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MEMORANDUM

Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone 703-385-6000

fax 703-385-6007

TO: Lisa Biddle and Paul Shriner, EPA

FROM: Kelly Meadows and John Sunda

DATE: January 7, 2014

SUBJECT: Summary of Materials on Evaporative Losses

In December 2012, Tetra Tech was tasked with reviewing materials regarding evaporative losses 

of water from cooling towers as compared to once-through cooling systems. A summary of the 

materials and analyses gathered to date are listed below, with some brief discussion about the 

information presented or conclusions reached.

Introduction

A common industry comment regarding closed-cycle cooling is the evaporative loss of water 

from a cooling tower. By design, a wet cooling tower evaporates water to dissipate the waste 

heat from the electricity generation process. Industry representatives claimed that this water is 

then removed from the waterbody and would not be available for other uses downstream. This

may be problematic in some areas, particularly those with drought conditions or other high water 

demands. EPA noted that once-through cooling also causes evaporation, as the heated plume 

discharged by the facility would also be subject to evaporative losses.1These evaporative losses 

of the source water would continue long downstream. EPA also noted that such losses are likely 

minimal where the source water is an ocean, but in the case of an inland water such as a river, 

where once-through cooling removes a huge portion of the river flow for cooling, such 

evaporative losses would theoretically approach the same losses of a cooling tower. The purpose 

of this memo is to compare the likely loss rates of water for the two cooling system types.

Materials on Evaporative Losses

Each source of information and data on evaporation is described briefly below. A table at the end 

of this section (where evaporation values reported are also converted to gallons per MWh 

 

1 Note that there are a number of site-specific variables that will affect evaporation, such as local climatological 

conditions, delta T of the condenser, and morphology and flow of the receiving stream. It is difficult to analyze on a 

national scale, so most approaches appear to use either generic scenarios or specific examples.
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(gal/MWh)) provides a summary of the range of values reported in a format that allows for direct 

comparison.

EPRI publication (2002) (DCN 10-6891): Water & Sustainability (Volume 3): U.S. Water 

Consumption for Power Production—The Next Half Century, EPRI, Palo Alto, CA: 2002. 

1006786. This document is often cited by other materials and was likely one of the only 

resources on the subject for some time. Recent efforts (such as the Tt modeling discussed below

and the Energy-Water Nexus research presentations) have greatly expanded upon the available 

data. EPRI estimated once-through fossil plant water consumption levels of 300-400 gal/MWh 

versus closed-loop water consumption levels of 480-720 gal/MWh. This resulted in a generally

cited rule of thumb that closed-cycle evaporative losses are approximately 2.5 times greater than 

those for once-through systems.

Tetra Tech memo to project file (2011): Tetra Tech conducted a water quality modeling 

analysis using data for two power plants; the model predicted evaporative losses in the 

downstream effluent plume. The modeling suggested that losses were approximately 0.3-0.5% of 

the cooling water flow, as compared to approximately 1.5-1.8% for cooling towers at the same 

sites. When converted to gal/MWh these values become 68-183 gal/MWh consumption for oncethrough and 480-620 gal/MWh for closed-cycle. The values are somewhat lower but are similar 

order of magnitude as the other estimates summarized here.

Diehl’s draft article Estimating Forced Evaporation from Surface Water: Presented at the 

Energy-Water Nexus. The paper stated that published estimates of forced evaporation range from 

“0.015 to 4.5 (2 – 1190 gal/MWh) liters per kilowatt-hour (L/kWh) for cooling ponds and 0.45 to 

12 L/kWh (119 – 3170 gal/MWh) for once-through cooling.” The presentation provides a new 

analysis and concludes that “forced evaporation from cooling ponds is estimated to be 1.2 to 1.4 

L/kWh (317 – 370 MWh) for fossil-fueled plants and from 1.5 to 1.8 L/kWh (396 – 476 MWh) 

for nuclear plants.”

