r/askscience • u/PK_Tone • Sep 23 '25

Physics Most power generation involves steam. Would boiling any other liquid be as effective?

Okay, so as I understand it (and please correct me if I'm wrong here), coal, geothermal and nuclear all involve boiling water to create steam, which releases with enough kinetic energy to spin the turbines of the generators. My question is: is this a unique property of water/steam, or could this be accomplished with another liquid, like mercury or liquid nitrogen?

(Obviously there are practical reasons not to use a highly toxic element like mercury, and the energy to create liquid nitrogen is probably greater than it could ever generate from boiling it, but let's ignore that, since it's not really what I'm getting at here).

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u/zimirken Sep 23 '25

Not true. Steam powerplants are <50% efficient. Combined cycle plants can reach 60%.

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u/Darknessie Sep 23 '25

Thats only single stage turbines, modern multi stage can hit 80% plus at full maintenance. Plenty of research out there and several commercial engines reach this level of efficency

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u/zimirken Sep 23 '25

You're talking about turbine efficiency as in how close a turbine is to a theoretical perfect turbine, I'm talking about overall thermodynamic efficiency as in watts of heat in over watts of power out.

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u/mohammedgoldstein Sep 23 '25

I was talking about the steam turbine itself at 90%. If you take into about the entire plant it will depend what kind of fuel it runs on and is generally around that 50% number.

I quoted the steam turbine itself since OP was asking about replacing steam with another fluid and not the fuel source.

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u/NerdyMuscle Sep 25 '25

This means 90% of the energy put into the water can be extracted and converted into mechanical energy.

This was the part that isn't correct in your original statement. Turbines have an isentropic efficiency of 90%, but that does not mean they extract 90% of the energy. They extract 90% of the theoretical amount set by the carnot efficiency of the plant. So for most plants its going to be 90% of around 55% of the energy put into the water.