Cold Air, Tall Plume

How cold air causes coal-fired power plants to operate at optimum efficiency. PLUS: An experiment you can try for yourself at home.

Have you ever noticed the cloud of water vapor rising from a chimney at a TVA coal-fired power plant, and how it climbs for miles in the winter and not in the summer? Did you know that the height of the cotton-looking plume reflects how efficiently the power plant is running? 

It’s all about physics — air density and humidity to be specific. There is fancy mathematical formula that explains why, but TVA’s PowerGeek makes it simple.

What Does Physics Have to Do with Efficiency?

Hot air rises, but it only through air that is cooler. Once rising hot air cools to ambient air temperature, the process stops. The colder it is outside, the longer it takes for warm exhaust air leaving a TVA chimney to cool — creating efficient natural draft.

Efficient combustion in a coal-burning furnace is a continuous balance between fuel and air flow. The more easily we can draw air into (and out of) the furnace, the more efficiently the power plant runs. This is because we don’t have to use additional energy to force air through the furnace.

As a result, during the winter, cold conditions cause the hot air to be shot from the furnace, rising quickly through the chimneystack. When you see the towering vapor cloud, it means our power plant is running at maximum efficiency.

Try It at Home

For this experiment, you will need a standard charcoal grill, a charcoal chimney, and a flat board or cookie sheet. Make sure to wear eye protection, long sleeves and thick leather gloves while conducting your experiment. You should always work with an adult.

Step One: Gather about a softball-sized mound of charcoal and light it. Make sure the damper on the bottom of the grill is fully open. This will allow air to flow through the grill like it does through a power plant’s furnace. Once the charcoal begins to turn gray, you are ready to begin the experiment.

Step Two: Make sure you have the grill rack in place above the flame, just as if you were grilling hotdogs or hamburgers.

Step Three: Place an empty charcoal chimney on the grill rack and keep the lid off the grill for the duration of the experiment. Position the chimney directly over the burning charcoal. You will see an immediate updraft. The charcoal chimney acts the same as a power plant chimneystack. The charcoal chimney creates a low-pressure area that acts as a vacuum, sucking air through the grill’s open damper, across the burning charcoal and through the chimney. Because of this hard draft, you should see your charcoal begin to glow red-hot. This represents wintertime efficiency.

Step Four: While wearing gloves and long sleeves, rest the board or cookie sheet gently atop your charcoal chimney. The smoke will no longer be pulled through the chimney, and the glowing charcoal will turn gray again because there is less draft across the coals. This represents summertime ambient air temperatures.

Repeat the experiment by moving the board or cookie sheet on and off the top of the charcoal chimney. As you study the affects the process has on your charcoal briquettes, you will understand why winter is the most efficient time for TVA to make power for the nearly 10 million people living in the Tennessee Valley.