieve social and environmental stress. The lowcost of solar cookers—many can be built locally for $2–10each—has made them available for purchase or donationin many impoverished areas.

In the developed world, most PV systems today areconnected to the regional electric grid. As a result, ownersof houses with PV systems can sell their excess solar en-ergy to their local power utility through a process calledtwo-way metering. The value of the power the consumersells to the utility is subtracted from the consumer’smonthly utility bill.

Finally, a major advantage of solar power over fossilfuels is that it does not pollute the air with greenhousegas emissions and other air pollutants. The manufac-ture of photovoltaic cells does currently require fossilfuel use, but once it is up and running, a PV system pro-duces no emissions. Consumers may be able to accessonline calculators to calculate the economic and envi-ronmental impacts of installing PV solar cells. For ex-ample, a calculator offered by BP Solar estimated thatinstalling a 5-kilowatt PV system in a home in Dallas,Texas, can provide 51% of annual power needs, save$391 per year on energy bills, and prevent the emissionof 9,023 lb of carbon dioxide from fossil fuel combus-tion. Even in overcast Seattle, Washington, a 5-kilowattsystem can produce 32% of energy needs, save $336 peryear, and prevent the emission of 6,419 lb of carbondioxide.


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Location and cost can be drawbacks forsolar power

Solar power currently has two major disadvantages. One isthat not all regions are sunny enough to provide adequatepower, given current technology. Although Earth as a wholereceives vast amounts of sunlight,not every location on Earthdoes (Figure 21.5). People in cities such as Seattle might findit difficult to harness enough sunlight most of the year to relyon solar power. Daily or seasonal variation in sunlight canalso pose problems for stand-alone solar systems if storagecapacity in batteries or fuel cells is not adequate or if backuppower is not available from a municipal electricity grid.

The primary disadvantage of current solar technol-ogy— as with other renewable sources—is the up-front costof investing in the equipment. The investment cost for solaris higher than that for fossil fuels, and indeed, solar powerremains the most expensive way to produce electricity. Pro-ponents of solar power argue that decades of governmentpromotion of fossil fuels and nuclear power—which havereceived many financial breaks that solar power has not—have made solar power unable to compete. Because the ex-ternal costs (� pp. 43–44) of nonrenewable energy have notbeen included in market prices, these energy sources haveremained relatively cheap. Governments, businesses, andconsumers thus have had little economic incentive to switchto solar and other renewables.

However, decreases in price and improvements inenergy efficiency of solar technologies so far are encour-aging, even in the absence of significant financial com-mitment from government and industry. At their adventin the 1950s, solar technologies had efficiencies of around6%, while costing $600 per watt. Recent single-crystalsilicon PV cells are showing 15% efficiency commer-cially and 24% efficiency in lab research, suggesting thatfuture solar technologies may be more efficient than anyenergy technologies we have today. Solar systems havebecome much cheaper over the years and now can oftenpay for themselves in 10–15 years. After that time, theyprovide energy virtually for free as long as the equipmentlasts. With future technological advances, some expertsbelieve that the time to recoup investment could fall to1–3 years.

Wind Energy

Wind energy can be thought of as an indirect form of so-lar energy, because it is the sun’s differential heating of airmasses on Earth that causes wind to blow. We can harnesspower from wind by using devices called wind turbines,

mechanical assemblies that convert wind’s kinetic energy,or energy of motion, into electrical energy.


Because somelocations receive more sunlightthan others, harnessing solarenergy is more profitable in someareas than in others. In the UnitedStates, many areas of Alaska andthe Pacific Northwest receive only3–4 kilowatt-hours per squaremeter per day, whereas most areasof the Southwest receive 6–7kilowatt-hours per square meterper day. Data from National

Renewable Energy Laboratory, U.S.

Department of Energy, 2005.

FIGURE 21.5

6.5–7.0

Solar radiation(kWh/m2/day)

6.0–6.55.5–6.05.0–5.54.5–5.0

4.0–4.53.5–4.03.0–3.5

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Wind has long been used for energy

Today’s wind turbines have their historical roots inEurope, where wooden windmills have been used for 800years. The Netherlands in particular is known for itswindmills, whose power has been used to pump water todrain wetlands and irrigate crops, and to grind grain intoflour. In each application, wind causes a windmill’s bladesto turn, driving a shaft connected to several cogs that turnwheels, which either grind grain or pull buckets from awell. In the United States, countless ranches in arid areasof the West and Great Plains feature windmills that drawgroundwater up to supply thirsty cattle.

The first wind turbine built to generate electricity wasconstructed in the late 1800s in Cleveland, Ohio, by inven-tor Charles Brush, who designed a turbine 17 m (50 ft) tallwith 144 rotor blades made of cedar wood. But technologyadvanced slowly during the 20th century, and it was not un-til after the 1973 oil embargo that wind energy was fundedby governments in North America and Europe. This mod-erate infusion of funding for research and developmentboosted technological progress, and the cost of wind powerwas cut in half in less than 10 years. Today wind power at fa-vorable locations generates electricity for nearly as little costper kilowatt-hour as do conventional sources, and modernwind turbines appear more like airplane propellers or sleeknew helicopters than romantic old Dutch paintings.

Modern wind turbines convert kineticenergy to electrical energy

Wind blowing into a turbine turns the blades of the rotor,which rotate the machinery inside a compartment called anacelle, which sits atop a tall tower (Figure 21.6). Insidethe nacelle are a gearbox and a generator, as well as

equipment to monitor and control the turbine’s activity.Most of today’s towers range from 40 to 100 m (131–328 ft)tall. Higher is generally better, to minimize turbulence(and potential damage) and to maximize wind speed.Most rotors consist of three blades and measure 42–80 m(138–262 ft) across. Turbines are designed to yaw, or ro-tate back and forth in response to changes in wind direc-tion, ensuring that the motor faces into the wind at alltimes. Turbines can be erected singly, but they are mostoften erected in groups called wind parks, or wind farms.The world’s largest wind farms contain several hundredor thousand turbines spread across the landscape.

Engineers have designed turbines to begin turning atspecific wind speeds to harness wind energy as efficientlyas possible. Some turbines create low levels of electricityby turning in light breezes. Others are programmed to ro-tate only in strong winds, operating less frequently butgenerating large amounts of electricity in short time peri-ods. Slight differences in wind speed can yield substantialdifferences in power output, for two reasons. First, the en-ergy content of a given amount of wind increases as thesquare of its velocity; thus if wind velocity doubles, energyquadruples. Second, an increase in wind speed causesmore air molecules to pass through the wind turbine perunit time, making power output equal to wind velocitycubed. Thus a doubled wind velocity actually results in aneightfold increase in power output.

Wind power is the fastest-growingenergy sector

Like solar energy, wind provides only a minuscule pro-portion of the world’s power needs, but wind power isgrowing fast—nearly 30% per year globally between 2000and 2004. Wind provided 3.9% of U.S. renewable electricity


A wind turbineconverts wind’s energy of motioninto electrical energy. Wind causesthe blades of a wind turbine tospin, turning a shaft that extendsinto the nacelle that is perchedatop the tower. Inside the nacelle,a gearbox converts the rotationalspeed of the blades, which can beup to 20 revolutions per minute(rpm) or more, into much higherrotational speeds (over 1,500 rpm).These high speeds provide adequatemotion for a generator inside thenacelle to produce electricity.

