Why Steam Plants Consume Water

Every thermal power plant obeys the second law: it must dump waste heat to a cold reservoir, and evaporating water is the most effective way to do that cheaply.

The unavoidable cold sink

A steam plant boils water, drives a turbine with the high-pressure steam, then must condense that steam back to liquid before pumping it around again. Condensing means removing the latent heat of vaporization. The second law of thermodynamics guarantees that a heat engine rejects a large fraction of its input heat to a cold reservoir; only the remainder becomes work.

For a plant with roughly one-third thermal-to-electric efficiency, about two units of heat are rejected for every unit of electricity produced. That rejected heat has to go somewhere, and the atmosphere is a poor conductor. Evaporating water carries heat away far more effectively than blowing air, because water's latent heat of vaporization is very large.

Consumption versus withdrawal

• Withdrawal: water drawn from a river, lake, or aquifer, some of which is returned
• Consumption: water permanently lost, mostly as vapor from a cooling tower
• Wet cooling towers withdraw less than once-through cooling but consume more, because the cooling mechanism is evaporation itself

Why it scales with power, not with fuel

The water draw of a steam plant depends on how much heat it rejects, which depends on its electrical output and its efficiency, not on the chemistry of its fuel. That is why gas, coal, and fission plants all share the same basic water burden: they are all steam plants underneath. A generator that never makes steam breaks this link entirely, which is the premise of the burner's direct energy conversion.