A cooling pond is a shallow reservoir having a large surface area for removing heat from water. The surface area exposed to the air may be increased through the use of spray nozzles. Cooling ponds are used where land is relatively inexpensive, cooling water is scarce or expensive, or where there are strict thermal loading restrictions in place. If cooling ponds are used, water in the pond can be reused, thus reducing the overall water-withdrawal requirement. A cooling tower is designed to remove heat by pumping water up into the tower and allowing it to fall down inside the tower.
Air comes in from the sides of the tower and passes by the falling water.
As the air passes the water, it exchanges some of the heat and evaporates some of the water. This heat and evaporated water flow out the top of the tower is in the form of a fine cloud-like mist. The cooled water is collected at the bottom of the tower and pumped back into the plant for reuse. Although there are many similarities between them, there are some important differences that affect how each plant uses water. In fossil fuel plants, coal, natural gas, or oil are burned to provide the heat necessary to turn the water into steam.