Thermal Pollution Control

Summary

Thermal pollution is heated water discharged into lakes and rivers. By raising the ambient water temperature, aquatic plant and wildlife are often threatened. The industry that dumps the greatest amount of heat into lakes and rivers is thermoelectric power, which consists of coal, oil, and natural gas-combusting plants, and nuclear reactors. Because of the laws of thermodynamics, this waste heat cannot be eliminated, but it can be kept from waterways by means of cooling towers or cooling ponds or used in cogeneration industrial parks.

Definition and Basic Principles

Thermal pollution is any increase of water temperature due to the discharge of warm or hot water into lakes or rivers. Most thermal pollution results from thermoelectric power plants using local water supplies to transfer heat from a condenser to the environment. This raises the ambient water temperature and can adversely impact aquatic life.

No machine is completely efficient, but heat engines, constrained by the second law of thermodynamics (which states that since not all energy within a heat engine can be transformed into useful mechanical energy, there will always be waste heat), are particularly inefficient. Since efficiency depends on the temperature difference between the input and output heat reservoirs, one would like the input temperature to be as high as possible and the output temperature to be as low as possible. The maximum temperature is constrained by the highest temperature and steam pressure the boiler can withstand, and the environment determines the output temperature. Typically, a fossil fuel plant operates with an input temperature of about 1,000 degrees Fahrenheit (537.78 degrees Celcius), the practical maximum. The output temperature is typically 100 degrees Fahrenheit (37.78 degrees Celcius), giving a theoretical efficiency of about 60 percent. In practice, this is impossible for several reasons. It is difficult to maintain the steam at 1,000 degrees Fahrenheit (537.78 degrees Celcius), turbines are less than 90 percent efficient, and the conversion of stored energy (such as coal) into heat is only about 88 percent efficient. Consequently, the actual efficiency of a fossil-fueled power plant is typically about 40 percent. Nuclear plants (operating at lower temperatures) are only 33 percent efficient. Two-thirds of the energy consumed is released as waste heat.

Background and History

Although US federal legislation governing waterways began in the nineteenth century, it was not until 1948 that the first law governing water quality, the federal Water Pollution Control Act, was enacted. Enforcement provisions were strengthened in 1956, expanded in 1965, and restructured under the Environmental Protection Agency (EPA) in 1972. With increased awareness of environmental issues, the Clean Water Act (CWA), was passed in 1972, with major amendments in 1977. This legislation enforced minimizing environmental impacts to water caused by contaminants. The EPA standards ensure that the best available technology is used to design cooling water structures and that large thermoelectric power plants upgrade existing facilities to be in compliance. Since this affects thousands of fossil-fuel plants as well as more than sixty nuclear power plants, the requirements are continuously being challenged by the electric power industry as being too stringent. Retrofitting or adding cooling towers to existing power plants incurs high costs.

How It Works

The Problem. Thermoelectric power plants (coal, oil, natural gas, and nuclear) use heat from combustion or a nuclear reactor to transform water into steam in a boiler. The steam impinges on the blades of a large turbine and then is converted back to liquid by a condenser on the output side of the turbine. Because of the huge pressure drop when the steam condenses, considerable force is exerted on the turbine blades, causing them to rotate. The rotational energy powers a generator that transforms the mechanical energy into electrical energy. The condenser, a large heat exchanger, removes heat by transferring it to the environment. The condensed steam is then pumped back into the boiler, where it is again vaporized. The heat removed from the condenser and dumped into the environment is termed "thermal pollution." The least expensive means of heat disposal is to draw water from a river or lake; by passing this water through the condenser, the heat required to condense the steam back to liquid is transferred to the water, raising its temperature by eight degrees to twenty-five degrees Fahrenheit (-3.78 degrees Celcius).

A 1,000-megawatt nuclear power plant operating at thirty-three percent efficiency generates 10.3 billion British thermal units (Btus) of heat energy every hour, of which 6.9 billion Btus are discharged as waste heat. Limiting the coolant temperature increase to twenty degrees Fahrenheit (-6.67 degrees Celcius) requires 700,000 gallons (2,649,788.25 liters) per minute of water to flow through the condenser. This is equivalent to a stream seventy-six feet wide and ten feet deep flowing at two feet per second. In contrast, a 1,000-megawatt coal-fired plant operating at forty percent requires 5.1 billion Btus to be discharged every hour, considerably less than the nuclear plant.

