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Thread: Tell me about Carnot

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    Tell me about Carnot

    I've heard that even the most advanced concepts for nuclear reactors rely on good old steam to turn turbines that in turn turn generators. And at decent efficiencies too. Why is steam the substance of choice in power generation? Are there any methods for skipping steam and making electrons to power generators?

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    Well, probably saying "making electrons" isn't exactly the right word, because it's more like "liberating electrons" I suppose, or "channeling electrons to where you want them to be." They exist already.

    With regard to the question, a steam-powered turbine is one popular way to create electricity, but there are in fact other ways. Essentially what a turbine does is change heat into mechanical power. If you have the mechanical power already, you don't need the steam. So for example, dams, tidal, and wind power don't require steam because you are getting the mechanical energy directly. And other type of power generation, photovoltaics, doesn't require it either because essentially you are using photons (light) to knock electrons out of a material and this creates a gradient that makes electricity. The steam-driven turbine is only used with sources that produce heat, i.e. coal, gas, fission reactors. It is possible to convert the energy directly into mechanical energy, as is done with the internal combustion engine in cars. You could have a car engine that turns a shaft that is used to generate electricity, and I'm not quite sure what the advantage of a steam turbine is over that. Maybe somebody else can comment on that.
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    An internal combustion engine such as a gasoline car or a diesel locomotive are still heat engines that are limited to the Carnot efficiency, which is really just another name for the 2nd law of thermodynamics. The general term for engines that extract useful mechanical work from two the potential energy of two heat reservoirs of different temperature is a heat engine. Brayton Cycle, Stirling Cycle, Rankine Cycle, and Otto Cycle are all examples of different thermodynamic cycles that can be used, but none can ever be more efficient than an ideal Carnot engine.

    Wind and hydro are heat engines, too, because the kinetic energy of the wind and falling water come from solar heating of the air and oceans.

    Steam is a convenient energy transfer medium because water is plentiful and a lot of energy can be transferred by a relatively low fluid flow rate. It is by no means the only choice used, though. For example, https://en.wikipedia.org/wiki/Mercury_vapour_turbine. There is a photo of a small solar-powered Stirling engine at https://en.wikipedia.org/wiki/Solar-...tirling_engine. It probably uses hydrogen or helium gas as its heat transfer fluid.

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    Quote Originally Posted by VQkr View Post

    Wind and hydro are heat engines, too, because the kinetic energy of the wind and falling water come from solar heating of the air and oceans.
    Yes, that's true. It's interesting I think that if you really think about it, all our energy is solar energy except for nuclear fission. Even petroleum is really thanks to chemicals that were synthesized using the power of photosynthesis.
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    Quote Originally Posted by cjackson View Post
    I've heard that even the most advanced concepts for nuclear reactors rely on good old steam to turn turbines that in turn turn generators. And at decent efficiencies too. Why is steam the substance of choice in power generation? Are there any methods for skipping steam and making electrons to power generators?
    Yes: thermionic and thermoelectric effects; the latter is used for NASA's RTG's.

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    Carnot was a steam engine designer and the first to realise that you could not convert all the heat into work. You have to reject heat at a lower temperature. His legacy is the second law and the Carnot efficiency. That is the top limit for converting heat into work and is the temperature difference between source and sink divided by the sink temperature. Steam includes the very large latent heat of vapourisation of water so as a working fluid it stores much more heat than air can. This advantage scales up when you come to design big machines. The turbine overtook the piston engine but the Carnot limit still applies so you want superheated steam (dry steam) and a low temperature condenser. The stage where the steam becomes wet steam is a tricky area for materials, it's corrosive.

    So where the source of energy is heat, the use of steam as a working fluid is still useful.
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    Quote Originally Posted by swampyankee View Post
    Yes: thermionic and thermoelectric effects; the latter is used for NASA's RTG's.
    Yes.

    There has been materials research for decades on these processes. Unfortunately, they have generally not achieved the efficiencies necessary for large scale power generation, and so have only been used in relatively niche applications (like RTGs).

    There is a nice review here about thermoelectrics (I'm much more familiar with them).

