BIGIDEAS
By2050solarpowercouldendU.S.dependenceon
foreignoilandslashgreenhousegasemissions
ByKenZweibel,JamesMasonandVasilisFthenakis
KEYCONCEPTS
■ Amassiveswitchfrom
coal,oil,naturalgasand
nuclearpowerplantstosolarpowerplantscouldsupply69percentoftheU.S.’s
electricityand35percent
ofitstotalenergyby2050.
■ Avastareaofphotovoltaic
cellswouldhavetobe
erectedintheSouthwest.
Excessdaytimeenergy
wouldbestoredascompressedairinunderground
cavernstobetappedduringnighttimehours.
■ L argesolarconcentrator
powerplantswouldbe
builtaswell.
■ Anewdirect-currentpow-
ertransmissionbackbone
woulddeliversolarelectricityacrossthecountry.
■ B ut$420billioninsubsi-
diesfrom2011to2050
wouldberequiredtofund
theinfrastructureand
makeitcost-competitive.
—The Editors
64
S C I E N T I F I CA M E R I C A N
SCHOTTAG/COMMERCIALHANDOUT/EPA/CORBIS
H
igh prices for gasoline and home heating oil are here to stay.
The U.S. is at war in the Middle East at least in part to protect
its foreign oil interests. And as China, India and other nations
rapidly increase their demand for fossil fuels, future fighting over
energy looms large. In the meantime, power plants that burn coal,
oil and natural gas, as well as vehicles everywhere, continue to pour
millions of tons of pollutants and greenhouse gases into the atmosphere annually, threatening the planet.
Well-meaning scientists, engineers, economists and politicians
have proposed various steps that could slightly reduce fossil-fuel use
and emissions. These steps are not enough. The U.S. needs a bold
plan to free itself from fossil fuels. Our analysis convinces us that a
massive switch to solar power is the logical answer.
Solar energy’s potential is off the chart. The energy in sunlight
striking the earth for 40 minutes is equivalent to global energy consumption for a year. The U.S. is lucky to be endowed with a vast resource; at least 250,000 square miles of land in the Southwest alone
are suitable for constructing solar power plants, and that land receives
more than 4,500 quadrillion British thermal units (Btu) of solar radiation a year. Converting only 2.5 percent of that radiation into electricity would match the nation’s total energy consumption in 2006.
To convert the country to solar power, huge tracts of land would
have to be covered with photovoltaic panels and solar heating
troughs. A direct-current (DC) transmission backbone would also
have to be erected to send that energy efficiently across the nation.
The technology is ready. On the following pages we present a
grand plan that could provide 69 percent of the U.S.’s electricity and
35 percent of its total energy (which includes transportation) with
solar power by 2050. We project that this energy could be sold to
consumers at rates equivalent to today’s rates for conventional power sources, about five cents per kilowatt-hour (kWh). If wind, biomass and geothermal sources were also developed, renewable energy could provide 100 percent of the nation’s electricity and 90 percent of its energy by 2100.
The federal government would have to invest more than $400 billion over the next 40 years to complete the 2050 plan. That investment is substantial, but the payoff is greater. Solar plants consume
little or no fuel, saving billions of dollars year after year. The infrastructure would displace 300 large coal-fired power plants and 300
more large natural gas plants and all the fuels they consume. The
plan would effectively eliminate all imported oil, fundamentally cutting U.S. trade deficits and easing political tension in the Middle East
A
SolarGrandPlan
and elsewhere. Because solar technologies are
almost pollution-free, the plan would also reduce greenhouse gas emissions from power
plants by 1.7 billion tons a year, and another 1.9
billion tons from gasoline vehicles would be displaced by plug-in hybrids refueled by the solar
power grid. In 2050 U.S. carbon dioxide emissions would be 62 percent below 2005 levels,
putting a major brake on global warming.
U.S.Planfor2050
PHOTOVOLTAICS
SolarPowerProvides . . .
