Janu a ry 2012 ASHRAE Jou rna l 65

EMERGING TECHNOLOGIES

Power Plant Water UseS everal recent Emerging Technologies columns have addressed

different forms of evaporative cooling, but have not closely

addressed the basic trade-offs involved in avoided electric energy

consumption vs. water consumption. In the December 2011 Emerging

Technologies column, we considered this from the residential consumer’s

point of view, comparing water costs incurred with evaporative cool-

ing with avoided electric energy costs. This column compares water

consumption of water evaporative cooling with the electric power plant

cooling water consumption that is avoided due to the reduced electric

energy consumption that results from evaporative cooling.

By Alissa Cooperman; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

Water/Electricity Trade-Offs in Evaporative Cooling, Part 2

Power generation in the United States is dominated by thermal-electric power plants, which use a heat-driven power cycle to generate shaft power, which is converted to electric power by electric generators.

A report published in 2003 found that 89% of the electricity consumed in the U.S. was generated at thermally driven, water-cooled power plants.

Nationwide, power generating fa-cilities were comprised of 89% ther-mal-electric plants, 9% hydroelectric facilities, and 2% renewable and other sources.1

A 2011 study, using data from 2010, reveals the breakdown of power genera-tion in the U.S., as shown in Table 1.

The first four categories (coal-fired, natural gas, nuclear, and petroleum fired) are thermal-electric power plants.

While there was an increase in power generation from renewable and other resources, thermal-electric power plants continue to supply 89% of the total power generated.2

Water Use at Power Plants Water is mainly used at power plants

for cooling. This is accomplished in one of two ways: once through or closed loop (recirculating) cooling systems. These systems both withdraw and consume water, but not all water withdrawn is consumed.

Once-through cooling draws water from a reservoir, river, ocean, or other body of water located near the power plant through a heat exchanger (usually a water-cooled condenser) and returns the water to that body of water, at a higher temperature.

Though little to no water is evapo-rated (consumed) in the heat exchanger, the increased temperature of the exiting water causes increased evaporation to occur from the receiving body of water.1

Closed-loop cooling systems usually use evaporative cooling in a cooling tower to reject the heat to the atmosphere and maintain the temperature of the cooling water loop.

Water consumption is the total of the water that is evaporated for cooling plus water lost to drift (unevaporated cooling water droplets that are entrained in the air passing though the cooling tower). These water losses plus blowdown must be replaced by freshwater withdrawals from the power plant’s water supply.

While once-through cooling with-draws more water, closed-loop cooling withdraws less water but consumes more.1

Once-through cooling systems typi-cally consume 4% of the water with-drawn while closed-loop systems con-sume 80% of the water withdrawn (with the remainder returned to the source body of water).

Though they consume more water, closed-loop systems are favored and supported by the Clean Water Act (2001) for their reduced impact on the environ-ment (less water withdrawn, no impact on the temperature of the source body of water).3

Hydroelectric power generation does not consume water for cooling, but the increased surface area of the storage reservoir results in increased evapora-tion, compared with the evaporation

This article was published in ASHRAE Journal, January 2012. Copyright 2012 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

1. ANSI/ASHRAE Standard 52.2-1999, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size*

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12. Guideline 1.1-2007, HVAC&R Technical Requirements for the Commissioning Process

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that would have occurred from the un-dammed river.

Based on estimates of the increased evaporation from the surface of the water storage reservoir, it consumes about 18 gallons/kWh (68 L/kWh).4 Hydroelec-tric power generation accounts for a relatively small fraction of the nations’ total electric output, so in this column we only consider the water consumption of thermal-electric power generation.

Table 2 (Page 68) compares the wa-ter consumption for several types of thermal-electric power plants, which on average consume 0.47 gallons/kWh (1.8 L/kWh).1

A study of power generation in the interior west shows that combined cycle natural gas-fired power plants consume less water regardless of cooling system type. Power generation in this region withdraws 650 million gallons per day (2.4 million kiloliters per day), or enough water for 4 million people. Only 15% of the cooling in this region is accomplished with once through systems, and two fa-cilities use dry cooling.4

Water Consumption by Cooling Technology

To examine the changes in water consumption for cooling at the site of energy consumption for cooling at the site of energy consumption and to drive cooling at the source of energy generation, we will again consider the two cases originally presented in the

