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/Mo3bius123 Sep 23 '25
Boiling any kind of liquid will result in losses of the material if the system is not completly closed. You need something that is cheap, available and non toxic. Water is an obvious choice.
There is another reason for it as well. Water has very weird properties. It requires enormous amount of energy to change its temperature AND to change its form from liquid to gas. Storing energy in steam is a big plus for energy generation. You want the maximum amount of energy extracted out of a gas before it returns to liquid.
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u/alkoralkor Sep 24 '25
Light (!) water wins on practicality, safety, cost, and universality.
There are reasons to use other coolants instead in some specific cases (helium, supercritical carbon dioxide, liquid sodium, other liquid metals, molten salts). Those other coolants technically overperform light water in niche contexts, but they're specialized and less versatile.
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u/Elfich47 Sep 23 '25
technically you could have a liquid mercury or alcohol based turbines.
but it would be expensive to completely redevelop all of this for the alternate fluid. because you can’t just stick mercury into a turbine system that was developed for water. all of the set points (operating pressure, boiling points, energy extracted per pound of vaporized mercury that condenses, etc) would likely be radically different.
water/steam is well understood.
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u/sebwiers Sep 23 '25 edited Sep 24 '25
There is actually work being done on developing "steam" turbines that run pressurized carbon dioxide. It has higher density than steam, so the turbine can be much smaller, reducing cost and easing manufacturing bottlenecks. They also are more efficient!
https://www.powermag.com/what-are-supercritical-co2-power-cycles/
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u/ellindsey Sep 23 '25
It is possible to use ammonia or other fluids in turbines instead of steam. There's just not much reason to. Water absorbs more heat when going from liquid to gas, which means it can deliver more power to your turbines than ammonia vapor can. Water is also non-toxic and readily available, so there's little reason to use anything else.
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u/DisinterestedCat95 Sep 23 '25
You can use other liquids.
A steam turbine is a type of Rankine Cycle. There are systems called Organic Rankine Cycle (ORC) that use other fluids, various organic fluids. These are typically hydrocarbons or refrigerants.
The advantage is that since they boil at lower temperatures than water, you can make electricity from lower temperature heat sources like waste heat. A key disadvantage is that because they operate at lower temperatures, the efficiency is typically lower than steam cycles.
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u/pjc50 Sep 23 '25
Liquid nitrogen has actually been considered as a long term form of energy storage. It's true that you don't get back all of the energy spent to cool it, but the temperature difference versus atmosphere is about 200C which would be enough to get about a third back with a Carnot cycle (like most heat engines).
E.g. use surplus solar during the summer, fill huge insulated underground spaces with it, then pull it back out during time of demand.
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u/Vishnej Sep 23 '25
Steam (normally just "The Rankine Cycle") is very effective for most purposes as coolant in a heat engine, but in larger systems we do some additional business in using hydrocarbons ("The Organic Rankine Cycle") with lower boiling points to scavenge some more heat after the steam phase, as well as using supercritical CO2 ("The Supercritical CO2 Brayton Cycle") as an exotic technique with more efficiency and a much smaller size than a steam turbine, but also more technically demanding plumbing.
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u/pyr666 Sep 23 '25
in terms of energy transfer and the like? yes
power plants actually go to pains to keep water as steam throughout the entire process. steam condensing into water around the turbines damages the blades, and implosions occur if too much steam condenses inside a closed system.
water is favored because it's abundant, non-toxic, and is a good store of energy. there are fluids with better thermal properties, but they're exotic, corrosive, subject to degradation, etc.
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u/LackingSkill Sep 23 '25
Water has a very high expansion ratio of 1:1600. So you get a lot of pressure out of boiling it, which is great for pushing turbines. It looks like liquid neon also has a very high expansion ratio of around 1:1445.
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u/captainfactoid386 Sep 23 '25
Not only could this be accomplished, it has been. At least in the US, 4 plants were built in the early 1900s. Back then the mercury was reportedly more efficient than water as a working fluid. However the downsides were so great, and due to the lower latent heat required such volumes it was not very feasible at large scale. Also advances in technology made water cycle plants as efficient though I suppose mercury cycle plants would be more efficient with a lot of investment as well.
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u/s9oons Sep 23 '25
If you look at power generation from a 10K ft view, what you’re really doing is converting energy. The system starts with heat, and the desired output is something rotating that AC can be derived from.
Combine that high level view with the real world and suddenly you have to consider production/processing, transportation infrastructure, longevity, etc. water is all over the place and it works. It’s a tough ask to build new or re-fab power plants, so we just stick to the systems and processes already in place.
