ASHRAE Journal - January 2012 - 65

EMERGING TECHNOLOGIES

Power Plant Water Use
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By Alissa Cooperman; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

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

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 cooling 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.
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 facilities were comprised of 89% thermal-electric plants, 9% hydroelectric facilities, and 2% renewable and other sources.1 A 2011 study, using data from 2010, reveals the breakdown of power generation 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.
January 2012

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 These systems both withdraw and consume water, but not all water withdrawn is consumed.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 evaporated (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 withdraws more water, closed-loop cooling withdraws less water but consumes more.1 Once-through cooling systems typically consume 4% of the water withdrawn while closed-loop systems consume 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 environment (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 evaporation, compared with the evaporation
ASHRAE Journal 65
ASHRAE Journal - January 2012

Table of Contents for the Digital Edition of ASHRAE Journal - January 2012
