FEASIBILITY OF ALTERNATIVE
COOLING SYSTEMS FOR POWER PLANTS
IN THE NORTHERN GREAT PLAINS
. - •wpn% - m, <
Environmental Protection Agency
Region VIII,
October 1974
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Feasibility of Alternative
Cooling Systems for Power Plants in
The Northern Great Plains
By
Dr. Bruce A. Tichenor
Thermal Pollution Branch
Pacific Northwest Environmental Research Laboratory
EPA
James W. Shaw
Co-Leader
Water Quality Subgroup
NGPRP
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TABLE OF CONTENTS
Page
List of Figures iii
List of Tables iv
I. Introduction 1
II. Power Plant Water Requirements 5
III. Operational/Engineering Considerations of Wet and
Dry Cooling Tower Systems 16
A. Operating Characteristics 16
B. Availability and Reliability 18
C. Energy Requirements 24
IV. Environmental Impacts of Closed-Cycle Cooling Systems. . 25
A. Wet Cooling Towers 25
B. Dry Cooling Towers 28
C. Wet/Dry Cooling Towers 29
V. Economics of Wet and Dry Cooling Towers 30
VI. Summary 35
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LIST OF FIGURES
Page
11-1 Schematic of Once-Through Cooling System 7
II-2 Cooling Water Requirements for Fossil Power Plants • • 8
11-3- Schematic of Closed-Cycle Cooling System 9
11-4 Types of Natural Draft Wet Towers 10
11-5 Crossflow Induced Draft Tower 11
II-6 Water Requirements of Selected Cooling Systems for
a 1000 MWe Power Plant in the NGP 14
III-1 Typical Large Mechanical Draft Wet Cooling Tower ... 19
111-2 Typical Natural Draft Wet Cooling Tower 21
111-3 Side Walls and Steam Headers at Direct-Type Air-
Cooled Condensing Unit - 20 MW Generating Station ... 22
II1-4 Prototype Mechanical Draft Wet-Dry Cooling Tower ... 23
VI-l Comparison of Alternative Cooling Systems 37
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LIST OF TABLES
Page
V-l Input Variables - Economic Analysis 32
V-2 Cost of Wet Versus Dry Cooling Tower Systems 33
V-3 Cost of Wet Versus Dry Cooling With Make-up Water
Cost Considered 34
iv
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I. Introduction
The Northern Great Plains, particularly the states of Montana,
North Dakota, and Wyoming, is a potential source of vast amounts of low
sulfur coal. Recent energy shortages, strengthened with plans such as
Project Independence, have increased national interest in the coal of
the NGP. Since much of the coal is in thick seams relatively near the
surface, ideal for surface mining, and low in sulfur, many people feel
it is the ideal fuel for meeting the nation's short term energy needs.
The NGP today is relatively undeveloped in the conventional sense
of the word. The present social and economic system is based primarily
on agriculture. Recreational areas of national value border most of the
coal country. In the western part of the coal region livestock ranching
predominates and the population density is less than two people per
square mile (compared to a national average of 56 people per square
mile). In the eastern part of the coal region there is more dry land
farming. Population in the area has not grown rapidly in the past.
Much of the area where coal is found is semi-arid to arid. Annual
precipitation over the coal areas averages about 14 inches, but some
areas receive considerably less. The water quality of the streams is
fair to good. Water quality data, however, are not available in many
areas of the NGP. When average flows prevail in streams within the
area, there are surpluses in the spring runoff period to all existing
uses within each tributary basin (much of the surplus has been appropriated).
When snowmelt and rainfall are deficient, however, many of the streams
immediately become short of water and unable to serve current needs.
Water short areas have developed impoundments to store seasonal runoff.
However, the Yellowstone and the Wind-Bighorn Rivers are the only rivers
in the NGP that have escaped having zero flows occasionally at at least
one flow recording station.
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The NGP is generally noted for its vastness. Visibility has always
been good, and coupled with the large open spaces has resulted in the
famous "Big Sky" image for the area. The "western" way of life is in
many ways dependent upon the concept of wide open spaces, clean air and
clean water. While local areas, primarily those around the few larger
towns, have some air and water quality problems, in general the area is
relatively unabused. An important exception may be those rural areas
that are overgrazed.
The local, state, and Federal governments which make land use and
resource planning decisions affecting the area face competing economic,
social, and environmental alternatives concerning coal development.
These interests have not always been well coordinated. Further, data on
which decisions can be based on .are not available for much of the area.
