What is thermal pollution?

What is thermal pollution

Among the environmental effects of energy, in addition to toxic and harmful chemical pollutants, atmospheric greenhouse effect, radioactive substances, etc., thermal pollution is also a kind of ecological environmental pollution in the process of energy utilization. Thermal pollution refers to environmental pollution caused by waste heat emitted by humans in various production and living activities that use energy extensively. Waste heat can pollute the atmosphere and water environment.

(1) Urban heat island effect
The so-called “urban heat island effect” refers to the phenomenon that due to the high population density, industrial concentration and consumption of large amounts of fuel in urban areas, all energy will eventually be converted into heat and enter the atmosphere, making the urban temperature significantly higher than the suburban temperature. The phenomenon of urban heat island was first discovered in London, England in the early 18th century. The urban heat island effect caused by this kind of atmospheric thermal pollution is one of the most typical characteristics of the impact of human activities on the climate of urban areas.

The heat island climate makes some cities that were originally very hot in summer even hotter. Excessive urban temperatures in summer can induce coronary heart disease, high blood pressure, stroke, etc., which directly damage human health. When there is a heat wave, the overall mortality rate tends to increase. For example, the heat wave in Chicago in 1995 and the extreme heat in France in the summer of 2003 caused many deaths in a short period of time. In the summer of 1980, the heat wave hit St. Louis and Kansas City in the United States. The death rates in the business districts of the two cities increased by 57% and 64%, respectively, while the nearby suburbs only increased by 10%. The urban heat island effect will naturally contribute to this kind of heat wave attack.

The urban heat island effect will also lead to the increase of clouds, fog and rainfall over the city. This is the so-called “rain island effect” and “fog island effect” brought about by the heat island effect. Fog in the city will seriously hinder water and land transportation and the take-off and landing of aircraft.

(2) Thermal pollution of water bodies
The large amount of wastewater discharged from thermal power plants, nuclear power plants, coal mines, metallurgy, petroleum, chemical industries and other industrial sectors that use rivers and lakes as cold sources will have a high temperature. Entering natural waters such as rivers and lakes will cause local water temperatures in these receiving water bodies. Rise, forming thermal pollution of the water body.

Thermal power plants and nuclear power plants are the main sources of thermal pollution in water bodies. Since all thermodynamic machines cannot directly use heat energy, they will have a thermal impact on the environment. For example, the principle of nuclear reactor power generation is to use the heat released by the fission reaction to generate water vapor, and then use the water vapor to drive a turbine connected to an alternator. The thermodynamic efficiency of this kind of system operation depends on the temperature difference between the heat source and the cold source. In France, the efficiency of using this type of nuclear reactor to generate electricity is 33%. This means that 2 GW of heat energy is wasted for every 1 GW of electricity produced. Generally, a 1000MW thermal power plant will emit 4,6TJ of heat per hour into natural waters. Power plants that use fossil fuels, biomass or waste have the same environmental problems.

The Breuer nuclear power plant at the mouth of the Giron River in France is equipped with four 900MW units, which produce 225m3 of warm water per second, which causes the water temperature within a few kilometers of the Gurón River mouth to rise by 5°C. The cooling water consumption of all enterprises in the United States is as high as 450 million cubic meters per day, and 80% of this is cooling water used by power plants.

Thermal pollution will increase the temperature of the receiving water body, affect the survival of aquatic organisms, destroy the ecological balance of natural waters, and endanger fishery production. As the water temperature rises, the dissolved oxygen content in the water will decrease and it will be in an oxygen-deficient state. At the same time, the increase of water temperature will cause the algae to multiply in the water, accelerate the eutrophication process of the water body, and will also increase the metabolic rate of all aquatic organisms and require more dissolved oxygen. As a result, the growth of fish and other plankton in the water will be affected. Increased water temperature is more harmful to fish that grow in lower water temperatures, and their spawning and hatching are affected, which leads to a decrease in reproduction rate.

In addition, the warming of the water body can easily breed some pathogenic microorganisms, causing the spread and epidemic of diseases. Thermal pollution of water bodies contributes to the occurrence of epidemic hemorrhagic fever, typhoid fever, malaria, dengue fever and other diseases.