    It can be shown that the maximum efficiency of a thermoelectric material depends on two terms. The first is the Carnot efficiency, for all heat engines can not exceed Carnot efficiency. The second is a term that depends on the thermoelectric properties, Seebeck coefficient, electrical resistivity and thermal conductivity. These material properties all appear together and thus form a new material property which we call zT, the Thermoelectric Figure of Merit.
    The other thing about steam engines is that humans have been perfecting steam generation and steam engines for centuries, and we've gotten very good at it.
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    Once you have dry steam there is not much extra to be gained at higher source temperature, because it is the latent heat that makes steam efficient, so in nuclear for example liquid sodium cycle is used to step down the temperature to the steam boiler, that's not in all reactors, some are water cooled. But if you could lower the sink temperature that would be good. It's also good rate of transfer. A flash boiler uses a hot plate and the water flashes into steam, this has a very high heat transfer rate, much higher than a liquid water boiler. The analysis of the heat transfer in flash boiling is very complex to model, all turbulent boundary layer stuff, but for large scale work it's hard to beat.
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    Quote Originally Posted by profloater View Post
    ... so in nuclear for example liquid sodium cycle is used to step down the temperature to the steam boiler, that's not in all reactors, some are water cooled. ...
    Almost all are water cooled - either BWRs or PWRs.

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    Quote Originally Posted by geonuc View Post
    Almost all are water cooled - either BWRs or PWRs.
    well not just the old Dounray UK but fast breeders in several countries, but I did say some are water cooled. I was adding points to the OP question of steam and nuclear. Some of those fast breeders are current projects but I take no position on the pros and cons, only that sodium can be used as a high temperature intermediate.
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    I should have added experimental, current commercial designs are indeed water cooled, I tend to think of experimental from old habits.
    sicut vis videre esto
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    Quote Originally Posted by profloater View Post
    Once you have dry steam there is not much extra to be gained at higher source temperature, because it is the latent heat that makes steam efficient,...
    I'm unclear what you mean by this. Hotter dry steam should be better because of the delta T gain, as you noted in post#6, right?

    If IIRC, the Carnot cycle reveals the max. efficiency for an engine, but it is too impractical for engines. Horsepower is the product of torque and rpm. The Carnot cycle should excell at torque (per unit of input energy) but its slightly better gain is lost with rpm. Other cycles mentioned can generate hp and be much lighter in weight, etc. I'd enjoy learning of actual Carnot engines in practical use, but I will guess rpm is not as big an issue.
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    Quote Originally Posted by George View Post
    I'm unclear what you mean by this. Hotter dry steam should be better because of the delta T gain, as you noted in post#6, right?

    If IIRC, the Carnot cycle reveals the max. efficiency for an engine, but it is too impractical for engines. Horsepower is the product of torque and rpm. The Carnot cycle should excell at torque (per unit of input energy) but its slightly better gain is lost with rpm. Other cycles mentioned can generate hp and be much lighter in weight, etc. I'd enjoy learning of actual Carnot engines in practical use, but I will guess rpm is not as big an issue.
    firstly the Carnot efficiency is the maximum work you can get from the heat, given the temperatures. Any real engine will be less efficient because of losses and the impossibility of getting all the working fluid to the high source temperature and low sink. However good engines can get passably close.

    The steam being generated starts and ends as wet steam carrying water, the water absorbs heat as it vaporises. Once all the water is steam it can be superheated as dry steam but now the heat absorbed is just the specific heat of the vapour with no further latent heat. The latent heat is large so you get most heat absorbtion in the wet steam phase. Similarly during cooling the max heat transfer is from the vapour turning into water, not from the vapour cooling.

    There is no Carnot engine, the principle applies to all heat engines with any working fluid or none, as in thermoelectric devices. Thats the value of the idea, the Carnot equation sets the limit for work out of heat. Power is the rate of doing work. So in your example the work is the torgue times the amount of rotation, the power is then that divided by time. There are various heat engine cycles as outlined in earlier posts, air engines are different from steam engines because of the latent heat. Petrol and diesel engines are air engines notably because the water vapour in them is exhausted, not condensed out.