66
S C I E N T I F I CA M E R I C A N
35%
69%
oftotalenergy
ofelectricity
PhotovoltaicFarms
B
y2050vastphotovoltaicarraysintheSouthwestwould
supplyelectricityinsteadoffossil-fueledpowerplantsand
wouldalsopowerawidespreadconversiontoplug-inelectricvehicles.Excessenergywouldbestoredascompressedairinundergroundcaverns.Largearraysthatconcentratesunlighttoheat
waterwouldalsosupplyelectricity.Anewhigh-voltage,direct-currenttransmissionbackbonewouldcarrypowertoregionalmarkets
nationwide.Thetechnologiesandfactorscriticaltotheirsuccess
aresummarizedattheright,alongwiththeextenttowhichthe
technologiesmustbedeployedby2050.Theplanwouldsubstantiallycutthecountry’sconsumptionoffossilfuelsanditsemission
ofgreenhousegases(below).Wehaveassumeda1percentannual
growthinnetenergydemand.Andwehaveanticipatedimprovementsinsolartechnologiesforecastedonlyuntil2020,withnofur— K.Z., J.M. and V.F.
thergainsbeyondthatdate.
ANNUAL U.S. FUEL CONSUMPTION
2007
ANNUAL U.S. FUEL CONSUMPTION
2050 (Existing energy path)
2007
2050 (Solar grand plan)
2050 (Existing energy path)
JENCHRISTIANSEN(graph);KENNBROWNANDCHRISWRENMondolithic Studios (illustration)
In the past few years the cost to produce photovoltaic cells and modules has dropped significantly, opening the way for large-scale deployment. Various cell types exist, but the least expensive modules today are thin films made of
cadmium telluride. To provide electricity at six
cents per kWh by 2020, cadmium telluride modules would have to convert electricity with 14
percent efficiency, and systems would have to be
installed at $1.20 per watt of capacity. Current
modules have 10 percent efficiency and an
installed system cost of about $4 per watt. Progress is clearly needed, but the technology is
advancing quickly; commercial efficiencies have
risen from 9 to 10 percent in the past 12 months.
It is worth noting, too, that as modules improve,
rooftop photovoltaics will become more costcompetitive for homeowners, reducing daytime
electricity demand.
In our plan, by 2050 photovoltaic technology
would provide almost 3,000 gigawatts (GW), or
billions of watts, of power. Some 30,000 square
miles of photovoltaic arrays would have to be
erected. Although this area may sound enormous, installations already in place indicate that
the land required for each gigawatt-hour of solar energy produced in the Southwest is less than
that needed for a coal-powered plant when factoring in land for coal mining. Studies by the
National Renewable Energy Laboratory in
Golden, Colo., show that more than enough
land in the Southwest is available without requiring use of environmentally sensitive areas,
population centers or difficult terrain. Jack
Lavelle, a spokesperson for Arizona’s Department of Water Conservation, has noted that
more than 80 percent of his state’s land is not
privately owned and that Arizona is very interested in developing its solar potential. The benign nature of photovoltaic plants (including no
water consumption) should keep environmental
concerns to a minimum.
The main progress required, then, is to raise
module efficiency to 14 percent. Although the
TECHNOLOGY
OIL2050 (Solar grand plan)
6.9
Billion barrels
OIL
6.9
Billion barrels
NATURAL GAS
Trillion cubic feet
NATURAL GAS
Trillion cubic feet
COAL
Billion tons
COAL
Billion tons
10.9
2.7
10.9
2.7
22.2
35.4
11.4
22.2
35.4
11.4
1.2
1.9
0.5
1.2
1.9
0.5
6.1
9.4
2.3
6.1
9.4
2.3
U.S. EMISSIONS
CARBON
DIOXIDE
U.S.