Therefore, EPA, the Departments of- the Interior, and Agriculture, and
the States of Montana, Nebraska, North Dakota, South Dakota, and Wyoming
have cooperatively set up the-Northern. Great Plains Resources Program
(NGPRP).
The primary objective of. the NGPRP is to provide an analytical and
information framework for policy and planning decisions at all levels of
government. Often objectives involve coordination of all data collection
and interpretation activities and the development'of a coordinating link
between data collection, research, planning, and. operational resource
management activities that exist within many different organizations in
the NGP.
The program has just completed its first year and has published an
interim report of its activities and findings to date.- The program
involved seven technical work groups which have also filed reports, some
interim and some final. In addition, several areas have been identified
which cross work group lines. In those instances, special reports, such
as this report, have been prepared.
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To facilitate analysis of possible impacts associated with coal
development, the NGPRP developed three "scenarios" or levels of coal
development that might occur. One is a base level of development,
consisting largely of present commitments. One is an intermediate level
and finally, one is a high level. The base level, for example, estimates
coal production in the NGP at 144 million tons per year by the year
2000; corresponding estimates for the intermediate and high scenarios
(by the year 2000) are 362 million tons per year and 977 millions tons
per year.
Two types of coal development have been assumed. Surface mining
and subsequent exportation of the coal for conversion near load centers
is one alternative. The second alternative is mining and conversion of
coal to other energy forms prior to exportation. Studies to date appear
to indicate that physical impacts associated within mining and exporta-
tion of coal would be overshadowed by the physical and socio-economic
impacts of large scale conversion facilities in the NGP. Conversion
facilities will require large labor forces, water, and land and will
have significant air emissions.
Water requirements, for development at the three scenario levels,
have been estimated by the Water Work Group of the NGPRP. The base
scenario shows a total of six power plants requiring 124,000 acre feet
of water. The intermediate scenario calls for 730,000 acre feet of
water annually to serve a mix of power plants and gasification plants.
Water requirements for the high scenario are estimated at 1,500,000 acre
feet annually, as the number of gasification plants would grow.
In the area where water needs to existing uses are not always met,
particularly in dry years, the impacts of supplying large amounts of
water for energy conversion must be closely assessed. In addition,
careful analysis of actual water needs is required. The above water
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estimates are based on a "requirement" of 19,000 acre feet of water per
year for a 1000 MW power plant and of 30,000 acre feet per year for a
250 x 10^ scf day gasification plant. Additional NGPRP studies have
placed the water requirements at 12,000 acre feet/year and 9,500 acre feet/
year respectively.
Some water certainly is available for additional use. When water
releases from Garrison Dam are considered, water is certainly available.
Of course, the location of water is not always convenient. Additional
impoundments, as well as adqueducts from existing impoundments, will be
necessary with an unknown impact on the existing aquatic ecosystem. The
cost, both to other water users and to the environment of removing water
below instream flow needs must be assessed. In some areas, ground water
may be available for industrial use. While it is appropriate to evaluate
all possible water sources for potential development, to better define
the cost and environmental impact of its use, it is also appropriate to
fully evaluate water "requirements".
The remainder of this report is a discussion of the feasibility of
alternate means of cooling coal conversion plants in the NGP. While
primary emphasis will be on thermal power plants, much of the discussion
will also be relevant to cooling requirements of coal gasification
plants. Since some cooling methods at power plants can represent over
95% of plant water requirements, it seems very appropriate to fully
evaluate the feasibility of alternate methods.
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11. Power Plant Water Requirements
Power plants which generate electric energy from coal require water
for a variety of purposes including:
a. Boiler feedwater--heated in the the boiler to make the steam
used to turn the turbine-generator.
b. Ash sluicing--used to wash away coal ash which falls from the
bottom of the furnace.
c. Service water--used by plant personnel and visitors for drinking,
Taundary, sanitary purposes, etc.
d. Stack gas cleaning—used to "scrub" air pollutants from the
stack effluent.
e. Cooling water—passed through the condenser to remove the
waste heat from the turbine exhaust steam.
Items a, b, and c require small amounts of water; approximately one
cfs for a 1000 megawatt power plant. Item d requires slightly more;
three to five cfs. By far the largest use of water in a power plant
concerns item e—cooling water.
The cooling water requirements of a power plant are determined to a
great extent by the type of cooling system used by the plant. The most
common cooling systems include:
a. Once-through cooling -- in this method the cooling water is
pumped from an adjacent water body (i.e., river, lake or
reservoir) through the condenser and discharged back to the
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water body (See Figure 11-1). This type of cooling system
requires huge quantities of water. For example, a modern coal
fired plant of 1000 megawatt capacity requires about 1000 cfs.