FIGURE 21.6

Blades

NacelleTower

Gearbox(increases rotational

speed of blades)

Generator(produceselectricity)

bre2ch21_619_645 12/16/05 4:09 PM Page 628

generation in 2004—a small amount but nearly 20times more than solar power. So far, wind energy produc-tion is geographically concentrated; only five nations ac-count for 82% of the world’s wind energy output (Figure21.7). California and Texas account for two-thirds of thewind power generated within the United States. Denmarkis a leader in wind power; there, a series of wind farmssupplies over 20% of the nation’s electricity needs. Ex-

perts agree that wind power’s rapid growth will continue,because only a very small portion of this resource is cur-rently being tapped. Meteorological evidence suggeststhat wind power could be expanded in the United Statesto meet the electrical needs of the entire country (see “TheScience behind the Story,” � p. 632).

Offshore sites can be promising

Wind speeds on average are roughly 20% greater over waterthan over land. There is also less air turbulence over watersurfaces than over land surfaces. For these reasons, offshorewind turbines are becoming popular Figure 21.8). Al-though costs to erect and maintain turbines in water arehigher, the stronger, less turbulent winds produce morepower and make offshore wind potentially more profitable.Currently, offshore wind farms are limited to shallow water,where towers are sunk into sediments singly or with a tri-pod configuration to stabilize them. However, in the futuretowers may be placed on floating pads anchored to theseafloor in deep water. At great distances from land, it maybe best to store the generated electricity as hydrogen andthen ship or pipe this to land (instead of building subma-rine cables to carry electricity to shore), but further re-search is needed on this option.

Denmark erected the first offshore wind farm in 1991.Over the next decade, nine more came into operation acrossnorthern Europe, where the North and Baltic Seas offerstrong winds. The power output of these farms increased by43% annually as larger turbines were erected. Several north-ern European nations are encouraging continued rapid


Most of the world’s fast-growing wind powergenerating capacity is concentrated in a handful of countries. TinyDenmark obtains the highest percentage of its energy needs fromwind, but the larger nations of Germany, the United States, andSpain have so far developed more total wind capacity. Data from

Global Wind Energy Council; and American Wind Energy Association.

2005. Global wind energy market report. AWEA.

FIGURE 21.7

Spain(16%)

Denmark(8%)

Rest of world(18%)

India(5%)U.S.

(16%)

Germany(37%)

More and more wind farmsare being developed offshore, becauseoffshore winds tend to be stronger yet lessturbulent. Denmark is a world leader inwind power, and much of it comes fromoffshore turbines. This Danish wind farm isone of several that provide over 20% of thenation’s electricity.

FIGURE 21.8

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growth in the near future. Wind advocates in Iceland areconsidering developing 240 offshore wind turbines in thenation’s waters to meet future electricity demand for itshydrogen economy.

Wind power has many benefits

Like solar power, wind produces no emissions once thenecessary equipment is manufactured and installed. As areplacement for fossil fuel combustion in the average U.S.utility generator, the U.S. Environmental ProtectionAgency has calculated that running a 1-megawatt windturbine for one year prevents the release of more than1,500 tons of carbon dioxide, 6.5 tons of sulfur dioxide,3.2 tons of nitrogen oxides, and 60 lb of mercury. Theamount of carbon pollution that all U.S. wind turbinestogether prevent from entering the atmosphere is greaterthan the cargo of a 50-car freight train, with each carholding 100 tons of solid carbon, each and every day.

Wind power appears considerably more energy-efficient than conventional power sources. One recentstudy, which compared the amount of energy that varioustypes of technology produce to the amount they consume,found that wind turbines produce 23 times as much asthey consume. For nuclear energy, the ratio was 16:1; forcoal it was 11:1; and for natural gas it was 5:1. Wind farmsalso use less water than do conventional power plants.

Wind turbine technology can be used on many scales,from a single tower for local use to fields of thousands thatsupply large regions. Small-scale turbine developmentcan help make local areas more self-sufficient, just as solarenergy can. For instance, the Rosebud Sioux Tribe of Na-tive Americans in 2003 set up a single turbine on theirreservation in South Dakota. The turbine is producingelectricity for 220 homes and brings the tribe an esti-mated $15,000 per year in revenue. Wind resources arerich in this region, and the tribe plans to develop a windfarm nearby in coming years.

Another societal benefit of wind power is thatlandowners can lease their land for wind development,which provides them extra revenue while also increasingproperty tax income for rural communities. A single largeturbine can bring in $2,000–4,500 in annual royaltieswhile occupying just a quarter-acre of land. Because eachturbine takes up only a small area, most of the land canstill be used for farming, ranching, or other uses.

Economically, wind energy involves up-front costs forthe erection of turbines and the expansion of infrastructureto allow electricity distribution, but over the lifetime of aproject it requires only maintenance costs. Unlike fossil fuelpower plants, the turbines incur no ongoing fuel costs.

Currently, startup costs of wind farms generally are higherthan those of fossil-fuel-driven plants, but wind farmsincur fewer expenses once they are up and running. Ad-vancing technology is driving down the costs of wind farmconstruction; as large wind farms become more efficient,the cost of each unit of electricity produced is dropping.

Wind energy has some downsides

Unlike power sources that can be turned off and on atwill, wind is an intermittent resource; we have no controlover when wind will occur. This poses little problem,however, if wind is only one of several sources contribut-ing to a utility’s power generation. Moreover, several tech-nologies are available to address problems posed byrelying on intermittent wind resources. For example, bat-teries or hydrogen fuel can store energy generated by windand release it later when needed.

Just as wind varies from time to time, it also varies fromplace to place. Some areas are simply windier than others.Global wind patterns combine with local topography—mountains, hills, water bodies, forests, cities—to create localwind patterns, and companies study these patterns closelybefore investing in a wind farm. Meteorological research hasgiven us information with which to judge prime areas for lo-cating wind farms. A map of average wind speeds across theUnited States (Figure 21.9a) shows that mountainous re-gions and areas of the Great Plains are best. Based on suchinformation, the young wind power industry has locatedmuch of its generating capacity in states with high windspeeds (Figure 21.9b), and is seeking to expand in the GreatPlains and mountain states. Provided that wind farms arestrategically erected in optimal locations, an estimated 15%of U.S. energy demand could be met using only 43,000 km2

(16,600 mi2) of land (with less than 5% of this land area ac-tually occupied by turbines, equipment, and access roads).

Good wind resources, however, are not always nearpopulation centers that need the energy. Thus, transmis-sion networks would need to be greatly expanded. More-over, when wind farms are proposed near populationcenters, local residents often oppose them. Turbines aregenerally located in exposed, conspicuous sites, and manypeople object to wind farms for aesthetic reasons, feelingthat the structures clutter the landscape. Although pollsshow wide public approval of wind projects in regionswhere wind power has already been introduced, new windprojects often elicit the so-called not-in-my-backyard

(NIMBY) syndrome. For instance, a proposal for NorthAmerica’s first offshore wind farm, in Nantucket Soundbetween Cape Cod and the islands of Nantucket andMartha’s Vineyard, has faced stiff opposition from wealthy

bre2ch21_619_645 12/16/05 4:09 PM Page 630


400–1,000

Wind power density at 10 m height(watts/m2)

300–400250–300200–250150–200

100–1500–100

1,000–2,100100–1,000

20–1001–20

0

WA(240)

(b) Wind generating capacity, 2004

(a) Annual average wind power

OR(263)

CA(2096)

WY(285)

MT(2)

CO(229) KS

(114)

OK(176)

MN(615)

IA(632)

WI(53)

IL(51)

MI(2)

OH(7)

NM(267)

TX(1293)

AK(1)

HI (9)

Wind power capacity(megawatts)

TN (29)

PA (129)

VT (6)

NY(48)

MA (1)

WV (66)

ND(66)

SD(44)

NE(14)

Wind’s capacity to generate power varies according to wind speed. Meteorologistshave measured wind speed to calculate the potential generating capacity from wind in differentareas. The map in (a) shows average wind power in watts per square meter at a height of 10 m (33 ft) above ground across the United States. Such maps are used to help guide placement of windfarms. The development of U.S. wind power so far is summarized in (b), which shows themegawatts of generating capacity developed in each state through the end of 2004. Sources: (a) Elliott,

D. L., et al. 1987. Wind energy resource atlas of the United States. Golden, CO: Solar Energy Research Institute;

(b) National Renewable Energy Laboratory, U.S. Department of Energy, 2005.