Environmental Effects. Although the temperature is not raised substantially, dumping waste heat into rivers or lakes can profoundly affect aquatic life. Every fish species has a lethal water temperature, which if encountered will kill the fish within an hour. Salmon, for example, prefer water with a temperature in the sixties; their lethal temperature is seventy-seven degrees Fahrenheit (25 degrees Celcius). Even a modest water temperature increase of fifteen degrees Fahrenheit (-9.44 degrees Celcius) could render a river uninhabitable for salmon. Also, an increased temperature in a river or near a lake shore could disrupt a species' spawning grounds. A power plant on the Columbia River in Washington State once raised the water temperature in its surroundings to such a degree that salmon could not swim upstream to lay their eggs. Additionally, as water temperature increases, the dissolved oxygen content is modified, which adversely affects various aquatic species. Furthermore, when a power plant goes off-line during winter for repairs, the sudden drop in water temperature has a biologically detrimental effect on fish. Finally, there are subtle effects such as a higher water temperature causing a small minnow to disappear, which deprives the larger species of fish that was relying on that minnow as a food source.

The extent to which a power plant affects the aquatic environment depends on how much water is used for cooling. For relatively narrow and shallow rivers, the entire flow of the river may be heated to a higher temperature, requiring a considerable downstream distance to be dissipated. Even a relatively modest increase in water temperature may ruin a river or lake by destroying natural habitats, particularly for fish that prefer cooler water. Higher water temperatures also can lower the dissolved oxygen content to the point that fish cannot experience respiration. Reduced oxygen content also encourages the unrestrained growth of aquatic plants and algae, which clogs the water and further lowers the oxygen content. Finally, warmer water may concentrate the lethal effect of any toxic chemicals present.

Traditional Solutions. Given the increasing demand for electricity requiring more power plants, most of the water used in the United States passes through a power plant and is heated by about 20 degrees Fahrenheit (-6.67 degrees Celcius), placing a severe demand on the aquatic environment. In 2017, water use by thermoelectric power plants reached around 52.8 trillion gallons (22 trillion liters) in the US. Although it is cost-effective to use a local body of water to dispose of waste heat, it is generally not an environmentally sound practice. Two other methods of disposing of the heat are cooling towers, which transfer heat directly to the atmosphere, and cooling ponds, artificial lakes that absorb the heat to create warm bodies of water at about 100 degrees Fahrenheit (37.78 degrees Celcius). The cooling pond slowly releases its heat into the atmosphere. There are advantages and disadvantages to each of these systems.

Applications and Products

Cooling Towers. There are two main types of cooling towers, wet towers and dry towers. Wet towers cool by evaporation and require constant water replacement. Warm water from the condenser is sprayed over baffles, and then unevaporated water drips to the foundation where it is collected and recirculated. Evaporation requires about 3 percent of the water to be replaced continuously.

Two different types of dry-cooling methods are possible: direct dry cooling using an air-cooled condenser or a water-cooled condenser, which transfers its heat to the atmosphere through finned tubes in the tower. Direct dry cooling functions like an automobile radiator: Air flowing past the condenser tubes absorbs heat by conduction and radiation. The generating plant uses less of the water required for a wet-cooled system, but the enormous fans employed consume up to 1.5 percent of the station's electrical output. Although the fans allow greater control over cooling, these systems are much less efficient than circulating water systems, thus a larger and more complex cooling plant is required. Closed water loop systems have no evaporative loss, but such systems are seldom employed because of their high initial price, increased operating costs, and reduced efficiency. Hybrid wet and dry towers are also used; a dry section is positioned above the wet section. This reduces the dry tower cost, lessens the evaporative water loss of a wet tower, and moderates the inefficiency of dry cooling.

Cooling towers have both beneficial and detrimental aspects. On the positive side, towers do not affect fish or terrestrial animal life, and they occupy a relatively small land area compared with cooling ponds. On the negative side, these structures are huge and costly to construct. Typically, power plants require more than one cooling tower, which increases the cost even more. The water lost to the atmosphere creates higher humidity, increasing fog, and winter icing. Economic considerations have established wet cooling towers as the preferred method of condenser cooling for thermoelectric plants in the United States constructed since 1970. By 2014, more than half of US thermoelectric plants were using recirculating wet cooling towers, as compared to about 3 percent using dry coolers. Hybrid and recirculating systems became increasingly common after 2000.