Figure 11-2 provides further information on cooling water
requirements for several condenser temperature rises.
b. Closed-Cycle cooling--this method requires an off-stream
cooling device to transfer the waste heat to the atmosphere
(see Figure 11-3). Several cooling devices are available for
use:
Met Cooling Towers are devices where the warm cooling water
flows over packing material and heat loss by evaporation and
convection occurs as air moves through the packing. Air
movement can be induced by a chimmney effect (natural draft
tower--see Figure 11-4 or by a fan (mechanical draft towers-
see Figure 11-5). In wet towers, the cooling water is recycled
back to the condenser. Water is lost by evaporation and a
small amount is discharged to waste (blowdown) to prevent a
buildup of undesirable dissolved materials. For a 1000
megawatt plant in the Northern Great Plains, a wet cooling
tower would require an annual average of 14 to 19 cfs depending
on the water treatment applied to the blowdown; under extreme
summer conditions the requirement could exceed 25 cfs.
Cooling Ponds are artifical lakes which use the natural heat
exchange processes of evaporation, convection, and radiation
to dissipate the power plant's waste heat. Since cooling
ponds also receive large amounts of energy from solar radiation,
the amount of heat dissipated from a cooling pond by evaporation
is generally higher than from a wet cooling tower designed to
serve the same power plant. In the Northern Great Plains, the
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FIGURE H-2
COOLING WATER REQUIREMENTS FOR
FOSSIL POWER PLANTS
\ ¦ 40%
IN-PLANT AND
STACK LOSSES
= 15%
AT = CONDENSER TEMP RISE
T|t * PLANT THERMAL EFFICIENCY
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Power
Plant
. 90°F r-i
rv
Figure 11-3. Schematic of Closed-cycle Cooling System
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Hot I ¦ V, -<7
Water
Distri-/ Drift
bution / Eliminator
UUtLUlI fa ,////
..w.o*, •• ' o;•;
Cold Water'
Basin
- v - . ^
'Cold Water
Basin
Counterflow
Crossflo
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Figure 11-4. Types of Natural Draft Wet Towers
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Air Out
Water In
Air In
Water\
Out
-Packing
Air In
Figure 11 - 5. Crossflow Induced Draft Tower
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amount of water required for a 1000 megawatt power plant
cooling pond would be about 20-25 cfs on an average annual
basis; under extreme conditions the water requirement could
exceed 50 cfs.
Spray Systems use fixed or floating nozzles or spinning discs
to produce spray droplets. The movement of the droplets
through the air provides cooling by evaporation and convection.
Spray system water requirements are similar to those for wet
cooling towers.
Dry cooling systems use only sensible heat transfer and are
appropriate in areas of little or no water. There are two
types of dry systems.
1. The direct air condenser where the turbine exhaust
steam is condensed by the air and no cooling water
is employed,
2. Indirect type dry systems where-direct spray condensers
(Heller type) are used and the cooling water and
steam are mixed, with the resultant hot water going
through an air heat exchanger. Thus, there is no
separate cooling water system. Recent studies
indicate that a standard surface condenser could be
used in place of a direct spray condenser.
Dry towers, as the name implies, require no evaporation and
thus no blowdown or makeup water.
Wet/dry cooling towers are constructed with both dry and
evaporative heat exchangers. Such towers can be designed and
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operated to reduce evaporative water loss by removing a large
amount of waste heat via convection from the dry portion of
the tower. Thus, water requirements of wet/dry towers fall in
between wet towers and dry towers.
Figure 11-6 shows the relationship between the total water require-
ments (including items a-e) of the various cooling systems for a 1000
megawatt coal fired power plant in the Northern Great Plains. It must
be emphasized that the data contained in this figure are approximate,
but they do shown the relationship between the various systems in terms
of water use.
The quality of water available for cooling has a large effect on
the amount of water required in a wet closed-cycle system. The higher
the dissolved solids content of the water, the greater the water require-
ment. This is true because at a high solids content a large "blowdown"
stream is required, while for extremely high quality fresh water, blowdown
requirements are minimal. The water use data in Figure II-6 are based
on average water quality for the Northern Great Plains study area.
A major consideration in the use of the Northern Great Plain's coal
reserves is the impact on the area's water resources, both in terms of
consumptive use and deterioration of quality. As is discussed above,
power plant water use can range from 1000 cfs for a once-through cooling
systems to 5 cfs for a dry cooling system, assuming a 1000 megawatt
power plant with air pollution scrubber equipment. Excessive water
requirements and the regulatory constraints of the Federal Water Pollution
Control Act Amendments of 1972 make once-through cooling an unlikely
alternative for much the Northern Great Plains. However, the use of
parts of the Missouri for oncethrough cooling is currently under study.
