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TACKLiNG CLimATE CHANGE:
Concentrating Solar to the Rescue
Why a nearly forgotten solar technology is emerging as
a promising weapon in the war on climate change.
By CHUCK KUTSCHER
Chuck Kutscher is a
principal engineer and
manager of the Thermal
Systems Group at the
National Renewable
Energy Laboratory.
He is a past ASES chair
and was chair of the
SOLAR 2006 conference, which resulted in
the ASES report, “
Tackling Climate Change in
the U.S.” (Free down-load at ases.org/
climatechange.) He
teaches a course at
the University of Colorado entitled “Climate
Change Solutions.”
The opinions expressed
here are solely those of
the author.
What comes to mind when you think of solar
electricity? If you’re like most people, you
think of photovoltaic (PV) modules. But
another type of solar technology generates electricity
in a way that is much like conventional power plants:
concentrating solar power, or CSP. As longtime solar
advocate Fred Morse puts it (in a takeoff on the old pork
industry ads), CSP is “the other white meat.”
CSP is simple enough. Mirrors concentrate solar
energy, producing the high temperatures needed to
efficiently run a thermodynamic heat engine. Because
diffuse sunlight can’t be focused, CSP plants work best
where skies are very clear, like the southwestern United
States. A study done for the Western Governors’ Association looked at the Southwest and filtered out land that
was already utilized or environmentally sensitive, had a
ground slope greater than 1 percent and had anything
less than the best solar resource ( 6.75 kilowatt-hour per
square meter per day of direct radiation). They concluded
that the remaining land could provide six times the current U.S. electric capacity.
CSP isn’t new. In the 1980s, the Israeli company Luz
constructed nine plants for a total of 354 megawatts
(M W) of CSP in the Mojave Desert, and these plants are
still operating successfully. They employ tracking parabolic trough reflectors to focus sunlight onto evacuated
tube receivers, through which a high-temperature heat-transfer fluid is pumped. The fluid transfers its heat to a
boiler, and the steam spins a turbine-generator.
After the last Luz plant was built in 1991, Luz went out
of business. The loss of financial incentives, low natural
gas prices and utility deregulation all conspired to kill the
industry. But in the last three years, CSP has experienced
a rebirth. Higher natural gas prices and a 30 percent federal investment tax credit, recently extended for eight
years, have made CSP attractive again. Renewable portfolio standards in 28 states have put pressure on utilities
to produce or buy electricity from renewable energy. And
utilities understand big steam-generating power plants.
In 2006, Solargenix (now Acciona, acciona.es) installed
America’s first new parabolic trough power plant in 15
years. Although only 1 MW in size, the Saguaro plant
outside Tucson provided the field experience needed to
build the 64-MW Nevada Solar One plant outside Las
Vegas only a year later. The Saguaro plant also gave Arizona Public Service (APS) experience integrating CSP
into their grid. When APS decided recently to order a
new power plant to service Phoenix’s growing population, they compared wind turbines, photovoltaics and
CSP to a new combined-cycle natural gas plant. They
chose CSP because it offers one key advantage: storage.
CSP plants generate heat, and storing heat is cheaper and
more efficient than storing electricity.
The 250-MW (net) Solana plant being built by Aben-goa ( abengoa.com) for APS will incorporate six hours of
thermal storage. A parabolic trough collector field will
be oversized so that, when the sun is shining, it will not
only generate electricity to send out to the grid but will
also heat tanks of molten salt. After the sun sets, the heat
from the molten salt will be transferred to the same fluid
that goes through the collectors, which can then continue to boil water for the steam turbines. Thus the utility
can meet high demand through the evening hours when
people get home from work. Solana is expected to create
1,500 construction jobs and 85 permanent jobs.
While the Solana plant will employ six hours of storage,
and new plants in Spain are using seven hours, analysts
at the National Renewable Energy Laboratory (NREL)
are looking into 12 hours or more of storage, allowing
CSP to compete in the base load power market, now
dominated by coal, the worst carbon emitter. Of course,
carbon price legislation will be needed to allow CSP to
compete economically against coal.
It is estimated that electricity from Solana will cost
about 14 – 15 cents per kilowatt-hour, after the tax credit,
compared to about 12 cents per kilowatt-hour for a new
combined-cycle plant. But the solar plant will eliminate
the risk associated with potential future price hikes in
natural gas. While further cost reductions are needed,
CSP technology is improving. The newest receiver tubes
being tested at NREL lose significantly less heat than
those at Nevada Solar One. New polymer reflector materials have the potential to replace heavy glass mirrors, thus
reducing the overall collector cost. One need only look
