Improving the performance of power plant cooling ponds

Improving the performance of power plant cooling ponds

Journal of Environmental Management 105 (2012) 90e95 Contents lists available at SciVerse ScienceDirect Journal of Environmental Management journal ...

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Journal of Environmental Management 105 (2012) 90e95

Contents lists available at SciVerse ScienceDirect

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Improving the performance of power plant cooling ponds S.A. Lowe* Civil and Environmental Engineering Department, Manhattan College, NY 10471, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 June 2011 Received in revised form 3 February 2012 Accepted 20 March 2012 Available online 25 April 2012

A study was conducted on the effectiveness of using vertical baffles to improve the thermal performance of power plant cooling ponds. A small scale physical model of a rectangular cooling pond was used. A base case was established using traditional horizontal baffles to create a serpentine flow pattern through the pond. The horizontal baffles were then replaced by a series of underflow weirs that spanned the pond. An improvement in cooling of over 30% was realized. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Cooling pond Thermal pollution Physical model Heat transfer

1. Introduction Many power plants use cooling ponds to reduce the temperature of the cooling water exiting the plant before it is discharged back into a receiving water body. Other plants recirculate the cooled water back through the plant. In the US the temperature of water discharging from the plant is subject to permit limitations at the point where it enters the receiving water body. Many power plants in the US make a significant amount of their annual profit over the hot summer months when they run at full capacity and when prices are high (US DOE, 2010). For plants with cooling ponds this often means their discharge temperatures reach their permit limits, forcing the plant to reduce output. Every extra degree of cooling a plant can achieve in the cooling ponds therefore translates directly to the plants output. For plants that recirculate cooling pond water back into the plant, the issue is: can they cool the water enough to operate in 100% recycle mode (also known as “closed cycle”)? On hot days, for example, they may need to bring in cooler river water to augment the cooling cycle. Therefore the more cooling that can be attained in the cooling ponds, the longer the facility can operate in closed cycle mode. The current regulatory environment in the US is also pressuring facilities to move away from “once through cooling” operations to more closed cycle modes. For facilities that have cooling ponds and * Tel.: þ1 9142632145. E-mail address: [email protected]. 0301-4797/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2012.03.046

operate in “once through” mode, an option is to reuse the cooled water, and move to a more closed cycle operation. Facilities that use once through cooling and do not have cooling ponds may have available land to construct cooling ponds. Cooling ponds are usually a cheaper alternative than constructing cooling towers. The extent to which these above options can be used is directly related to how much cooling is realized in the ponds. A facility that can operate in a closed cycle mode for most of the year may not be subject to withdrawal and discharge regulations, the EPA 316(a) and 316(b) rules. US Energy Information Administration (EIA) data for 2010 shows 110 power plants (12% of all plants) use cooling ponds (US DOE, 2012). About 50 of these plants operated in once through mode and 60 used a closed cycle when able. The number of plants currently using once through cooling without cooling ponds is 344 (37% of all plants). It is not known how many of these could potentially install cooling ponds. It should also be noted that although the focus of this study was cooling ponds at power plants, this is not the only industry to use cooling ponds. Other examples include the food and beverage processing industry; steel mills; paper mills and chemical processing plants. From an ecological perspective it is desirable to reduce the temperature that is discharged into natural receiving waters at all times of the year. The heated discharge water represents an anthropogenic impact on the ambient water conditions. The greatest impacts may not occur during the summer months, when

S.A. Lowe / Journal of Environmental Management 105 (2012) 90e95


Fig. 1. Plan view of the experimental cooling pond in the base case configuration.

ambient waters also have elevated temperatures, but in winter when the temperature differential between discharge and ambient is the highest (Cooke et al., 2000). Alternatively there may be sensitive fish or other aquatic biota whose spawning season would benefit from cooler discharges. The design of cooling ponds was studied extensively in the 1960’s and 1970’s, with research interest waning by the early 1980’s (Tatinclaux et al., 1975; Jain, 1980; Jirka and Watanabe, 1980a). It was well understood that ponds should use internal baffles to increase the actual hydraulic detention time and hence increase cooling (Ryan et al., 1974; Jirka and Watanabe, 1980b). This use of baffles usually resulted in a serpentine flow path through the pond. The studies previously done on internal baffles focused exclusively on the horizontal positioning of the baffles. All baffles used in previous studies extended full depth of the pond in the vertical (i.e. from the bottom to above the water surface).

In recent years a cost effective method for constructing internal baffles has been the use of geotextile fabric supported intermittently by piles (Thaxton et al., 2004). This system works because there is negligible hydraulic load on the baffle, as there is water on either side. Using this method it is easy to construct a baffle that does not extend to the bottom, that is, create an underflow weir. In fact it is preferable to have the geotextile sit above the bottom because: 1. It is easier to install, 2. It does not require a custom shape to fit the bottom profile of the pond, 3. The fabric will not rub on the bottom, which causes wear and tear. In this study the effect of vertical baffles on the performance of a cooling pond was examined. The study used a small scale physical model of a cooling pond.