The relatively high water needs of cooling ponds makes wet cooling
towers a more attractive alternative. From the standpoint of minimum
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Figure 11-6. Water Requirements* of Selected
Cooling Systems for a 1000 MWe
Power Plant in the Northern Great Plains
* All plants are assumed to have S0X removal equipment and a base H2O requirement of 5 cfs.
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water use, dry towers are the most acceptable cooling system. Thus, to
minimize water use, the most viable cooling alternatives for thermal
power plants in the Northern Great Plains are wet cooling towers (or
spray systems), dry cooling towers, and wet/dry cooling towers. The
remainder of this report will discuss the engineering, environmental and
economic aspects of these alternative cooling systems.
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III. Operational/Engineering Considerations of Wet and Dry Cooling
Tower Systems.
Operating Characteristics
Wet Cooling Towers, both mechanical and natural draft, dissipate
the majority (approximately 75%) of the waste heat in the cooling water
by latent heat transfer (evaporation) with,the remainder lost by sensible
heat transfer (conduction-convection). The wet-bulb temperature of the
ambient air is the minimum temperature to which the water can be cooled.
The tower is designed to cool the water to some specified temperature
above the wet-bulb temperature.
The approach of the tower is defined as the difference between the
cool outlet water and the wet-bulb temperature. Approaches of 15-20°F
are generally used. The cooling range is the difference between the hot
inlet water temperature and the cool outlet water temperature. In a
closed-cycle system, the range equals the condenser temperature rise and
is generally between 25 and 40°F. Thus, given information on the approach,
range, and wet-bulb temperature, one can determine the inlet (hot leg)
and outlet (cold leg) tower temperatures. For a given range, the lower
the approach the larger and more expensive the tower, but the cooler the
water and the more efficient the power plant. Thus, an optimal approach
must be determined by balancing capital and operating expenditures.
In evaluating tower performance, one must differentiate between
design conditions and average conditions. Design wet-bulb temperature
is often designated as not to be exceeded more than a fixed percentage
(say 5%) of the time during the summer months. It represents a severe
condition. Average conditions will be much more moderate. For example,
a particular tower may be designed for a 15°F approach, a 30°F range,
and 75°F wet-bulb temperature. This would provide a 120°F tower inlet
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temperature with a 90°F outlet temperature. Under more moderate weather
conditions much cooler temperatures will be realized.
Dry Cooling Towers can also be designed as either natural or mechan-
ical draft. As discussed above, all of the waste heat is discharged to
the atmosphere via convective heat exchange. Thus, in theory, the
minimum temperature to which the water can be cooled is the dry-bulb
temperature. In actual practice, the tower is designed to cool the
water to some specified temperature above the dry-bulb temperature.
The primary design variable for dry towers is the Initial Temperature
Difference (ITD), which is the difference between the incoming hot water
and the ambient air dry-bulb temperature. The selection of the ITD
requires an evaluation of four major cost items—capital cost, auxiliary
power cost, cost to the loss of capacity, and-fuel cost. At large ITD's
the cooling system is highly efficient and thus compact and relatively
inexpensive. The auxiliary power requirements are also relatively
small. On the other hand, the loss of power plant capability and the
increased fuel cost due to lower power plant efficiencies caused by high
cooling water temperatures are large. Thus, the capital cost and auxiliary
power cost are low at high ITD's while the cost due to the loss of
capacity and fuel costs are low at low ITD's. The optimal ITD for a
given region is consequently dictated by the combined effects of all
four cost factors. In studies conducted to evaluate dry cooling towers
for thermal power plants in the North.ern Great Plains, design ITD values of
60-65°F were used.
Wet/Dry Cooling Towers have received intensive study in recent
years by several manufacturers. These systems, as mentioned previously
are constructed with both dry and evaporative heat exchangers. The
normal design provides initial cooling water passage through a dry heat
exchanger with the water then falling through conventional wet cooling
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tower packing. Other configurations are also possible, such as separate
closed-loop cooling circuits for the wet and dry sections.
The purposes of utilizing wet/dry. towers are two-fold:
1. To reduce or eliminate.the visible plume emission by (a)
decreasing the moisture content of the vapor discharge and (b)
heating the plume to allow it to hold more water vapor before
becoming saturated.
