Harper, W. L.

Waldrop, W. R.

English

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V-B-69
AN OPERATIONAL PROCEDURE FOR PREDICTING THE MOST ECONOMICAL USE
OF CONDENSER COOLING MODES
W. L. Harper and W. R. Waldrop
Water Systems Development Branch, Tennessee Valley Authority
Norris, Tennessee USA
ABSTRACT
A technique is presented for estimating the most economical use of the
cooling towers at the Tennessee Valley Authority (TVA) Browns Ferry
Nuclear Plant while complying with the applicable thermal water quality
standards of the State of Alabama. This plant is designed to operate in
either of three condenser cooling modes: Open Cycle in which the condens-
er cooling water is discharged directly into the river through submerged
diffusers; Helper Mode in which the cooling water is routed through the
cooling towers prior to being discharged into the river through the dif-
fusers; and Closed Cycle in which the cooling tower effluent is returned
to the intake for reuse as condenser cooling water. The energy required
for pumping and the loss of plant operating efficiency is minimized by
computing the least amount of cooling tower operation required for each
hour to comply with the Alabama thermal standards for the Tennessee River.
The procedure requires projections for the heat transfer rate from the
condenser to the cooling water, the rate of cooling from the cooling
towers, and the dispersion of heated effluent in the river. Plant per-
formance curves and projected generating levels are used to estimate the
condenser heat disposal rate.- Cooling tower performance estimates,
projected meteorology, and Incoming water temperatures are used for esti-
mating cooling tower effluent temperatures. Projected operation of
upstream and downstream hydroelectric plants is used to compute river
flows in the vicinity of the diffuser. These flows, along with upstream
ambient river temperature, the temperature of the effluent, and results
of laboratory and theoretical investigations of diffuser-induced mixing
are used to compute the downstream temperature in the river. This program
is used daily by the personnel of the Browns Ferry Nuclear Plant to pre-
dict the most efficient cooling mode for each hour throughout the follow-
ing 24 hour period.
INTRODUCTION
TVA's Browns Ferry Nuclear Plant, situated on Wheeler Reservoir of the
Tennessee River in north Alabama (Figure 1), was designed to generate
3456 MW. This plant was originally designed to operate in Open Mode for
condenser cooling. In this mode, the plant pumps 4410 cubic feet per
second (cfs) of water from the river and through the steam condenser
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V-B-70
where the cooling water is heated approximately 25°F before being dis-
charged through submerged multiport diffusers in the river.
The river flow past the submerged diffuser is primarily determined by the
discharges from Wheeler Dam 19 miles downstream and Guntersville Dam 55
miles upstream of the nuclear plant. The mean annual flow rate of the
river is 45,000 cfs. Since discharges from these dams are normally used
for hydroelectric generation at periods of peak power demand, the flow in
Wheeler Reservoir is often unsteady. As a result, flows near the plant
site usually change drastically throughout the day and may, for an hour or
so, reverse due to a "sloshing" effect of the reservoir. During such
periods of low flow, there is an insufficient supply of cool river water
available for mixing with the diffuser discharge; consequently, compliance
with the thermal water quality standards is not possible when operating in
the Open Mode of condenser cooling during low flows. These water quality
standards permit a maximum plant-induced mixed temperature rise of 5°F and
a maximum downstream temperature of 86°F. TVA maintains three permanent
water temperature monitors downstream of the mixing zone of the diffusers
to demonstrate compliance with these standards.
After delineation of the possible environmental consequences of this
method for disposing of the excess heat from the condensers, and later the
enactment of the thermal water quality standards, the nuclear plant was
retrofitted with six mechanical draft cooling towers, two per unit. This
provides the plant operators with the option of cooling the condenser
cooling water when the thermal discharge of the Open Mode can contribute
to a violation of the thermal standards. The effluent from the cooling
towers can either be routed to the diffusers for discharge into the river
(Helper Mode), or routed to the plant intake channel for reuse as condens-
er cooling water (Closed Mode). These three possible modes of operation
of the Browns Ferry Nuclear Plant cooling system are'illustrated in
Figure 2.
