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The
future availability of water is a big question in and of itself but it
is also a big question for energy producers. If worsening droughts
associated with global climate change do not entirely impede access to
adequate water, they will certainly make it a more precious – and
therefore more expensive – commodity.
Will the added expense of water alter assumptions about which form of electricity generation is the best?
Concentrating
Solar Power Commercial Application Study: Reducing Water Consumption of
Concentrating Solar Power Electricity Generation; Report to Congress,
from the U.S. Department of Energy’s National Renewable Energy
Laboratory (NREL), is aimed at answering that question as it pertains
to solar power plant technologies.
Want to skip the details?
Here’s the bottom line: The question of water puts more nails in the
coffins of coal and nuclear as viable future power generators and
offers yet more evidence that New Energy is the only sensible way to
build.
In the details, the NREL report offers important
suggestions about currently competing solar power plant concepts based
on their water needs and what those needs mean to the cost of solar
power plant-generated electricity.
Bottom line. (click to enlarge)
COMMENTARY
The
report is in answer to a requirement by the Energy Independence and
Security Act of 2007 for the Secretary of Energy to report to Congress
on ways to cut the water use of concentrating solar power (CSP)
systems.
The reason for requiring such a study is pretty
obvious: The sun on the wide-open spaces of the U.S. Southwest is an
energy asset the nation can no longer ignore. Concentrating solar power
is the way to harvest that's sun generation. But some forms of CSP are
water intensive and water in the Southwest is as precious as sun is
abundant.
In late 2008, Congress extended the 30% investment tax
credit (ITC), beginning in 2009, to the full cost of solar systems and
made utilities and big power producers eligible for it for the first
time. Other recession-easing incentives were also implemented. Despite
the economic downturn, those incentives have proven too tempting to
resist. Some solar power plants have started construction and many more
are being planning.
There are now 400+ megawatts of installed
CSP, some having performed at utility scale for more than 15 years.
More than 4000 megawatts are in planning stages around the world.
click to enlarge
There are 4 main CSP technologies: parabolic troughs, linear Fresnel, power towers, and dish/engine.
Parabolic
troughs are the most tested technology. Power tower technologies are
beginning to be put into use in Spain and just being tested in the U.S.
Linear Fresnel systems have agreements with utilities in California and
pilot projects are under construction.
Parabolic troughs, linear
Fresnel and power towers focus the sun to heat a liquid that flows to
conventional Rankine steam cycle turbines like those heated by coal and
natural gas combustion and nuclear energy. Steam cycle power plants
require cooling to condense the steam and complete the cycle. It can be
water cooling, air cooling or a combination.
click to enlarge
Dish/Stirling
engine systems use curved mirroring to focus the sun’s heat to drive a
small engine. The most common ones use Stirling cycle engines with
hydrogen as the working fluid. They are air-cooled and only require
water for mirror washing.
All 4 CSP designs use a small amount
of water for mirror washing. The first three operate a steam cycle and,
like fossil and nuclear plants, require water for steam makeup and,
when they are water-cooled, a substantial amount of water for cooling.
Though the use of the steam cycle gives the 3 water-consuming CSP
technologies the same disadvantages it gives fossil and nuclear plants
in terms of water use, it gives them 2 advantages: (1) Utility managers
are familiar with the power generating system; and, (2) Storage systems
can be integrated, allowing the dispatch of electricity as it is needed
and the ability to produce electricity into the night.
Thermoelectric
fossil fuel combustion and nuclear power plants are water-cooled by one
of two methods: (1) Once-through cooling and (2) Recirculating
evaporative cooling.
Once-through cooling withdraws large
volumes (23,000 to 27,000 gallons per megawatt-hour) from a water
source and returns it to the source at an elevated temperature. That
causes further evaporative loss from the water source.
click to enlarge
Recirculating
evaporative cooling withdraws a lesser amount (500 to 750 gallons per
megawatt-hour) but evaporates most of the water directly. Once-through
cooling ultimately consumes less water but is restricted in use because
it potentially impacts the environment and aquatic habitat of the water
source.
Air cooling blows steam cycle heat directly into the
air. A fossil power plant using air cooling withdraws water only for
the steam cycle and housekeeping uses and, therefore uses less than 10%
of the water used by a water-cooled plant.
At present, water
cooling is cheaper in fossil fuel, nuclear and CSP plants. Less capital
is required for construction and it manages temperature variations more
efficiently regardless of seasonal temperature fluctuations because
water body temperature fluctuates less than air temperature. Air
cooling is quite inefficient when it is hot because the air temperature
is nearer the plant’s temperature and provides little cooling.
