POWER September 2012 - 57

WATER MANAGEMENT
As a consequence of the efficiency penalty
for dry cooling, the plant requires more
fuel to produce the same amount of electrical
energy compared with once-through cooling
or cooling towers; therefore, the emissions
of air pollutants and CO2 per unit of electrical
energy delivered (pounds of pollutant per
MWh) are greater.
There are other practical consequences
for operators of dry cooling systems. Dry
cooling systems are significantly larger than
traditional wet cooling towers and require
additional land. The large number of cooling
fans can create noise issues, which can be alleviated
with the purchase of low-speed, lownoise
fans-but those add to costs.
Also, high winds can stall airflow in leading
edge fans, causing a sudden drop in ACC
performance and translating into higher
backpressure on the steam turbine. The control
system will generally reduce steam flow
to the turbine as the backpressure alarm point
is approached, which reduces unit generation
capacity. Under gusty wind conditions, there
have been cases where the controls and system
components did not respond fast enough
to limit backpressure, and the units tripped
off line to protect the turbines from damage.
Hybrid systems combine wet and dry
cooling towers (Figure 8). This arrangement
allows most of the heat to be rejected to the
atmosphere on cooler days, avoiding the
use of cooling water, while maintaining the
power plant's thermal efficiency during hot
days, with the wet tower taking part of the
cooling load.
Hybrid cooling is becoming more popular
because the tower sizes can be minimized to
reduce additional costs, and performance is
better than with air-cooling only. There are
many ways to optimize such a system, depending
on the goals of the plant design and
the available water sources. EPRI guidelines
are available to assist plant designers with
this optimization process.
Increased Thermal Conversion Efficiency.
High-efficiency natural gas combined
cycle power plants have a large benefit from
7. Hot day power loss. The performance of a typical 500-MW coal-fired plant using dry
cooling is highly temperature-dependent; therefore, expected plant performance will vary considerably,
based on location. Source: EPRI
Yuma
525
500
475
450
425
400
375
350
-50
-25
25
50
Ambient temperature (F)
8. Hybrid cooling system. A hybrid cooling system combines the best features of the
wet cooling tower and the air-cooled condenser. Source: EPRI
Turbine
75
100
125
Jacksonville
Bismarck
Burlington
St. Louis
a water conservation standpoint, as shown in
Figure 6. Because the combustion turbines
produce roughly two-thirds of the power,
cooling water consumption is reduced by
an equivalent amount. In addition, the steam
from the turbine can be condensed by ACCs,
further reducing water use. The ACC will be
smaller, as it is only cooling the steam turbine,
about one-third of the combined cycle plant's
power output, which minimizes hot day efficiency
penalties. Combustion turbines will
also sustain a hot day capacity penalty. This
penalty, however, can be mitigated with inlet
sprays or evaporators, or with inlet chillers
that draw power.
Water Recycling. Power plants in operation
today already employ many practices to
reuse water. Water is typically " cascaded " from
one use to another, depending on the quality of
water that is needed for each process.
Another potential place for recycling is
stack gas. A significant amount of water is
lost through power plant stacks (flue gas from
fossil fuels contains 8% to 13% moisture
as a byproduct of the combustion process)
and cooling tower plumes. Several research
groups, including EPRI, are funding work to
develop technologies to economically recapture
water from plumes, but these technologies
are not yet commercially available.
In the meantime, to the extent that renewable
technologies can economically penetrate
the generation mix, water use can be
reduced. Renewable energy from wind, solar
photovoltaic, geothermal (when using brine
for cooling water), hydroelectric, marine,
and hydrokinetic sources all require little to
no water consumption. Solar thermal is the
one notable exception.
Fans
Water Conservation Options for
Existing Power Plants
Options for existing plants are more limited
than for new ones, as the available conservation
methods usually require major modifications
to the boiler and steam turbine and may
have other environmental and permitting
constraints. The associated cost increases
may limit the economic viability of the plant.
Some of the options that have been suggested
or considered for water conservation at existing
power plants are examined below.
Retrofit Closed-Cycle Cooling (Wet
Wet-cooling tower
To boiler
September 2012 | POWER
www.powermag.com
Air-cooled condenser
Cooling Towers). Although water withdrawals
would be reduced by over 95% with wet
cooling towers, the evaporative losses would
approximately double. EPRI has a significant
body of research estimating costs and other
impacts of retrofitting cooling towers to all
U.S. plants using once-through cooling.
The retrofit of wet cooling towers will result
in negative consequences, such as lower
thermal efficiency and correspondingly in57
Plant
output (MW)

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POWER September 2012

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