2. To reduce water consumption. The tower designer can specify
what proportion of the waste heat load must be rejected by the
dry section and design the tower accordingly (i.e., 50% dry,
50% wet; 30% dry, 70% wet; etc.). In general, the dry section
will have a larger heat rejection capacity than the wet section
when water conservation is the goal.
The operation of the wet/dry tower will depend on meteorological,
hydrological, and pi ant-load factors. For example, during meteorological
conditions conducive to fog problems, maximal use of the dry section
will reduce or eliminate the visible plume. During other weather condi-
tions, the tower may operate primarily as an evaporative cooler. Low
availability of make-up water will also require maximal use of the dry
section, with due consideration to the effect of high cooling water
temperature on the plant's capacity and efficiency.
Availability and Reliability
Wet Cooling Towers have proven reliability and are available to
meet the cooling demands of any power plant size. U.S. experience with
mechanical draft towers (see Figure 111-1) is extensive and several
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Figure III-l
Typical Large Mechanical Draft Cooling Tower
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manufacturers are available to meet any reasonable specifications.
Natural draft towers (see Figure 111-2) have been employed by U.S.
electric utilities for over 10 years and are becoming increasingly
popular for large power plants. Their reliability has proven to be very
high. In the Northern Great Plains, either wet mechanical draft or
natural draft towers would be a reasonable choice for a thermal power
plant closed-cycle cooling system.
Dry Cooling Towers have been proven to be reliable devices for
removing waste heat from power plants. Several dry cooling systems are
operating and are under construction in Europe and South Africa; however,
the only U.S. experience with dry cooling for power plants is at the
Simpson Station in Wyodak, Wyoming. This 20 megawatt unit employs a
direct air condenser (see Figure 111-3). An additional 330 megawatt
unit using a dry cooling system is being added to the Simpson Station.
The economic and technical feasibility of dry cooling systems has
been widely reported. The major obstacle to their use on large power
plants in the U.S., appears to be the lack of suitable high back pressure
steam turbines. A wider use of dry towers in the U.S. awaits the success-
ful demonstration of a large prototype. The most obvious use of dry
systems is at fuel-rich and water-poor locations, with the Northern
Great Plains being a prime candidate.
Wet/Dry Cooling Towers have reached the stage of full-scale opera-
tion, with several U.S. manufacturers competing for the available market.
As yet, no U.S. power plant operates completely on wet/dry towers, and
to date only mechanical draft prototype units have been tested (see
Figure III-4). The lack of experience with wet/dry towers makes it
difficult to assess their reliability, although no technical impediments
exist.
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Figure II1-2
Typical Natural Draft Viet Cooling Tower
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Figure 111-3
Side Walls Erected Around Direct-Type Air-Cooled
Condensing Unit 20 MW Generating Unit
Steam Headers and Hail Screens Direct-Type
Air-Cooled Condensing Unit 20 MW Generating Unit
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Figure 111-4
Prototype Mechanical Draft Wet-Dry
Cooling Tower
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Energy Requirements
The "laws" of thermodynamics dictate-that the lower the temperature
of the incoming cooling water, the more efficient the power plant.
Since closed-cycle cooling generally results in a higher temperature for
the incoming cooling water, a decrease in power plant efficiency can be
experienced resulting in increased fuel requirements for producing a
constant supply of power. In addition, increased pumping power (due to
the requirement to lift the water to the cooling tower packing) and the
need for fan power for mechanical draft towers cause increases in power
consumption. Also, on hot summer days, the power plant may experience a
loss in capacity due to excessively high cooling water temperatures.
The above factors combine to provide an increase in energy require-
ments for a plant with closed-cycle cooling. On an average annual
basis, a power plant with wet cooling towers will require less than 1%
more energy than an equivalent plant (i.e., one with the same power
output) with once-through cooling. A plant with dry towers will require
about 3%o more energy than its equivalent with once-through cooling. The
additional energy requirements of wet/dry cooling will range from 1-3%
depending on the proportion devoted to dry cooling.
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IV. Environmental Impacts of
Closed-Cycle Cooling Systems
Wet Cooling Towers
In addition to the consumption of water, wet cooling towers can
have potentially adverse side effects due to vapor plumes, drift, and
blowdown.
Cooling Tower Vapor Plumes have the potential for causing or increasing
local fogging conditions. The key word here is potential, since in most
cases, no such problems will occur. Fog is defined here as a condition
where vision is obstructed.
Cooling towers do produce visible plumes; however, plumes are
normally not a problem unless they reach the ground. Under normal
conditions, cooling tower plumes rise due to their initial velocity and
buoyancy and rarely intersect the ground before they are mixed with the
ambient air and dissipated. However, under adverse climatic conditions
(i.e., high humidity and low temperature), the moisture could produce a
fog condition if it were trapped in the lower levels of the atmosphere,
such as during a period of high atmospheric stability (i.e., an inversion).
