Appraising Our Future Cooling Water Options

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 https://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.

Technologies to Address Water Shortages

Specific tower components have been developed and continue to undergo improvement as a direct result of decreasing water availability and increasing difficulty in obtaining permits for new cooling towers.

For example, high-efficiency film fills maximize tower thermal performance, and modern drift eliminator designs can limit drift loss to as low as 0.0005% of the circulating water flow rate. Cooling tower fill is used to achieve the most economical heat transfer rate by providing increased contact surface area between the water and air.

Two types of cooling tower fill are used today: splash and film.

Splash fill consists of elements that break the recirculating water into droplets, while film fill utilizes thin sheets of PVC that enable the water to spread into a thin film. Both approaches increase the amount of water surface area that is exposed to cooling air, although film fill typically allows manufacturers to use a smaller cooling tower than would be required to achieve the same amount of cooling with splash fill.

Unfortunately, the use of film fill is subject to water quality limitations because the presence of suspended solids and other contaminants can impede water flow channels in the fill. To minimize the amount of fill surface area required when poor quality water is used, manufacturers are developing designs that incorporate the characteristics of film fill along with design elements that allow for the use of poor quality water.

Another concern is drift. The amount of drift produced in a cooling tower is a function of tower component design, air and water flow patterns, and tower operation. Manufacturers can limit drift to 0.0005% of the circulating water flow rate with the use of state-of-the-art drift (water carryover) eliminators. The discharge air is forced to change direction while exiting through the drift eliminators, facilitating inertial separation of the drops and air. Current drift eliminators are reaching their physical limit. Drift eliminators in the near future may require higher velocities, thereby leading to high pressure drops and increased fan power requirements.

Until recently, dry cooling was the only alternative at sites with insufficient water for wet cooling. In a dry cooling system, heat is transferred from the process fluid (water or steam) to the air using extended surfaces or fin tube bundles. The major difference in performance between a dry and wet cooling system concerns the ambient dry bulb temperature (DBT) and wet bulb temperature (WBT) respectively. The performance of dry cooling systems is a strong function of ambient DBT of the air. The downside to dry cooling is the increased auxiliary load to operate the large number of fans required and the commensurate increase in capital cost. Also, a dry cooling system has a larger footprint than an equivalent sized evaporative cooling tower. However, when the WBT is lower than the DBT, evaporative cooling systems are more efficient and are less expensive for the same design conditions when water is available.

If dry cooling your only option, then there are three basic types of dry cooling system available: direct, indirect, and hybrid cooling.

Direct Dry Cooling or Air-Cooled Condensers. Air-cooled condensers (ACCs) are the most commonly used direct dry cooling system. They have been installed and operated successfully in all parts of the world, even in desert areas with high ambient DBTs during the summer. The ACC tube bundles have a relatively large tube side cross section and are usually arranged in an A-frame configuration, as depicted in Figure 2, resulting in a high heat-exchange-surface-area/plot-area ratio.

2. The direct air-cooled condensing system. Source: Bechtel Power Corp.

Indirect Dry Cooling: The Heller System. The Heller system is an indirect dry cooling technology that requires a separate condenser and circulating water pump (Figure 3).

3.The indirect Heller system with direct contact jet condenser and dry cooling tower. Source: Bechtel Power Corp.

The Heller system (see sidebar “Case Study 1”) utilizes a dry tower with water-to-air heat exchangers, typically in a natural draft configuration, although a mechanical draft configuration is also available. The tower may be equipped with a peak cooling system that sprays water on part of the heat exchanger bundles during hot ambient conditions for peak-shaving purposes. Most Heller systems are located in regions with limited supplies of makeup water. A direct contact jet condenser is typically used, because it is characterized by low terminal temperature difference values. Because Heller systems are indirect, there is no need for a large-diameter steam duct between the steam turbine and condenser.

The use of seawater not only affects the physical properties of water but also results in higher scaling, corrosion, and biological fouling. Calcium carbonate (CaCO3) scale is observed in most cooling applications. The presence of scale should be avoided on any heat exchange surface because a CaCO3 thickness of as little as 0.1 inch can reduce heat transfer by up to 40%. Scale inhibitors are designed to prevent the deposition of CaCO3. Typical inhibitors are phosphonates and polymers. Higher salinity levels make scale inhibitors less effective, and the operation of seawater cooling towers requires advanced scaling control. Proper acid addition can help to minimize scale inhibitor requirements.