CSP
plants need water for cooling, for steam, for condensing steam, and for
mirror washing. Yet the Southwest, where solar resources are ideal for
CSP, has no extra water and importing or purifying water would be
expensive. There are, however, ways to increase the water efficiency of
CSP.
click to enlarge
New
fossil and nuclear power plants use evaporative water cooling at ~500
gallons of water per megawatt-hour, as do solar power towers. A
combined-cycle natural gas plant uses ~200 gallons per megawatt-hour. A
water-cooled parabolic trough plant uses ~800 gallons per
megawatt-hour, 2% for mirror washing. Dish/engine systems only require
~20 gallons per megawatt-hour for mirror washing.
When there
are water limitations and environmental regulations, new fossil and
nuclear plants and CSP power tower and trough facilities can use air
cooling to cut water use dramatically. New fossil and nuclear plants
with air cooling technology eliminate 90% of their water use. A
dry-cooled parabolic trough plant in the Mojave Desert reduced output
5% per year and increased the cost 7-to-9% but eliminated 90% of its
water use. In New Mexico, the cost increase was only 2% because daytime
temperatures were not as high as in the Mojave.
The potential of the Southwest is incredible. (click to enlarge)
Different
technologies lose different levels of output with air cooling. Average
loss for CSP trough plants was 4.6% of the electricity output. Average
power tower loss was only 1.3%. So many factors are part of the total
output equation, from field size to local ambient temperatures, that
conclusive comparisons are almost too generalized to be meaningful.
Also relevant are local water conditions and the cost of peak demand
electricity.
Other cooling systems are also used. There are
hybrid wet/dry cooling systems which balance water use and output
losses. Newer plants can use a parallel cooling system (PCS) that uses
both air and water cooling and balances them more efficiently. A
computer model for PCS in a parabolic trough CSP power plant showed 50%
water savings with only a 1% drop in output or 85% water savings with a
3% output drop. PCS keeps the increased cost of electricity down to 5%,
instead of the 7-to-9% cost increase in a purely air cooled CSP
parabolic trough plant.
click to enlarge
Linear
Fresnel CSP has not yet been evaluated. Dish/Stirling engine systems
use almost no water except for mirror washing but are the newest and
most untested form of CSP.
Clearly, air cooling and hybrid
cooling systems are viable ways to reduce the water needs of CSP
80-to-90% while only increasing the costs of the electricity they
generate by 2-to-10%. Air cooling and hybrid cooling systems also
reduce the water needs of fossil fuel and nuclear power plants to
comparable levels.
So, if water use is comparable, why not use
Old Energies instead of experimenting with CSP technology? First,
because Old Energy is dirty. Coal is filthy, the single biggest
energy-generating contributor to global climate change. Nuclear
represents a battery of dangers, from weapons proliferation to the
radioactive waste for which nobody has provided a safe way disposal
plan. But there’s a more important reason to choose CSP.
Not too many accidents with sun and wind. (click to enlarge)
To
incorporate the air cooling and hybrid cooling systems, the Old Energy
facilities have to be NEW. No NEW nuclear plant has been built in the
U.S. since 1978 for a simple reason: It’s just too damned expensive and
the risk of an accident, while remote, is just too potentially costly.
And the public in the U.S. and Western Europe simply won’t stand for
NEW coal plants anymore. Where new coal or nuclear plants manage to
fight their way to the construction stage, they take huge amounts of
capital and tie it up for 6-to-12 years or longer, invariably coming in
far behind schedule and over budget.
Meanwhile, plans are being
made and construction is underway for CSP installations all over the
Southwest and in hot dry regions from the Maghreb of North Africa to
Australia’s Great Outback to China’s empty western expanses. They go up
in 1-to-2 years and promptly start paying back on the relatively
available amounts of capital they tie up.
So, of course, the
people who have the money are not putting it into coal and nuclear
plants and are ever more interested in solar power plant technologies.
Adding all the costs, sun and wind are by far best buys. (click to enlarge)
QUOTES
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From the NREL study: “Peak power demand, particularly in California,
Nevada and Arizona, is approaching system capacity and growing rapidly.
It is expected that renewable energy sources will increasingly be
tapped to meet market and regulatory demands. Many of the Southwestern
states have established renewable portfolio standards (RPS) that
encourage the development of technologies like CSP…”
From the NREL
study: “Utilities are showing increasing interest in the deployment of
concentrating solar power plants to meet the requirements of state
renewable electricity standards…”
posted by Herman K. Trabish
Concentrating
Solar Power Commercial Application Study: Reducing Water Consumption of
Concentrating Solar Power Electricity Generation; Report to Congress
July 2009 (U.S. Department of Energy/National Renewable Energy Laboratory)