EMISSIONS
Billion tons
CARBON DIOXIDE
Billion tons
COMPRESSED-AIR
ENERGYSTORAGE
(withphotovoltaic
electricity)
CONCENTRATED
SOLARPOWER
DCTRANSMISSION
CRITICALFACTOR
2007
2050
Landarea
10sqmiles
30,000sqmiles
Thin-filmmoduleefficiency
10%
14%
Installedcost
$4/W
$1.20/W
Improvementsinmoduleefficiency;gainsfromvolumeproduction
Electricityprice
16¢/kWh
5¢/kWh
Followsfromlowerinstalledcost
Totalcapacity
0.5GW
2,940GW
0
535billioncuft
Installedcost
$5.80/W
$3.90/W
Economiesofscale;decreasingphotovoltaicelectricityprices
Electricityprice
20¢/kWh
9¢/kWh
Followsfromlowerinstalledcost
Totalcapacity
0.1GW
558GW
Nationalenergyplan
10sqmiles
16,000sqmiles
Policiestodeveloplargepubliclandareas
13%
17%
Fluidsthattransferheatmoreeffectively
Installedcost
$5.30/W
$3.70/W
Single-tankthermalstoragesystems;economiesofscale
Electricityprice
18¢/kWh
9¢/kWh
Followsfromlowerinstalledcost
Totalcapacity
0.5GW
558GW
Nationalenergyplan
500miles
100,000–
500,000miles
Volume
Landarea
Solar-to-electricefficiency
Length
ADVANCESNEEDED
Policiestodeveloplargepubliclandareas
Moretransparentmaterialstoimprovelighttransmission;moredensely
dopedlayerstoincreasevoltage;largermodulestoreduceinactivearea
Nationalenergyplanbuiltaroundsolarpower
Coordinationofsitedevelopmentwithnaturalgasindustry
Newhigh-voltageDCgridfromSouthwesttorestofcountry
efficiencies of commercial modules will never
reach those of solar cells in the laboratory, cadmium telluride cells at the National Renewable
Energy Laboratory are now up to 16.5 percent
and rising. At least one manufacturer, First Solar in Perrysburg, Ohio, increased module efficiency from 6 to 10 percent from 2005 to 2007
and is reaching for 11.5 percent by 2010.
PressurizedCaverns
The great limiting factor of solar power, of
course, is that it generates little electricity when
skies are cloudy and none at night. Excess power must therefore be produced during sunny
hours and stored for use during dark hours.
Most energy storage systems such as batteries
are expensive or inefficient.
Compressed-air energy storage has emerged
as a successful alternative. Electricity from photovoltaic plants compresses air and pumps it
into vacant underground caverns, abandoned
mines, aquifers and depleted natural gas wells.
The pressurized air is released on demand to
turn a turbine that generates electricity, aided by
burning small amounts of natural gas. Compressed-air energy storage plants have been operating reliably in Huntorf, Germany, since
1978 and in McIntosh, Ala., since 1991. The turbines burn only 40 percent of the natural gas
they would if they were fueled by natural gas
alone, and better heat recovery technology
would lower that figure to 30 percent.
Studies by the Electric Power Research Institute in Palo Alto, Calif., indicate that the cost
of compressed-air energy storage today is about
half that of lead-acid batteries. The research indicates that these facilities would add three or
four cents per kWh to photovoltaic generation,
bringing the total 2020 cost to eight or nine
cents per kWh.
Electricity from photovoltaic farms in the
Southwest would be sent over high-voltage DC
transmission lines to compressed-air storage
facilities throughout the country, where turbines would generate electricity year-round.