Hybrid Cooling: Parallel Condensing System. A partial dry cooling technology can be used for heat sinks with limited cooling water supply or uncertain long-term supply. One such hybrid cooling system is a parallel condensing (PAC) system. In a PAC system, the turbine exhaust steam is condensed in both the steam surface condenser (SSC) and the ACC, as shown in Figure 4. This system consumes about half of the water consumed by a wet system, although the amount is very dependent on the fraction of the total duty handled by the wet evaporative cooling system.

4. An air-cooled condenser in parallel with a wet cooling tower. Source: Bechtel Power Corp.

The heat load between the SSC and ACC will automatically adjust based on the size and heat rejection capacity of the SSC and ACC, operation mode, and ambient conditions. The turbine backpressure can be controlled by adjusting the PAC system capacity, using both the ACC and cooling tower fans. The cooling tower evaporation rate can be controlled by adjusting the wet cooling capacity. In a water-saving mode of operation, the ACC should be set at maximum capacity and the heat load to the cooling tower can be reduced by turning off the cooling tower fans whenever possible.

Managing the Quality of Various Water Sources

As freshwater becomes less and less available, new supplies from marginal or impaired sources need to be identified and developed. Sources that have been used include seawater and gray water. Water quality affects tower design in several ways:

  • The presence of salts, oils, and grease affects the physical properties of water in such a way that performance in the tower is decreased.
  • The presence of certain chemical components can increase the corrosion rate in certain metals, requiring careful (and often expensive) materials and coatings selections. In severe cases, increased maintenance or replacement could become necessary.
  • Biological slime and inorganic scale can foul heat exchanger surfaces, and the water must be treated accordingly.

The limited availability of freshwater for today’s power plants makes using salt water for a circulating cooling water system’s makeup supply increasingly common at sites located near the sea. Although this makeup water source is less than ideal, a closed cooling system utilizing a wet cooling tower can perform adequately with salt water, provided the impacts of the water are properly considered.

The high concentration of salts changes the physical properties of water, affecting the thermal performance of a cooling tower. Proper tower design can typically compensate for these changes through measures such as an increase in tower size or fan motor power consumption.

Cooling tower performance is affected when seawater or water with high total dissolved solids content is used in cooling towers. Dissolved salt lowers the vapor pressure of the water, which means that the salt water will evaporate less readily than pure water, effectively reducing the thermal performance of a cooling tower. The presence of salt in water also reduces the water’s specific heat, decreasing its heat-absorbing capacity, which again reduces thermal performance. Density increases with salinity, which increases thermal performance; however, this increase is insufficient to offset the effects of the lowered vapor pressure and specific heat. Therefore, for a given heat load, salt water towers are larger than freshwater towers because more heat exchange area is needed to compensate for the salt-related performance loss. A larger cooling tower demands higher fan horsepower to move the required air volume through the larger facility.

The use of seawater not only affects the physical properties of water but also results in higher scaling, corrosion, and biological fouling. Calcium carbonate (CaCO3) scale is observed in most cooling applications. The presence of scale should be avoided on any heat exchange surface because a CaCO3 thickness of as little as 0.1 inch can reduce heat transfer by up to 40%. Scale inhibitors are designed to prevent the deposition of CaCO3. Typical inhibitors are phosphonates and polymers. Higher salinity levels make scale inhibitors less effective, and the operation of seawater cooling towers requires advanced scaling control. Proper acid addition can help to minimize scale inhibitor requirements.

Certain water characteristics can also contribute to increased rates of corrosion. Water temperature, chemistry, halogen residuals, and dissolved oxygen are some of the factors that affect corrosion rates. Corrosion inhibitors such as azoles, phosphates, and zinc can be used to lessen the risk of steel corrosion.

Bacteria and algae that are present can form micro slimes and films that impede heat transfer. Aquatic species including barnacles, bryozoa, oysters, and mussels can contribute to macro-fouling. Some of the chlorines, bromine, and nonoxidizing biocides can control both micro- and macro-biological fouling. Chlorine discharge levels are often regulated by permit requirements. Adding the proper amount of sodium bisulfite can eliminate excess chlorine.