FIGURE 21.9

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Where does windtranslate into energy?

In Idaho, where resourceplanners decided that the state’spower future lies in generating elec-tricity from wind. Now scores ofIdahoans, from small farmers toNative American tribes, have joinedin the search for gusts with energypotential.

By handing out wind-measuringdevices to interested landowners,Idaho has turned its citizens into“wind prospectors” who pinpointpotential areas for wind farms.Idaho first launched the public windprospecting program in 2001, afterjoining several other northwesternstates in a regional research effort.People who join the program mustcollect data on wind speed and di-rection, share that data with thestate, and agree to make it public.

A promising wind farm site re-quires some infrastructure, such asroads for erecting wind turbines andtransmission lines for sending outpower the turbines generate. But thesingle most important factor is thespeed and frequency of the wind.Effective commercial wind farmshave a steady flow of wind justabove ground level, with regulargusts of at least 21 km/hr (13 mi/hr)at a height of about 50 m (164 ft).

Determining whether a sitemerits further study starts with ana-lyzing existing data. In many partsof the developed world, decadesworth of weather information havebeen compiled into computerizedwind maps that indicate generalwind conditions. In Idaho, energyplanners provide prospectors withstarter maps that divide the stateinto seven wind “classes” and reveal

which general areas might haveenough wind to make a wind farmworthwhile. Areas listed as “Class 3”or higher, with wind speeds ofabout 23 km (14.3 mi) per hour at50 m (164 ft) above ground, offerthe best possibilities.

Such maps, however, may notprovide enough detail about a spe-cific location. A piece of property,for example, may sit in a Class 3area but be sheltered by a small hillthat blocks the wind. Knowing thatkind of detail requires site-specificon-the-ground research.

To make such research possible,Idaho loans landowners in areaslisted as Class 3 or higher devicescalled anemometers (see the figure),which measure wind speed and di-rection. The Idaho program uses acommon cup anemometer, with an

array of three or four hollow cupsset to catch the wind and rotatearound a vertical rod. The force ofwind on the cups causes them to ro-tate at a speed proportional to thewind speed; the greater the wind,the faster the cups rotate. Wind di-rection is measured by a vane thatturns on a vertical axis pointing di-rectly into the wind. The cup wheeland wind vane are connected elec-trically to speed and direction dials,which relay wind data.

More than 80 landowners bor-rowed anemometers from the statein the Idaho program’s first year, andsent data to the state every 60 daysfor review by energy planners andfor subsequent posting online. Thestudies have generated new fundingand wind farm plans in the state. Inthe fall of 2003, one farm near IdahoFalls won a $500,000 federal grant tohelp build a 1.5-megawatt wind farmthat could supply power for approxi-mately 500 homes.

In eastern Idaho, five anemome-ters set up on Shoshone-Bannocktribal lands have revealed goodprospects for a commercial windfarm on two Native American reser-vations. The research effort hasshown that the lands are “world-class sites” for wind power, accord-ing to a state energy official. Withaverage wind speeds in the 29 km/hr(18 mi/hr) range, further study ofthe tribal lands revealed possiblesites for large-scale commercialwind farms, which could mean jobsand revenue for the reservations.Similar wind prospecting programsare now under way on other reser-vations, as well as in other states,including Utah, Oregon, Virginia,and Missouri.

TheScience

behind

theStory

Idaho’s Wind Prospectors

To determine where to build windfarms, Idaho’s wind prospectors useanemometers, which collect and relaywind data. Cup wheels rotate to indicatewind speed, and a vane turns on a verti-cal axis to reveal wind direction.

bre2ch21_619_645 12/21/05 3:59 PM Page 632


area residents, even though many of these residents like tothink of themselves as environmentalists.

Wind turbines are also known to pose a threat to birdsand bats, which can be killed when they fly into the rotatingblades. At California’s Altamont Pass wind farm, which is lo-cated in a region with one of the densest populations ofgolden eagles in the country, turbines killed many eagles andother raptors during the 1990s. Studies since then at othersites have suggested that bird deaths may be a less severeproblem than was initially feared. It has been estimated thatroughly one to two birds are killed per turbine per year—farfewer than the millions already being killed annually by tel-evision, radio, and cell phone towers, tall buildings, automo-biles, domestic cats, pesticides, and other human causes. Batmortality may be higher, but more research is needed. Thekey for protecting birds and bats seems to be selecting sitesthat are not on flyways or in the midst of prime habitat forspecies that are likely to fly into the blades.

Weighing the Issues:

Wind and NIMBY

If you could choose to get your electricity from a wind

farm or a coal-fired power plant, which would you

choose? How would you react if the electric utility

proposed to build the wind farm that would generate

your electricity atop a ridge running in back of your

neighborhood, such that the turbines would be clearly

visible from your living room window? Would you

support or oppose the development? Why? If you would

oppose it, where would you suggest the farm be located?

Do you think anyone might oppose it in that location?

Geothermal Energy

Geothermal energy is one form of renewable energy thatdoes not originate from the sun. Instead, it is generatedfrom deep within Earth. The radioactive decay of ele-ments amid the extremely high pressures deep in the inte-rior of our planet generates heat that rises to the surfacethrough magma (molten rock, � p. 206) and through fis-sures and cracks. Where this energy heats groundwater,natural spurts of heated water and steam are sent up frombelow. Terrestrial geysers and submarine hydrothermalvents (� pp. 106–107) are the surface manifestations ofthese processes. Iceland is built from magma that ex-truded above the ocean’s surface and cooled—magmafrom the Mid-Atlantic Ridge (� pp. 209, 471–472), the area

of volcanic activity along the spreading boundary of twotectonic plates. Because of the geothermal heat in this re-gion, volcanoes and geysers are numerous in Iceland. Infact, the word geyser originated from the IcelandicGeysir, the name for the island’s largest geyser, which re-cently resumed its periodic eruptions after many years indormancy.

Geothermal power plants use the energy of naturallyheated water to generate power. Rising underground wa-ter and steam are harnessed to turn turbines and createelectricity. Geothermal energy is renewable in principle(its use does not affect the amount of heat produced inEarth’s interior), but the power plants we build to use thisenergy may not all be capable of operating indefinitely. Ifa geothermal plant uses heated water at a rate faster thanthe rate at which groundwater is recharged, the plant willeventually run out of water. This is occurring at TheGeysers, in Napa Valley, California, where the first genera-tor was built in 1960. In response, operators have beguninjecting municipal wastewater into the ground to replen-ish the supply. More and more geothermal power plantsthroughout the world are now injecting water, after it isused, back into aquifers to help maintain pressure andthereby sustain the resource. A second reason geothermalenergy may not always be renewable is that patterns ofgeothermal activity in Earth’s crust shift naturally overtime, so an area that produces hot groundwater now maynot always do so.

