The combustion of fossil fuels always produces heat, sometimes as a primary, desired product, and sometimes as a secondary, less-desired by-product. For example, families burn coal , oil, natural gas , or some other fuel to heat their homes. In such cases, the production of heat is the object of burning a fuel. Heat is also produced when fossil fuels are burned to generate electricity. In this case, heat is a byproduct, not the main reason that fuels are burned.
Heat is produced in a number of other common processes. For example, electricity is also generated in nuclear power plants , where no combustion occurs. The decay of organic matter in landfills also releases heat to the atmosphere .
It is clear, therefore, that a vast array of human activities result in the release of heat to the environment . As those activities increase in number and extent, so does the amount of heat released. In many cases, heat added to the environment begins to cause problems for plants, humans, or other animals. This effect is then known as thermal pollution .
One example of thermal pollution is the development of urban heat islands. An urban heat island consists of a dome of warm air over an urban area caused by the release of heat in the region. Since more human activity occurs in an urban area than in the surrounding rural areas, the atmosphere over the urban area becomes warmer than it is over the rural areas.
It is not uncommon for urban heat islands to produce measurable climate changes. For example, the levels of pollutants trapped in an urban heat island can reach 5–25% greater than the levels over rural areas. Fog and clouds may reach twice the level of comparable rural areas, wind speeds may be reduced by up to 30%, and temperatures may be 0.9–3.6°F (0.5°–2°C) higher than in surrounding rural areas. Such differences may cause both personal discomfort and, in some cases, actual health problems for those living within an urban heat island.
The term thermal pollution has traditionally been used more often to refer to the heating of lakes, rivers, streams, and other bodies of water, usually by electric power-generating plants or by factories. For example, a one-megawatt nuclear power plant may require 1.3 billion gal (4.9 billion L) of cooling water each day. The water used in such a plant has its temperature increased by about 30.6°F (17°C) during the cooling process. For this reason, such plants are usually built very close to an abundant water supply such as a lake, a large river, or the ocean.
During its operation, the plant takes in cool water from its source, uses it to cool its operations, and then returns the water to its original source. The water is usually recycled through large cooling towers before being returned to the source, but its temperature is still likely to be significantly higher than it was originally. In many cases, an increase of only a degree or two may be "significantly higher" for organisms living in the water.
When heated water is released from a plant or factory, it does not readily mix with the cooler water around it. Instead, it forms a stream-like mass known as a thermal plume that spreads out from the outflow pipes. It is in this thermal plume that the most severe effects of thermal pollution are likely to occur. Only over an extended period of time does the plume gradually mix with surrounding water, producing a mass of homogenous temperature.
Heating the water in a lake or river can have both beneficial and harmful effects. Every species of plant and animal has a certain range that is best for its survival. Raising the temperature of water may cause the death of some organisms, but may improve the environment of other species. Pike, perch, walleye, and small mouth bass, for example, survive best in water with a temperature of about 84°F (29°C), while catfish, gar, shad, and other types of bass prefer water that is about 10°F (5.5°C) warmer.
Spawning and egg development are also very sensitive to water temperature. Lake trout, walleye, Atlantic salmon , and northern pike require relatively low temperatures (about 48°F/9°C), while the eggs of perch and small mouth bass require a much higher temperature, around 68°F (20°C).
Clearly, changes in water temperature produced by a nuclear power plant, for example, is likely to change the mix of organisms in a waterway.
Of course, large increases in temperature can have disastrous effects on an aquatic environment. Few organisms could survive an accident in which large amounts of very warm water were suddenly dumped into a lake or river. This effect can be observed especially when a power plant first begins operation, when it shuts down for repairs, and when it restarts once more. In each case, a sudden change in water temperature can cause the death of many individuals and lead to a change in the make-up of an aquatic community. Sudden temperature changes of this kind produce an effect known as thermal shock. To avoid the worst effects of thermal shock, power plants often close down or re-start slowly, reducing or increasing temperature in a waterway gradually rather than all at once.
One inevitable result of thermal pollution is a reduction in the amount of dissolved oxygen in water. The amount of any gas that can be dissolved in water varies inversely with the temperature. As water is warmed, therefore, it is capable of dissolving less and less oxygen. Organisms that need oxygen to survive will, in such cases, be less able to survive.
Water temperatures can have other, less-expected effects as well. As an example, trout can swim less rapidly in water above 66°F (19°C), making them less efficient predators. Organisms may also become more subject to disease in warmer water. The bacterium Chondrococcus columnaris is harmless to fish at temperatures of less then 50°F (10°C). Between temperatures of 50°–70°F (10°–21°C), however, it is able to invade through wounds in a fish's body. It can even attack healthy tissue at temperatures above 70°F (21°C).
