POWER September 2012 - 58

WATER MANAGEMENT
creased air pollutant and CO2
emissions per
unit of electrical energy (MWh) generated,
cooling tower drift, and blowdown issues. In
addition to the cost of purchasing and constructing
cooling towers for the plant, the
designer would need to consider whether to
reoptimize the remainder of the power plant
to reduce the efficiency penalty.
Retrofitting to cooling towers may also require
water pretreatment (softening/clarification),
scale and corrosion inhibition additions,
and blowdown treatment for discharge regulations,
including possible zero liquid discharge.
All of these treatment processes (plus the cost
of the cooling towers) will add millions of dollars
of infrastructure as well as significant operational
and maintenance expenses.
Retrofit Dry Cooling. Dry cooling retrofits
are very difficult for existing plants, and
the high retrofit costs may render the plant
uneconomic to operate. The balance of the
plant would have to be redesigned to work
with the dry cooling system, because it operates
at such different conditions than wet
cooling towers for five distinct reasons:
■ Dry cooling will cool to the dry bulb temperature,
whereas a wet cooling tower
cools the water to the wet bulb temperature.
Turbines designed for new plants to
be equipped with dry cooling are normally
modified to accommodate the expected
higher turbine backpressure.
■ Air-cooled systems work most effectively
when the steam is ducted straight to the ACC.
There are two options for converting a watercooled
plant to air cooling: use indirect dry
cooling, where the steam is condensed by
recirculating water and the heated water is
routed to an air-cooled heat exchanger, or
remove the condenser and route the steam
directly to an ACC. Because the ACC has a
lower efficiency impact (by eliminating the
temperature drop across the condenser), it
would be the preferred choice over indirect
dry cooling. Removing the condenser and
circulating water lines and routing large-bore
steam piping past the turbine pedestal and to
the ACC would be difficult and expensive,
especially below grade level.
■ Air-cooled systems are typically used on
combined cycle plants for the reasons presented
earlier. This is one reason why, in recent
years, the predominant plant built in the
western U.S. (where dry cooling is needed)
has been natural gas combined cycle plants.
■ In addition to the efficiency penalties
discussed above, hot day operation with
ACCs can result in capacity loss, which
usually coincides with peak system load
(air conditioning) and the highest power
prices. This occurs because as turbine
backpressure rises to near trip points, the
operators must reduce steam flow to prevent
a unit trip (Figure 9).
■ Retrofitting ACCs to an existing unit
would affect plant operation and internal
assets. For example, iron corrosion and
transport is typically an issue with units
on ACCs. It would be even worse with the
mixed metallurgy found in conventional
boilers and steam cycles.
Convert to Partial- or Full-Degraded
Water Use. This method is probably the
easiest water conservation technique to adopt
for existing plants. Its advantages are that the
thermodynamics of the plant and the power
9. Lost hot day performance. The main disadvantage of the air-cooling tower is that
on hot days, turbine back pressure rises more quickly than in the typical once-through or wet
condenser cooling system. Rising condenser pressure means lost steam turbine performance
and efficiency. Source: EPRI
20
18
16
14
12
10
8
6
4
2
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
Turbine exhaust pressure (in. HgA)
58
www.powermag.com
block equipment remain largely unchanged.
Depending on the distance of the source water
from the power plant, transportation costs
(building pipelines and pump stations) can be
one of the largest expenses, but there may also
be costs associated with water and wastewater
treatment systems and chemical additives.
The plant would also need to be evaluated to
ensure that construction materials are compatible
with the expected water quality.
Water Shortage Sharing Agreements.
EPRI is currently implementing a variety of
tools to assist electric power companies with
water issues. For example, WaterPRISM is a
modeling tool that allows stakeholders in a
given watershed to evaluate water reduction
and sharing options. This can even include
special reductions during drought periods-
such as providing revenue streams to support
shifting agricultural land to non-irrigated
crops or non-use for the drought periods.
Looking Forward
Water availability could become a major issue
for the electric power industry over the
next decade and beyond. Siting of new plants
is already constrained by access to cooling
water, especially freshwater. Electric power
is frequently assigned the lowest priority for
water allocation after residential, commercial,
industrial, and agricultural uses. With limited
freshwater supplies and increasing demands,
it is critical to examine options for reducing
anticipated demand within the electric sector
given its critical role in powering the U.S.
economy. This demand must be viewed in light
of weather/climate variability and new technologies
expected to enter the marketplace.
The electric power industry is changing rapidly
in response to new environmental policy
and regulatory requirements, natural gas prices,
and the replacement of aging infrastructure-
among other issues. Although water conservation
practices can be implemented for existing
plants, the larger conservation opportunities
using advanced cooling technologies are more
easily and cost-effectively achieved in the design
and construction of new generating plants.
EPRI and its members are working with
stakeholders and researchers to improve existing
technologies by, for example, reducing the cost
and energy penalties associated with air-cooled
condensers and treating wastewater for reuse.
EPRI is also examining and testing a suite of
new water conservation technologies. These include
thermosyphon coolers, dew point cooling,
and nanoparticles, all of which show promise to
further reduce water use in electricity generation
while preserving overall plant efficiency. ■
-Kent D. Zammit (kezammit@epri.com) is
senior program manager for Water & Ecosystems
programs for the Electric Power
Research Institute.
POWER | September 2012
Loss in turbine output (%)

http://www.powermag.com

POWER September 2012

Table of Contents for the Digital Edition of POWER September 2012