Plant Type % of Total Power

Generated

Coal-Fired 44.9

Natural Gas 23.8

Nuclear 19.6

Petroleum-Fired 0.9

Hydroelectric 4.5

Renewable & Other 6.3

Table 1: Power generated (by plant type) in the U.S. in 2010.2

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EMERGING TECHNOLOGIES EMERGING TECHNOLOGIES

Plant Type And Cooling System

Withdrawal (cooling & process)

gallons/kWh

Consumption (cooling)

gallons/kWh

Steam (Once-through) 20-50 0. 3

Steam (Recirculating) 0.3 – 0.8 0.24 – 0.64

Steam (Dry Cooling) 0.04 0

NG Combine Cycle (Once-through) 7.5-20 0.1

NG Combined Cycle (Recirculating) 0.23 0.18

NG Combined Cycle (Dry cooling) 0.04 0

Coal Combined Cycle (Recirculating) 0.38 0.2

Table 2: Water consumed based on thermal power plant cooling technology.4

Power Plant Type

Power Provider Thermoelectric (gallons/kWh)

Hydroelectric (gallons/kWh)

Western Interconnect 0.38 12.4

Eastern Interconnect 0.49 55.1

Texas Interconnect 0.44 0.0

U.S. Aggregate 0.47 18.0

Table 3: Water consumed by electricity generation.1

December 2011 Emerging Technologies column:

• Directevaporativecooling,asinthetypical evaporative cooler (sometimes called a swamp cooler), outdoor air is contacted with water reducing the tem-perature from the dry-bulb temperature to a lower temperature approaching the wet-bulb temperature. For this example, it is assumed that the outdoor dry- and wet-bulb temperatures are 90°F (32°C) and 60°F (16°C), respectively, with air delivered to the conditioned space at 65°F (18°C) dry-bulb temperature.

• A water-cooled condenser operat-ing at AHRI standard conditions (95°F [35°C] dry bulb and 75°F [24°C] wet bulb).

As previously calculated, direct evaporative cooling consumes 3 gallons per ton·hour (3.23 L/kWh) of cooling and about 250 Wh per ton·hr of cooling. Whereas, a SEER 13 central air-condi-tioning system consumes 1200 Wh of electricity per ton·hr (341 Wh per kWh) of cooling at AHRI standard conditions, and an air-conditioning system with an evaporatively precooled condenser consumes 980 Wh and 2.5 gallons of water per ton·hr (2.69 L/kWh) of cool-ing. Based on a consumer’s location, the amount of source water consumed for power generation per kWh varies.

Table 3 outlines the average water consumption per kWh consumed for three major power regions in the U.S.

Table 4 uses the average of 0.47 gallons of water consumed by thermal-electric power plants per kWh generated to compare the decrease in power plant source water consumption attributable to reduced electric power consumption via evaporative cooling with the water consumed by evaporative cooling.

Cooling Technology Site Electricity Consumed(Wh/ton·hr cooling)

Site Water Consumed(gallons)

Source Water Consumed at Power Plant

(gallons)

Direct Evaporative Cooler 250 3.0 0.12

Evaporative Condenser 980 2.5 0.46

Central AC (SEER 13) 1200 0 0.56

Table 4: Cooling site electricity and water consumption vs. source water consumption.

The bottom line is that the water consumed for evaporative cooling is not offset significantly by power plant water consumption savings.

There is a favorable trade-off with evaporative cooling between energy and water consumption for the consumer, at least based on typical U.S. electric energy and water rates.

References1. Torcellini, P., N. Long, and R. Judkoff,

2003. “Consumptive Water Use for U.S. Power Production.” NREL Technical Report NREL/TP-550-33905. http://tinyurl.com/Torcellini2003.

2. U.S. Energy Information Administra-tion, DOE. 2011. “Electric Power Monthly.”

http://tinyurl.com/ElectricPower2011.3. U.S. DOE. 2010. “Water Vulnerabili-

ties for Existing Coal-fired Power Plants.” DOE/NETL-2010/1429. http://tinyurl.com/WaterVulnerabilities.

4. Clean Air Task Force. 2003.“The Last Straw: Water Use by Power Plants in the Arid West.” The Hewlett Foundation. http://

tinyurl.com/LastStrawWater.

Alissa Cooperman is a technologist and John Dieckmann is a director in the Mechanical Systems Group of TIAX LLC, Lexington, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C.

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