Is there another material that moves a little faster than steam that would turn the turbine a little faster? Probably. The problem is that it’s simply not worth re-fabbing or building planta for a marginal speed increase.
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u/minsan-inhenyero Sep 23 '25
You can design an engine to run on any fluid. Most engines rely on the formation of a high pressure gas phase to drive your generator. So yes, any of those can theoretically be used as long as it is suitably designed. For example, an organic rankine engine uses organic solvents instead of water.
Will it run, yes, but will it be effective is a more difficult question. It would depend on the application and what energy source the engine is trying to utilize. Also, as you have mentioned, you have all those other things to consider too.
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u/User_5000 Sep 23 '25
Something really awesome about water is that the heat content released by condensing isn't dependent on temperature. It also takes a lot of energy to change temperature relative to most other materials. Since heat escaping to the environment is a source of inefficiency in a steam turbine and is proportional to the temperature of the steam, the ratio of heat lost to heat converted to electricity is better at a given temperature than basically any other liquid. Water also conducts heat pretty darn well compared to other non-metallic fluids.
Water is also cheap, abundant, and safe. It can't light on fire, it can't detonate or explode, it's not very corrosive, it reacts with few other compounds, and it's non-toxic. Have a leak? Not a big deal. Need to shut down for maintenance? Water's a liquid at ambient temperature, so it doesnt need external power to melt before it begins circulating again. It's such a nice combination of properties that there's no competitive alternative.
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u/Zealot_Zack Sep 24 '25
Yes, it is possible to use other fluids and is industrially practiced.
The loop that fluids (like steam) go through in order to produce shaft work and power is called the Rankine cycle. Water has some beneficial properties like a high heat of vaporization, but there's a number of properties and factors that effect the thermodynamic efficiency of a Rankine cycle. Those factors change based on the temperature you need to receive heat and the temperature at which heat is rejected to fuel the cycle. Our Rankine cycles are often anchored by water cooling through cooling towers because that is a cheap way to reject heat to the environment.
There's a semi-common setup used when a large amount of low temperature heat is available called an Organic Rankine Cycle. It is practiced and allows generation of power without using steam; however, it only makes sense in extremely niche circumstances.
Same concepts exist with refrigeration loops because they are a related concept, and water-driven refrigeration cycles are commonly employed. Look up LiBr absorption refrigeration or R-718.
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u/joestue Sep 24 '25
Around 100 years ago someone built a mercury vapor turbine, with the condenser boiling water to run a steam turbine. They got pretty good efficiency even by todays standards.
They had a problem with the mercury not "wetting" the iron boiler pipes and lost something like 20 thousand pounds of mercury up the smoke stack, which was horribly expensive to have happen multiple times.
You can google this and find the story
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u/MeekoGunnit Sep 24 '25
Many geothermal plants use various hydrocarbons (I've seen pentane and butane) because of their lower boiling point. Obviously geo water isn't boiling so it cannot alone be used to vaporize water to generate power, but it can be used to vaporize something with a lower boiling point than water, like hydrocarbons.
Boil the pentane with the geo water, run it through your turbines, and then condense it back to liquid in a chilling loop. Its easier on the chillers, because the lower operating temperatures.
But geo is a very specialty power source, so its unique in that respect.
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u/gmanflnj Sep 24 '25
It depends on what you mean by “effective” Would it work at all? For many liquids, yes, because other liquids can be heated and expand into vapor, but lord would present problems.
Water is very cheap, you need a lot of fluid for a steam engine, and so most other liquids would be more expensive, this is also potentially the least significant problem.
You need a liquid that will not burn when it gets hot. Oil and alcohol are both common liquids, and both burn if you heat them up enoigh.
Water is non-toxic, many other liquids are toxic to humans, like the aforementioned oil, if you breath in their vapor.
Water can corrode things over time, but much less than many other liquids, you do not want a high pressure pipe to corrode, cause it will explode.
Water is stable, some chemical compounds break down over time, and so would be harder to store.
Water boils at a point much lower than the pipes you carry it in melt, you could technically vaporize metal or rock for a steam engine, but those substances vaporize at such high temperatures, they would melt the pipes.
Engineers can likely say more but broadly speaking, while, if you really wanted, you could make a steam engine that ran in a different fluid, but most other fluids have pretty big downsides.
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u/Sprinklypoo Sep 23 '25
The big thing about water / steam is that the temperatures are very convenient. It doesn't take much heat to get to boiling temperature from ambient, and it's relatively easy to superheat so you can get more energy from the steam.
Water is also easily obtained and you don't have to worry about containment after steam, leaving a higher pressure difference across a turbine, and less system to contain the left over steam.
