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hour for a typical nuclear power plant). With
dry-cooling or hybrid-cooling systems, the water
demands can be less than 100 gallons per megawatt-hour, with most of it used for steam makeup
water and mirror washing.
It’s possible to design a CSP system to consume a negligible amount of water. Hank Price
at the National Renewable Energy Laboratory
(NREL) calculates that CSP trough steam-electric plants with wet-cooling towers have 13 to15
percent solar-to-electricity conversion efficiency.
At the peak of the summer day, a dry-cooled CSP
steam plant can lose as much as 5 to 8 percent
of its power output, compared to the same plant
using a wet-cooled tower. (The efficiency drop
is due primarily to the massive release of latent
energy when water is evaporated.)Thus, an air-cooled steam-electric CSP system has an 8 percent higher levelized cost of electricity than a
wet-cooled system. According to a 2006 report
commissioned by NREL, the systems reach
cost-parity when the price of water rises roughly
tenfold. Of course if water is unavailable at any
price, the wet-cooled system can’t compete at
all. Where water is scarce, a hybrid system can
use wet cooling during peak summer days, when
electricity is in highest demand, and dry cooling
the rest of the year.
world market share in solar water-heating equipment. Hawaii now mandates solar water heating
for most newly constructed homes as do most
EU countries, many of which have also added
mandates for schools and commercial buildings.
From thermodynamic and greenhouse gas perspectives, solar water heating just makes sense
as an alternative to using high-quality forms of
energy, such as natural gas and electricity, to create relatively low volumes of a low-quality energy
resource — hot water.
Solar Can Help
Other forms of solar energy can relieve
water-shortage issues. Some CSP technologies
don’t use steam cycles: Dish-heated stirling
engines consume less than 10 gallons of water
per megawatt-hour and offer conversion efficien-cies up to 25 percent, well above commercially
available non-concentrating photovoltaic (PV)
panels. PV and concentrating PV cells generate
electricity using no water at all, except for occasional rinsing of dust, leaves and bird droppings
from the panels. Wind is ultimately solar-driven
convection in the atmosphere, and wind turbines
use no water.
Higher-grade solar-heated water, at up to
120°C (248°F) and several atmospheres of
pressure, can provide the heat for CO2-capture
systems at coal power plants. This could be a
beneficial use of solar energy during a transition
from fossil fuel-produced to renewable electricity. When capturing CO2 from the flue or from
pre-combustion gases, using chemical solvents
such as amines, the cool solvent first absorbs
the CO2. Then the loaded, or “rich,” solvent is
regenerated by heating it to release the CO2,
making the solvent ready again to absorb more
CO2. Nominally, the heat required for solvent
regeneration will be taken from the middle of
the steam cycle. This steam would otherwise
generate electricity, and therefore, the power
output from the plant is decreased by 10 to 15
percent. Using concentrating solar energy systems to provide regeneration heat could restore
full power output.
Solar energy can power desalination. The
Economist reports that desalination capacity may
double by 2015, with approximately one-third of
new plants driven by thermal systems. Concentrated solar energy could provide the heat for
this desalination. Australia has already installed
wind farms that are contractually coupled to
desalination plants.
Future Impact Is Hard To Predict
Solar water-heating systems, closed systems
that lose no water to the atmosphere, are often
the most economical use of solar energy. Almost
4 percent of U.S. electricity is used to heat water
in the residential sector alone. Heating water
in the commercial and residential sectors using
electricity and natural gas causes the release of
approximately 200 million metric tons of CO2
annually. Israel and China are the world leaders in the use of solar water heating for homes,
and China accounts for over 75 percent of the
Solar water heating is already economical,
and PV systems are nearly cost-competitive with
other electric power sources. Increasing use of
these solar energy systems will put downward
pressure on both the water intensity of electricity generation and the use of conventional fuels
to heat water. As explained above, the cost of
bringing water to CSP plants will dictate their
choice of cooling system, wet or dry. However,
if CSP developers are forced to use dry-cooling
systems by lack of water resources, there will still
likely be overall benefits from the solar electricity. To make use of the vast solar resource of the
Even if water-free
technologies are more
expensive, investing in
mirrors in the Southwest with
no water will be more
profitable than putting those
same mirrors in the
relatively wetter and cloudier
Midwest and East.
solartoday.org SOLAR TODAY January/February 2010 27
American Southwest, we’ll need to build plants
that don’t require significant water access. Even
if water-free technologies are more expensive,
investing in mirrors in the Southwest with no
water will be more profitable than putting those
same mirrors in the relatively wetter and cloudier
Midwest and East.
If a price is placed on carbon emissions, water
resources won’t constrain solar development
unless dry-cooling technologies prove too costly
or burdensome in the desert environments of the
Southwest. In that case, it may prove economical for CSP steam-cycle developers to desalinate
brackish groundwater for cooling.
The Earth’s climate has been governed by the
solar energy-water nexus since it was formed.
Sunlight and water have enabled the building blocks of life upon which the food chain
depends. The fossil fuels of our industrial age
are the result of ancient sunlight converted into
decaying biomass in water filled swamps. Our
challenge now is finding ways to best use real-time sunlight and water to fuel and quench our
needs today, without degrading our potential to
make use of them in the future. From low-grade
heating applications, such as drying, to industrial
steam and clean electricity, solar power has some
answers and some challenges within the energy-water nexus. But these challenges are worth the
focus of our effort in avoiding detrimental conflicts while making best use of our most fundamental energy resource, the sun, and our most
precious life-bearing resource, our water. ST
