Cooling towers: what are they and how do they work?

If you’ve ever had a window seat flying out of or into Raleigh, N.C., on a clear day, most likely you spotted in the distance, a tower with what appears to be smoke coming from it, but do you really know what it is?

It’s Harris Nuclear Plant’s natural draft cooling tower and even though it appears small in stature from the sky, the 523-foot-tall structure is almost as tall was the Washington Monument, which stands at 555 feet.

The cloud leaving the top of the Harris cooling tower is often visible from miles away too. However, it’s not smoke billowing from the top. It’s simply clean water vapor that results from the cooling process.

Cooling tower at Harris Nuclear Plant in New Hill, North Carolina

So, what purpose does the cooling tower serve? It depends. Thermoelectric power plants, like Harris, that use heat to make steam to spin a turbine generator need a system to cool steam back to water. They also provide an energy efficient and environmentally friendly way to remove heat before the water is returned to the lake.

With a surface area of 4,100 acres, Harris Lake was constructed specifically to provide cooling water for the Harris plant. Water is pumped from the lake into the cooling tower basin. This water then circulates through the plant’s condenser tubes, absorbs heat from the steam and travels back to the cooling tower. The water is then sprayed through the hollow core of the cooling tower onto a grid in the center of the tower. Cool air flows up from the hollow center, and passes the warm falling water, causing some of it to evaporate. Evaporation removes heat from the water, cooling it. Water in the cooling tower never comes in contact with water in the nuclear reactor, which is a closed system.

To complete the cycle, a pipe returns water to Harris Lake within a degree or two of the lake’s normal temperatures. This ensures there is no harm to the aquatic life in Harris Lake. The lake is popular for sport fishing, boating and water skiing.

Cooling tower at Catawba Nuclear Station in York, S.C.

Within the Duke Energy nuclear fleet, only two plants -- Harris and the Catawba Nuclear Station -- use cooling towers. McGuire Nuclear Station, Oconee Nuclear Station and Robinson Nuclear Plant all use large manmade lakes to cool steam back to water. Instead of a lake, Brunswick Nuclear Plant uses the Cape Fear River for cooling.

Because these bodies of water are used for other purposes, including drinking water, irrigation and industrial uses, Duke Energy established environmental protection programs decades ago (before the nuclear plants were built), including actively monitoring the water quality, plankton, fisheries and more throughout the year. The company submits annual reports to the state summarizing the important environmental monitoring results.

Cooling Tower Facts

• Cooling towers are constructed for plant cooling and to protect aquatic environments.
• The shape of most cooling towers is a hyperboloid. They are built this way because the broad base allows for greater area to encourage evaporation, then narrows to increase air flow velocity. It then widens slightly to aid in mixing the moisture laden air into the atmosphere.
• The cloud at the top of cooling tower is clean water vapor resulting from cooling water in a system that is totally separate from the nuclear reactor. The water in the reactor stays in a closed system, never coming into contact with the water in the cooling tower.
• There are more than 250 cooling towers for power plants across America, and fewer than 100 at nuclear plants.