From: Scientific American, Jan. 1, 2008


By 2050 solar power could end U.S. dependence on foreign oil and slash
greenhouse gas emissions

By Ken Zweibel, James Mason and Vasilis Fthenakis

High 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

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, bio-mass 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 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.

Photovoltaic Farms

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 cost-competitive 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 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.

Pressurized Caverns

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

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, and it would be reasonable for the natural
gas industry to invest in such a network.

Hot Salt

Another technology that would supply perhaps one fifth of the solar
energy in our vision is known as concentrated solar power. In this
design, long, metallic mirrors focus sunlight onto a pipe filled with
fluid, heating the fluid like a huge magnifying glass might. The hot
fluid runs through a heat exchanger, producing steam that turns a

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 first 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 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, 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.

Direct Current, 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, direct-current (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.

Stage One: Present to 2020

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 mass-production 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 construction
would rise to 5 GW apiece, helping firms 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.

Stage Two: 2020 to 2050

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
plug-in 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 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 the suitable
Southwest land. Most of that land is barren; there is no competing use
value. And the land will not be polluted. We have assumed that only 10
percent of the solar capacity in 2050 will come from distributed
photovoltaic installations--those on rooftops or commercial lots
throughout the country. But as prices drop, these applications could
play a bigger role.

2050 and Beyond

Although it is not possible to project with any 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

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.

Who Pays?

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 financial 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 subsidies would end from 2041 to 2050. The HVDC transmission
companies would not have to be subsidized, because they would finance
construction of lines and converter stations just as they now finance
AC lines, earning revenues by delivering electricity.

Although $420 billion is substantial, the annual expense would be less
than the current U.S. Farm Price Support program. It is also less than
the tax subsidies that have been levied to build the country's high-
speed telecommunications infrastructure over the past 35 years. And it
frees the U.S. from policy and budget issues driven by international
energy conflicts.

Without subsidies, the solar grand plan is impossible. Other countries
have reached similar conclusions: Japan is already building a large,
subsidized solar infrastructure, and Germany has embarked on a
nationwide program. Although the investment is high, it is important
to remember that the energy source, sunlight, is free. There are no
annual fuel or pollution-control costs like those for coal, oil or
nuclear power, and only a slight cost for natural gas in compressed-
air systems, although hydrogen or biofuels could displace that, too.
When fuel savings are factored in, the cost of solar would be a
bargain in coming decades. But we cannot wait until then to begin
scaling up.

Critics have raised other concerns, such as whether material
constraints could stifle large-scale installation. With rapid
deployment, 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 materials
that cells require. And in the long term, old solar cells can largely
be recycled into new solar cells, changing our energy supply picture
from depletable fuels to recyclable materials.

The greatest obstacle to implementing a renewable U.S. energy system
is not technology or money, however. It is the lack of public
awareness that solar power is a practical alternative--and one that
can fuel transportation as well. Forward-looking thinkers should try
to 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
national solar plan.


** A massive switch from coal, oil, natural gas and nuclear power
plants to solar power plants could supply 69 percent of the U.S.'s
electricity and 35 percent of its total energy by 2050.

** A vast area of photovoltaic cells would have to be erected in the
Southwest. Excess daytime energy would be stored as compressed air in
underground caverns to be tapped during nighttime hours.

** Large solar concentrator power plants would be built as well.

** A new direct-current power transmission backbone would deliver
solar electricity across the country.

** But $420 billion in subsidies from 2011 to 2050 would be required
to fund the infrastructure and make it cost-competitive.

--The Editors

Plentiful Resource:

Solar radiation is abundant in the U.S., especially the Southwest. The
46,000 square miles of solar arrays required by the grand plan could
be distributed in various ways.


** Foreign oil dependence cut from 60 to 0 percent

** Global tensions eased and military costs lowered

** Massive trade deficit reduced significantly

** Greenhouse gas emissions slashed

** Domestic jobs increased


** Subsidies totaling $420 billion through 2050

** Political leadership needed to raise the subsidy, possibly with a
carbon tax

** New high-voltage, direct-current electric transmission system built
profitably by private carriers


The Terawatt Challenge for Thin Film Photovoltaic. Ken Zweibel in Thin
Film Solar Cells: Fabrication, Characterization and Applications.
Edited by Jef Poortmans and Vladimir Arkhipov. John Wiley & Sons,

Energy Autonomy: The Economic, Social and Technological Case for
Renewable Energy. Hermann Scheer. Earthscan Publications, 2007.

Center for Life Cycle Analysis, Columbia University:

The National Solar Radiation Data Base. National Renewable Energy
Laboratory, 2007.;data/ns

The U.S. Department of Energy Solar America Initiative:


In the 2050 plan vast photovoltaic farms would cover 30,000 square
miles of otherwise barren land in the Southwest. They would resemble
Tucson Electric Power Company's 4.6-megawatt plant in Springerville,
Ariz., which began in 2000. In such farms, many photovoltaic cells are
interconnected on one module, and modules are wired together to form
an array. The direct current from each array flows to a transformer
that sends it along high-voltage lines to the power grid. In a thin-
film cell, the energy of incoming photons knocks loose electrons in
the cadmium telluride layer; they cross a junction, flow to the top
conductive layer and then flow around to the back conductive layer,
creating current.

Underground Storage

Excess electricity produced during the day by photovoltaic farms would
be sent over power lines to compressed-air energy storage sites close
to cities. At night the sites would generate power for consumers. Such
technology is already available; the PowerSouth Energy Cooperative's
plant in Mcintosh, Ala. (left), has operated since 1991 (the white
pipe sends air underground). In these designs, incoming electricity
runs motors and compressors that pressurize air and send it into
vacant caverns, mines or aquifers (right). When the air is released,
it is heated by burning small amounts of natural gas; the hot,
expanding gases turn turbines that generate electricity.

Concentrated Solar

Large concentrated solar power plants would complement photovoltaic
farms in the Southwest. The Kramer Junction plant in California's
Mojave Desert, using technology from Solel in Beit Shemesh, Israel,
has been operating since 1989. Metallic parabolic mirrors focus
sunlight on a pipe, heating fluid such as ethylene glycol inside. The
mirrors rotate to track the sun. The hot pipes run alongside a second
loop inside a heat exchanger that contains water, turning it to steam
that drives a turbine. Future plants could also send the hot fluid
through a holding tank, heating molten salt; that reservoir would
retain heat that could be tapped at night for the heat exchanger.


Ken Zweibel, James Mason and Vasilis Fthenakis met a decade ago while
working on life-cycle studies of photovoltaics. Zweibel is president
of PrimeStar Solar in Golden, Colo., and for 15 years was manager of
the National Renewable Energy Laboratory's Thin-Film PV Partnership.
Mason is director of the Solar Energy Campaign and the Hydrogen
Research Institute in Farmingdale, N.Y. Fthenakis is head of the
Photovoltaic Environmental Research Center at Brookhaven National
Laboratory and is a professor in and director of Columbia University's
Center for Life Cycle Analysis.

Copyright Scientific American
Blogged with the Flock Browser

Tags: ,

Leave a Reply