This paper describes a computer model for analyzing the heat rejection rate
of the plant and the pertinent meteorological and river conditions for the
purpose of advising the plant operators of the most economical method of
routing the condenser cooling water while assuring compliance with the
thermal water quality standards.
FACTORS INFLUENCING CHOICE OF COOLING MODES
The possible environmental consequences of thermal discharges are well
documented and the thermal water quality standards were promulgated to
prevent adverse effects upon the aquatic ecosystem. The environmental
advantages between the Open and Helper Modes are variable; the discharge
rates are comparable, 4410 cfs for Open Mode versus 3675 cfs for Helper
Mode. However, the discharge temperature for the Helper Mode, which
depends upon meteorology, is normally cooler than that of Open Mode.
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The discharge rate for Closed Mode is only about 110 cfs; therefore, the
discharge during Closed Mode has a negligible effect on the downstream
temperature of the river.
The environmental effects of plant intakes are also of concern. Fish may
be trapped within intake structures or canals and become impinged against
the intake screens. Fish too small to be impinged on the intake structures
may be entrained into the cooling system of the plant. Although impinge-
ment and entrainment of fish must be evaluated for each plant, the number
of fish affected is usually proportional to the flow rate of condense**
cooling water pumped from the river. Therefore, the environmental effects
at the plant intake are approximately the same for Helper Mode and Open
Mode operation. The Closed Mode of operation is superior with respect to
intake environmental effects since only a small quantity of water (approx-
imately 220 cfs) is pumped from the river for "makeup."
The cooling tower lift pumps, fans and peripheral equipment require approx-
imately 56 MW for normal operation. This is power which would be included
in the net output of the plant in Open Mode, but which must now be obtained
from other sources. In Closed Mode, there is an additional loss of net
power generated due to the decreased efficiency which results from increased
cooling water temperature. The precise loss in efficiency varies as a
function of meteorology, but a conservative estimate of this loss, based
on a Carnot cycle, is one percent of total generation or 35 MW. This
results in a total loss of 91 MW in Closed Mode.
The cost of the power required to recover these losses varies with the
source from which the power is obtained. The additional expense of Helper
and Closed Mode operation will range from $560 per hour to $3,640 per hour
or more, depending upon the source of the replacement power. This does
not include depreciation of the cooling tower pumps, fans, etc. It is
obvious, therefore, that unnecessary cooling tower operation can result in
considerable cost to TVA and its consumers.
The relative environmental and economic advantages and disadvantages of
each mode of cooling system operation are summarized in Table 1.
PROGRAM PARAMETERS AND COMPUTATIONS
The mixed temperature in the river downstream of the plant depends upon
the performance of several subsystems of the plant cooling system. These
will be discussed individually.
Temperature Increase Across Condensers
The condenser rise was determined by calibration tests which equated heat
rejection (BTU/sec) with plant generation levels (MW). For a known con-
denser flow rate, the increase in temperature was easily computed.
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V-B-72
Cooling Tower Performance
The cooling tower effluent temperature is primarily a function of the wet
bulb temperature of the air and the temperature of the hot water from the
condenser. The wet bulb temperature is computed from the dry bulb temper-
ature and the dew point. An example of the tower performance curves is
presented in Figure 3.
Multiport Piffuser Mixing
The mixed temperature of the river downstream of the plant depends upon
the flow rate and temperature of both the thermal discharge and the river.
The discharge conditions depend upon the mode of cooling and the condenser
and, if applicable, cooling tower performance.
River flows over the diffuser are computed with a one-dimensional, unsteady
finite-difference flow model. Hourly releases from Guntersville and
Wheeler Dams are input as boundary conditions. A water temperature monitor
[Ref. 1] upstream of the plant provides an ambient river temperature at
initiation of a computer run. Subsequent river temperatures are computed
by superimposing a statistical diurnal and annual cycle.
The mixing induced by the high velocity (approximately 10 ft/sec) jets of
the submerged multiport diffuser is highly dependent upon the flow rate in
the river. Model studies of this diffuser [Ref. 2] and theoretical tech-
niques for this general class of diffuser [Ref. 3] were used to generate
diffuser mixing curves such as those presented in Figure 4.
