POWER September 2012 - 55

WATER MANAGEMENT
over the outside of a heat exchanger. There
are generally two types of dry cooling: direct
and indirect. In direct dry cooling, steam is
ducted to the air-cooled condenser (ACC),
where it is condensed (Figure 4). This cooling
process has been used at dozens of gasfired
combined cycle plants, largely located
in the western U.S. With indirect dry cooling,
the steam is condensed in a traditional
condenser by cooling water that is then
pumped to an air-cooled heat exchanger.
Hybrid systems combine wet and dry cooling
processes. To date, there has been limited
application of indirect dry cooling in
the U.S., and it has not been retrofitted to
existing power plants.
Current Water Use in Power Plants
According to the U.S. Geological Survey
(USGS), thermoelectric power plant cooling
accounts for approximately 40% of U.S.
freshwater withdrawals (Figure 5). Agricultural
irrigation accounts for approximately
the same amount. Most of the water withdrawn
by thermoelectric generation is discharged
to the water body (once through
cooling), so only approximately 3% of total
U.S. freshwater consumption is used for
electricity generation (Figure 5).
After 1995, the USGS stopped reporting
water consumption values; water use numbers
were reported for 2000 but have not
changed substantially since 1995 and have
possibly decreased with reduced operation of
the nation's coal-fired plants in recent years.
However, percentages of withdrawal and consumption
vary widely throughout the U.S.
The increasing application of recirculating
cooling systems (for example, wet cooling towers),
in addition to freshwater conserving measures,
reduced the electric power industry's water
withdrawals per unit of electric power generated
by a factor of three, from 63,000 gal/MWh in
1950 to 21,000 gal/MWh in 2000. Over the
same period, power output increased by a factor
of 15. The net result is a five-fold increase in
water withdrawals by the electric power industry
over this 50-year period. All of this increase
occurred before 1980, and water use rates for
power generation have steadily declined since
then. With the recent increase in combined cycle
plant operation, reductions in water withdrawals
should continue in the future.
Power plant water use varies with the type
of generation. Figure 6 depicts average water
use values for closed cycle cooling (wet cooling
towers) for each type of plant, based on
actual U.S. plant data.
4. Air-cooled condenser. Exhaust
steam from the turbine is condensed on
the inside of the ACC by air flowing past the
outside of the tubes. The condensed water
is then returned to the condenser. Unlike
once-through cooling and wet cooling tower
options, the air-cooled condenser requires
no cooling water, although the system requires
additional auxiliary power to run the
cooling fans and suffers higher efficiency
penalties at elevated ambient temperatures.
Source: EPRI
Exhaust steam
from turbine
Ambient air
Ambient air
Condensed
steam to boiler
Pumps  Valves  Systems
Congratulations to Lodi Energy Center
as Power Magazine's 2012 Top Gas Plant
Award winner.
KSB, Inc. is proud to have supplied and
commissioned the boiler feed pumps.
As a global supplier of engineered
critical service pumps and high pressure
valves for the Power Industry, KSB is
committed to providing quality and
reliable products.
KSB, Inc. · 4415 Sarellen Road · Henrico, VA 23231
Tel.: (804) 222-1818 · Fax: (804) 226-6961
E-mail: sales@ksbusa.com · www.ksbusa.com
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