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### MEMORANDUM Tetra Tech, Inc. 10306 Eaton Place, Suite 340 Fairfax, VA 22030 phone 703 - 385 - 6000 fax 703 - 385 - 6007 DATE: January

16, 2009 TO : Paul Shriner and Jan Matuszko, EPA FROM : John Sunda, SAI C and Kelly Meadows, Tetra Tech SUBJECT: Helper C ooling Towers Cooling towers are one technology used to discharge waste heat from electric generators and, depending on their configuration, can also reduce the volume of cooling water withdrawn from the source waterbody. EPA tasked Tetra Tech and SAI C w ith describing the prevalence and characteristics of “ helper ” cooling towers, which serve a different purpose at power plants. Cooling System Configuration Cooling water sy stems at p ower pl ants that utilize cooling towers may be capable of operating in o ne or more of the following flow configurations:

 Once

- through m ode (non

- recirculating)

– A facility may elect to by

pass its cooling towers and operate in once

- through mode as a cost saving measure. There is no reduction in intake flow or heat rejection while operating in this mode.

 Closed

- cycle m ode (recirculating)

– W arm water exiting the steam condensers is passe d through cooling towers and is then return ed to the cool water inlet of the steam condensers. This configuration minimizes the volume of co oling water that must be withdrawn and rejects most of the waste heat to the atmosphere
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1 Additionally, there may be two sets of circulation pumps with one each for the condenser and cooling tower portions of the circuit. Or there may be a single se t of circulation pumps that pump water through the condensers and then through the cooling towers or that pump water to the top of the cooling tower which is set at an elevation such that tower effluent flows by gravity through the condenser.2

 Helper m ode (non

- recirculating)

– Cooling water flows through a once - through cooling s ys tem with warm water effluent from the steam condensers passing through c oolin g towers prior to being discharged
. There is no reduction in intake flow, but there is a reduction in the waste heat discharged to the receiving waterbod y.

 Combination m ode

– Portions of the cooling water flow in more than one of the above configurations
. As described above , cooling towers can be operated in either the closed

- cycle or helper modes ( or sometimes not at all )
. At most plants , cooling towers and the associated piping are configured to operate in either the closed

- cycle mode or helper mode b ut not both. 2 Helper Tower Use in the Phase II Industry Of the 220 Phase II facilities that reported information 3 on cooling sy stem configuration, only 12 (approximately

5.5%) plants indicated that they used a cooling s ys tem that was “ Once
- T hrough With N on - R ecirculating Cooling Towers , ” another name for helper towers
. By

comparison, 81 of the 526 Phase II facilities (15%) reported using recirculating cooling towers either in closed cycle or combination mode. 4 Helper Mode Typical Usage and Design C oo ling towers operating in the helper mode are intended to serve onl y one purpose : rem ov ing enough waste heat to ensure that the heat rejected to the receiving water meets NPDES discharge limits
. These limits ty picall y include a maximum increase in water te mperature and a maximum discharge temperature ; in some cases, they may also include a maximum discharge heat load (ty picall y measured in BTU)
. In other words , helper cooling towers generall y only function to help facilities meet discharge requirements ass ociated with Section 316(a). Helper towers are most commonly used during warmer summer periods when discharge temperature limits are more difficult to meet due to higher ambient water temperatures
. Helper towers also allow a plant to avoid reduc ing its p ower generation (derate) to meet heat load

- related discharge limits. Since helper towers serve a different purpose, they are often sized differently than cooling towers used for closed cycle cooling. Helper towers are ty pically sized to transfer onl y eno ugh waste heat to the atmosphere to ensure that the discharge limits are met. Thus, they

may be designed to cool only a portion of the total cooling water flow

2 Cooling sys tems can be designed such that the cooling water flow can be alternated between all of the mo des described above. Such systems are referred to as mixed mode or multimode cooling systems. 3 Data extracted from the 2000 detailed industry questionnaire; Ques tion 1d in Part 2. 4 Note that this data does not reflect changes to cooling systems installed after 2000, such as the helper tower system at Harllee Branch described below
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volume ; ty

picall y, the helper tower co ol water discharge is mixed with the remaining once - throu gh cooling water prior to final discharge. Typical Operation Because the co o ling towers require energy to operate , 5 facilities usually minimize helper tower operation , with the periods of tower operation and pumping rates being adjusted to meet the NPDES temperature limits. For example , G eorgia Power’s Harllee Branch Power Plant installed a helper cooling to wer in 2001 that was capable of cooling only about half of the total cooling water flow. The tower operation starts and stops when the plant's flow

- weighted discharge temperature exce eds or falls below 94 o F . T he tower is capable of reducing the temperature of the water p assing through it b y about 20 o F (Blankenship 2001). While they

may be sized differentl y, there is nothing inherent in the design o f helper cooling towers that would prevent them from being used in a cooling s ys tem that was converted to operat e in a closed cycle or mixed mode configuration
. However, depending on the flow and heat rejection capacity , they may not be capable of providi ng full recirculation without construction of additional towers
. Issues associated with converting helper cooling towers to operate in closed cycle or mixed mode are discussed in greater detail below. E f fects on Biota Cooling towers operating in the helper mode may provide little benefit with regard to impingement mortality

or entrainment reduction and may possibl y be detrimental with regard to a reduction in survival rate of entrained organisms due to an increase in mechanical injury

resulting from the additional passage of the cooling water through the tower. However, they

may also provide some benefits with regard to reduced heat stress and improved dissolved ox ygen for entrained organisms in the cooling water that has passed through the towers and, o f course, for organisms in the receiving water. Mixed Mode Systems As described above, the pumping and piping s ys tem for cooling s ys tems utilizing cooling towers can be configured such that the sy stem can alternate between once - through, closed cycle, helper, and combination modes
. Such mixed mode s ys tems offer greater flexibility for meeting 316(a) and 316(b) permit conditions while optimizing power generation
. S uch flexibility require s the installation of a flow and temperature monitoring and control s ys tem and the establishment of decision rules that guide the timing and selection of specific operating modes. Such decisions will be guided b y the environmental standard requirements that must be met and the overall design of the cooling s ys tem.

