Thermal pollution refers to the degradation of water quality caused by any process that changes its ambient temperature. Thermal power plants are a significant source of this pollution. They often use water as a coolant and then discharge it back into the environment at a higher temperature, disrupting aquatic ecosystems.
How Power Plants Generate Heat
Thermal power plants convert heat energy into electrical energy by burning fossil fuels or through nuclear fission. This process generates intense heat, boiling water to produce high-pressure steam that drives turbines. These turbines are connected to electrical generators, converting rotational motion into electricity.
The conversion of heat into electricity is inherently inefficient, with a substantial amount of energy lost as waste heat. Even efficient plants, like combined cycle natural gas plants, might only achieve efficiencies exceeding 60%, while coal-based plants typically range from 35% to 40% efficiency. This waste heat must be removed from the system to maintain operational integrity and efficiency.
The Role of Cooling in Power Plants
After the steam drives the turbines, it exits at a lower pressure but still retains considerable heat. For continuous and efficient operation, this exhausted steam must be condensed back into liquid water. Condensing the steam creates a low-pressure zone, maximizing energy extraction and increasing the plant’s overall thermal efficiency.
This conversion also allows the water to be reused in the boiler, forming a closed-loop system that conserves water and maintains water chemistry. The condensation process requires a cooling medium to absorb heat from the steam. Large volumes of water circulate through condensers, absorbing heat and causing the steam to change back into liquid. Without effective cooling, the steam would not condense efficiently, reducing turbine performance and plant productivity.
Methods of Heat Transfer to Water Bodies
The absorbed heat from the steam must ultimately be dissipated into the environment. Power plants employ various cooling methods, each with distinct mechanisms for transferring this heat to natural water bodies.
Once-through cooling systems
Once-through cooling systems draw large volumes of water from a nearby natural source, such as a river, lake, or ocean. This water passes through the plant’s condensers, absorbing heat. The now-heated water, which can be approximately 10°C warmer than the intake, is then discharged directly back into the same natural source. This direct discharge of elevated temperature water is a primary contributor to thermal pollution, as it can significantly raise the ambient temperature of the receiving water body.
Cooling towers
Cooling towers transfer most waste heat to the atmosphere rather than directly to water bodies. In these systems, water heated by the condensers is sent to large towers where it is cooled primarily through evaporation. As a portion of the water evaporates, it carries heat away, reducing the temperature of the remaining water, which is recirculated back to the condensers. While cooling towers reduce direct thermal discharge, they require a continuous supply of “makeup” water to replace what is lost through evaporation. Additionally, cooling towers produce “blowdown,” a concentrated stream of water containing dissolved solids and chemicals that must be periodically purged and can be discharged into water bodies, potentially contributing to chemical or thermal changes.
Cooling ponds or artificial lakes
Cooling ponds or artificial lakes are another method used to dissipate heat. These are man-made or modified natural water bodies where heated water from the power plant is held. Heat dissipates from the surface through natural processes such as evaporation, convection, and radiation. After cooling, the water can be reused by the plant or discharged into a natural water source. While less direct than once-through systems, these ponds can still raise the temperature of connected natural water bodies or affect local groundwater if not properly managed.