Sufficient water must be added to the circulating water system to make up for evaporation, blowdown, drift, and any other losses, which deplete the recirculating water inventory and/or concentrate the solids in this water. The required makeup rate to hold a given solids concentration ratio may then be computed based on an assumed cooling tower cycles of concentration. For details on the calculation procedure, see “Strategies to Reduce Sulfuric Acid Usage in Evaporative Cooling Water Systems” (POWER, March 2010).

Component Selection Considerations

The quality of makeup water can potentially impact every part of a wet cooling tower. Figure 5 depicts components in both a cross-flow and counter-flow mechanical draft cooling tower configuration. These components can be selected specifically to accommodate the use of poor quality water. Optimization studies must be performed to determine the best balance of water consumption, tower materials, tower cost and size, and output for the site conditions of a particular plant. For example:

  • Larger motors may be required to offset the poor heat transfer properties of seawater. Larger motors may also be required to offset increased pressure drops associated with high-efficiency drift eliminators.
  • The level of total suspended solids (TSS) in the water source must be considered for proper selection of the heat exchange surface in a wet cooling tower (the fill).
  • High-efficiency drift eliminators can be incorporated to minimize emissions.
  • More expensive hardware material may be considered. Copper alloys can easily erode by contact with suspended solids. Other alloys, such as titanium, are free of corrosion products and reduce the number of sites for potential TSS entrapment but are much more expensive (see sidebar “Case Study 2”).
  • Structural materials and applicable coatings need to be selected to avoid corrosion, surface damage, and breakdown. Properly mixed cement, plastics, and ceramics are all suitable options.
  • Any mechanical equipment fabricated from cast iron should be heavily coated, or corrosion-resistant metallurgy must be used.

Expected Future Developments

Beyond cooling tower designs that reduce water consumption, users should also expect increased regulation of siting and of cooling tower emissions such as plumes and noise.

Plume Abatement. A plume is heated air and water vapor that exits the cooling tower. Under certain ambient conditions, the plume becomes visible when moist, heated air is cooled past its dew point. Its presence is a concern for many reasons, including the fact that the plume can be viewed negatively when the public associates it with stack emissions or nuclear power plants. The plume can also create dangerous fogging and icing conditions and must often be abated if the towers are located near airports or roads.

To combat these problems, plume-abated (PA) towers have been developed (Figure 6). To avoid creating a visible plume under certain ambient conditions, saturated air from the wet section is mixed with hot and dry air in the dry section of the tower. Because some of the heat in the incoming water is dissipated in the dry section, not as much cooling has to take place via evaporation in the wet section; thus, water savings can be realized with the PA towers. However, given the special constraints on the dry section, the savings potential is limited. The main purpose of PA towers, where the dry and wet sections are integrated in the same system, is to avoid a visible plume, and water conservation is an incidental consequence.

5. The cross-flow and counter-flow cooling tower. Source: Bechtel Power Corp.

Site Area Restrictions. Cooling towers must be oriented properly to minimize factors that negatively impact tower thermal performance. One consideration is to ensure that cooling towers are adequately spaced so that hot discharge air from one tower is not drawn into the intake of another tower. Towers must also be located a sufficient distance from other structures in a power plant. Tall turbine and boiler buildings impede airflow into cooling towers, and tower drift can cause unacceptable salt deposits on switchyards. However, some sites simply do not have sufficient area available to address all of these concerns or to allow for enough linear cooling tower area to dissipate the required heat load. Unique layouts have been employed to ensure that towers incorporate all of the required features while being located within site boundaries.

Cooling towers must also be placed correctly with respect to the prevailing wind. The air inlets should be aligned parallel to the prevailing wind coincident with the highest ambient temperatures. This minimizes recirculation, where some of the saturated exiting air is induced back into the tower air inlets, raising the inlet WBT and resulting in a higher cold water temperature for a given ambient condition (see sidebar “Case Study 3”).

Resourceful Designs Required

Given impending water resource shortages, power plant designers will need to become more resourceful in designing cooling systems. As noted here, many innovative and hybrid technologies have been implemented already, and we expect many new applications will evolve in the coming years.

—Natasha Jones and Christopher Kaplan are mechanical engineers, and Ram Narula (rnarula@bechtel.com) is vice president, chief technology officer emeritus, and a Bechtel Fellow for Bechtel Power Corp.