Geothermal energy is harnessed forheating and electricity

Geothermal energy can be harnessed directly from geysersat the surface, but most often wells must be drilled downhundreds or thousands of meters toward heated ground-water. Generally, water at temperatures of 150–370 �C(300–700 �F) or more is brought to the surface and con-verted to steam by lowering the pressure in specializedcompartments. The steam is then employed in turningturbines to generate electricity (Figure 21.10).

Hot groundwater can also be used directly for heat-ing homes, offices, and greenhouses; for driving indus-trial processes; and for drying crops. Iceland heats mostof its homes through direct heating with piped hot wa-ter. Iceland began putting geothermal energy to use inthe 1940s, and today 30 municipal district heating sys-tems and 200 small private rural networks supply heatto 86% of the nation’s residences. Other locales are ben-efiting in similar ways; the Oregon Institute of Technol-ogy heats its buildings with geothermal energy for12–14% of the cost it would take to heat them with

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(b) Nesjavellir geothermal power station, Iceland

(a) Geothermal energy

Magma heatsgroundwater

1

Where naturalfissures or cracksappear, heatedwater or steamsurfaces ingeysers or hot springs

2 Wells tap undergroundheated water or steamto turn turbines andgenerate power

3

Steam is cooled, condensed, and injected back into the aquifer to maintain pressure

4

Fault

Geyser

Turbine and generator

Steam

RechargeareaCooling

tower

Injectionwell

Heat source(magma)

Impermeablerock

Impermeablerock

Confinedaquifer

With geothermal energy(a) magma heats groundwater deep in theearth (1), some of which is let offnaturally through surface vents such asgeysers (2). Geothermal facilities tap intoheated water below ground and channelsteam through turbines in buildings togenerate electricity (3). After being used,the steam is often condensed and pumpedback into the aquifer to maintain pressure(4). At Nesjavellir geothermal powerstation in Iceland (b), steam is piped fromfour wells to a condenser at the plantwhere cold water pumped from lakeshorewells 6 km (3.7 mi) away is heated. Thewater, heated to 83 �C (181 �F), is sentthrough an insulated 270-km (170-mi)pipeline to Reykjavik and environs,where residents use it for washing andspace heating.

FIGURE 21.10

natural gas. Such direct use of naturally heated water ischeap and efficient, but it is feasible only in areas suchas Iceland or parts of Oregon, where geothermal energysources are available and near where the heat must betransported.

Thermal energy from water or solid earth can also beused to drive a heat pump to provide energy. Geothermalground source heat pumps (GSHPs) use thermal energyfrom near-surface sources of earth and water rather thanthe deep geothermal heat for which utilities drill.Roughly half a million GSHPs are already used to heatU.S. residences. Compared to conventional electric heat-ing and cooling systems, GSHPs heat spaces 50–70%

more efficiently, cool them 20–40% more efficiently, canreduce electricity use by 25%–60%, and can reduceemissions by up to 72%. These pumps work because soildoes not vary in temperature from season to season asmuch as air does. The pumps heat buildings in the win-ter by transferring heat from the ground into buildings;they cool buildings in the summer by transferring heatfrom buildings into the ground. Both types of heat trans-fer are accomplished by a single network of undergroundplastic pipes that circulate water. Because heat is simplymoved from place to place rather than being producedusing outside energy inputs, heat pumps can be highlyenergy-efficient.

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Use of geothermal power is growing

Geothermal energy provides less than 0.5% of total pri-mary energy used worldwide. It provides more powerthan solar and wind combined, but only a small fractionof the power from hydropower and biomass. Geothermalenergy in the United States provides enough power tosupply electricity to over 1.4 million homes. At the world’slargest geothermal power plants, The Geysers in northernCalifornia, generating capacity has declined by more than50% since 1989 as steam pressure has declined, but TheGeysers still provide enough electricity to supply a millionresidents. Currently Japan, China, and the United Stateslead the world in use of geothermal power.

Geothermal power has benefits and limitations

Like other renewable sources, geothermal power greatlyreduces emissions relative to fossil fuel combustion.Geothermal sources can release variable amounts of gasesdissolved in their water, including carbon dioxide,methane, ammonia, and hydrogen sulfide. However, thesegases are generally in very small quantities, and it has beenestimated that geothermal facilities on average releaseonly one-sixth of the carbon dioxide produced by plantsfueled by natural gas. Geothermal facilities using the latestfiltering technologies produce even fewer emissions. Byone estimate, each megawatt of geothermal power pre-vents the emission of 7.8 million lb of carbon dioxideemissions and 1,900 lb of other pollutant emissions fromgas-fired plants each year.

On the negative side of the ledger, geothermal sources,as we have seen, may not always be truly sustainable. Inaddition, the water of many hot springs is laced withsalts and minerals that corrode equipment and pollutethe air. These factors may shorten the lifetime of plants,increase maintenance costs, and add to pollution.

Moreover, use of geothermal energy is limited to ar-eas where the energy can be tapped. Unless technology isdeveloped to penetrate far more deeply into the ground,geothermal energy use will remain more localized thansolar, wind, biomass, or hydropower. Places such as Ice-land are rich in geothermal sources, but most of theworld is not. In the United States, geysers exist in someareas, such as Yellowstone National Park, and hotgroundwater and steam exist in various locations in thewestern part of the country. Nonetheless, many hy-drothermal resources remain unexploited around theworld, awaiting improved technology and governmentalencouragement of their development.

Ocean Energy Sources

The oceans are home to several underexploited energysources. Each involve continuous natural processes thatcould potentially provide sustainable energy for ourneeds. Of the three approaches developed so far, two in-volve motion and one involves temperature.

We can harness energy from tides and waves

Just as dams on rivers use flowing freshwater to generatehydroelectric power, some scientists, engineers, busi-nesses, and governments are developing ways to use themotion of ocean water to generate electrical power. Twotypes of kinetic energy show the most promise so far: theenergy of wave motion and the energy of tidal motion.

The rising and falling of ocean tides twice each day atcoastal sites throughout the world can move large amountsof water past any given coastal point. Differences in heightbetween low and high tides are especially great in long,narrow bays such as Alaska’s Cook Inlet or the Bay ofFundy between New Brunswick and Nova Scotia. Such lo-cations are best for harnessing tidal energy, which is ac-complished by erecting dams across the outlets of tidalbasins. The incoming tide flows through sluices past thedam, and as the outgoing tide passes through the dam, itturns turbines to generate electricity (Figure 21.11). Somedesigns allow for generating electricity from water mov-ing in both directions. The world’s largest tidal generatingfacility is the La Rance facility in France, which has oper-ated for over 30 years. Smaller facilities now operate inChina, Russia, and Canada. Tidal stations release few orno pollutant emissions, but they can have impacts on theecology of estuaries and tidal basins.

Wave energy could be developed at a greater variety ofsites than could tidal energy. The principle is to harnessthe motion of wind-driven waves at the ocean’s surfaceand convert this mechanical energy into electricity. Manydesigns for machinery to harness wave energy have beeninvented, but few have been adequately tested. Some de-signs are for offshore facilities and involve floating devicesthat move up and down with the waves. Wave energy isgreater at deep-ocean sites, but transmitting the electricityproduced to shore would be expensive.

