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gray_sky_guy t1_iulj54s wrote

Reply to comment by TheNoobsauce1337 in Engineering question: With today's material sciences, why don't ocean liners use pressurized steam to power electric engines? by [deleted]

so to charge it on land you need some other source of energy. if you're just trying to use the steam as a weird kind of battery, then you'd have to look at energy density of steam, and also consider how well you could insulate the thing. you're probably not gonna get across an ocean with stored steam energy.

for the generators on board, again, how are they generating the electricity? it doesn't come from nowhere

(i'm just gonna ignore the fact that it makes way more sense to use the electrical power you have stored to directly drive an electric motor that drives the propeller)

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TheNoobsauce1337 t1_iulkk3z wrote

That's basically the idea. Use water from the ocean to periodically refill the boilers (since water below the waterline automatically generates a positive pressure towards the inside of the ship).

Basically think of it like this:

Water replenished periodically from outside > Boilers > Steam > Generators > Engines and boilers on separate circuits > Process repeats so long as you can keep replenishing water from the ocean into the boilers that are already heated.

The key thing would be to build generators that could generate enough electricity to maintain the heating of the boilers and run the engines simultaneously.

The volume of water in the ocean would be the source of power, provided you could generate enough electricity to keep the water boiling to keep the generators running.

A self-sustaining reaction once started.

Does that make sense? I feel like people keep thinking about this like a Tesla that has to be recharged with a cord. It's not that.

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thegagis t1_iullan1 wrote

Its nowhere near self sustaining.

Turning water into steam USES absolutely massive amounts of energy. You need to constantly produce amazing amounts of heat to be able to do that.

Replenishing the water is not an issue, you can just direct the condensed water from the steam cooling back down back to the system.

This is how power plants work. They burn some fuel or run a nuclear reactor to convert massive amounts of energy into heat to heat water which turns into steam and that steam rotates a turbine with its pressure and the turbine rotates a generator.

Steam is not a source of energy, it is a local temporary storage for energy.

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TheNoobsauce1337 t1_iullo5z wrote

Makes sense. So the amount of electricity to maintain a boiling temperature would be unrealistic with today's generator technology?

Again, not saying this to troll or be passive aggressive. Literally throwing an idea out in front of other scientific minds and seeing what they have to say and critique.

The water would be the source of the energy. Steam would just be a conversion to transfer that energy to a generator that could then keep the boilers AND engine running.

Basically two primary circuits: 80-90% of the electricity goes to the engine, 20-10% goes back to the boilers to keep the water boiling and steam incoming, reduced to high pressure for high RPMs on the generator.

Not possible or feasible then?

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thegagis t1_iulmo3x wrote

Its unrealistic with physics. If entropy was not a thing, you could at best get exactly as much energy back as you used to boil the water with. In reality some of the energy is always wasted and depending on how good your generator is you get some % below 100% out as electricity.

Water is simply not a source of energy. Boiling water requires more energy than you can get by using the pressure of boiled water to drive a generator. Steam is just a way to move that energy from point A to point B.

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SLR_ZA t1_iulmsij wrote

No, it's possible to boil the water to make steam

It is impossible to recover nearly as much energy from that that steam as it would take to boil it.

It's not a 'today's technology' thing it's a law of thermodynamics thing.

Water is not a source of energy, it is a medium of energy transfer.

Every kg of water at boiling point being converted to steam takes say 2133 kJ of energy.

You cannot then power a generator with that steam and get more than 2133 kJ of energy out of it. Your actual efficiency of a steam generator is 10-40%

So now you have 2133 x 0.4 = 853 kJ of electric energy.

So you take 20% to the boiler and you have 170 kJ of energy. Which is not enough to boil a kg of water.

Even if you had an impossible 100% efficiency boiler, steam system, turbine and battery system you'd never be able to pull even 1 kJ out without decreasing your steam volume.

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