Thermal pollution

DEFINITION: Adverse environmental effect caused by heat, particularly waste heat from steam-electric power plants

When thermal effluent is discharged into waterways from steam-electric power plants, the resulting increases in water temperature can cause damage to the aquatic ecosystems of those waterways.

The overwhelming majority of heat in industrialized countries comes not from factories but from steam-electric power plants such as coal-burning and nuclear power plants. Steam-electric power plants convert thermal energy from the combustion of fossil fuels or nuclear reactions into mechanical work and then into electrical energy. While the generators that convert the mechanical work into electrical energy in such a plant are nearly 100 percent efficient, the rest of the plant is subject to maximum efficiencies imposed by the laws of thermodynamics and determined by the highest and lowest temperatures of the plant. Steam-electric power plants typically have efficiencies of about 40 percent or less, which means that 40 percent of the heat is converted into electrical energy, while the other 60 percent becomes unusable waste heat that must be removed.

As an example, consider a large electric power plant producing 1,000 megawatts (MW) of electricity. If its efficiency is 40 percent, the plant will have to produce 2,500 MW of heat (since 1,000 MW is 40 percent of 2,500 MW) to maintain this output. Waste heat will be produced at the rate of 2,500 MW − 1,000 MW = 1,500 MW. At a coal-burning plant, perhaps 200 MW of heat will be lost in and around the boilers and the rest of the plant, leaving 1,300 MW to be removed. Nuclear power plants usually run at lower maximum temperatures, so they have lower efficiencies and correspondingly produce more waste heat; in addition, less heat is lost around the plants themselves, so more of the waste heat needs to be removed. Common methods of disposing of this heat include dumping it into rivers or lakes and or using it to evaporate water in cooling towers. Such disposal methods can have adverse effects on aquatic ecosystems or generate fog and ice.

There are three major methods of removing waste heat. The least expensive is known as once-through cooling, in which water from a stream, lake, or other body of water is used to cool the steam, after which the water is returned to its source at a higher temperature. This method may result in temperature increases of several degrees in the body of water.

A second method is the use of artificial lakes or cooling ponds, which may be up to several square kilometers in size. The heated water from the power plant is discharged into one end of a pond, while water to be used for cooling is drawn from the bottom of the pond at the other end. The water in the pond cools naturally by evaporation; therefore, the pond’s water source must be continuously replenished.

A third method is the use of cooling towers, either evaporative or nonevaporative. Evaporative towers, as their name suggests, cool water from the power plant by promoting evaporation. Some evaporative towers produce natural drafts, while others use fans to induce drafts mechanically. Natural draft towers may be more than 100 meters (328 feet) high, while mechanical draft towers are often much smaller. Nonevaporative cooling towers allow moving air to cool pipes containing the heated water from the power plants; these kinds of towers are less commonly used than evaporative towers because they are expensive to build and operate.

Since most cooling methods ultimately lead to the evaporation of substantial amounts of water, large power plants are usually located adjacent to rivers, which provide a source of water. The major ecological effects of thermal occur in natural rivers and lakes and involve fish and other aquatic organisms. These organisms typically thrive when the temperature remains within a narrow range and may die if the water changes to lower or higher temperatures. For example, if a of largemouth bass that are acclimated to a water temperature of 20 degrees Celsius (68 degrees Fahrenheit) are exposed to temperatures as low as 4.4 degrees Celsius (40 degrees Fahrenheit) or as high as 32 degrees Celsius (90 degrees Fahrenheit) for one or two days, about 50 percent will die. In addition, the sudden changes in temperature that are encountered by fish swimming into thermal effluent can produce thermal shock and almost instantaneous death if the changes are sufficiently large.

All chemical reactions are increased by heat, so can lead to more rapid physiological processes in fish and other aquatic organisms. In certain circumstances this can cause increased growth rates and shorter life spans, leading to decreased populations and less in the ecosystem; in other circumstances it may lead to increased populations and biomass, or it may extend the growing system. Ecologists have established that a temperature change of a few degrees can have significant effects, both short-term and long-term, on aquatic ecosystems.

In addition, evaporative cooling methods inject large amounts of water vapor into the atmosphere. During humid weather this may lead to fog, which can cause dangerous visibility issues on nearby roads; during cold weather it may lead to damage by icing roads, trees, and buildings.

Bibliography

Camp, William G., and Thomas B. Daugherty. “Water Pollution.” In Managing Our Natural Resources. 6th ed. Cengage, 2015.

Goudie, Andrew. “The Human Impact on the Waters.” In The Human Impact on the Natural Environment: Past, Present, and Future. 8th ed. Wiley-Blackwell, 2018.

Hill, Marquita K. “Water Pollution.” In Understanding Environmental Pollution. 4th ed. New York: Cambridge University Press, 2020.

Laws, Edward A. “Thermal Pollution and Power Plants.” In Aquatic Pollution: An Introductory Text. 4th ed. New York: John Wiley & Sons, 2017.

Madden, Allie. "What is Thermal Pollution?" Colorado Clean Energy Fund, 5 Mar. 2023, cocleanenergyfund.com/what-is-thermal-pollution/. Accessed 24 July 2024.