ELI5• Why you cannot use methanol or even ammonia inbpower plants instead of water. water takes more energy to boil and thus requires more coal burnt. You can condense the methanol by passing it through a waterbody of some sort so it doesn’t get lost in the atmosphere like steam does?
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Using flammable liquids to cool a reactor would generally be a bad idea due to the possibility of reaching its flash point. Furthermore, water also acts as a radiation shield, which other liquids may not. Overall, there are probably more reasons why water is the best choice, but these are the ones that come to mind.
you are aware that the “lost” steam is the exhaust of the last cooling loop which *is* just done by using a river in a bunch of power plants.
the working steam isnt just vented.
and the water isnt heated from 0, but goes through the water-steam phase change and not much temp variation outside of that to use the steam expansion.
its also not as questionable as using a burnable solvent as a high temperature working fluid….
Converting heat to electricity requires a working fluid to be boiled then condensed. We use water because it is chemically stable and physically well understood. Steam is allowed to escape to cool the loop. An alternative is cooling in some body of water such as a river or bay.
You can. You can use almost whatever you want. So why don’t we?
The heat used to boil the water isn’t lost in the process. That energy comes out to do useful work in the heat engine. In fact, in the rankine cycle, the high heat of vaporization actually improves efficiency.
1) The heat of vaporization isn’t wasted energy. Most steam engines rely on the condensor to create a significant vacuum with regards to ambient pressure, in order to produce full power. That is, to remove spent low pressure steam from the last stage. Therfore work is performed by the latent heat of vaporization contained in the cool, low pressure steam by means of transferring that heat into the condensor walls, thus dramatically reducing the volume of the steam.
2) Anhydrous ammonia is pretty toxic. It’s still used as a refrigerant in some large industrial freezers. (Which are in a sense, steam engines that operate in reverse.) Although new systems generally avoid it’s use because nontoxic refrigerants (like CO2) require far fewer safety measures and less regulatory compliance concerns.
There have been a good number of incidents over the decades where ammonia leaks from refrigerant systems or chemical plants have resulted in deaths. Anhydrous ammonia can be extremely dangerous because high volume leaks can cause people exposed to become confused then lose consciousness in a matter of seconds. Then, asphyxiatiate and die faster than emergency services can get there.
I remember reading about one case where a freezer had a crack in a refrigerant line that slowly grew so workers had just became accustomed to the smell or smell blind. The cracked refrigerant line suddenly broke, one worker in the freezer collapsed, the other worker outside saw him on the floor, then rushed in and was overcome almost immediately, bringing the death toll to 2.
Ammonia may be a good choice for heat engines that operate in space past the orbit of jupiter, or on the surface of mars for example. Specifically, small lightweight nuclear powerplants. Toxicity is a minor concern in space, it has a very low freezing point and the moderate increase in performance means weight savings. Such equipment would operate only at mildly elevated temperatures of a few hundred C in the reactor core and would take advantage of very cold, subzero temperatures on the condensor side to produce high efficiency. The low core temperature would mean thinner lighter weight materials could be used.
3) Methanol has a tendency to pyrolize at high temperature forming a number of other products like dimethyl ether, aromatic hydrocarbons, tar, and formaldehyde.
So while it could be used, the fact that the max operating temperature and thus max pressure is limited to reasonably below the pyrolysis temp, would mean less efficiency than H2O.
Water is what is used to transfer the energy from furnace to the turbine&generator.
The more energy you can store in the water, the better it is. Water comes in at 40C degrees and leaves the furnace at about 600 C or more. That’s the effort they put into make water to carry as much energy as possible.
If they want to ever replace water, they would use something that stores even more energy while it boils.
The reason for using water is that it’s free and it’s enough to carry the energy. We wish to have something as cheap as water that does boil higher. Unfortunately you need a molecule that’s pure, like pure water. You can’t use compounds,mixes, oil, metals or whatever, because at those temperatures they will clog the pipes with residues, or corrode something. Steam is incredibly neutral toward the equipment, at least, relatively to the temperatures and speeds. Pure enough water doesn’t leave any trace behind when evaporates, and does not age, because it’s just pure water. Because it does not react with things around it, it does not expire.
So, it’s just a fortunate case that something as cheap as pure water is also this good at the job. Maybe not the absolute best, but very close to it.
Other liquids can be used in power plants but they’re more dangerous/toxic/expensive. Any extra efficiency benefits are outweighed by those factors
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