Other designs are for coastal onshore facilities. Someof these designs funnel waves from large areas into narrowchannels and elevated reservoirs, from which water is thenallowed to flow out, generating electricity as hydroelectricdams do. Other coastal designs use rising and falling

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High tide: Ocean levelrises while water levelin basin remains low

1

Sluice gates open;water leaves basinand flows past turbineto generate power

4

Tide recedes;water level in basinremains high

3

Sluice gates open andwater fills basin; turbinedoes not generate power

2

Basin

Ocean

Basin

Ocean

Basin Ocean

Basin Ocean

Sluicegateclosed

Sluicegateclosed

Sluicegateclosed

Sluicegateclosed

Barrage

Barrage

Barrage

Barrage

Turbine

Turbine

Turbine

Energy can be extracted from the movement ofthe tides at coastal sites where tidal flux is great enough. One wayof doing so is involves using bulb turbines in concert with theoutgoing tide. At high tide, ocean water is let through the sluicegates, filling an interior basin. At low tide, the basin water is let outinto the ocean, spinning turbines to generate electricity.

FIGURE 21.11

waves to push air into and out of chambers, turning tur-bines to generate electricity (Figure 21.12). No commer-cial wave energy facilities are operating yet, but some havebeen deployed as demonstration projects in several west-ern European nations.

The ocean stores thermal energy

Besides the motion of tides and waves, other oceanic en-ergy sources we have not yet effectively tapped include themotion of ocean currents, chemical gradients in salinity,and the immense thermal energy contained in the oceans.The concept of ocean thermal energy conversion (OTEC)

has been most fully developed. Each day the tropicaloceans absorb an amount of solar radiation equivalent tothe heat content of 250 billion barrels of oil—enough toprovide 20,000 times the electricity used daily in theUnited States. The ocean’s sun-warmed surface is higher intemperature than its deep water, and OTEC approachesare based on this gradient in temperature.

In the closed cycle approach, warm surface water ispiped into a facility to evaporate chemicals, such as am-monia, that boil at low temperatures. These evaporatedgases spin turbines to generate electricity. Cold waterpiped in from ocean depths then condenses the gases sothey can be reused. In the open cycle approach, the warm

bre2ch21_619_645 12/16/05 4:09 PM Page 636


surface water is evaporated in a vacuum, and its steamturns the turbines and then is condensed by the cold wa-ter. Because the ocean water loses its salts as it evaporates,water can be recovered, condensed, and sold as desalin-ized freshwater for drinking or agriculture. Research onOTEC systems has been conducted in Hawaii and otherlocations, but costs remain high, and as of yet no facility iscommercially operational.


Your Island’s Energy?

Imagine you have been elected the president of an island

nation the size of Iceland, and your nation’s congress is

calling on you to propose a national energy policy. Unlike

Iceland, your country is located in equatorial waters. Your

geologists do not yet know whether there are fossil fuel

deposits or geothermal resources under your land, but

your country gets a lot of sunlight and a fair amount of

wind, and broad, shallow shelf regions surround its

coasts. Your island’s population is moderately wealthy

but is growing fast, and importing fossil fuels from

mainland nations is becoming increasingly expensive.

What approaches would you propose in your energy

policy? What specific steps would you urge your congress

to fund immediately? What trade relationships would you

seek to establish with other countries? What questions

would you ask of your economic advisors? What questions

would you fund your country’s scientists to research?

Hydrogen

All the renewable energy sources we have discussed can beused to generate electricity more cleanly than can fossil fu-els. As useful as electricity is to us, however, it cannot bestored easily in large quantities for use when and where itis needed. This is why vehicles rely on fossil fuels for power.The development of fuel cells and hydrogen fuel showpromise to store energy conveniently and in considerablequantities and to produce electricity at least as cleanly andefficiently as renewable energy sources.

In the “hydrogen economy” that Iceland’s leaders andmany energy experts worldwide envision, hydrogen fuel,together with electricity, will serve as the basis for a clean,safe, and efficient energy system. This system will use as afuel the universe’s simplest and most abundant element.In this system, electricity generated from renewablesources that are intermittent, such as wind or solar energy,can be used to produce hydrogen. Fuel cells can then em-ploy hydrogen to produce electrical energy as needed topower vehicles, computers, cell phones, home heating,and countless other applications.

Fuel cell technology has been used since the 1960s inNASA’s space flight programs. Basing an energy system onhydrogen could alleviate dependence on foreign fuels andhelp fight climate change. For these reasons, governmentsare funding research into hydrogen and fuel cell technology,and automobile companies are investing in research anddevelopment to produce vehicles that run on hydrogen.

Coastal facilities canmake use of energy from the motion ofocean waves. As waves are let into andout of a tightly sealed chamber, the airinside is compressed and decompressed,creating air flow that rotates turbinesto generate electricity.

FIGURE 21.12

Incoming wavesenter chamber

1

The rise and fall ofwater level within thechamber compressesand decompressesthe column of airabove it

2

Air flow in bothdirections drivesthe turbine,generatingpower

3Incoming waves

Oscillatingwater column

Columnof air Turbine and

generator

bre2ch21_619_645 12/16/05 4:09 PM Page 637

Hydrogen fuel may be produced fromwater or from other matter

Hydrogen gas (H2) does not tend to exist freely on Earth;rather, hydrogen atoms bind to other molecules, becomingincorporated in everything from water to organic mole-cules. To obtain hydrogen gas for fuel, we must force thesesubstances to release their hydrogen atoms, and this re-quires an input of energy. Several potential ways of produc-ing hydrogen are being studied (see “The Science behindthe Story,” above). In electrolysis, the process being pur-sued by Iceland, electricity is input to split hydrogenatoms from the oxygen atoms of water molecules:

2H2O → 2H2 + O2

Electrolysis produces pure hydrogen, and it does so with-out emitting the carbon- or nitrogen-based pollutants of

fossil fuel combustion. However, whether this strategy forproducing hydrogen will cause pollution over its entire lifecycle depends on the source of the electricity used for theelectrolysis. If coal is burned to create the electricity, thenthe entire process will not reduce emissions compared withreliance on fossil fuels. If, however, the electricity is pro-duced by some less-polluting renewable source, then hy-drogen production by electrolysis would create much lesspollution and greenhouse warming than reliance on fossilfuels. The “cleanliness” of a future hydrogen economy inIceland or anywhere else would, therefore, depend largelyon the source of electricity used in electrolysis.

The environmental impact of hydrogen productionwill also depend on the source material for the hydrogen.Besides water, hydrogen can be obtained from biomassand fossil fuels. Obtaining hydrogen from these sourcesgenerally requires less energy input, but results in emissions


As scientists searchfor new ways to gener-

ate energy, some are look-ing past wind farms and solarpanels to an unlikely powersource—pond scum. Algae are be-ing studied as an innovative way togenerate large amounts of hydrogento move society toward a more sus-tainable energy future.

Hydrogen’s benefits hinge onhow hydrogen fuel is produced.Some methods release substantialamounts of carbon dioxide, andother, nonpolluting, processes canbe costly. These drawbacks havekept scientists searching for newhydrogen sources.

At the University of Californiaat Berkeley, plant biologist Anastasios Melis thought one pos-sible hydrogen source might be a single-celled aquatic plant knownto be a capable, if sporadic, hydro-gen producer. The algaChlamydomonas reinhardtii was

known to emit small amounts ofhydrogen for brief periods of timewhen deprived of light.

Melis hypothesized that the algamight be encouraged to producehydrogen in large amounts. He set

up an experiment with energyexperts at the National RenewableEnergy Laboratory in Colorado,aiming to develop ways to tweak thealga’s basic biological functions sothat the plant produced greaterquantities of hydrogen.