    The man Carnot was working on piston steam engines, early ones had the steam condense in the cylinder to produce the pressure change, later better steam engines, thanks to his insights, emphasised the separate cold condenser where the low pressure was achieved. I am very fond of the Stanley steamer car, which was a sophisticated steam engine, worth a quick study
    this link is to the working diagram:
    http://www.stanleymotorcarriage.com/...%20Diagram.jpg
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by profloater View Post
    Once all the water is steam it can be superheated as dry steam but now the heat absorbed is just the specific heat of the vapour with no further latent heat. The latent heat is large so you get most heat absorbtion in the wet steam phase. Similarly during cooling the max heat transfer is from the vapour turning into water, not from the vapour cooling.
    Your heat of vaporization point is a good one, and I'm very rusty with triple points. When we see 1000 C temperatures for steam, is this at a pressure where the temp. is just above that point? I was thinking heating continued but perhaps not.
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    Quote Originally Posted by George View Post
    Your heat of vaporization point is a good one, and I'm very rusty with triple points. When we see 1000 C temperatures for steam, is this at a pressure where the temp. is just above that point? I was thinking heating continued but perhaps not.
    The heating of dry steam is still useful as a working fluid just as air can be heated but the key difference for steam is the large contribution of latent heat in wet steam.The latent heat part of the enthalpy is around 2257 J/g, the specific heat of water is just 4.2 J/g/K and of water vapour is just about 2 J/g/K So (you need steam tables to do this calculation properly) 1000 degree steam might add another couple of thousand Joules per gram but the big change is converting the liquid phase to vapour. Also at the heating and cooling surfaces the rate of heat transfer is orders of magnitude higher for wet steam, this is a vital part of the scaling of a heat engine.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by profloater View Post
    The heating of dry steam is still useful as a working fluid just as air can be heated but the key difference for steam is the large contribution of latent heat in wet steam.The latent heat part of the enthalpy is around 2257 J/g, the specific heat of water is just 4.2 J/g/K and of water vapour is just about 2 J/g/K So (you need steam tables to do this calculation properly) 1000 degree steam might add another couple of thousand Joules per gram but the big change is converting the liquid phase to vapour. Also at the heating and cooling surfaces the rate of heat transfer is orders of magnitude higher for wet steam, this is a vital part of the scaling of a heat engine.
    I think we are in agreement, though I meant to say a temperature a little above the press - temp line which the triple point is found is ideal. In other words, the line between vapor and liquid for any given pressure.
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    Right now, the temperature limit for steam plants is about 1100F (set by material proprties, mostly due to corrosion) with a pressure of about 2500 to 3500 psia. Nuclear plants run lower steam conditions; for a pressurized water reactor the steam going to the turbines is at only about 600F, as it's necessary to keep temperature inside the core low enough to prevent boiling.

    Superheated -- "dry" -- steam is partly used to minimize condensation in the turbine, as water droplets hitting turbine blades at 1000 ft/sec is erosive.
    Last edited by swampyankee; 2017-May-19 at 09:35 AM.

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    Quote Originally Posted by swampyankee View Post
    Right now, the temperature limit for steam plants is about 1100F (set by material proprties, mostly due to corrosion) with a pressure of about 2500 to 3500 psia. Nuclear plants run lower steam conditions; for a pressurized water reactor the steam going to the turbines is at only about 600F, as it's necessary to keep temperature inside the core low enough to prevent boiling.

    Superheated -- "dry" -- steam is partly used to minimize condensation in the turbine, as water droplets hitting turbine blades at 1000 ft/sec is erosive.
    yes exactly the condensation phase is taken into a separate chamber in a steam turbine. Superheated steam is quite dangerous stuff too, I remember being drilled about not tightening small fittings under pressure, with safety films about small parts stripping their threads and acting as bullets plus steam follow through. In the old days as now, boilers are dangerous and tightly regulated components.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    And remember: we get a lot of power in a closed system when we cool and condense the steam after turbine :
    "Cooling the condensate puts a partial vacuum on the exhaust port of the low-pressure turbine, making it work harder for the same energy input. It is thermodynamically more efficient than not doing so. The colder the condensate, the better. " from "Engineering 360",'to name one.

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    Quote Originally Posted by profloater View Post
    yes exactly the condensation phase is taken into a separate chamber in a steam turbine. Superheated steam is quite dangerous stuff too, I remember being drilled about not tightening small fittings under pressure, with safety films about small parts stripping their threads and acting as bullets plus steam follow through. In the old days as now, boilers are dangerous and tightly regulated components.
    Most steam turbines are expanded to the point that the steam in the last few stages is below the saturation point for that temperature and pressure. Steam tends to become less superheated on expansion, unlike some other working fluids

    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



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