Cooling Ponds. Cooling ponds, because of their warm temperature, can be stocked with fish having a high lethal temperature, such as catfish, which can be harvested. Additional positive aspects of these ponds compared with cooling towers are that evaporative losses are significantly reduced, ponds are less costly to construct, and the environmental impact is considerably diminished. The unfortunate aspect of these ponds is that they require a substantial land area. Cooling ponds typically consumer between 0.2 and 2.7 cubic meters of water per megawatt-hour. The warm water could also cause more regional fog. Cooling ponds are most often used in the Southwest, where land is available and humidity is low. Sometimes the water is mechanically sprayed into the air to encourage evaporative cooling, which reduces the pond size.

Repurposing of Waste Heat. Although waste heat can be disposed of through cooling towers or cooling ponds, the heat ultimately still enters the environment. Rather than merely dumping the heat, several methods may be employed to use the heat in more environmentally benign manners. Waste heat can be used for desalination plants, which evaporate saltwater to obtain distilled water. Heated water could also be used to irrigate crops, helping to extend growing seasons. Waste-treatment plants could use the heated water to evaporate water from sewage more rapidly, and certain rivers, such as the St. Lawrence, could be kept ice-free all winter from the heat emanating from a series of riverside power plants. Finally, if the power plant were associated with an industrial park, the waste heat could be piped to nearby buildings as a source of winter heat—a process termed cogeneration.

Careers and Course Work

Controlling thermal pollution from electric utility plants while containing costs will require new, more efficient designs for cooling towers as well as new technologies to alleviate the problem by more cost-effective means. Those interested in working in these areas will need a technical background, best achieved by a college major in physics or engineering. If one is more interested in the environmental effects of thermal pollution, the obvious college major would be biology or environmental studies. The EPA also needs people with legal training, such as lawyers and paralegals, because of the complicated legislation that needs to be interpreted and applied to industries that pollute the aquatic environment.

There is an increasing need to provide cooling for thermoelectric power plants without compromising the environment. Creative people with technological training will be necessary to fulfill future demands to develop new water-treatment technologies for purifying the cooling water needed for thermoelectric plants from contaminated sources such as reclaimed wastewater, agricultural runoff, salty groundwater, mine water, or storm water.

Social Context and Future Prospects

Coal-fired power plants, requiring more than 3 million tons of coal per year for each 1,000 megawatts of generating capacity, must be located relatively close to a fuel supply, typically inland. (However, coal-fired power plants are gradually being replaced by those powered by natural gas because of the latter's high water efficiency and other environmental benefits.) Since nuclear power plants are not subject to this constraint, they can be situated in coastal regions and employ once-through seawater cooling. Coastal nuclear power plants also have the advantage of being able to use the waste heat for desalination. Countries in North Africa and the Middle East already employ waste heat from fossil fuel plants for this purpose.

A 2008 DOE/NETL study estimating future power plant water consumption based on a projected increase of generated power and anticipated technological changes indicates that future nuclear plants would have higher thermal efficiency, thus requiring less water. Although future coal plants would also employ new technologies to reduce water usage, the mandated removal of sulfur compounds (which create sulfuric acid) would increase water consumption. If greenhouse-gas emissions were also to be limited, fifty to ninety percent more water would be required, making fossil fuel plants considerably more water intensive than nuclear plants.

Since oceans are an immense heat sink, it has been proposed that power plants be built several miles offshore where they would scarcely be visible, and detrimental biological effects should be minimal. Offshore plants would also allow power to be generated in the vicinity of large coastal energy users, thus minimizing transmission line losses.

As urbanization and population growth continues to increase demand, and as water scarcity is expected to worsen from climate disruption, finding better ways to conserve water and prevent thermal pollution will become paramount to the thermoelectric generation industry. Some innovative water-saving, energy-efficient technologies under investigation include dew-point cooling, thermosyphon coolers, and nanoparticles. Additional areas of interest include recycling steam, recapturing water from cooling tower plumes, conversion to degraded water, and lowering costs while maintaining energy efficiency for retrofits of existing power plants. Moreover, interest has increased in a type of nuclear power plant with a water-free cooling system, the molten salt reactor (MSR). This technology eliminates the use of water in nuclear reactors by replacing it with molten salt, thereby preventing thermal pollution.

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