The loss of a single aquatic species or the change in the structure of an aquatic community can have far-reaching effects. Each organism is part of a food chain/web . Its loss may mean the loss of other organisms farther up the web who depend on it as a source of food.
The water heated by thermal pollution has a number of potentially useful applications. For example, it may be possible to establish aquatic farms where commercially desirable fish and shellfish can be raised. The Japanese have been especially successful in pursuing this option. Some experts have also suggested using this water to heat buildings, remove snow, fill swimming pools, use for irrigation , de-ice canals, and operate industrial processes that have modest heat requirements.
The fundamental problem with most of these suggestions is that waste heat has to be used where it is produced. It might not be practical to build a factory close to a nuclear power plant solely for the purpose of using heat generated by the plant. As a result, few of the suggested uses for waste heat have actually been acted upon.
There are no easy solutions to the problems of thermal pollution. To the extent that industries and utilities use less energy or begin to use it more efficiently, a reduction in thermal pollution should result as a fringe benefit.
The other option most often suggested is to do a better job of cooling water before it is returned to a river, lake, or the ocean. This goal could be accomplished by enlarging the cooling towers that most plants already have. However, those towers might have to be as tall as a 30-story building, in which case they would create new problems. In cold weather, for example, the water vapor released from such towers could condense to produce fog, creating driving hazards over an extended area.
Another approach is to divert cooling water to large artificial ponds where it can remain until its temperature has dropped sufficiently. Such cooling ponds are in use in some locations, but are not very attractive alternatives because they require so much space. A one-megawatt plant, for example, would require a cooling pond with 1,000-2,000 acres (405-810 ha) of surface area. In many areas, the cost of using land for this purpose would be too great to justify the procedure.
Some people have also used the term thermal pollution to describe changes in the earth's climate that may result from human activities. The large quantities of fossil fuels burned each year release a correspondingly large amount of carbon dioxide to the earth's atmosphere. This carbon dioxide, in turn, may increase the amount of heat trapped in the atmosphere through the greenhouse effect . One possible result of this change could be a gradual increase in the earth's annual average temperature. A warmer climate might, in turn, have possibly far-reaching and largely unknown effects on agriculture, rainfall, sea levels, and other phenomena around the world.
[David E. Newton ]
Harrison, R. M., ed. Pollution: Causes, Effects, and Control. Cambridge: Royal Society of Chemistry, 1990.
Langford, T. E., ed. Ecological Effects of Thermal Discharges. New York: Elsevier Science, 1990.
Hudson, J., and J. B. Cravens. "Thermal Effects." Water Environment Research 64 (June 1992): 570–81.
The broadest definition of thermal pollution is the degradation of water quality by any process that changes ambient water temperature. Thermal pollution is usually associated with increases of water temperatures in a stream, lake, or ocean due to the discharge of heated water from industrial processes, such as the generation of electricity. Increases in ambient water temperature also occur in streams where shading vegetation along the banks is removed or where sediments have made the water more turbid . Both of these effects allow more energy from the sun to be absorbed by the water and thereby increase its temperature. There are also situations in which the effects of colder-than-normal water temperatures may be observed. For example, the discharge of cold bottom water from deep-water reservoirs behind large dams has changed the downstream biological communities in systems such as the Colorado River.
The production of energy from a fuel source can be direct, such as the burning of wood in a fireplace to create heat, or by the conversion of heat energy into mechanical energy by the use of a heat engine. Examples of heat engines include steam engines, turbines , and internal combustion engines. Heat engines work on the principal of heating and pressuring a fluid, the performance of mechanical work, and the rejection of unused or waste heat to a sink . Heat engines can only convert 30 to 40 percent of the available input energy in the fuel source into mechanical energy, and the highest efficiencies are obtained when the input temperature is as high as possible and the sink temperature is as low as possible. Water is a very efficient and economical sink for heat engines and it is commonly used in electrical generating stations.
The waste heat from electrical generating stations is transferred to cooling water obtained from local water bodies such as a river, lake, or ocean. Large amounts of water are used to keep the sink temperature as low as possible to maintain a high thermal efficiency. The San Onofre Nuclear Generating Station between Los Angeles and San Diego, California, for example, has two main reactors that have a total operating capacity of 2,200 megawatts (MW). These reactors circulate a total of 2,400 million gallons per day (MGD) of ocean water at a flow rate of 830,000 gallons per minute for each unit. The cooling water enters the station from two intake structures located 3,000 feet offshore in water 32 feet deep. The water is heated to approximately 19°F above ambient as it flows through the condensers and is discharged back into the ocean through a series of diffuser -type discharges that have a series of sixty-three exit pipes spread over a distance of 2,450 feet. The discharge water is rapidly mixed with ambient seawater by the diffusers and the average rise in temperature after mixing is less than 2°F.