Tidwell/Sandia Laboratory presentation Value of Water in Energy Production (DCN 12-

6851): Presented at the Energy-Water Nexus. The presentation has a lot of data on water 

consumption, including data by fuel type. Consumption ranged from approximately 400 to 700 

gal/MWh for recirculating systems at coal fired plants and 100 – 250 gal/MWh for once-through 

cooling coal systems, and 600 – 800 gal/MWh for cooling ponds.

NETL (2008) (DCN 10-6871): Estimating Freshwater Needs to Meet Future Thermoelectric 

Generation Requirements. DOE/NETL-400/2008/1339, September 2008. The analysis estimates 

the effects on national water use for various scenarios of water use for power plants. 

Unfortunately, the analysis does not account for downstream evaporative losses, so this data is 

not further useful in this comparison.

GAO publication (2009) (DCN10-6870): ENERGY-WATER NEXUS: Improvements to Federal 

Water Use Data Would Increase Understanding of Trends in Power Plant Water Use. United 

States Government Accountability Office, October 2009. The document primarily discusses 

material that is not already known, with a focus on the available data that could be collected by 

various federal agencies to better understand the water use patterns.
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DOE (2006) (DCN 12-6852): Energy Demands on Water Resources: Report to Congress on the 

Interdependency of Energy and Water. December 2006. The document provides a broad view of 

water and energy, with specific data on evaporative consumption. Values for fossil plants range 

from 100-300 gal/MWh for once-through cooling systems to 180-480 gal/MWh for cooling 

towers. Values for nuclear facilities range from 400 gal/MWh (once-through) to 720 gal/MWh 

(cooling towers). Values are also provided for cooling ponds.

NSF (2013) (DCN 12-6847): Workshop Report: Developing a Research Agenda for the Energy 

Water Nexus. December 2013. The document presents materials resulting from a National 

Science Foundation grant to establish research priorities for the water-energy nexus. The 

analysis uses 300 gal/MWh for once-through cooling and 480 gal/MWh for closed-cycle.

International Energy Agency (2012) (DCN 12-6853): World Energy Outlook 2012. The 

document provides a broad view of energy use and water withdrawals and consumption. Prior 

years’ editions did not address consumptive losses. The ranges presented for evaporative 

consumption were collected from documents summarized above.

GAO publication (2012) (DCN 12-6854): ENERGY-WATER NEXUS: Coordinated Federal 

Approach Needed to Better Manage Energy and Water Tradeoffs. United States Government 

Accountability Office, September 2012. The document discusses relevant general concepts but 

provides no data on evaporative consumption rates.

USGS (2013) (DCN 12-6855): Methods for Estimating Water Consumption for Thermoelectric 

Power Plants in the United States. United States Geologic Survey, 2013. Scientific 

Investigations Report 2013-5188. This document provides a thorough analysis of modeling for 

evaporative losses, but does not provide any explicit data on consumption rates.
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Summary of the Range of Estimated Once-through and Closed-Cycle Evaporation

Cooling System Type EPRI 2002 Tetra Tech 

2011

Diehl 2012 

(Pub. Est.)

Diehl 2012 

(New)

Tidwell

2012

DOE 

2006

NSF 

2013

gal/MWh gal/MWh gal/MWh gal/MWh gal/MWh gal/MWh gal/MWh

Once-through 300 - 400 68 - 183 119 - 3,170 100 - 250 100 - 400 300

Closed-cycle 480 - 720 480 - 620 400 - 700 180 - 720 480

Cooling Pond 4 - 1,190 600 - 800

Cooling Pond- Fossil 317 - 370 480

Cooling Pond- Nuclear 396 - 476 720

Note: One MWh of energy would evaporate approximately 420 gallons of water. Most generating systems are >32% efficient. In a 

generating system that was 33% efficient where all waste heat was transferred to cooling water, the evaporation could be no greater 

than 840 gal/MWh. The actual value will be lower because not all waste heat ends up in the cooling system or as evaporation (e.g., 

some exits via flue gas, some heats the air). Accordingly, the upper end of the range of published values in the DOE document 

represent very inefficient generating systems.
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General Review

Prior to this collection of data, industrial sources cited evaporative losses of cooling towers as 10 

times that of once through cooling. The data identified here show the relative losses of cooling 

towers are at most a factor of 2.5 times that of once-through, and on average would be 2 or less. 