The key is to find adequate sites. Mapping by
the natural gas industry and the Electric Power
Research Institute shows that suitable geologic
formations exist in 75 percent of the country,
often close to metropolitan areas. Indeed, a
compressed-air energy storage system would
look similar to the U.S. natural gas storage system. The industry stores eight trillion cubic feet
of gas in 400 underground reservoirs. By 2050
our plan would require 535 billion cubic feet of
storage, with air pressurized at 1,100 pounds
per square inch. Although development will be
a challenge, plenty of reservoirs are available,
Photovoltaics
Inthe2050planvastphotovoltaic
farmswouldcover30,000square
milesofotherwisebarrenlandin
theSouthwest.Theywould
resembleTucsonElectricPower
Company’s4.6-megawattplantin
Springerville,Ariz.,whichbegan
in2000(left).Insuchfarms,many
photovoltaiccellsareinterconnectedononemodule,andmodules
arewiredtogethertoforman
array(right).Thedirectcurrent
fromeacharrayflowstoatransformerthatsendsitalonghighvoltagelinestothepowergrid.In
athin-filmcell(inset),theenergy
ofincomingphotonsknocksloose
electronsinthecadmiumtelluride
layer;theycrossajunction,flowto
thetopconductivelayerandthen
flowaroundtothebackconductivelayer,creatingcurrent.
68
S C I E N T I F I CA M E R I C A N
J a n u a r y2 0 0 8
TUCSONELECTRICPOWERCOMPANY
By2100
renewable
energycould
generate
100percent
oftheU.S.’s
electricityand
morethan
90percentof
itsenergy.
and it would be reasonable for the natural gas
industry to invest in such a network.
SOURCEFORMAP:COURTESYOFNATIONALRENEWABLEENERGYLABORATORY;DONFOLEY(illustrations)
HotSalt
Another technology that would supply perhaps
one fi fth of the solar energy in our vision is
known as concentrated solar power. In this
design, long, metallic mirrors focus sunlight
onto a pipe fi lled with fluid, heating the fluid
like a huge magnifying glass might. The hot fluid runs through a heat exchanger, producing
steam that turns a turbine.
For energy storage, the pipes run into a large,
insulated tank filled with molten salt, which retains heat efficiently. Heat is extracted at night,
creating steam. The molten salt does slowly
cool, however, so the energy stored must be
tapped within a day.
Nine concentrated solar power plants with a
total capacity of 354 megawatts (MW) have
been generating electricity reliably for years in
the U.S. A new 64-MW plant in Nevada came
online in March 2007. These plants, however,
do not have heat storage. The fi rst commercial
installation to incorporate it— a 50-MW plant
with seven hours of molten salt storage — is
being constructed in Spain, and others are being designed around the world. For our plan,
16 hours of storage would be needed so that
PlentifulResource
SolarradiationisabundantintheU.S.,
especiallytheSouthwest.The46,000
squaremilesofsolararrays(white
circles)requiredbythegrandplan
couldbedistributedinvariousways;
oneoptionisshownheretoscale.
AverageDailyTotalRadiation
(kWh/sqm/day)
8
NOTE:ALASKAANDHAWAIINOTSHOWNTOSCALE
7
6
5
4
3
2
PAYOFFS
electricity could be generated 24 hours a day.
Existing plants prove that concentrated solar
power is practical, but costs must decrease.
Economies of scale and continued research
would help. In 2006 a report by the Solar Task
Force of the Western Governors’ Association
concluded that concentrated solar power could
provide electricity at 10 cents per kWh or less by
2015 if 4 GW of plants were constructed. Finding ways to boost the temperature of heat exchanger fluids would raise operating efficiency,
Foreignoildependencecut
from60to0percent
■
Globaltensionseasedand
militarycostslowered
■
Massivetradedeficit
reducedsignificantly
■
Greenhousegasemissions
slashed
■
Domesticjobsincreased
■
Photovoltaicarray
Junctionbox
Electricity
delivered
tothegrid
Su
n li
gh
t( p
h
ot o
ns
)
Current
Electronflow
createscurrent
Transparent
conductivelayer
Cadmiumsulfide
semiconductor
Junction
Cadmiumtelluride
semiconductor
Powerconditioner
andtransformer
w w w. S c i A m . c o m
Conductivemetal
Glass
S C I E N T I F I CA M E R I C A N
69
DirectCurrent,Too
The geography of solar power is obviously different from the nation’s current supply scheme.