In most all cases, natural draft towers are less likely to cause fogging
problems than mechanical draft towers. Mechanical draft towers may
cause problems, but in most cases fogging would be on-site (i.e., within
1000-2000 ft of the tower). Also the limited vertical mixing occurring
during neutral stability conditions could limit plume dispersion.
During sub-freezing weather, fogging and drift conditions may
result in icing. As with fog, experience with large power plant cooling
towers has not resulted in major icing problems. Methods of predicting
the accumulation of ice due to cooling tower plumes are not widely
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reported. In general, icing caused by plumes will be a low-density
accumulation of granular ice tufts and is unlikely to cause damage due
to its weight. A problem to be considered would be associated with the
danger of icy roads. It should be noted that icing caused by the plume
will be generally limited to vertical surfaces, and icing of horizontal
roadways will be less severe.
As the water falls down through the tower packing and below, it is
possible for small droplets to become entrained in the air stream moving
out through the tower top. These droplets called drift have the same
chemical characteristics as the cooling water in the system. The use of
drift eliminators above the packing can reduce the drift loss substanti-
ally. Draft eliminators are louver-type baffles which intercept the
drift particles before they can exit from the tower. State-of-the-art
design can be used to obtain drift losses of 0.005% of the circulating
flow rate for mechanical draft units and 0.002% for natural draft towers.
In no case should the drift loss exceed 0.01% for modern, well-designed
towers.
For cooling towers in the Northern Great Plains, drift should not
be a problem. Except for isolated instances of electrical arcing between
transmission lines, drift from freshwater cooling towers has not caused
environmental problems. Separating cooling towers and switchyards by
500-1000 ft. should prevent such problems.
As discussed above, blowdown is discharged to the receiving water
to prevent dissolved solids build-up in the cooling water system. For
the Northern Great Plains, the original concentration of materials in
the make-up water will be increased about four times prior to discharge
in the blowdown. After an appropriate mixing zone, such concentration
should cause no environmental problems, although discharge of blowdown
in certain areas with salinity problems may be restricted. The Four-
Corners area is an example of an area where salinity problems dictate
no discharge of blowdown to surface waters.
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A potential problem connected with cooling tower blowdown is the
concentration of residual chlorine. Chlorine is added to the circulating
cooling water to prevent biological fouling of heat transfer surfaces in
the condenser and to inhibit biological growths on the packing in the
cooling tower. The chlorine is normally added intermittently at relatively
high concentrations. This periodic addition of chlorine is an effective
method of control which has been widely used in the power industry.
However, the toxic effects of chlorine or chlorine derivatives to aquatic
organisms require minimization and close control of these constituents
in effluent discharges. Fortunately, the control of residual chlorine
is achievable in an economic fashion by using one or a combination of
approaches:
1. Practicing split stream chlorination, i.e., treating one
condenser at a time, thus providing greater dilution and lower
chlorine concentrations prior to discharge.
2. Reducing the chlorine feed period.
3. Combining a discharge stream with another in-plant stream
which has a high chlorine demand, such as the treated sanitary
washes, ash sluice water, etc.
4. Discontinuing blowdown during periods when residual chlorine
is present in the cooling tower sump.
5. Decreasing the rate of chlorine addition during feed, in
proportion to the reduction of chlorine demand of recirculating
water in closed-cycle systems. This method maintains a constant
residual chlorine level at the condenser discharge.
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6. Adding sodium sulfite, sodium bisulfite, or sulfur dioxide to
blowdown to reduce chlorine at costs ranging from 0.001 to
0.014 mills/KWH.
An alternative to chlorination is mechanical cleaning of condenser
tubes which, from an environmental standpoint, is the preferred method
because no chemicals are employed. Balls (Amertap System) or brushes
(MAN system) are passed through the condenser tubes periodically to
cleanse them of biological growth. Mechanical cleaning is being used in
numerous instances, particularly in new plants where it can be incorporated
into the initial design. Mechanical cleaning may promote better heat
transfer efficiency; disadvantages include installation and maintenance
problems and potentially higher costs than chemical cleaning. Typical
total costs are 0.01-0.02 mills/KWH. Also, the condenser alone is
treated whereas chemical cleaning affects the entire cooling system.