Green algae, like terrestrialgreen plants, photosynthesize,drawing in carbon dioxide and wa-ter, absorbing energy from light thatconverts those nutrients into food,and then expelling oxygen as awaste product. Additional nutrientsfrom soil or water, and catalystscalled enzymes within the plant,keep this process running smoothly.To conduct photosynthesis effec-tively, Chlamydomonas reinhardtii

needs the element sulfur as a nutri-ent. The alga also contains an en-zyme called hydrogenase, which cantrigger the alga to stop producingoxygen as a metabolic by-productand start releasing hydrogen in-stead.

TheScience

behind

theStory

Algae as a Hydrogen Fuel Source

Could green algae such as this providehydrogen for our energy needs?

bre2ch21_619_645 12/21/05 3:59 PM Page 638

of carbon-based pollutants. For instance, extractinghydrogen from the methane (CH4) in natural gas entailsproducing one molecule of the greenhouse gas carbondioxide for every four molecules of hydrogen gas:

CH4 + 2H2O → 4H2 + CO2

Thus, whether a hydrogen-based energy system is envi-ronmentally cleaner than a fossil fuel system depends onhow the hydrogen is extracted.

In addition, some new research suggests that leakageof hydrogen from the production, transport, and use ofthe gas at Earth’s surface could potentially deplete strato-spheric ozone and lengthen the atmospheric lifetime of thegreenhouse gas methane. Research into these questions isongoing, because scientists do not want society to switchfrom fossil fuels to hydrogen without first knowing thepossible risks from hydrogen.


Precaution over Hydrogen?

Some environmental scientists have recently warned

that we do not yet know enough about the

environmental consequences of replacing fossil fuels

with hydrogen fuel. An increase in tropospheric

hydrogen gas would deplete hydroxyl (OH) radicals, they

hypothesize, possibly leading to stratospheric ozone

depletion and global warming from increased

concentrations of methane. Some scientists say such

effects will be small; others say there could be further

effects that are presently unknown. Do you think we should

apply the precautionary principle to the development of

hydrogen fuel and fuel cells? Or should we embark on

pursuing a hydrogen economy before knowing all the

scientific answers? What factors inform your view?


Hydrogenase is normally activeonly after Chlamydomonas rein-

hardtii has been deprived of light.When the alga is deprived of light,the light-dependent reactions ofphotosynthesis ebb, little oxygen isproduced, and hydrogenase is acti-vated. When light returns and thealga begins producing oxygen again,hydrogenase is promptly deacti-vated, and its associated hydrogenrelease stops.

Melis’s team wanted to activatehydrogenase so that more hydrogenwould be produced. But simplykeeping the algae in the dark wouldnot escalate hydrogen productionbecause the alga’s metabolic func-tions slowed without light, result-ing in small amounts of releasedhydrogen.

The researchers decided to trylimiting the alga’s oxygen outputanother way, by putting it on asulfur-free, bright-light regimen.The lack of sulfur would hinder

photosynthesis, limiting oxygenoutput enough to activate hydroge-nase and trigger hydrogen produc-tion. The presence of light wouldkeep the algae metabolically activeand releasing large amounts of by-products.

The researchers cultured largequantities of the algae in bottles inlabs. Then they deprived the cul-tures of sulfur but kept the algae ex-posed to light for long periods oftime—in some cases up to 150hours. After the sustained light ex-posure, gas and liquids were ex-tracted from the bottles andanalyzed.

The analysis supported theteam’s hypothesis. Without sulfur orphotosynthesis, the algae were notproducing oxygen. This low-oxygen,or anaerobic, environment had in-duced hydrogenase, which spurredthe algae to begin splitting watermolecules and releasing gas. Theplants had released amounts of

hydrogen that were substantial rela-tive to the size of the algal cultures.Hydrogen also dominated the alga’semissions—in gas collection analy-sis, approximately 87% of the gaswas hydrogen, 1% was carbon diox-ide, and the remaining 12% was ni-trogen with traces of oxygen. Theresearch teams published their find-ings in the journal Plant Physiology

in 2000.Many questions remain about

algae-derived hydrogen, particularlyhow much fuel can be harvestedcontinuously using this photo-

biological process. Nevertheless, theresearch results so far are helping tofuel the momentum of a future hy-drogen economy.

Within 30 years, some federalenergy experts predict that photobi-ological methods for generating hy-drogen could be commonplace—meaning cars on future freewaysmight just be powered by pondscum.

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Hydrogen fueldrives electricity generation in afuel cell, creating water as a wasteproduct. Atoms of hydrogen arefirst stripped of their electrons (1).The electrons move from anegative electrode to a positiveone, creating a current andgenerating electricity (2).Meanwhile, the hydrogen ions pass through a proton exchangemembrane (3) and combine with oxygen to form watermolecules (4).

FIGURE 21.13

In the negativeelectrode,hydrogen is stripped of its electrons, leavinghydrogen ions(protons, H+)

1

The electronsmove from thenegative electrodeto the positive electrode,creating a current andgenerating electricity

2

In the positiveelectrode, wateris formed when oxygen combineswith the protons and electrons thatflow from thenegative electrode

4

Meanwhile,the protonstraverse themembrane

3

Hydrogen(fuel)

Proton (H+)exchangemembrane

H2 H2O

H+

H+

Oxygen

Positiveelectrode

Water (H2O)(waste)

Negativeelectrode

– –

– – – –

– –

Fuel cells produce electricity by joininghydrogen and oxygen

Once hydrogen gas has been isolated, it can be used as afuel to produce electricity within fuel cells. The chemicalreaction involved in a fuel cell is simply the reverse of thatshown for electrolysis; an oxygen molecule and two hy-drogen molecules each split so that their atoms can bindand form two water molecules:

2H2 + O2 → 2H2O

The way this occurs within one common type of fuel cellis shown in Figure 21.13. Hydrogen gas (usually com-pressed and stored in an attached fuel tank) is allowedinto one side of the cell, whose middle consists of twoelectrodes that sandwich a membrane that only protons(hydrogen ions) can move across. One electrode, helpedby a chemical catalyst, strips the hydrogen gas of its elec-trons, creating two hydrogen ions that begin movingacross the membrane. Meanwhile, on the other side ofthe cell, oxygen molecules from the open air are split intotheir component atoms along the other electrode. Theseoxygen ions soon bind to pairs of hydrogen ions travelingacross the membrane, forming molecules of water thatare expelled as waste, along with heat. While this is oc-curring, the electrons from the hydrogen atoms havetraveled to a device that completes an electric current be-tween the two electrodes. The movement of the hydro-

gen’s electrons from one electrode to the other creates theoutput of electricity.

Hydrogen and fuel cells have many benefits

As a fuel, hydrogen offers a number of benefits. We willnever run out of hydrogen; it is the most abundant ele-ment in the universe. It can be clean and nontoxic to use,and—depending on the source of the hydrogen and thesource of electricity for its extraction—it may producefew greenhouse gases and other pollutants. Pure waterand heat may be the only waste products from a hydrogenfuel cell, along with negligible traces of other compounds.In terms of safety for transport and storage, hydrogen cancatch fire, but if it is kept under pressure, it is probably nomore dangerous than gasoline in tanks.

Hydrogen fuel cells are energy-efficient. Dependingon the type of fuel cell, 35% to 70% of the energy re-leased in the reaction can be used. If the system is de-signed to capture heat as well as electricity, then theenergy efficiency of fuel cells can rise to 90%. Theserates are comparable or superior to most nonrenewablealternatives.