OPERATION OF THE COMPUTER MODEL
The computer program is used daily by personnel of the TVA Load Control
Center in Chattanooga, Tennessee, to predict the optimum mode of cooling
for the following 24 hours at the Browns Ferry Nuclear Plant. As input,
the program uses ambient river temperature and projected hourly values of
(a) Browns Ferry Nuclear Plant generation (MW); (b) flow releases from
Guntersville Dam; (c) flow releases from Wheeler Dam; and (d) air temper-
ature and dew point. The program uses these inputs to compute the mixed
temperature in the river downstream of the diffusers for each of the
following 24 hours. For each hour, computations are first performed for
the most economical mode (Open Cycle), but if the thermal water quality
standards cannot be satisfied, the program automatically cycles to Helper
Mode and finally to Closed Mode if necessary. A flow chart for this com-
puter program is presented in Figure 5.
Predicted mixed downstream river temperature, plant-induced heating and
recommended cooling modes for each of the next 24 hours are transmitted
to the operators at the Browns Ferry Nuclear Plant. Within practical
limits, the nuclear plant operators follow these recommendations. If it is
necessary to deviate significantly from any of the projections, the program
WLH -4-
V-B-73
is rerun to determine if changes in the recommended mode of cooling are
required.
Table 2 presents typical results of a computation during a period of low
river flows. Releases from Guntersville Dam varied between 0 and 45,000
cfs and releases from Wheeler Dam varied between 0 and 58,000 cfs. The
resulting flows near the Browns Ferry Nuclear Plant were quite unsteady,
as shown in Table 2. The net generation of the plant was a constant 2900
MW throughout this period. Predicted mixed temperatures (TM) and plant-
induced temperature rises (AT^ j) in the river immediately downstream of the
diffuser correspond to the mode of cooling recommended for that particular
hour.
The column showing the approximate cumulative cost incurred by cooling
tower operation during this period is not included in routine computations.
These costs are based upon purchasing this power from outside utilities at
an estimated rate of $40 per MW-hr. At those rates, the total cost of
operating the towers for the minimum time suggested was $23,000. Without
a sophisticated computation scheme of this type, it is likely that the
cooling system of this nuclear plant would be operated in the Closed Mode
throughout most of this 24-hour period. The cost of Closed Mode cooling
for 24 hours at the maximum estimated rate is $87,000. However by using
this predictive technique, TVA could effect a savings of as much as $64,000
while placing no undue thermal stress upon the aquatic ecosystem of the
Tennessee River.
CONCLUSIONS
The condenser cooling system of the TVA Browns Ferry Nuclear Plant is
designed to operate in three modes: Open, Helper and Closed. Open Mode
discharges the cooling water directly to the river; Helper Mode routes the
cooling water through cooling towers prior to discharge; and Closed Mode
recycles the cooling tower effluent to the intake for reuse in condenser
cooling.
Much of the time the plant can operate in the Open Mode without violating
the thermal water quality standards and overstressing the aquatic eco-
system. However, because the river flows near the plant are often unsteady
as a result of releases from upstream and downstream hydroelectric plants,
the Helper and Closed Modes must be used periodically. The power required
to operate these towers, and, for the case of Closed Mode, the loss in
plant efficiency resulting from warmer temperatures of condenser cooling
water, dictate that the cooling towers should be operated no more than
necessary to protect the environment.
The factors influencing the choice of cooling mode are complex and include
computations of heat rejection rate of the condenser, cooling tower per-
formance, river flow and temperature, and submerged diffuser mixing. A
computer program has been developed which analyzes hourly projections of
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V-B-74
plant generations, river flows, ambient river temperatures, and meteorology
for the following 24 hours, and predicts the most efficient mode of cooling
which will assure compliance with the thermal water quality standards for
each hour. This program is run daily at TVA's Load Control Center in Chat-
tanooga and results are transmitted to the Browns Ferry Nuclear Plant. An
example of a typical 24-hour operation demonstrated that using the cooling
towers only when necessary could save as much as $64,000 per day when com-
pared with Closed Mode cooling for the same period. This was accomplished
without violating the thermal water quality standards.