Today coal, oil, natural gas and nuclear power
plants dot the landscape, built relatively close
to where power is needed. Most of the country’s solar generation would stand in the Southwest. The existing system of alternating-current (AC) power lines is not robust enough to
carry power from these centers to consumers
everywhere and would lose too much energy
over long hauls. A new high-voltage, directcurrent (HVDC) power transmission backbone would have to be built.
Studies by Oak Ridge National Laboratory
indicate that long-distance HVDC lines lose far
less energy than AC lines do over equivalent
spans. The backbone would radiate from the
Southwest toward the nation’s borders. The
lines would terminate at converter stations
where the power would be switched to AC and
sent along existing regional transmission lines
that supply customers.
The AC system is also simply out of capacity,
leading to noted shortages in California and
other regions; DC lines are cheaper to build
and require less land area than equivalent AC
lines. About 500 miles of HVDC lines operate
in the U.S. today and have proved reliable and
efficient. No major technical advances seem to
be needed, but more experience would help refine operations. The Southwest Power Pool of
Texas is designing an integrated system of DC
and AC transmission to enable development of
10 GW of wind power in western Texas. And
TransCanada, Inc., is proposing 2,200 miles of
HVDC lines to carry wind energy from Montana and Wyoming south to Las Vegas and
beyond.
70
S C I E N T I F I CA M E R I C A N
PINCHPOINTS
Subsidiestotaling$420
billionthrough2050
■
Politicalleadershipneeded
toraisethesubsidy,
possiblywithacarbontax
■
Newhigh-voltage,
direct-currentelectric
transmissionsystembuilt
profitablybyprivate
carriers
■
StageOne:Presentto2020
We have given considerable thought to how the
solar grand plan can be deployed. We foresee
two distinct stages. The first, from now until
2020, must make solar competitive at the massproduction level. This stage will require the
government to guarantee 30-year loans, agree
to purchase power and provide price-support
subsidies. The annual aid package would rise
steadily from 2011 to 2020. At that time, the
solar technologies would compete on their own
merits. The cumulative subsidy would total
$420 billion (we will explain later how to pay
this bill).
About 84 GW of photovoltaics and concentrated solar power plants would be built by
2020. In parallel, the DC transmission system
would be laid. It would expand via existing
rights-of-way along interstate highway corridors, minimizing land-acquisition and regulatory hurdles. This backbone would reach major
markets in Phoenix, Las Vegas, Los Angeles
and San Diego to the west and San Antonio,
Dallas, Houston, New Orleans, Birmingham,
Ala., Tampa, Fla., and Atlanta to the east.
Building 1.5 GW of photovoltaics and 1.5
GW of concentrated solar power annually in the
first five years would stimulate many manufacturers to scale up. In the next five years, annual
J a n u a r y2 0 0 8
POWERSOUTHENERGYCOOPERATIVE
too. Engineers are also investigating how to use
molten salt itself as the heat-transfer fluid, reducing heat losses as well as capital costs. Salt
is corrosive, however, so more resilient piping
systems are needed.
Concentrated solar power and photovoltaics
represent two different technology paths. Neither is fully developed, so our plan brings them
both to large-scale deployment by 2020, giving
them time to mature. Various combinations of
solar technologies might also evolve to meet demand economically. As installations expand,
engineers and accountants can evaluate the pros
and cons, and investors may decide to support
one technology more than another.
construction would rise to 5 GW apiece, helping fi rms optimize production lines. As a result,
solar electricity would fall toward six cents per
kWh. This implementation schedule is realistic;
more than 5 GW of nuclear power plants were
built in the U.S. each year from 1972 to 1987.
What is more, solar systems can be manufactured and installed at much faster rates than
conventional power plants because of their
straightforward design and relative lack of environmental and safety complications.
