What nuclear power plants towers really are

Natural and forced draft cooling towers in Westfalen, Germany

When I was a child, I asked myself many times what those towers are, as I associated them with nuclear power plants. I thought that hot steam flew in them, making a nice gigantic turbine spin to create the energy that made my PS2 turn on. That was just black magic for a curious 8 years old.

Only serveral years later I discovered that those only serve the pourpose of cooling water. What a disappointment for a kid. But that’s not all.

Let’s start now from the bigger picture. A thermal power plant (nuclear, geotermal or fuel powered) basically generates steam by transferring heat to water, and uses the steam pressure to run a turbine that’s connected directly to a generator, that uses his mechanical movement to produce electrical power.

Turbines and generators from a thermal power plant. They are BIG.

We’ll focus on the work liquid, that is often plain water. First, we want the water to be as close as possible as distilled, since any fixed residue would damage our really (really) expensive equipment. Second, we need a lot of it. For a 120 MW powerplant, roughly 100 kg of steam per second are needed to run the turbines at their ideal rates, and that’s 100 kg of steam per second that we need to condensate and cool down to make the energy production more efficient. We could think about building an open cycle power plant, where we take in water from a river and put it back at the end of the cycle, but this has too many drawbacks (starting from the enviromental and refining ones) to be considered a feasible solution.

So it’s time for our black magic towers to shine.

The hot water (after getting condensed) enters the cooling tower and gets sprayed down by arrays of nozzles to the exchange surfaces. It drops down to the cold water basin and then gets pumped back again to the combustion chamber. But wait a minute, why would the water be colder?

Nice question. Think about when you are wet and wind blows on you, it’s CHILL. That’s the same principle, it’s called evaporative cooling. Water needs a lot of heat to evaporate, and when hot water gets sprayed by the nozzles on the surfaces, some of it evaporate by subtracting heat to near... water, actually coooling it.

We need a lot of fresh dry air to make it happen and that’s what that stylish tower shape is for. Hot and wet air gets less dense, floating on colder air and raising out of the tower, creating a vacuum that sucks dry and cold air from the bottom of the tower. The “smoke” that can be seen coming out of those tower it’s just steam, nothing less, nothing more. It’s about the 3–5% of the water that we just cooled, that we need to reintegrate before the next cycle starts.