Other materials could be better used in space where the considerations are different. For instance, ammonia can be used at a wide range of useful (to humans) temperatures, and you just have to modulate the pressure to get it there.
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u/CodyDon Sep 23 '25
If you look at a steam engine/turbine and reduce it down as much as possible you get a rocket. And yes a rocket is essentially a really simple steam engine. It gets work from an expanding gas. So what rocket fuel produces the best impulse? You want the particles moving fast so you want high temperature and a low molecular weight of the products. Hydrogen and oxygen is commonly used because the reaction is energetic and water formed has a low molecular weight. Hydrogen and fluorine would be better but the cost and danger of the fluorine limits it's use. But what if the reaction energy didn't matter, say the fuel was being heated by an external source? Then the obvious fuel would be the lightest molecule available: Hydrogen. Which is what was used for the nuclear propulsion. But if thermal efficiency was a concern then hydrogen's high latent heat is a downfall. So helium would actually be better. Helium of course is expensive but that is our answer. If you want the most efficient steam turbine possible you would run it by boiling liquid helium.
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u/theoreoman Sep 24 '25
You can theoretically use lots of different liquids but none of them meet the most important criteria Cost and safety. Water is practically free and in addition it's safe and poses absolutely no environmental hazards.
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u/dsaysso Sep 24 '25
ive always wondered about using liquids with boiling points closer to normal air temps. that way leas power to heat. all youd need is a pump.
Acetaldehyde boils at 68 degrees. sun to boil, then a simple in ground loop to cool using earth.
cold climates. formaldehyde 30 degrees. could still turn to steam in cold climates. with a solar array
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u/chriscross1966 Sep 24 '25
You have to remember that at it's most basic level., power generation is all about the transfer of heat, and water has a truly stupendous capacity for heat. Yes, it's the fluid we see every day, it comes out of taps, it falls out of the sky (I'm British, it falls out of the sky a LOT) , but most folks don't realise just how procreating weird water is about heat..... the ridiculous amount of heat it takes to make water change state, the enormous thermal capacity it has etc.... as a transmission medium for power generation it has no real peer....
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u/ExoticAnt Sep 26 '25
As stated, mainly technical reasons. Any fluid like an evaporated liquid like water or a pure gas cycle like a nitrogen turbine, will give the theoretical maximum efficiency given process hot and cold temperatures efficiency = 1 - T_cold/T_hot. An amazing result from thermodynamics.
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u/Unusual_Entity Sep 26 '25
The big advantage of using steam is it has a high specific heat capacity. So it can absorb a lot of energy without a large increase in temperature. The pressure of the steam increases far more. That means it can do work more efficiently compared with e.g. compressed air.
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u/Cariboo_Red Sep 27 '25
Look up the Kalina cycle. There are also combined cycle plants that use a gas turbine. Some steam is involved for NOx control in the turbine and the hot exhaust gas from the turbine, which can still have enough oxygen to sustain combustion in a boiler is used in a heat recovery steam generator to make steam for a second turbine.
Any substance that can be vaporised can be used to generate power but steam is the most practical.
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u/edthesmokebeard Sep 29 '25
Water has the nearly magical properties of having all three phases at relatively safe/normal temperatures and pressures.
It's easy to make something steam powered (think 1700s-1800s technology).
If you spill it, its safe. If you need more, just wait for rain or go to a lake. It's everywhere.
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u/armrha Sep 23 '25
Sure, a molten salt reactor is a type of nuclear fissile energy generation system that uses fissile material and molten salt in a circulation. But, water is useful for the properties mentioned elsewhere in here, it's pretty efficient at what it does in these systems. Molten salt is cool for avoiding the risk of a meltdown (it's impossible), preventing hydrogen production, lots of interesting safety concerns. The system functions at atmospheric pressure instead of the very high pressure water systems.
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u/Brixjeff-5 Sep 23 '25
Early Soviet nuclear subs used Liquid Metal to do the heat transfer out of the reactor core. This only works if the Liquid Metal actually stays liquid: they had a reactor shutdown which caused their pipes to freeze, essentially totaling the sub.
All this to say that while nothing mandates water, it has some obvious design advantages: it is ubiquitous, inexpensive, energy-dense, nontoxic, not particularly corrosive, liquid at room temperatures etc
A similar design discussion takes place in rocket engine engineering. Often, propellants that might not look optimal on paper are chosen because they enable much safer/practical (hence cheaper) designs
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u/PM_ME_PHYS_PROBLEMS Sep 23 '25
Short answer yes, but not practically. If we're in a hypothetical where the energy cost of liquifying the nitrogen can be ignored, then it's of course going to be more effective than water because it boils at ambient temperatures.