DONFOLEY
StageTwo:2020to2050
It is paramount that major market incentives
remain in effect through 2020, to set the stage
for self-sustained growth thereafter. In extending our model to 2050, we have been conservative. We do not include any technological or
cost improvements beyond 2020. We also
assume that energy demand will grow nationally by 1 percent a year. In this scenario, by
2050 solar power plants will supply 69 percent
of U.S. electricity and 35 percent of total U.S.
energy. This quantity includes enough to supply
all the electricity consumed by 344 million plugin hybrid vehicles, which would displace their
gasoline counterparts, key to reducing dependence on foreign oil and to mitigating greenhouse gas emissions. Some three million new
Underground
Storage
Excesselectricityproducedduring
thedaybyphotovoltaicfarms
wouldbesentoverpowerlinesto
compressed-airenergystorage
sitesclosetocities.Atnightthe
siteswouldgeneratepowerfor
consumers.Suchtechnologyisalreadyavailable;thePowerSouth
EnergyCooperative’splantinMcIntosh,Ala.(left),hasoperated
since1991(thewhitepipesends
airunderground).Inthesedesigns,
incomingelectricityrunsmotors
andcompressorsthatpressurize
airandsenditintovacantcaverns,
minesoraquifers(right).Whenthe
airisreleased,itisheatedbyburningsmallamountsofnaturalgas;
thehot,expandinggasesturn
turbinesthatgenerateelectricity.
w w w. S c i A m . c o m
domestic jobs — notably in manufacturing solar
components — would be created, which is several times the number of U.S. jobs that would be
lost in the then dwindling fossil-fuel industries.
The huge reduction in imported oil would
lower trade balance payments by $300 billion a
year, assuming a crude oil price of $60 a barrel
(average prices were higher in 2007). Once solar
power plants are installed, they must be maintained and repaired, but the price of sunlight is
forever free, duplicating those fuel savings year
after year. Moreover, the solar investment would
enhance national energy security, reduce financial burdens on the military, and greatly decrease the societal costs of pollution and global
warming, from human health problems to the
ruining of coastlines and farmlands.
Ironically, the solar grand plan would lower
energy consumption. Even with 1 percent annual growth in demand, the 100 quadrillion Btu
consumed in 2006 would fall to 93 quadrillion
Btu by 2050. This unusual offset arises because
a good deal of energy is consumed to extract and
process fossil fuels, and more is wasted in burning them and controlling their emissions.
To meet the 2050 projection, 46,000 square
miles of land would be needed for photovoltaic
and concentrated solar power installations. That
area is large, and yet it covers just 19 percent of
[THEAUTHORS]
KenZweibel,JamesMasonand
VasilisFthenakismetadecade
agowhileworkingonlife-cycle
studiesofphotovoltaics.Zweibel
ispresidentofPrimeStarSolarin
Golden,Colo.,andfor15yearswas
manageroftheNationalRenewableEnergyLaboratory’sThin-Film
PVPartnership.Masonisdirector
oftheSolarEnergyCampaignand
theHydrogenResearchInstitutein
Farmingdale,N.Y.Fthenakisis
headofthePhotovoltaicEnvironmentalResearchCenteratBrookhavenNationalLaboratoryandis
aprofessorinanddirectorof
ColumbiaUniversity’sCenterfor
LifeCycleAnalysis.
Brilliant?
Far-fetched?
For a discussion with
the authors about the solar grand
plan, please visit our Community
page at http://sciencecommunity.SciAm.com; click on
Discussions, then Technology.
Electricity
tothegrid
Electricityfrom
photovoltaicfarm
Generator
Naturalgas–fueled
combustionchamber
Exhaustheat
Recuperator
(pre-heatsair)
Water-coolingtower
Compressors
Motor
C av
ern
Highpressure
turbine
e d
mp
p u v e r n
r
i
A ca e
g
o
i n t s to ra
for
Lowpressure
turbine
sed
l e a t e
e
r
a
Air ener y
g
t o t r i c i t
c
e
l
e
S C I E N T I F I CA M E R I C A N
71
exactitude 50 or more years into the future, as
an exercise to demonstrate the full potential of
solar energy we constructed a scenario for 2100.