Dry Cooling Towers
By the nature of their operation, dry cooling systems will not
cause fogging, icing, drift, or blowdown problems. There is some contro-
versy over what the overall environmental effects of the warm air discharge
from a dry cooling tower might be. A beneficial effect may be to increase
ventilation in inversion prone areas. The potential modification of
local meteorology, such as triggering the formation of cumulus clouds,
requires further study. Also the overall meteorological consequences of
large heat releases should be assessed; however, this problem should be
considered in the broad content of all large heat sources. In general,
dry towers can be expected to be good environmental "neighbors." More
research is needed to identify possible noise problems.
A secondary environmental consequence due to the use of dry towers
is the potential increase in air pollution due to increases in stack gas
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emissions. Since dry towers cause a decrease in power plant efficiency,
more fuel (about 3%) will be consumed to generate the required electric
power. The air pollution potential of a 3% increase in fuel use should
be minimal, especially if adequate stack gas cleaning technology is
employed.
Wet/Dry Cooling Towers
The fogging and icing potential of wet/dry towers is a function of
design and operations. Such problems can be eliminated by using the
proper proportions of dry and wet heat exchange during the winter.
Drift is caused by "mechanical" forces and is not affected by the
heat exchange in a tower. Therefore, if all the water is circulated
through the wet section of a wet/dry tower, one would not expect the
drift rate and subsequent deposition to be much different than for a
conventional wet tower.
Blowdown volume will be reduced as the amount of waste heat dissipated
via the dry heat exchanger is increased. The concentration of dissolved
materials in the blowdown will be the same as for a conventional wet
tower. Thus the blowdown from a wet/dry tower will have a smaller
impact than if a standard wet tower were used.
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V. Economics of Wet and Dry Cooling Towers
A multitude of factors must be considered in assessing the cost of
a wet or dry cooling tower system, including:
1.
Meteorological conditions
2.
Fuel costs
3.
Interest rates
4.
Material and labor costs
5.
Plant load factor
6.
Steam turbine characteristics.
In determining the cost of a power plant cooling system, both
capital and operating costs are evaluated. Costs can be expressed in
total cost, ($), annual cost ($/year), or production cost (mills/Kwhr,
often called busbar cost). Since the cost is ultimately borne by the
power consumer, translating the costs into increases in consumer costs
provides the truest picture of the economic impact.
An evaluation of the costs of wet and dry cooling towers for thermal
power plants in the Northern Great Plains was performed by EPA's Thermal
Pollution Branch located at the Pacific Northwest Environmental Research
Laboratory in Corvallis, Oregon. The results of their study are contained
in a staff report by Drs. Tichenor and Shirazi, "Engineering and Economic
Aspects of Wet and Dry Cooling Systems." This report discusses the
analyses conducted as well as the results obtained. Table V-l contains
a summary of the assumptions and input values used.
Results of the economic analysis are summarized in Table V-2, where
the difference in cost between wet and dry cooling tower systems is
given for various fixed charge rates, fuel costs, and peaking season.
For summer peaking, gas turbine peaking units are required to make up
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lost capacity during periods of excessively high air temperatures. Since
most of the power to be generated in the Northern Great Plains will be
exported, summer peaking could be provided by the end user (e.g., a
Chicago utility) instead of at the main power plant site (e.g., Gillette).
For systems that have a peak in the winter where cool temperatures
preclude the necessity to make up capacity with gas turbine peaking
units, dry cooling is more economically attractive.
The results given in Table V-2 are for the Gillette site. The
costs given indicate the economic penalty of selecting dry towers over
wet towers. It is emphasized that these data are based on an analysis
of feasibility and should not be considered hard design costs. They do
reflect, however, reasonable assumptions and valid economic and engineering
judgements.
The costs provided in Table V-2 show the added expense to a utility
to provide dry towers. They were computed without including the cost of
water for make-up to the wet towers. Table V-3 provides cost estimates
similar to those in Table V-2, but with the cost of make-up water considered.
Also, the increased cost to a typical residential consumer is indicated.
The effect on consumer cost was calculated assuming a residential electric
cost of 20 mills/Kwhr and a monthly consumption of 1000 Kwhr. It was
also assumed that the consumer purchases 100% of his power from a plant
with dry towers. The data in Table V-3 indicate that the average resi-
dential cost of electricity due to the use of dry towers in the Northern
Great Plains ranges between $7.32 per year and a savings of $0.48 per
year depending on water cost and other economic factors.
Met/dry towers were omitted from the economic analysis because no
reliable data on their costs are available. It can be assumed, however,
that the cost of a wet/dry cooling tower system would fall between the
costs of a dry and wet tower system.
