June 1, 2010

Appraising Our Future Cooling Water Options

By Natasha Jones, Christopher Kaplan, and Ram Narula

Ensuring the availability of water for power plants is a matter of both water quantity and quality. As freshwater becomes less available for power plant use, new supplies from marginal or impaired sources will require new cooling technologies. We look at cooling equipment options and how water availability and quality affect cooling system design and cost.

Water is an essential element used in the process of producing electricity. A study by the U.S. Government Accountability Office and reported on in “Running Dry at the Power Plant” (EPRI Journal, Summer 2007) predicts that water shortages will be experienced in the U.S. within the next 10 years under average climate conditions, and the situation will be more severe under drought conditions. POWER has also written extensively on the need to develop more efficient cooling water technologies. Examples (all available at http://www.powermag.com) include “Determining Carbon Capture and Sequestration’s Water Demands” (March 2010), “Conserve Water by Improving Cooling Tower Efficiency” (January 2009), and “Costlier, Scarcer Supplies Dictate Making Thermal Plants Less Thirsty” (January 2008).

In most plants, water is “used” and returned to the source in a once-through cooling water system. Currently, an estimated 43% of thermoelectric generating capacity in the U.S. uses once-through cooling, according to “New Coal Plant Technologies Will Demand More Water” (POWER, April 2008). In a once-through (open loop) system, cooling water from a large water source (such as an ocean, river, or lake) is “used” to remove heat from the condenser, and then that warmer water is discharged directly back to the source. The cooling takes place naturally in the source.

In plants that employ evaporative cooling (closed loop) systems, water is “consumed” (via evaporation losses) in the process of cooling condenser water. If new power plants continue to be built with evaporative cooling, water consumption for power production could more than double—from 3.3 billion gallons per day in 1995 to 7.3 billion gallons per day by 2030, according to “Energy Demands on Water Resources,” a 2006 U.S. Department of Energy report that explores the interdependency of energy and water. That rate of growth for cooling water use alone will not be sustainable in the future.

Our purpose is writing this article is to acknowledge the need to employ cooling water processes that will require less water. We explore several new alternative cooling technologies that may reduce water use or consumption and then assess their potential for future power plant designs.

Water Is Everywhere, But Not Always Usable

In 2001 the U.S. Environmental Protection Agency (EPA) implemented Section 316(b) of the Clean Water Act, which requires that the location, design, construction, and capacity of cooling water intake structures reflect the best technology available for minimizing adverse environmental impact. According to the EPA, once-through cooling systems can negatively impact marine life, causing direct kills of fish and eggs by entrainment and the destruction of aquatic ecosystems as a result of the elevated water temperatures near plant discharge. [For more information on implementing Section 316(b), see “Cooling Water Intake Structure Regulations” (October 2009), “Alternative Cooling Water Intake Analysis Under CWA Section 316(b)” (February 2008), and “Fish and Cooling Water Intakes: Debunking the Myths” (February 2005).]

Recirculating or closed loop systems, such as wet cooling towers, are likely to become more prevalent should the requirements of Section 316(b) be fully implemented. In a closed loop configuration, the cooling water used to reject the steam heat is sent from the condenser to the cooling towers, which cool the water primarily through evaporation (Figure 1). As of this writing, the EPA has suspended the Cooling Water Intake Structure Phase II Regulation for existing large power plants in response to the 2nd Circuit Court of Appeals decision in Riverkeeper, Inc. v. EPA. (See “Looking Downstream after the Cooling Water Case,” POWER, June 2009.)

1. A recirculating or closed loop water system using a cooling tower. Source: Bechtel Power Corp.

Even plants with sufficient water to supply a closed loop cooling system face many additional air and water permitting issues. For example, some of the circulating water may be entrained in the air stream and carried out of the tower as droplets or drift. These droplets contain the same chemical impurities as the recirculating water. Particulate matter (PM) components, defined as solid or liquid particles found in air, has been classified as an air emission by the EPA. PM is generated when the drift droplets evaporate and leave behind crystallized dissolved solids. PM10 (particles that are 10 micrometers or less) have been determined by the EPA to pose significant health risks to humans due to their small size and ability to reach the lower respiratory tract. Permit requirements in regard to PM10 and other emissions are getting stricter, requiring that lower and lower drift values be achieved from a cooling tower.

Cooling tower particulate emissions can be calculated using emission factors compiled in USEPA AP 42, Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources, Chapter 13: Miscellaneous Sources, which categorizes a wet cooling tower as a miscellaneous source for particulate emissions. The PM can be conservatively estimated using equations in AP 42.