By that time, based on our plan, total energy
demand (including transportation) is projected
to be 140 quadrillion Btu, with seven times
today’s electric generating capacity.
To be conservative, again, we estimated how
much solar plant capacity would be needed under the historical worst-case solar radiation
conditions for the Southwest, which occurred
during the winter of 1982–1983 and in 1992
and 1993 following the Mount Pinatubo eruption, according to National Solar Radiation
Data Base records from 1961 to 2005. And
again, we did not assume any further technological and cost improvements beyond 2020,
even though it is nearly certain that in 80 years
ongoing research would improve solar efficiency, cost and storage.
Under these assumptions, U.S. energy demand could be fulfilled with the following capacities: 2.9 terawatts (TW) of photovoltaic power
going directly to the grid and another 7.5 TW
dedicated to compressed-air storage; 2.3 TW of
concentrated solar power plants; and 1.3 TW
of distributed photovoltaic installations. Supply
would be rounded out with 1 TW of wind farms,
0.2 TW of geothermal power plants and 0.25
TW of biomass-based production for fuels. The
model includes 0.5 TW of geothermal heat
pumps for direct building heating and cooling.
The solar systems would require 165,000 square
miles of land, still less than the suitable available
area in the Southwest.
In 2100 this renewable portfolio could generate 100 percent of all U.S. electricity and more
than 90 percent of total U.S. energy. In the
spring and summer, the solar infrastructure
would produce enough hydrogen to meet more
than 90 percent of all transportation fuel demand and would replace the small natural gas
supply used to aid compressed-air turbines.
Adding 48 billion gallons of biofuel would cover the rest of transportation energy. Energy-related carbon dioxide emissions would be reduced 92 percent below 2005 levels.
Concentrated
Solar
Largeconcentratedsolarpower
plantswouldcomplementphotovoltaicfarmsintheSouthwest.The
KramerJunctionplantinCalifornia’s
MojaveDesert(left),usingtechnologyfromSolelinBeitShemesh,Israel,hasbeenoperatingsince1989.
Metallicparabolicmirrorsfocussunlightonapipe,heatingfluidsuchas
ethyleneglycolinside(right).The
mirrorsrotatetotrackthesun.The
hotpipesrunalongsideasecond
loopinsideaheatexchangerthat
containswater,turningittosteam
thatdrivesaturbine.Futureplants
couldalsosendthehotfluid
throughaholdingtank,heating
moltensalt;thatreservoirwould
retainheatthatcouldbetappedat
nightfortheheatexchanger.
72
S C I E N T I F I CA M E R I C A N
J a n u a r y2 0 0 8
COURTESYOFNREL
the suitable Southwest land. Most of that land is
Although barren;
there is no competing use value. And the
$420billionis land will not be polluted. We have assumed that
10 percent of the solar capacity in 2050 will
substantial, only
come from distributed photovoltaic installaitislessthan tions— those on rooftops or commercial lots
the country. But as prices drop, these
theU.S.Farm throughout
applications could play a bigger role.
PriceSupport
2050andBeyond
program. Although
it is not possible to project with any
➥ MORETO
DONFOLEY
WhoPays?
dies would end from 2041 to 2050. The HVDC
transmission companies would not have to be
subsidized, because they would fi nance conTheTerawattChallengeforThin
struction of lines and converter stations just as
FilmPhotovoltaic.KenZweibelin
they now fi nance AC lines, earning revenues by
Thin Film Solar Cells: Fabrication,
Characterization and Applications.
delivering electricity.
