A Solar Grand Plan
January, 2008 - Ken Zweibel, James Mason and Vasilis
Fthenakis - Scientific American
By 2050 solar power could end U.S. dependence on
foreign oil and slash greenhouse gas emissions.
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Image:
Schott AG/Commercial Handout/EPA/Corbis |
Key Concepts
* 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
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 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 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, 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 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 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 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.
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.
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