Open cooling water systems in thermal power plants draw very large quantities of water from large water bodies like lakes or a sea. This is discharged back into these water bodies at a higher temperature. This can result in environmental issues and destruction of delicate ecosystems.
Circulating Water (CW) system is an important aspect of a thermal power plant that affects the investment decision, plant siting, and the performance of the plant.
Water is the main requirement for life on the planet. Any shortage, pollution, or change in its conditions affect the human, animal and plant life. Circulating Water requirement in a thermal power plant is a major water resource issue and has tremendous effect on the surrounding environment, population, and animal and aquatic life.
Problems are more severe in open cycle systems where the quantity of water requirement is very high. In closed cycle systems the effect is less severe since the raw water quantity requirement is only 5 % of the open cycle system.
Open cycle cooling system water sourced from fresh water like a river or lake has a direct effect by restricting the availability of water for human and agricultural consumption. Regulation in India and many other countries do not allow new power plants to be constructed with an open cycle system using fresh water from rivers or lakes.
In an open cycle systems the most important environmental effect is the discharge temperature. As the cooling water passes through the condenser, it picks up heat. The amount of temperature rise depends on the amount of water flow. Higher water flow results in lower temperature rise but higher investment and operation cost, due to larger capacity pumps.
- Regulations vary in different countries from 3 to 7 °C.
- Also regulations do not allow water to be discharged to estuaries and mangroves that are breeding place for aquatic life.
- A higher discharge temperature is detrimental to the aquatic life, especially the smaller species and fish eggs. This in turn has a snowball effect on the entire ecosystem.
Intake and Outfall Design.
Another main problem in open cycle systems is at the suction point.
- Tons of fish and other smaller species get sucked into the inlet of the cooling system to eventually be killed by impact on the screens, or by the chemical dosing. Chemical dozing in CW systems is done to remove the organic matter, otherwise it will form hindering growth on the pipes, condenser tubes and other equipment. This requires specially designed low velocity filters to avoid entrapment of aquatic life at the suction point.
The outfall or the discharge point also has to be designed properly so that flow patterns do not create problems for aquatic life and breeding grounds. Improperly designed outfalls can cause formation of sand banks, erosion of sea faces, and undue thermal gradients in the water.
The third problem is the chemicals that are added as biocides in the Circulating Water Systems.
- Seawater Open Cycle Systems use Sodium hypochlorite produced by electrolysis from the seawater itself. This is very cost effective since only the conversion cost is required. Higher amounts of this chemical will change the pH of the discharge water which is again detrimental to aquatic life.