Fig. 2. Pond with base case configuration, viewed from the inlet end.


S.A. Lowe / Journal of Environmental Management 105 (2012) 90e95

Fig. 3. Pond viewed from outlet end, showing fan location and outlet weir collection box.

2. Experimental configuration 2.1. Base case A physical model of a cooling pond was constructed. There is no typical physical layout of a real cooling pond as they are sized to fit the available land, or arranged to fit a particular intake or discharge point. A rectangular shape was used in this study. The pond dimensions used were 2.9 m  0.75 m. The pond was sized to fit into available laboratory space. A depth of 5.1 cm was maintained by using an outlet weir. A schematic of the pond is shown in Fig. 1 which shows the inlet pipe, the outlet weir and the internal baffling. Figs. 2 and 3 are pictures taken of the base case set up. A series of five, 0.66 m long internal baffles were used to create a serpentine flow path. This arrangement constitutes the base case. This represents a pond that is already hydraulically optimized to prevent short circuiting. It is against this configuration that performance results for alternative configurations will be compared. Temperature

measurements were recorded throughout the pond, beginning at the inlet tube and ending at the outlet weir. Wind induced cooling is an important component in real ponds. Typically winds above 1 m/s occur >95% of the time at most locations in the US (Stewart and Essenwanger, 1978). In order to incorporate this effect a small fan was placed at the outlet end of the pond, as shown in Fig. 3. The air was directed along the main axis of the pond. The fan produced a velocity of 3 m/s half way down the pond when the fan was at its’ maximum setting. This was used for all runs. A series of five runs were completed with this set up. For all the runs a flowrate of 3.8 L/min was used. This equates to a detention time of approximately 30 min. The pond was initially allowed to fill and then run for 30 min before readings were begun. In all runs the inlet temperatures ranged between 39.4  C and 42.2  C at the start of recording. Data were recorded for 90 min at 5 min intervals. During this time the inlet temperatures gradually fell, and ranged between 35.4  C and 39.4  C at the completion of the runs. The information for individual runs is listed in Table 1. In all runs the air temperature was 25  C  0.25  C.

Table 1 Results of all runs. Configuration

Run #

Initial inlet Temp. ( C)

Final inlet Temp. ( C)

Avg. inlet Temp. ( C)

Initial outlet Temp. ( C)

Final outlet Temp. ( C)

Avg. outlet Temp. ( C)

Avg. Temp. drop ( C)

Base case

1 2 3 4 5 Average

39.8 40.9 41.9 40.4 39.7 40.6

37.1 37.8 39.3 35.4 37.4 37.4

38.4 39.5 40.4 37.9 38.5 38.9

34.0 35.2 36.2 34.8 33.9 34.8

31.8 32.6 33.8 30.0 31.7 32.0

32.9 34.1 34.8 32.4 32.8 33.4

5.5 5.4 5.6 5.4 5.7 5.5


1 2 3 4 5 Average

38.9 40.2 39.7 41.0 39.6 39.9

36.3 38.2 37.5 38.2 37.1 37.4

37.4 39.3 38.6 39.6 38.3 38.7

34.3 33.2 32.7 33.6 32.6 33.3

28.2 30.2 29.9 30.7 29.5 29.7

30.2 31.7 31.3 32.2 31.1 31.3

7.2 7.6 7.3 7.4 7.2 7.4

S.A. Lowe / Journal of Environmental Management 105 (2012) 90e95


Fig. 4. Plan view of the underflow weir configuration.

Fig. 5. Elevation section of the underflow baffle arrangement.

While it would have been ideal to maintain a constant temperature throughout the experiments, this was not an available option as the project had to work with the constraints of the building hot water system. However as the drop in temperature was consistent for every run, including those described in the next section, the comparison between configurations is still valid. Likewise real cooling ponds are also never at steady state as the rate of cooling is affected by meteorological variables (wind speed/direction, cloud cover, humidity, air temperature, precipitation); the diurnal solar cycle; and the flowrate and temperature of the incoming water.

elevation view. The weirs protruded above the water surface and terminated 0.32 cm above the bottom. A series of five runs were made using this configuration. The same measurement protocol used in the base case runs was followed. The inlet temperatures at the beginning of the runs ranged between 38.9  C and 41.1  C, and ended between 36.1  C and 38.3  C. The information for individual runs is listed in Table 1. The flow and other conditions (air speed, air temperature) were the same as the base case. The same start up procedure was followed.

2.2. Underflow weir experimental configuration

Two other configurations were tested but yielded no noticeable change from the base case results. The first of these was similar to the underflow baffle arrangement, except that the baffles began at the bottom and ended just under the water surface (see Fig. 6). The baffles extended across the full width of the pond.

In this set up the internal baffles from the base case were replaced by a series of five underflow weirs that extended across the pond. This is shown in plan view in Fig. 4. Fig. 5 shows an

2.3. Other configurations

Fig. 6. Elevation section of the overflow baffle arrangement.