EditedbyJefPoortmansand
Although $420 billion is substantial, the anVladimirArkhipov.JohnWiley&
nual
expense would be less than the current U.S.
Sons,2006.
Farm Price Support program. It is also less than
the tax subsidies that have been levied to build
EnergyAutonomy:TheEconomic,
SocialandTechnologicalCasefor
the country’s high-speed telecommunications
RenewableEnergy.Hermann
infrastructure over the past 35 years. And it
Scheer.EarthscanPublications,2007.
frees the U.S. from policy and budget issues
driven by international energy conflicts.
C
enterforLifeCycleAnalysis,
Without subsidies, the solar grand plan is imColumbiaUniversity:
www.clca.columbia.edu
possible. Other countries have reached similar
conclusions: Japan is already building a large,
TheNationalSolarRadiation
subsidized solar infrastructure, and Germany
DataBase.NationalRenewable
has
embarked on a nationwide program. AlEnergyLaboratory,2007.
though the investment is high, it is important to
http://rredc.nrel.gov/solar/old_
data/nsrdb
remember that the energy source, sunlight, is free.
There are no annual fuel or pollution-control
T heU.S.DepartmentofEnergy
costs like those for coal, oil or nuclear power, and
SolarAmericaInitiative:
only
a slight cost for natural gas in compressedwww1.eere.energy.gov/solar/
air systems, although hydrogen or biofuels could
solar_america
displace that, too. When fuel savings are factored
in, the cost of solar would be a bargain in
coming decades. But we cannot wait unSunlight
til then to begin scaling up.
Critics have raised other conPipe
cerns, such as whether material
filled
constraints could stifle large-scale
with
ethylene
installation.
With rapid deployglycol
ment, temporary shortages are
possible. But several types of cells
exist that use different material combinations. Better processing and recycling
are also reducing the amount of maHeatexchanger
terials that cells require. And in the long term,
Electricityto
old solar cells can largely be recycled into new
thegrid
solar cells, changing our energy supply picture
from depletable fuels to recyclable materials.
Superheated
The greatest obstacle to implementing a rewaterflow
newable U.S. energy system is not technology
or money, however. It is the lack of public
Return
awareness that solar power is a practical alterwaterflow
native — and one that can fuel transportation as
Steam
condensation
well. Forward-looking thinkers should try to
unit
inspire U.S. citizens, and their political and scientific leaders, about solar power’s incredible
potential. Once Americans realize that potential, we believe the desire for energy self-sufficiency and the need to reduce carbon dioxide
emissions will prompt them to adopt a nationg
al solar plan.
Our model is not an austerity plan, because it
includes a 1 percent annual increase in demand,
which would sustain lifestyles similar to those
today with expected efficiency improvements in
energy generation and use. Perhaps the biggest
question is how to pay for a $420-billion overhaul of the nation’s energy infrastructure. One
of the most common ideas is a carbon tax. The
International Energy Agency suggests that a carbon tax of $40 to $90 per ton of coal will be
required to induce electricity generators to adopt
carbon capture and storage systems to reduce
carbon dioxide emissions. This tax is equivalent
to raising the price of electricity by one to two
cents per kWh. But our plan is less expensive. The
$420 billion could be generated with a carbon
tax of 0.5 cent per kWh. Given that electricity
today generally sells for six to 10 cents per kWh,
adding 0.5 cent per kWh seems reasonable.
Congress could establish the fi nancial incentives by adopting a national renewable energy
plan. Consider the U.S. Farm Price Support program, which has been justified in terms of national security. A solar price support program
would secure the nation’s energy future, vital to
the country’s long-term health. Subsidies would
be gradually deployed from 2011 to 2020. With
a standard 30-year payoff interval, the subsi-
Parabolictrough
Ethylene
glycolflow
Futureplan:
heat-holdingtank
(moltensalt)
Steamturbine
Generator
w w w. S c i A m . c o m
EXPLORE
S C I E N T I F I CA M E R I C A N
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