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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, SAIC and Kelly Meadows, Tetra Tech

SUBJECT: Helper Cooling 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withdrawn from the source waterbody.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 SAIC with describing 

the prevalence and characteristics of “helper” cooling towers, which serve a different 

purpose at power plants.

Cooling System Configuration

Cooling water systems at power plants that utilize cooling towers may be capable of 

operating in one or more of the following flow configurations: 

 Once-through mode (non-recirculating) – A facility may elect to bypass 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 mode (recirculating) – Warm water exiting the steam condensers is 

passed through cooling towers and is then returned to the cool water inlet of the 

steam condensers. This configuration minimizes the volume of cooling 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must be withdrawn and rejects most of the waste heat to the atmosphere.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 set 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.
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 Helper mode (non-recirculating) – Cooling water flows through a once-through 

cooling system with warm water effluent from the steam condensers passing 

through cooling 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 

waterbody.

 Combination mode – 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 but not both.2

Helper Tower Use in the Phase II Industry

Of the 220 Phase II facilities that reported information3on cooling system configuration, 

only 12 (approximately 5.5%) plants indicated that they used a cooling system that was 

“Once-Through With Non-Recirculating 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

Cooling towers operating in the helper mode are intended to serve only one purpose: 

removing enough waste heat to ensure that the heat rejected to the receiving water meets

NPDES discharge limits. These limits typically include a maximum increase in water 

temperature and a maximum discharge temperature; in some cases, they may also include 

a maximum discharge heat load (typically measured in BTU). In other words, helper 

cooling towers generally only function to help facilities meet discharge requirements 

associated 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 reducing 

its power 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 typically sized to 

transfer only enough 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 systems can be designed such that the cooling water flow can be alternated between all of the 

modes described above. Such systems are referred to as mixed mode or multimode cooling systems.

3 Data extracted from the 2000 detailed industry questionnaire; Question 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; typically, the helper tower cool water discharge is mixed with the remaining 

once-through cooling water prior to final discharge.

Typical Operation

Because the cooling towers require energy to operate,

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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, Georgia Power’s Harllee Branch 

Power Plant installed a helper cooling tower 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 exceeds or falls below 94 oF. The tower is 

capable of reducing the temperature of the water passing through it by about 20oF 

(Blankenship 2001).

While they may be sized differently, there is nothing inherent in the design of helper 

cooling towers that would prevent them from being used in a cooling system that was 

converted to operate in a closed cycle or mixed mode configuration. However, 

depending on the flow and heat rejection capacity, they may not be capable of providing 

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.

Effects on Biota

Cooling towers operating in the helper mode may provide little benefit with regard to 

impingement mortality or entrainment reduction and may possibly 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 oxygen for entrained organisms in the cooling water that has 

passed through the towers and, of course, for organisms in the receiving water.

Mixed Mode Systems

As described above, the pumping and piping system for cooling systems utilizing cooling 

towers can be configured such that the system can alternate between once-through, closed 

cycle, helper, and combination modes. Such mixed mode systems offer greater flexibility

for meeting 316(a) and 316(b) permit conditions while optimizing power generation.

Such flexibility requires the installation of a flow and temperature monitoring and control 

system and the establishment of decision rules that guide the timing and selection of 

specific operating modes. Such decisions will be guided by the environmental standard 

requirements that must be met and the overall design of the cooling system.

 

5 The energy penalty associated with helper towers includes only the parasitic component 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. 
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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 

built with the capability to operate in both the helper and closed-cycle mode and this 

capability was unique among power stations at that time. The case study provides an 

analysis of the multimode operation at the Browns Ferry Nuclear plant, evaluating mixed

mode operation and the capability of computer models to predict river temperatures. The 

goal was to optimize power generation while meeting 316(a) limits.

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 The study did not 

address flow reduction. 

A cursory review of the model output graphs provides the following rough estimates 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 analysis estimates that 

while operating cooling towers in full helper mode, 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 capacity. Thus, the case study 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 year and selected the optimum mode of operation for optimizing power 

generation while meeting 316(a) limitations. Closed-cycle operation was selected by the 

model as optimum power generation mode for less than 10 days of the year. Depending 

on the selected scenario, helper mode operation was selected for roughly 30 to 40 days of 

the year. 

Converting Helper Cooling Towers to Recirculating or Mixed Mode Cooling Towers

As noted above, only 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. In existing helper cooling towers, warm 

water that exits the condensers is typically 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 systems,

the intake pumps, pipes, and condenser cooling system are hydraulically 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 system 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 no more than 5 o

F and a 

maximum downstream river temperature of 86 oF

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 total closed cycle penalty of 

3.7% (Sunda, 2008).
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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 channels. In many cases, the elevation 

of the cooling tower basins will be high enough to allow return flow to occur by gravity,

or installation of control structures and a return channel connecting the discharge channel 

to the intake channel could provide for return flow. Either type of system would not 

require an additional set of pumps. 

The costs of converting a helper tower system to a closed-cycle or mixed mode system 

will include the costs of additional piping, channels, valves and control system upgrades. 

The magnitude of these construction costs may vary greatly 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 drivers in determining 

the magnitude of the project capital costs. No matter the solution, such a system 

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.
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References

Blankinship, Steve. “Georgia Power's New Cooling Tower Design Reduces 

Environmental Impact.” Power Engineering. September 2001. 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. MIT 

Energy Laboratory Report No. MIT-EL 79-036. January 1979.

Accessed on December 19, 2008 at website:

http://dspace.mit.edu/bitstream/handle/1721.1/35204/MIT-EL-79-036-

09510127.pdf?sequence=1

Sunda, J., SAIC. Memo: Lost Generation - Construction Downtime and Energy Penalty. 

December 5, 2008.