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Table V-l
Input Variables - Economic
Analysis
Fuel cost - 16 and 19 <£/10^ BTU
Fixed Chart Rate* - 12, 15, and 18%
Average Annual Plant Capacity Factor - 70%
Meteorological Data for: Colstrip, Gillette and Stanton
Steam Turbine: Cross-compound, 3600/1800
RPM, 3500 psig, 1000/1000°F
6 flow, 38 inch last stage
blades (General Electric)
*Fixed charge rate is the percent of total capital cost paid each year
and includes interest, taxes, amortization, etc.
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Table V-2
Cost* of Wet Versus Dry Cooling Tower Systems
Fi xed
Charge
Rate (%)
Fuel
Cost c
((£/10 BUT)
Peaking
Season
Mills
Kwhr
s-
>1
,
Total**
$
12
16
summer
0.53
3,200,000
27,000,000
18
16
summer
0.74
4,500,000
25,000,000
18
19
summer
0.78
4,800,000
27,000,000
12
16
wi nter
0.30
1,800,000
15,000,000
18
16
winter
0.41
2,500,000
14,000,000
18
19
winter
0.46
2,800,000
16,000,000
*Cost = dry cost - wet cost for a 1000 MWe power plant with a capacity factor
of 70% (water cost not included).
**Total $ = Total capital cost, including 0&M costs.
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Table V-3
Cost of Wet Versus Dry Cooling with Make-up
Water Cost* Considered
Fixed Fuel Water
Charge Cost Peaking Cost
Rate (X) U/10 BTU) Season ($/ac-ft)
Increase in
Power Cost
(miHs/Kwhr)
Increase in Consumer
Cost
{%) ($/year)
12
16
summer
150
0.36
1.8
4.32
12
16
summer
300
0.19
0.9
2.28
18
16
summer
150
0.57
2.9
6.84
18
16
summer
300
0.40
2.0
4.80
18
19
summer
150
0.61
3.1
7.32
18
19
summer
300
0.44
2.2
5.28
12
16
winter
150
0.31
0.7
1.56
12
16
winter
300
-0.04
-0.2
-0.48
18
16
winter
150
0.24
1.2
2.88
18
16
winter
300
0.07
0.3
0.84
18
19
winter
150
0.29
1.5
3.48
18
19
winter
300
0.12
0.6
1.44
*Based on average annual make-up requirements for a plant with wet towers of 17
cfs and a 5 cfs base water requirement for a plant with dry towers. This gives
an average annual differential water requirement of 12 cfs.
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VI. Summary
For power plants in the Northern Great Plains, closed-cycle cooling
with wet, dry, or wet/dry towers are all reasonable alternatives. A
comparison of the three types of towers can be made by evaluating five
factors:
1. Operating experience: Wet towers rate highest followed by dry
towers (a distant second at best) and, last, wet/dry towers.
2. Economic penalties: Generally wet towers are the most economical
with wet/dry towers next followed by dry towers; however, this
order could change depending on the make-up water cost and
other cost associated with depleting stream flows.
3. Environmental impact: Dry towers have the lowest potential
for environmental problems followed closely by wet/dry towers
with wet towers last.
4. Water use: Dry towers consume the least water, followed by
wet/dry towers, then wet towers.
5. Energy requirements: Wet towers require the smallest amount
of energy, followed by wet/dry towers, with dry towers last.
Figure VI-1 provides the above information in a matrix format. The
numerical ratings contained in the matrix reflect the relative merit of
each tower type, with a rating of 3 being the most favorable. It is
emphasized that these comparisons are given only in a general way;
specific conditions at specific sites will dictate the eventual choice
of the type of cooling system.
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In order to enable a more precise and accurate comparison of alter-
native cooling system, additional studies are required to provide:
a. Complete and accurate cost data on dry tower heat exchangers.
b. Complete and accurate data on cost and performance of high
back pressure steam turbines in the 500-1000 MWe size range.
c. Information on wet/dry tower cost and performance.
These information needs can be satisfied by industrial and government
sponsored research.
In summary, three alternative closed-cycle cooling systems are
available for use on thermal power plants in the Northern Great Plains.
The selection of a specific system will depend on site related factors.
Further study will enable such selections to be made with increased
confidence as both better cost information is developed for hardware and
operating expenses and additional costs associated with water use in
arid areas are factored into analyses.
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Figure VI-1
Comparison of Alternative Cooling Systems
Wet Tower Met/Dry Tower Dry Tower
Operating Experiences 3 1 2
Economic Penalties 3 2 1
Environmental Impact 1 2 3
Water Use 1 .2 3
Energy Requirements 3 2 1
TOTALS 11 9 10
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