Fuel cells are also silent and nonpolluting. Unlike bat-teries (which also produce electricity through chemicalreactions), fuel cells will generate electricity whenever

bre2ch21_619_645 12/16/05 4:09 PM Page 640


Hydrogen and Renewable EnergyIs establishing a “hydrogen economy,” as Iceland is trying to do, the best way to

reduce the use of fossil fuels?VIEWPOINTS

The Role of Renewable Energy for the Hydrogen Economy

Abundant, reliable, and affordable energyis an essential component of a healthyeconomy. Because hydrogen can be pro-duced from a wide variety of domesticallyavailable resources and can be used inheat, power, and fuel applications, it isuniquely positioned to contribute to our

growing energy demands, particularly for resource-constrained communities. However, if we are to realize the true benefits of a hydrogen economy, other renewablesmust play a substantial role in the efficient and affordableproduction of the hydrogen.

Several renewable options could make a substantialimpact in the production of hydrogen: electrolysis pow-ered by wind, photovoltaic, solar-thermal electric, hy-dropower, and geothermal energy; use of microorganismsand semiconductors to split water; and the thermal andbiological conversion of biomass and wastes. Researchersaround the globe are working on improving these renew-able technologies. As a result, costs continue to drop. Tech-nologies for renewable hydrogen production, coupledwith advances in hydrogen production equipment (e.g.,electrolyzers) can supply cost-competitive hydrogen andwill ultimately play a substantial role in our energy supply.

In addition to the potential supply of affordable hydro-gen, these technologies also offer a wide variety of opportu-nities for developing new centers of economic growth. Mostinvestments in renewable energy are spent on materials andworkmanship to build and maintain the facilities, ratherthan on costly energy imports. Therefore, funds are usuallyspent regionally and even locally, leading to new jobs andinvestments in local economies. Because of this synergisticrelationship, the shift toward a hydrogen economy will natu-rally facilitate the advancement of renewable energy. Bydiversifying our energy supply, we will not only reduce ourdependence on imported fuels, but also will benefit fromcleaner technologies and investment in our communities.

Susan Hock directs the Electric and Hydrogen Technologiesand Systems Center of the National Renewable EnergyLaboratory. The center conducts research activities in fourareas: distributed power systems integration, hydrogentechnologies and systems, geographicinformation system analysis, and solarmeasurements and instrumentation.

Is Hydrogen the Answer?We’ll never use the last drop of oil, thelast chunk of coal, the last cubic foot ofnatural gas, or the last pound of uranium.Eventually though, these fossil andnuclear fuels will become too expensive toextract, or politics will make one or moreof them unavailable, leaving us to ask

how we’ll satisfy our voracious appetite in the future.We should immediately apply all practical energy con-

servation strategies. Mother Nature is out there makingmore fossil fuels as we speak, but we don’t have time towait the few million years that will take. The short list ofrenewables: solar, wind, hydro, biomass, geothermal,waves, tides, and ocean thermal energy conversion. Theseare all relatively benign and abundant.

An alternative: hydrogen. It can either be burned orelectrochemically used in fuel cells to provide usefulenergy. The by-product or “exhaust” is water. You start withwater, get some energy, and end up with water, making itrenewable. Another form of hydrogen energy is fusion,hydrogen atoms fusing to form helium plus a lot of energy,the way the sun does it. The catch? It takes about as muchenergy to extract hydrogen gas from water (by electrolysis)as you get back from your energy conversion device. Untilit becomes cheaper (economically and in physical terms),fossil fuels will continue to rule the energy world. Thebreakthrough may involve using our renewable energyresources to separate hydrogen from other molecules.

Arguably, to reduce our dependence on fossil fuels, thepriority list for this country should be:

1. Energy conservation2. Wind3. Passive solar4. Biomass5. Active solar6. Hydrogen (chemical)7. Hydroelectricity8. Hydrogen (fusion)9. Others (geothermal, tides, waves, ocean thermal)

Daryl Prigmore has studied energy and the environment sincethe late 1960s. After receiving bachelor and master of sciencedegrees in mechanical engineering from Colorado State

University, he spent 10 years in industry with acompany developing solar, geothermal, and low-pollution automotive power systems. He hastaught energy science classes for the past 23 years,20 at the University of Colorado (Colorado Springs).

Explore this issuefurther by accessingViewpoints at

www.aw-bc.com/withgott.

bre2ch21_619_645 12/16/05 4:09 PM Page 641


Hydrogen tanks1

Fuel cellsupply unit

2

Fuel cellstacks

3Coolingunits

4

Air conditioningunit

5

Electric motor6

Watervapor exhaust

7

The hydrogen-fueled Citaro buses operating in Reykjavik and other Europeancapitals are designed by Mercedes-Benz and Daimler-Chrysler. Hydrogen is stored in nine fuel tanks(1) The fuel cell supply unit (2) controls the flow of hydrogen, air, and cooling water into the fuelcell stacks (3). Cooling units (4) and the air conditioning unit (5) dissipate waste heat produced bythe fuel cells. Electricity generated by the fuel cells is changed from direct current (DC) toalternating current (AC) by an inverter, and it is transmitted to the electric motor (6), which powersthe operation of the bus. The vehicle’s exhaust (7) consists simply of water vapor.

FIGURE 21.14

hydrogen fuel is supplied, without ever needing recharg-ing. For all these reasons, hydrogen fuel cells are beingused to power vehicles, including the buses now operatingon the streets of Reykjavik and many other European,American, and Asian cities (Figure 21.14).

Conclusion

The coming decline of fossil fuel supplies and the increas-ing concern over air pollution and global climate changehave convinced many people that we will need to shift torenewable energy sources that will not run out and willnot pollute. Renewable sources with promise for sustain-ing our civilization far into the future without greatly de-grading our environment include solar energy, windenergy, geothermal energy, and ocean energy sources.

Moreover, by using electricity from renewable sources toproduce hydrogen fuel, we may be able to use fuel cells toproduce electricity when and where it is needed, helpingconvert our transportation sector to a nonpolluting, re-newable basis.

Most renewable energy sources have been held backfor a variety of reasons, including little funding for re-search and development, and artificially cheap marketprices for nonrenewable resources that do not includeexternal costs. Despite this, renewable technologies haveprogressed far enough to offer hope that we can shiftfrom fossil fuels to renewable energy with a minimum ofeconomic and social disruption. Whether we can alsolimit environmental impact will depend on how soonand how quickly we make the transition and to what ex-tent we put efficiency and conservation measures intoplace.

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You should now be able to:

Outline the major sources of renewable energy and

assess their potential for growth

� The “new renewable” energy sources include solar, wind,geothermal, and ocean energy sources. They are nottruly “new,” but rather are in a stage of rapid develop-ment. (pp. 621–622)

� The new renewables currently provide far less energyand electricity than the conventional renewables,hydropower and biomass energy—and only a smallfraction of the energy and electricity we obtain fromfossil fuels. (pp. 621–622)

� Use of new renewables is growing quickly, and thisgrowth is expected to continue as people seek to moveaway from fossil fuels. (pp. 622–623)

Describe solar energy and the ways it is harnessed, and

evaluate its advantages and disadvantages

� Energy from the sun’s radiation can be harnessed usingpassive methods or by active methods involving poweredtechnology. (pp. 623–624)

� Major solar technologies include solar panels, mirrors to concentrate solar rays, and photovoltaic cells.(pp. 624–626)

� Solar energy is perpetually renewable, and solar technol-ogy creates no emissions and allows for decentralizedpower. (p. 626)

� Solar radiation varies in intensity from place to placeand time to time, and harnessing solar energy remainsexpensive. (p. 627)

Describe wind energy and the ways it is harnessed, and

evaluate its advantages and disadvantages

� Energy from wind is harnessed using wind turbinesmounted on towers. (pp. 627–628)

