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°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 TowersAccording 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 OptionsCooling 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."
Cooling towers are used frequently because they allow the user to reject heat from a system or process without consuming excessive quantities of water or thermally polluting a body of surface water. There are minor drawbacks, which are usually accepted as the price of the larger benefit, according to David Hutton, P.E., process and power market manager (international) for Baltimore Aircoil Company. These drawbacks are:
- Higher operating temperatures than would be obtained from surface water or municipal water; resulting in some loss of thermodynamic efficiency;
- Cost of electricity to run fans;
- Drift, the discharge of minute water droplets which contain minerals and other impurities;
- Cost of water treatment to prevent corrosion, scale, and biological fouling of equipment; and
- Risk of dispersion of airborne pathogens from poorly maintained cooling towers.
"Cooling towers solve the problem by shifting the rejection of heat from bodies of water to the atmosphere, which has a much greater ability to absorb and dissipate the heat input without adverse effects," added Hutton.
Due to energy and environmental concerns, new power plants are often being built with permanent cooling towers. A power plant that incorporates cooling towers from the beginning of the design process can be better optimized with the other systems and components. Zammit states this might include using a two-pass condenser instead of a one-pass condenser, lower cooling water flow rates and higher temperature differentials, and installing a turbine that is designed for higher backpressure. The cooling tower would also be designed to cool the entire flow.
It would be difficult to install that type of equipment on a retrofit basis, though. "Plants that are forced to retrofit typically compromise the design," said Zammit. "They'll come up with something that's not quite as expensive to install, but they live with a pretty severe energy penalty for the rest of the life of that plant. That, of course, increases their costs and decreases their ability to bid power to a deregulated energy market."
For these situations, temporary cooling towers may still make the most sense.
Closed Loop and Hybrid OptionsSome power plants around the country have no choice but to go to closed-loop (recirculating) cooling systems. Depending on the area, it may be due to a local moratorium on withdrawing water from a river or lake. This happened recently to a major chemical refinery on the Gulf Coast, which elected to discontinue using river water for all of its plant cooling applications.
"They ended up installing Baltimore Aircoil closed-circuit cooling towers, eliminating approximately 500 million Btuh (146 MW) of thermal input into the river," said Hutton. "Using closed-circuit cooling towers also eliminated frequent outages due to fouling of their heat exchangers."
There are drawbacks associated with closed-loop systems as well. Recirculating wet-cooling towers have an energy penalty associated with the additional pumps, fans, and auxiliary equipment and can also require more extensive water treatment.
According to a recent report from the National Energy Technology Laboratory, power plants with recirculating cooling systems require from .5 to 1.25 gallons per kW to operate, while a plant using once-through cooling withdraws about 30 to 40 times more water on a gallon per kW basis.
Although once-through systems withdraw significantly greater amounts of water (a 500-MW unit can withdraw approximately 450 million gallons per day), consumptive losses are significantly less, about 10% of the consumption of a similar unit equipped with a wet-cooling tower.
Due to these energy consumption concerns, some power plants are opting for hybrid systems, which incorporate a recirculating cooling system in combination with a once-through cooling system. The Brayton Point Station (1600 MW) in Somerset, MA, for example, will address thermal discharge and fish protection issues through the installation of an enhanced multi-mode (EMM) system.
"By utilizing the unique configuration of the cooling system, they are able to use a single 20-cell tower to cool Units 3 and 4 in a recirculated mode, or Units 1 and 2 in a helper mode," said Zammit.
Other Sources of WaterIn many parts of the country, competing demands for fresh water have forced power plants to consider alternative cooling water supplies. These "degraded" water sources can include treated sewage effluent, contaminated or high TDS groundwater produced from energy production, or reclaimed water. When used in conjunction with a cooling tower, there are different issues that have to be addressed, such as loss of heat transfer, fouling of fill media, and corrosion. There are also potential issues concerning the exposure of cooling tower operators to harmful agents.
"There are definite questions about the issues you get with effluent," said Zammit. "You may have to treat that effluent to be able to manage it within the plant. If you have a high nitrogen or phosphorous loading, you have to be able to treat it to minimize any biofouling in a system, because that can contribute to fouling, corrosion, and a lot of other problems."
The follow-up issue, Zammit points out, is what happens to the blowdown? "You eventually have to blowdown part of that water to keep your system clean enough to operate without scaling it or fouling it to the point where it impacts operations."
In many cases, the treatment facility will accept the blowdown back, adding it to the front end of the treatment works or adding it to the excess effluent being discharged. In some cases, it is necessary to treat the blowdown to remove any of the disagreeable components. It may even be necessary to use brine concentrators to reduce it down to dry salt for disposal, depending on permit requirements.
These issues all have to be evaluated when alternative water sources are used. Zammit said that EPRI just finished a project that investigated the formation of Trihalomethane (THM) compounds, which can form when organically contaminated waters (e.g., wastewater effluent) are treated with chlorine or bromine for biofouling control. "It's well known that if you brominate or chlorinate these organically contaminated waters you can form THM, a family of compounds that are known carcinogens in sufficient concentrations."
There was a possibility of exposure that workers in and around the cooling tower in question may be exposed to the carcinogens. Fortunately, the testing showed that the formation rate of the carcinogenic compounds is very low and the dissipation is very high. "We don't believe, from the test results that we have so far, that it's going to be an exposure issue, even for the operators who are in close proximity," said Zammit.
That's good news, because from energy penalties to treatment issues associated with degraded water, there are definitely some challenges to using cooling towers in a power plant. But most experts agree that cooling towers are often the quickest, most effective solution to thermal discharge issues. ES