In summary, the studies show:

 The differences in evaporative losses are minimal in terms of gallons lost and in most 

cases are minor compared to river flow.

 In areas where water resources are limited (e.g. the desert southwest or the recently 

drought-stricken southeast), once-through cooling may not be a prudent option for new 

facilities and it may be a liability for existing facilities. EPA observed this in its site 

visits, where several facilities retrofitted to closed-cycle cooling in spite of drought 

conditions.2EPA found that such facilities could not obtain enough water to support 

once-through cooling, and the evaporative losses become a moot point.

 Similarly, for facilities located on smaller waterbodies, evaporative losses from oncethrough cooling will be significantly higher since the effluent comprises a larger 

percentage of the receiving stream, won’t mix as quickly, and will remain heated longer, 

all leading to additional evaporation. Such heated water remains exposed to the 

environment downstream, and is thus exposed to evaporative forces for much longer than 

a similar volume of water passing through a cooling tower. Smaller receiving streams are 

also more likely to be affected by thermal discharges from the perspective of 316(a), 

which requires that the discharge not affect the “balanced indigenous population.”

 Dry cooling and hybrid (wet/dry) cooling are available technologies that significantly

reduce evaporative losses. Dry cooling systems require virtually no water withdrawals 

and hybrid systems consume about 15% less water through evaporation. EPA has 

recognized the availability and applicability of these systems for reducing evaporative 

losses for over 30 years.3

 The analysis presented above uses techniques that are broadly applicable to all parts of 

the United States, regardless of climate, waterbody type, fuel type, or other facilityspecific factors.

 While EPA did not attempt to identify or quantify the meteorological effects, the water 

vapor in the evaporative plumes does not simply disappear; it will be incorporated into 

the atmosphere and may return to the original watershed in the form of precipitation.

 Cooling water withdrawals are a very small component of consumptive uses nationwide. 

As noted in EPA’s Closed-cycle Cooling Systems for Steam-electric Power Plants: A 

State-of-the-art Manual, consumptive water uses by the steam electric sector was 1.2% of 

consumptive uses nationwide in 1975; agriculture was 85%, drinking water was 7% and 

mining was 7%. The Nuclear Energy Institute presented similar data, noting that a 

closed-cycle power plant typically consumes 23 gallons of water per day per household 

 

2

See the site visit reports for McDonough, Yates, Williams, and Nearman Creek (DCNs 10-6536, 10-6538, 10-

6533, and 10-6524). While these facilities did not explicitly convert to cooling towers over 316(b) concerns, each 

facility concluded that, despite challenging conditions for evaporative losses (and other factors), closed-cycle 

cooling was the best technology.

3

See Closed-cycle Cooling Systems for Steam-electric Power Plants: A State-of-the-art Manual, DCN 10-6845F.
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served with electricity, while the same average household uses 94 gallons per day for 

domestic uses, indicating that a conversion to closed-cycle cooling will lead to nominal 

increases in water consumption.4 As a whole, a small increase in the consumption by

closed-cycle cooling will be virtually indiscernible on a national scale.5”

Therefore, in general, this parameter was not deemed to pose a significant factor in deciding 

whether to install or retrofit closed-cycle cooling for most facilities.

 

4 Nuclear Energy Institute fact sheet “Water Use and Nuclear Power Plants,” May 2010. Available at 

http://www.nei.org/filefolder/Water_Use_Fact_Sheet_0510_1.pdf. 

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See DCN 10-6845F, page 316.