� Turbines are often erected in arrays at wind farms lo-cated on land or offshore. Wind farms are developed inlocations with optimal wind conditions. (pp. 628–632)

� Wind energy is renewable, turbine operation creates noemissions, wind farms can generate economic benefits,and the cost of wind power is nearly competitive withthat of electricity generated from fossil fuels. (p. 630)

� Wind is an intermittent resource and occurs at adequatestrengths only in some locations. Turbines kill somebirds and bats, and wind farms can face oppositionfrom local residents. (pp. 630–633)

Describe geothermal energy and the ways it is

harnessed, and evaluate its advantages and

disadvantages

� Energy from radioactive decay in Earth’s core rises toward the surface and heats groundwater. Energy from this heated water and steam is harnessed at thesurface or by drilling at geothermal power plants.(pp. 633–634)

� Use of geothermal energy for direct heating of water, aswell as for electricity generation, can be efficient, clean,and renewable. (pp. 633–635)

� Geothermal sources occur only in certain areas and maybecome exhausted if too much water is pumped outwithout being replenished. (p. 635)

Describe ocean energy sources and the ways they can

be harnessed, and evaluate their advantages and

disadvantages

� Major ocean energy sources include the motion oftides and waves and the thermal heat of ocean water.(pp. 635–637)

� Tidal and wave energy is perpetually renewable andholds much promise, but so far technologies have seenonly limited development. (pp. 635–637)

Explain hydrogen fuel cells and assess future options

for energy storage and transportation

� Hydrogen can serve as a fuel to store and transportenergy, so that electricity generated by renewablesources can be made portable and used to power vehicles. (p. 637)

� Hydrogen can be produced through electrolysis, but itmay also be produced by using fossil fuels—in whichcase its environmental benefits are greatly reduced.(pp. 638–639)

� There is some concern that releasing excess hydrogencould have negative impacts on the atmosphere.(p. 639)

� Fuel cells create electricity by controlling an interactionbetween hydrogen and oxygen, and they produce onlywater as a waste product. (pp. 640, 642)

� Hydrogen can be clean, safe, and efficient. Fuel cells aresilent, are nonpolluting, and do not need recharging.(pp. 640, 642)

REVIEWING OBJECTIVES

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Of the new renewable energy alternatives discussed in thischapter, photovoltaic conversion of solar energy is the onethat most areas of the United States could most easilyadopt. The influx of solar radiation varies with time of day,time of year, and location, so all areas are not equally well

suited. Today’s photovoltaic technology is approximately10% efficient at converting the energy of sunlight into elec-tricity, but new technologies under development may in-crease that efficiency to as much as 40%.

INTERPRETING GRAPHS AND DATA

1. About how much of our energy now comes from re-newable sources? What is the most prevalent form of re-newable energy we use? What form of renewable energyis most used to generate electricity?

2. What is causing renewable energy sectors to expand?What renewable source is experiencing the most rapidgrowth?

3. Describe how passive solar heating works. How doesactive solar heating work? Give examples of each.

4. Describe the photoelectric effect. Describe a photo-voltaic (PV) cell, and explain one way these are used.

5. What are the environmental and economic advantagesand disadvantages of solar power?

6. How do modern wind turbines generate electricity?How does wind speed affect the process?

7. What are the environmental and economic benefits ofwind power? What are its disadvantages?

8. Define geothermal energy and explain how it is ob-tained and used. In what ways is it renewable, and inwhat way is it not renewable?

9. List and describe three approaches to obtaining energyfrom ocean water.

10. How is hydrogen fuel produced? Is this a clean process?What factors determine the amount of pollutantshydrogen production will emit?

TESTING YOUR COMPREHENSION

1. Why might a hydrogen economy be closer than we think?Why might it instead not come to pass? Do you think wa-ter could be “the coal of the future”? Why or why not?

2. For each source of renewable energy discussed in thischapter, what factors are standing in the way of an expe-dient transition from fossil fuel use?

3. Explain how the use of new renewable energy sourcescan reduce fossil fuel emissions.

4. Do you think development and implementation of re-newable energy resources to replace fossil fuels can bemoved forward without great social, economic, and en-vironmental disruption? What steps would need to betaken? Will market forces alone suffice to bring aboutthis transition? Do you think such a shift will be goodfor the economy?

5. Iceland is giving itself many years to phase in itsplanned hydrogen economy. Do you think the United

States could transition to a hydrogen economy morequickly, less quickly, or not at all? Why? What stepscould the United States take to accelerate such atransition?

6. Imagine you are the CEO of a company that developswind farms. Your staff is presenting you with threeoptions, listed below, for sites for your next development.Describe at least one likely advantage and at least onelikely disadvantage you would expect to encounter witheach option. What further information would you liketo know before deciding on which to pursue?� Option A. A remote rural site in North Dakota� Option B. A ridge-top site among the suburbs of

Philadelphia� Option C. An offshore site off the Florida coast

SEEKING SOLUTIONS

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Average per capita residential use of electricity in the UnitedStates in 2004 (red line) and average influx of solar radiation persquare meter for Topeka, Kansas (blue line). The dashed linesrepresent the yearly average values for each. Data from Renewable

Resource Data Center, National Renewable Energy Laboratory, U.S.

Department of Energy (DOE); and Energy Information Administration.

2005. Annual energy review 2004. DOE.

1. Given a 10% efficiency for photovoltaic conversion ofsolar energy, approximately how many square meters ofphotovoltaic cells would be needed to supply oneperson’s residential electrical needs for a year, based onthe yearly average values? How many square meterswould be needed if efficiency were improved to 40%?

2. Given the same 10% conversion efficiency, approxi-mately how many square meters of photovoltaic cellswould be required to supply one person’s residentialelectrical needs during the month of April? During July?How many square meters would be required to supplythe average U.S. household of four people for each ofthose months?

3. Commercially available photovoltaic systems of this ca-pacity cost approximately $20,000. The average cost ofelectricity in the United States is approximately 9¢ perkilowatt-hour. At these prices, how long would it takefor the PV system to generate $20,000 worth of electric-ity? Calculate a combination of PV system cost and elec-tricity cost at which the system would pay for itself in 10 years.

Per capita electricity useSolar radiation/m2

100

200

300

400

500

0Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Kilo

wat

t-ho

urs

per

mon

th

Month

Assume that average per capita residential consumption ofelectricity is 12 kilowatt-hours per day, that photovoltaiccells have an electrical output of 10% incident solar radia-tion, and that PV cells cost $800 per square meter. Now re-fer to Figure 21.5 on p. 627, and estimate the area and costof the PV cells needed to provide all of the residential elec-tricity used by each group in the table.

CALCULATING ECOLOGICAL FOOTPRINTS

Area of Cost of

photovoltaic cells photovoltaic cells

You 25 $20,000

Your class

Your state

United States

Take It FurtherGo to www.aw-bc.com/withgott or the student CD-ROM where you’ll find:

� Suggested answers to end-of-chapter questions� Quizzes, animations, and flashcards to help you study� Research Navigator™ database of credible and reliable

sources to assist you with your research projects

� tutorials to help you master how to interpretgraphs

� current news articles that link thetopics that you study to case studies from your regionto around the world

1. What additional information do you need in order toincrease the accuracy of your estimates for the areas inthe table above?

2. Considering the distribution of solar radiation in theUnited States, where do you think it will be most feasi-ble to greatly increase the percentage of electricity gen-erated from photovoltaic solar cells?

3. The purchase price of a photovoltaic system is consider-able. What other costs and benefits should you consider,in addition to the purchase price, when contemplating“going solar”?

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