A solution for thermal pollution

Engineered Systems, Sept, 2004 by Joanna R. Turpin

Using nearby water for cooling in industrial applications and then returning that water to the source is a traditional maneuver, but it can also damage local aquatic life and violate EPA regulations. Now more than ever, cooling towers are being used to precool that water before discharge, diverting rejected heat into the atmosphere instead. Learn the costs and benefits of a few configurations, along with tips for dealing with blowdown and effluent within the facility itself.

Warm water can be found in a number of different areas in nature--for example, hot springs or water warmed by volcanic activity. Warm water can become a problem, however, when it is created by man and introduced into nature. An example of this is when the water used to cool power plants or other industrial applications is discharged into streams, rivers, and lakes.

This is known as thermal pollution or thermal discharge, and it is the introduction of waste heat into bodies of water that support aquatic life. The addition of heat reduces the water's ability to hold dissolved gases, including the oxygen required for aquatic life. If the water temperature is greater than 95[degrees]F, the dissolved oxygen content may be too low to support some species. If the differential temperature is too large, the difference can also stress some species.

As a result, thermal pollution can wreak havoc on native fish species, such as trout, that require cold water with high levels of dissolved oxygen. When the water becomes warmer, other non-native fish that thrive in the warmth can take over habitats from native fish. In addition, warmer water allows bacterial populations to increase and thrive, and algae "blooms" may occur.

Regulators and lawmakers in the United States long ago recognized that thermal pollution is a problem and addressed the issue in Section 316(a) of the EPA Clean Water Act. States and other regulatory agencies use those guidelines to require power plants and industries to limit warm water discharges back into surface waters, sometimes by way of cooling towers.

GROWING NEED FOR TOWERS

According to the U.S. Geological Survey, about 48% of all freshwater and saline-water withdrawals for 2000 were used for thermoelectric power. Most of this water was derived from surface water and used for once through cooling at power plants. About 52% of fresh surface water withdrawals and about 96% of saline water withdrawals were for thermoelectric power use.

This large amount of water is needed by power plants due to the fact that over the years, there has been an ever-increasing need for electricity. This means power plants are expected to run at near maximum output for a large part of the year. The cheapest and easiest method for power plants to operate has always been to withdraw water from a nearby body of water, pass it through the plant, and return the heated water to the same body of water.

These once-through cooling systems now require very strict environmental permits, issued in accordance with the National Pollution Discharge Elimination System. The permits vary from state to state and location to location and may have different requirements. For example, some permits require that the plant must discharge the water within a temperature differential limit over the temperature of the intake water. Other permits have an ultimate limit; in other words, they can't ever exceed a specific temperature. Still other permits have both differential and ultimate limits.

Depending on the permit, restrictions are often magnified during low river or lake levels, or drought conditions. That's because most utilities see their peak loads in the summer months, when air conditioning loads are high. In addition, water temperatures are at their highest in the summer, which can make it difficult for power plants to comply with permit requirements.

Cooling towers provide one way in which power plants can follow permit restrictions. "Cooling towers are the surest way to solve thermal pollution problems, because the cooling results can be predicted with a high degree of accuracy prior to installing the towers," said Robert C. Brink, president and CEO of Tower Tech, Inc. in Oklahoma City.

Since the need for cooling towers can be seasonal, some power plants rent cooling towers as needed. In months where plants are bumping their thermal limits, they can install rental cooling towers to cool the effluent, provided there is reasonable access. "These facilities pump all or part effluent through the cooling tower and then back to the discharge to diminish the ultimate temperature before it reaches the surface water," said Kent Zammit, manager for cooling water technologies at Electric Power Research Institute (EPRI) in Palo Alto, CA.

These installations are easier in the presence of a discharge canal that provides access to the heated effluent and diffusion of the cooled water back into the effluent.

ONCE-THROUGH OPTIONS

Cooling towers use evaporation to cool water, and their ability to cool is driven largely by the difference between the wetbulb temperature and the desired cold water temperature. "Of course," Brink said, "a cooling tower uses other variables to provide a certain amount of cooling, such as fill media, fans, motors, tower size, but it is this temperature differential that enables a cooling tower to cool water by evaporation."