ABSTRACT
In 1999, more than 60% of the utility power-generating capacity in the U.S. (382,270 MW) utilized the steam electric process[3]. At nuclear and fossilfuel power plants, electricity is produced by heating purified water to create high-pressure steam. The steam is expanded in turbines, which drive the generators that produce electricity. After leaving the turbines, the steam passes through a condenser with multiple tubes and a large surface area. A high volume of cool water circulates through the tubes, absorbing heat from the steam. As the steam cools and condenses, the temperature of the cooling water rises. Most power plants use either once-through cooling or closed-cycle cooling. Once-through cooling systems withdraw large volumes of water, typically in the range of tens of millions to billions of gallons per day, from a river, lake, estuary, or ocean. The water is pumped through the condenser and finally returned to the same or a nearby water body. Closed-cycle cooling systems recirculate cooling water to a cooling tower and basin, cooling pond, or cooling lake before returning it to the condenser. Because evaporation and planned cooling-tower blowdown remove cooling water from the evaporative system, regular additions of “makeup” cooling water are needed. At many plants, the makeup water is withdrawn from surface water bodies. Makeup volumes are much lower than daily once-through volumes and may range from hundreds of thousands to millions of gallons per day. The most commonly used type of closed-cycle cooling systems employs wet cooling towers, where water rejects heat to the atmosphere through evaporation and sensible heat transfer to the ambient air flowing through the tower. The air flow through the tower is maintained by fans (mechanical draft) or by convective currents created by the shape of the tower (natural draft). Some stakeholders have advocated the dry cooling tower method because it requires even less makeup water than a wet tower. Dry towers remove heat to the atmosphere only by sensible heat transfer. They do not rely on evaporation and, therefore, require little makeup water. Few dry towers have been installed in power-plant-sized applications (typically such units have a capacity of several hundred megawatts) to date because of cost and practical thermodynamic heat transfer limitations.