The primary effects of thermal pollution are direct thermal shock , changes in dissolved oxygen, and the redistribution of organisms in the local community. Because water can absorb thermal energy with only small changes in temperature, most aquatic organisms have developed enzyme systems that operate in only narrow ranges of temperature. These stenothermic organisms can be killed by sudden temperature changes that are beyond the tolerance limits of their metabolic systems. The cooling water discharges of power plants are designed to minimize heat effects on local fish communities. However, periodic heat treatments used to keep the cooling system clear of fouling organisms that clog the intake pipes can cause fish mortality. A heat treatment reverses the flow and increases the temperature of the discharge to kill the mussels and other fouling organisms in the intake pipes. Southern California Edison had developed a "fish-chase" procedure in which the water temperature of the heat treatment is increased gradually, instead of rapidly, to drive fish away from the intake pipes before the temperature reaches lethal levels. The fish chase procedure has significantly reduced fish kills related to heat treatments.
Small chronic changes in temperature can also adversely affect the reproductive systems of these organisms and also make them more susceptible to disease. Cold water contains more oxygen than hot water so increases in temperature also decrease the oxygen-carrying capacity of water. In addition, raising the water temperature increases the decomposition rate of organic matter in water, which also depletes dissolved oxygen. These decreases in the oxygen content of the water occur at the same time that the metabolic rates of the aquatic organisms, which are dependent on a sufficient oxygen supply, are rising because of the increasing temperature.
The composition and diversity of communities in the vicinity of cooling water discharges from power plants can be adversely affected by the direct mortality of organisms or movement of organisms away from unfavorable temperature or oxygen environments. A nuclear power-generating station on Nanwan Bay in Taiwan caused bleaching of corals in the vicinity of the discharge channel when the plant first began operation in 1988. Studies of the coral Acropora grandis in 1988 showed that the coral was bleached within two days of exposure to temperatures of 91.4°F. In 1990 samples of coral taken from the same area did not start bleaching until six days after exposure to the same temperature. It appears that the thermotolerance of these corals was enhanced by the production of heat-shock proteins that help to protect many organisms from potentially damaging changes in temperature. The populations of some species can also be enhanced by the presence of cooling water discharges. The only large population of sea turtles in California, for example, is found in the southern portion of San Diego Bay near the discharge of an electrical generating station.
The dilution of cooling water discharges can be effectively accomplished by various types of diffuser systems in large bodies of water such as lakes or the ocean. The only thermal effects seen at the San Onofre nuclear generating station are the direct mortality of planktonic organisms during the twenty-five-minute transit through the cooling water system. The effectiveness of the dilution systems can be monitored by thermal infrared imaging using either satellite or airborne imaging systems. The use of cooling towers has been effective for generating stations located on smaller rivers and streams that do not have the capacity to absorb the waste heat from the cooling water effluent . The cooling towers operate by means of a recirculating cascade of water inside a tower, with a large column of upwardly rising air that carries the heat to the atmosphere through evaporative cooling. Cooling towers have been used extensively at nuclear generating stations in both the United States and France. The disadvantages of cooling towers are the potential for local changes in meteorological conditions due to large amounts of warm air entering the atmosphere and the visual impact of the large towers.
see also Electric Power; Energy; Fish Kills; Visual Pollution; Water Pollution
brown, richard d.; ouellette, robert p.; and chermisinoff, paul n. (1983). pollution control at electric power stations: comparisons for u.s. and europe. boston: butterworth-heinemann.
henry, j. glenn, and heinke, gary w. (1996). environmental science and engineering. upper saddle river, nj: prentice-hall.
hinrichs, roger a., and kleinbach, merlin. (2001). energy: its use and the environment, 3rd edition. monterey, ca: brooks/cole publishing company.
langford, terry e. (1990). ecological effects of thermal discharges. new york: elsevier applied science.
larminie, james, and dicks, andrew. (2000). fuel cell systems explained. new york: john wiley & sons.
liu, paul ih-fei. (1997). introduction to energy and the environment. new york: john wiley & sons.
ristinen, robert a., and kraushaar, jack j. (1998). energy and the environment. new york: john wiley & sons.
slovic, paul. (2000). the perception of risk. london: earthscan publications ltd.
california energy commission. "energy-related environmental research." available from http://www.energy.ca.gov/pier/energy/energy_aquatic.html.
Thermal pollution from power plants in Florida turned out to be a lifesaver for the state's threatened manatee population. The ecology changed when irrigation wells and diversion channels that support Florida's agricultural development severely impacted the natural springs that moderate river-water temperatures. Manatees cannot survive in cold water and naturalists feared that irregular cold snaps would put the sea mammals at risk. Manatees, however. discovered the power-plant discharge zones and today, naturalists take advantage of cold weather to tally manatee population as the herds gather at local power plants.