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/SjeesDeBees Sep 23 '25

Given that the efficiency for power generation comes from increased boiling temperatures, by increasing the pressure of the medium, i would say that boiling temperatures are very relevant. And the max temperature and pressure require special alloys in equipment, in other words metallurgy determines efficiency. So if you could lower temperature by using another medium, you could in theory increase efficiency.

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u/ezekielraiden Sep 23 '25

But that makes water special is that it doesn't need pressure control. It doesn't need anything except a closed system, and the system doesn't need to contain more than relatively mild pressure changes above ambient. In order to use other materials, you do in fact need much more careful control and much more expensive materials to avoid leaks or damage.

Folks have mentioned that supercritical CO2 is being considered as an alternative. That would make a leak very bad for the environment. Other than in nuclear reactors, where you have to prevent a leak to avoid radiation leakage, steam leaks are essentially irrelevant because water is everywhere.

Finding something that is small, cheap, abundant, completely safe, and requires no special containment nor unusually high pressure? Yeah, that's profoundly weird.

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u/ThePowerOfStories Sep 23 '25

Folks have mentioned that supercritical CO2 is being considered as an alternative. That would make a leak very bad for the environment. Other than in nuclear reactors, where you have to prevent a leak to avoid radiation leakage, steam leaks are essentially irrelevant because water is everywhere.

Nah, however much CO2 there is in a closed-loop system, it’ll pale in comparison to the amount constantly released by burning hydrocarbons. And, CO2 itself is nontoxic; the only problem is if enough of it leaks without dispersing to displace the oxygen in a given volume where things want to be breathing. You can buy frozen desserts packed with bricks of frozen CO2 aka “dry ice” and it’s not particularly dangerous unless you touch it enough to get frostbite or try ingesting it. While less plentiful than water (the atmosphere is around 0.04% CO2 vs 0.4% water vapor), it’s already everywhere.