What Comes Out of Those Enormous Concrete Chimneys? Water Vapour and Nothing Else
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The curved towers at a power station are not chimneys and emit no smoke, being devices that cool water by evaporating a small part of it. Their shape is determined entirely by how air moves through them.
What they are actually doing
A thermal power station must reject a large quantity of waste heat, and it does so by passing warm water over a large surface in a moving stream of air. A small proportion of that water evaporates, and because evaporation absorbs a great deal of energy, it cools the remainder substantially, which is exactly the mechanism by which sweating cools a body. The cooled water returns to the condenser to absorb more heat and the cycle repeats. What leaves the top is warm humid air, in which the water vapour condenses into a visible cloud as it meets cooler air outside, which is why the plume appears on cold days and is nearly invisible on warm dry ones.
Why the shape
The distinctive waisted profile is functional rather than decorative:
- •A chimney effect draws air in at the base and up through the tower
- •Warm humid air is less dense than the outside air, which drives the flow
- •Narrowing at the waist accelerates the rising air
- •Widening above slows it again, which reduces losses at the top
- •The curved shell is structurally efficient in thin concrete
- •Height increases the draught and disperses the plume above ground level
The misunderstanding they cause
These structures are photographed constantly to illustrate air pollution and emit nothing but water vapour, which is a persistent and consequential confusion. Actual combustion products leave through a separate, far narrower chimney that appears in the same photographs and attracts no attention, and a nuclear station has towers and no combustion at all, so the plume there is unambiguously just steam. Editors reach for the image because it is visually dramatic. The confusion matters because it shapes public understanding of what emissions look like, and several media organisations have adopted guidance discouraging the use of such pictures for stories about greenhouse gases.
The alternatives to using one
Rejecting heat is unavoidable and several arrangements do it differently. Once-through cooling draws water from a river, lake or the sea, passes it through the condenser once and returns it warmer, which needs no tower at all and consumes almost no water, at the cost of raising the temperature of the receiving water and killing organisms drawn in with it. Dry cooling uses air blown over finned tubes with no evaporation, which consumes no water and is far less effective, so a station using it loses efficiency and needs a larger structure, and it is used where water is genuinely unavailable. Hybrid systems switch between the two according to conditions. And using the waste heat rather than rejecting it removes the problem entirely where a customer exists nearby.
The problems they do cause
The genuine environmental issues are different from the imagined one. Water consumption is substantial, since the cooling depends on evaporating it, and a large station consumes a significant quantity continuously, which matters in dry regions and during droughts. The concentrated water left behind must be discharged and carries accumulated salts and treatment chemicals. Warm damp conditions inside can support the bacterium causing Legionnaires' disease, and several serious outbreaks have been traced to industrial cooling systems, which is why they are regulated and treated. Plumes affect local weather slightly and cause fog and icing on nearby roads. And the structures are enormous, visually dominant and expensive to demolish.
The takeaway
Warm water is cooled by evaporating a small part of it, and what leaves the top is warm humid air whose vapour condenses into a visible plume in cool weather. The waisted shape accelerates the rising air at the throat and slows it above, with the whole flow driven by the density difference rather than by fans. The plume is water and the actual emissions leave a separate narrow chimney.