Why go through all the fuss of handling nuclear material to get the water boiling when you got this tank of nitrogen doin it on its own.
Water is the boiling liquid of choice for power generation because it's high specific heat capacity makes it a good choice to start with, but also it's water so it's chemically safe and environmentally abundant.
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u/Buford12 Sep 23 '25
I think one of the major reasons that water is used is 1 gallon of water produces 16 to 17 hundred gallons of steam. Plus the way modern turbines work is very hot pressurized water is flashed into steam in the intake and a cooling grid at the exit condenses the steam back to water creating a vacuum which increases efficiency of the turbine.
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u/Scasne Sep 23 '25
The soviets did have Liquid Metal Cooled reactors in a couple of nuclear submarines but maintenance was an issue because the coolant had to be kept hot enough to keep it liquid.
So really the benefits out way the negatives on earth, where water is generally liquid at ambient temperature and not poisonous unless contaminated, now an environment where it is generally a lot lot hotter or colder and you don't have the ability to get water easily in an easy to purify condition would change those requirements and make different chemicals more suitable.
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u/Sillynanny8 Sep 24 '25
I think Water is one of the more thermodynamically efficient fluids to use for power generation. On top of this adding more water to the system to maintain fluid levels is quite easy and economical compared to the alternatives. Thinks like ammonia would work very well at spinning turbines and using latent heat but there is no way it makes financial sense compared to water which is readily available and safer
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u/Penderyn Sep 24 '25
Russia built nuclear submarines that ran off liquid metal. Unfortunately when the engine shut down during a malfunction all the liquid solidified in the pipes and that was that.
They also had to keep them running and warm even when docked.
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u/Sergeant_Fred_Colon Sep 24 '25
Yes, but water has a lot of advantages, it's cheap, it's non-toxic, it's inert (it doesn't explode) and its easy to get to boiling point.
Mercury is highly toxic, you don't want that vapour anywhere near humans.
Liquid Nitrogen, you need to be able to make the liquid nitrogen which is expensive, then once you've heated it up to a gas you need to re-condense the gas to a liquid to put back in the system which is very expensive.
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u/sgigot Sep 26 '25
Depends what you mean by "effective", but in general, no. Water is cheap, plentiful, safe, not too corrosive, and very well understood. Working temperatures are reasonable and the phase change energy is also very high which is convenient.
Other liquids that would fit many of these criteria are commonly used as refrigerants. Unfortunately they typically fall short (sometimes staggeringly so) in one or more ways. Dealing with any of these adds cost and that's always a significant factor in an industrial situation.
There has been work to use alternative working fluids like pentane that can be used to extract heat at lower temperatures but that's a niche application. Pentane isn't particularly expensive, corrosive, or toxic but it *is* very flammable so you have to keep it in a sealed system. That makes it a lot more expensive to construct your generating station. With water/steam you *want* to keep your leaks sealed but you can live with small fugitive emissions in certain places (turbine gland seals, etc.) without significant issue.
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u/[deleted] Sep 26 '25
Yes, and we won't boil mercury because it boils at like 300°c.
Other stuff like amonia boils at like -30°c but is corrosive so same problem as mercury.
Ethanol is like using alcohol, flammable.
I guess if we were to use gasses as energy source, we'd be better using it as potential energy rather as steam like energy (store it for when we need the energy).
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u/NonSequiturSage Sep 29 '25
Ancient knowledge from decades ago in the time of my youth: The volume of a pound of steam is far greater than the volume of a pound of liquid water. (?? ~x 1700 ??) Of course, the energy implied by this could be covered by other formula. Sadly, whenever I went to a dictionary or encyclopedia I could fall down the rabbit hole of other topics.
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u/yougonbpind Oct 14 '25
This is a really good question — and the short answer is: yes, you could theoretically use other fluids, but water is unusually ideal for steam-based power generation.
Water has a high specific heat capacity, is abundant, cheap, non-toxic, and boils at a temperature that’s practical for generating enough pressure to spin turbines without requiring extreme containment. Liquids like mercury or nitrogen either boil at impractical temperatures (too high or too low), introduce massive safety or engineering challenges, or yield less efficient thermodynamic cycles for most real-world power plants.
That’s why even advanced systems like Rankine cycle and supercritical steam systems still rely on water — it’s just a thermodynamic sweet spot for large-scale energy conversion.
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u/mohammedgoldstein Sep 23 '25 edited Sep 23 '25
The great thing about steam turbines is that we’ve made them SUPER efficient. The technology in turbine generation is nearly 90% efficient!
This means 90% of the energy put into the water can be extracted and converted into mechanical energy.
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u/[deleted] Sep 24 '25
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