WLH -6-
V
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V-B-76
TABLE 2: PREDICTED RESULTS FOR TYPICAL
24-HOUR OPERATION OF THE BROWNS
FERRY COOLING SYSTEM
Hour River Flows Tm
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
0100
0200
0300
0400
0500
0600
0700
0800
0900
1000
1100
1200
Icfs)
33,650
28,899
19,543
17,221
15,147
25,034
29,313
35,668
30,422
30,028
26,825
24,204
17,315
12,361
10,121
8,571
. 12,591
18,098
18,384
34,005
41,125
43,226
39,272
39,981
40,728
38.8
39.3
40.8
39.8
40.
40.
39.
39.
39.
39.
39.
40.
39.
40.
36.
36.4
40.0
38.
38.
38.
38.1
38.0
38.3
38.4
38.4
.5
.0
.5
.0
.4
.4
.7
.0
.1
.2
.3
.7
.9
.5
Cooling Cumulative Estimated
Jm Mode Cost?IT
2.6
3.0
4.5
3.4
4.1
3.5
3.0
2.5
2.9
3.0
3.3
3.7
2.9
4.0
0.2
0.4
4.2
2.9
3.1
2.7
2.2
2.0
2.2
2.2
2.2
Open
Open
Open
Helper
Helper
Open
Open
Open
Open
Open
Open
Open
He! per
Helper
Closed
Closed
Helper
Hel per
Helper
Open
Open
Open
Open
Open
Open
(Dollars)
0
0
0
0
$ 2,240
4,480
4,480
4,480
4,480
4,480
4,480
4,480
6,720
8,960
12,600
16,240
18,480
20,720
22,960
22,960
22,960
22,960
22,960
22,960
22,960
V-B-77
REFERENCES
1. Driver, E. E. and W. R. Waldrop, "The Tennessee Valley Authority's
Program for Monitoring Water Temperature in the Vicinity of Thermal
Power Plants," Proceedings of the XVIth Congress of IAHR. Vol. 3,
pp 556-563, Sao Paulo, Brazil, July 1975.
2. Harleman, D.R.F., L. C. Hall, and T. G. Curtis, "Thermal Diffusion of
Condenser Cooling Water During Steady and Unsteady Flows with Applica-
tion to the TVA Browns Ferry Nuclear Plant," MIT, Report No. 111.
Cambridge, MA. September 1968.
3. Jirka, G. and D.R.F. Harleman, "The Mechanics of Submerged Multiport
Diffusers for Buoyant Discharges in Shallow Water," MIT. Report No. 169,
Cambridge, MA. March 1973.
BROWNS FERRY
NUCLEAR PLANT
10 20 30 miles
DECATUR,
ALABAMA
GUNTERSVILLE DAM-
Figure l: Location of Browns Ferry Nuclear Plant
COOLING
TOWERS i
, HELPER. CLOSED I POWERHOUSE
TINTAKE
:" INTAKE
HANNEL
Figure 2 Schematic of Three Condenser Cooling Modes
of Browns Ferry Nuclear Plant
V-B-79
UJ
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QC
UJ
O
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35
IT 30
•
"«25
O
7 20
10
100
WET BULB TEMPERATURE =
105 110 115 120 125
CONDtNSER DISCHARGE TEMPERATURE (THQT) ~ °F
Figure 3: Cooling Tower Performance at Design Flow Rate
UJ
IE
oc
tr
UJ
a.
i-
o
UJ
u
RIVER TEMPERATURE = 50°F
DISCHARGE TEMPERATURE =75.8eF
20 40 60 80
RIVER FLOW AT PLANT SITE X IO"3 (cfs)
100
Figure 4: Mixed Temperature Increase Downstream
of Diffuser
V-B-80
WRITE
TIME
FLOWS
/PLANT LOADS/
COOLING
MODES,
Figure 51 Flow Chart of Browns Ferry River Temperature Rise Prediction Program


An operational procedure for predicting the most economical use of condenser cooling modes

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