S.A. Lowe / Journal of Environmental Management 105 (2012) 90e95

Fig. 7. Plan view of the combination base case and underflow baffle configuration.

The other configuration used the base case set up with underflow weirs at the end of the horizontal baffles, as shown in Fig. 7. 3. Results and discussion The results for the base case and underflow weir runs are given in Table 1. The amount of cooling achieved (DT) is shown in Fig. 8. The results show a significant improvement in cooling using the underflow weirs. The average temperature drop for all runs was 5.5  C for the base case, and 7.4  C for the underflow weirs. This represents an improvement of 1.9  C or 34.5%. The results are consistent across all the runs. It is thought that the mechanism causing the improvement is that the coolest water, being the densest, is passing under the weirs. The hottest water, being lighter, is held longer in each cell, allows it to cool further. For a power plant this improvement could represent a significant benefit. Many power plants in the US make a significant amount of their annual profit over the hot summer months when they run at full capacity and when prices are at a premium. For plants with cooling ponds this often means during the summer months their discharge temperatures are pushing their permit limits, forcing the plant to reduce output in order to not exceed the limit. In simple terms every extra degree of cooling they can achieve in their ponds translates directly to the plants output. Ecologically it is desirable to reduce the temperature that is discharged into natural receiving waters at all times of the year. At various times of the year different benefits may be realized to the ecosystem biota. This may include spawning season effects; decreased mortality in larval or juvenile life stages; an increase in species abundance, and many more (Lessard and Hayes, 2003).

4. Conclusions This study investigated the use of vertical baffling to increase the cooling obtained in a power plant cooling pond. A physical model of a cooling pond was constructed. A fan provided air movement over the pond. The previous body of work on cooling pond performance had focused on using horizontal baffles to improve hydraulic, and ultimately, thermal performance. The outcomes of previous studies invariably lead to the placement of baffles that create a serpentine pattern in a pond. In this study this arrangement was used as the base case, against which comparisons were made. Advances in construction techniques and materials now make the option of creating vertical baffles viable. In this study both overflow and underflow weirs were examined. It was found that overflow weirs did not improve the performance compared to the serpentine base case. This was true even when used in conjunction with underflow weirs to produce an over and underflow pattern. A series of runs were made using multiple underflow weirs that replaced the horizontal baffles and extended across the pond. The results indicated an average improvement in cooling of nearly 2  C over the base case (7.4  C versus 5.5  C). This represents an improvement of over 30%. This improvement was considerably more than was expected and proved the concept has potential. Based on this initial success, ongoing work is planned to test the robustness of the method. A series of runs will be made varying the parameters in the study. This would include changing the fan induced wind direction and speed, and changing the water depth in the pond. Acknowledgements This work was funded in part by HDR Engineering. The physical model was constructed by Hernane De Almeida and Kurt Paxton, undergraduate students in the Civil and Environmental Engineering Department at Manhattan College. Many of the runs were made by Steven Cruz, also an undergraduate student in the same department. References

Fig. 8. Comparison of cooling effectiveness for different pond configurations.

Cooke, S.J., Bunt, C.M., McKinley, R.S., 2000. Winter residency of smallmouth bass in a thermal discharge canal: implications for tempering pump operation. N. Am. J. Fish. Manag. 20, 288e295. Jain, S.C., 1980. Density currents in the sidearms of cooling ponds. ASCE J. Energy Division 106 (1), 9e21. Jirka, G.H., Watanabe, M., 1980a. Steady-state estimation of cooling pond performance. ASCE J. Hydraulics Division 106 (6), 1116e1123. Jirka, G.H., Watanabe, M., 1980b. Thermal structure of cooling ponds. ASCE J. Hydraulics Division 106 (5), 701e715.

S.A. Lowe / Journal of Environmental Management 105 (2012) 90e95 Lessard, J.L., Hayes, D.B., 2003. Effects of elevated water temperature on fish and macroinvertebrate communities below small dams. River Res. Appl. 19 (7), 721e732. Ryan, P.J., Harleman, D.R.F., Stolzenbach, K.D., 1974. Surface heat loss from cooling ponds. Water Resour. Res. 10 (5), 930e938. Stewart, D.A., Essenwanger, O.M., 1978. Frequency distribution of wind speed near the surface. J. Appl. Meteorol. 17, 1633e1642. Tatinclaux, J.C., Sayre, W.W., Jain, S.C., 1975. Hydraulic modeling of shallow cooling ponds. ASCE J. Power Division 101 (1), 43e53.


Thaxton, C.S., Calantoni, J., McLaughlin, R.A., 2004. Hydrodynamic assessment of various types of baffles in a sediment retention pond. Trans. Am. Soc. Agric. & Biol. Eng. 47 (3), 741e749. US Dept of Energy e Energy Information Administration, 2010. Electric Sales, Revenue and Average Price 2009. (June 23, 2010). US Dept of Energy e US Energy Information Administration, 2012. Annual Electric Generator Report 2010. Released Nov 2011.