5 The energy penalty associated with helper towers includes only the parasitic com ponent for pumping water to the top of the towers and for operating the fans but does not include the turbine efficiency loss associated with closed cycle cooling towers.4

In the past , the environmental standards of concern were primarily associated with 316(a) , as discussed in a case study of TVA's Browns Ferry

Nuclear Plant (Stolzenbach et al 1979)
. In 1979, the existing cooling towers at the Browns Ferry Nuclear Plant were bui lt with the capability

to operate in both the helper and closed

- cycle mode and this capability was unique among power stations at that time
. T he case study provides an anal ys is of the multimode operation at the Browns Ferry Nuclear plant , evaluating mixed mode operation and the capability of computer models to predict river temperatures . T he goal was to optimiz e power generation while meeting 316(a) limits
. 6 The study did not address flow reduction. A cursory review of the model output graphs provides the following rough estimate s of the relative magnitude of energy penalties for operating cooling towers in the helper and closed cycle mode in comparison to once

- through mode. The anal ys is estimates that while operating cooling towers in full helper mod e, total plant output would drop roughly 60 to 85 MW or 1.8% to 2.6% of the 3300 MW capacity . And that while operating cooling towers in closed cycle mode, total plant output would drop roughly 110 to 150 MW or 3.3% to 4.5% of the 3300 MW capacit y. Thus, the case stud y estimates that the parasitic energy

penalty associated with cooling towers operated in either the helper or closed cycle mode would range from 1.8% to 2.6% and that the turbine efficiency penalty (i.e., difference between helper and closed cycle) would range from 1 .5% to 2.0%. 7 The case study ’ s model predicted the net power generation for each mode of operation throughout the ye ar and selected the optimum mode of operation for optimizing power generation while meeting 316(a) limitations
. C losed

- cycle operation was selected by the model as optimum power generation mode for less than 10 day s of the ye ar . D epending on the selected scenario , helper mode operation was selected for roughl y 30 to 40 days of the year. Converting Helper Cooling Towers to Recirculating or Mixed Mode Cooling Towers As noted above, onl y about 5.5% of Phase II facilities reported using helper cooling towers in 2000. Thus, the availability of this technology

for conversion to closed

- cycle or mixed mode operation is somewhat limited. Inexisting helper cooling towers , warm water that exits the condensers is ty pically pumped from either the condenser outlet conduit or the discharge channel up to the top of the helper cooling tower and then flows by gravity back into the discharge conduit or channel in a location downstream of the withdrawal point or is discharged through a separate discharge conduit. In other sy stems , the intake pumps, pipes, and condenser cooling s ystem are hydraulicall y designed such that the intake pumps are capable of pumping water through the condensers and then up to the top of the cooling towers. Either s ys tem could be converted to one that operates in a multi

- mode fashion capable of recirculating the volume that the tower is designed to

6 The thermal discharge limits were an increase in downstream river temperature of n o more than 5 o F and a maximum downstream river temperature of 86 o F 7 These values compare favorably to EPA’s estimates used in the economic analysis of an average parasitic penalty of 1.7%, an average turbine efficiency penalty of 2.0%, and an average to tal closed cycle penalty of 3.7% (Sunda, 2008).5

handle , which may

involve only partial closed cycle (combination) mode if helper tower flow capacity

is limited. To convert helper cooling towers to closed

- cycle mode , facilities would need to return the cooled water from the tower basins back to the intake canal/channel or the condenser cooling water circulation pump well. Depending on the location of the cooling tower basins and/or discharge channel in relation to the intake, various engineering solutions may be available to construct return flow pipes or ch annels. In m an y cases , the elevation of the cooling tower basins will be high enough to allow return flow to occur by gravit y , or installation of control structures and a return channel connecting the discharge channel to the intake channel could provide for return flow. Either ty pe of s ys tem would not require an addition al set of pumps. The costs of converting a helper tower s ys tem to a closed

- cycle or mixed mode sy stem will include the costs of additional piping, channels, valves and control s ys tem u pgrades. The magnitude of thes e construction costs may vary greatl y depending on site

- specific conditions. While site

- specific challenges may exist with regard to the placement and design of return piping or channels, in most cases engineering solutions should be available and the difficulties and distances involved will be major driver s in determining the magnitude of the project capital costs. No matter the solution, such a s ystem modification would provide for an intake flow reduction associated with closed cycle cooling while avoiding the substantial cost component associated with construction of new cooling towers.6

References Blankinship , Steve. “ Georgia Power's New Cooling Tower Design Reduces Environmental Impact .” Power Engineering. September 2 001. Article accessed on December 19, 2008 at website: http://pepei.pennnet.com/articles/article_display .cfm?article_id=121621 Stolzenbach, Keith D. Freudberg, Stuart A. Ostrowski, Peter and Rhodes, John A. Operational Issues Involving Use of Supplementary Cooling Towers to Meet Stream Temperature Standards with Application to the Browns Ferry Nuclear Plant. MI T Energy L aboratory

Report No. MIT
- EL 79
- 036. January 19 79. Accessed on December 19, 2008 at website: http://dspace.mit.edu/bitstream/handle/1721.1/35204/MI T
- EL
- 79
- 036
- 09510127.pdf?sequence=1 Sunda, J., S AI C. Memo: Lo st Generation

- Construction Downtime and Energy Penalt y . December 5, 2008.