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[Thermodynamics] How big and heavy do steam turbines need to be?

  1. Oct 12, 2013 #1
    I'm curious how large(dimensions) and how heavy/massive a steam(or other gas) turbine needs to be to extract a given fraction and amount of the energy from the fluid. For example if a power plant is producing 1GW of thermal power, how much of that can realistically be converted to electrical power, and what kind of turbines would it take? Could the turbines be made more compact and lighter if they were to go on a submarine or ship (and how would that impact efficiency)?

    -FireStorm
     
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  3. Oct 12, 2013 #2

    Andrew Mason

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    The efficiency of a thermal power plant depends on the temperatures that it operates between. The theoretical maximum efficiency would be with a Carnot cycle: η = 1-Th/Tc where Th is the temperature of the thermal source and Tc is the ambient temperature at which heat flow from the plant is discharged. In practice thermal efficiency does not get much better than 1/3 except some nuclear plants that achieve very high operating temperatures where it might approach 40%. Turbines are used because they are efficient at converting heat flow into useful work.

    AM
     
  4. Oct 12, 2013 #3
    The efficiency can be about 60%, when using combining a turbine with a steam power plant (a combined cycle plant). A single gas turbine could get 40%.

    The efficiency of nuclear power plants is much lower, because they work with liquid water as coolant, so the Th can't be higher than the critical temperature of water (647 K)
     
  5. Oct 12, 2013 #4
    Thanks for the answers. That Carnot equations tells quite a bit and mostly answers the efficiency part. Any idea on how much one of those turbines weigh though?
     
    Last edited: Oct 12, 2013
  6. Oct 12, 2013 #5

    SteamKing

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    The thermal efficiency of the turbine by itself is relatively high. It's not unusual for turbines to have efficiencies of about 85%. The efficiency of the combined plant is much lower, as noted above.

    Of course, there is more than one steam turbine in a power plant. The smallest turbines use the highest pressure steam, and as steam pressures drop, the size of the turbine increases because a given amount of steam occupies more volume as it expands.

    It's hard to say what turbines weigh, because they come in different sizes and have different ratings. A small turbine capable of producing 1000 kW might weigh a couple hundred kg with the rotor and casing included. A large power plant turbine can weigh hundreds of tonnes.

    To give you an idea of what a large power plant turbine looks like, here is a brochure from Siemens:

    http://www.energy.siemens.com/hq/po...eam-turbines/SST-9000/SST-9000_Data_sheet.pdf
     
  7. Oct 12, 2013 #6

    AlephZero

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    Of course they can, and your OP missed the biggest motivation for weight reduction in gas turbines, namely aircraft engines. As others have said the efficiency depends mostly on the thermodynamic cycle of the engine, not its weight.

    Actually the question is a bit backwards. A stationary engine in a power station doesn't have to be light weight. Very high reliability and low maintenance costs are more important., and a "lower tech" heavy machine is "better" for that than a high tech light one.

    As an extreme example, some military jet engines need overhaul every 200 hours of running time, and at any one time maybe 25% or even more of those planes are being maintained and not available for use. But you wouldn't want to operate a power station that had to be shut down for maintenance for four days every two weeks!
     
  8. Oct 12, 2013 #7
    It's impressive the range of designs you guys have pointed out. I hadn't really considered jet engines since I was mainly considering extracting thermal energy from a fluid, rather than adding it, but in hindsight, I suppose aircraft engines are relevant.

    I guess the take away would be that it's all about trade-offs. Mass & Size vs. Maintenance vs. Efficiency vs. Power Output etc. I'm sure operating temperature and pressure factors in as well.

    (How much) would it make a difference if the working fluid is, for example, LH2 or LOX in a rocket turbo-pump, or the high temperature air in a jet engine?(as compared to the steam turbines we've mainly been discussing)
     
  9. Oct 12, 2013 #8

    SteamKing

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    It's not clear what you mean here. For a steam turbine, the working fluid (water) absorbs the heat of combustion of fuel in a boiler, which converts liquid water to steam. The turbine extracts heat energy from the steam and turns that into work. This is an external combustion arrangement.

    In a gas turbine, there are three key components: 1. a compressor to raise the pressure of the combustion air, 2. a combustion section where fuel is mixed with the compressed air and burned, generating hot exhaust gasses, and 3. the actual turbine section, where the heat energy of the combustion gasses drives a turbine.

    In aircraft gas turbines, the work produced by the turbine is used to drive the compressor. The rest of the heat energy of the hot gasses is turned into thrust by passing these gasses through a nozzle as they exit the turbine.

    Both turbines extract work from a fluid: a steam turbine uses steam produced by burning a fuel, a gas turbine uses the hot gasses from burning fuel directly, without the use of another fluid.

    Now, gas turbines can be modified by adding turbine sections to extract more work from the hot gasses of combustion, and consequently reducing the thrust produced. The additional turbine sections are connected to an output shaft, which can turn a propeller or a generator. A gas turbine driving an aircraft propeller is called a turbo-prop. It works just as well with a ship propeller, with the addition of a reduction gear to the turbine so the propeller is driven at its most efficient speed, which is in the range of 100 RPM or so.

    One of the limiting factors in the design of a gas turbine is the material used to construct the turbine blades. These blades must be very strong, so they do not break when being spun at high RPM, and they must remain strong while they are very hot. Due to various factors, it's not practical to use any cooling system for the blades.
     
  10. Oct 12, 2013 #9

    SteamKing

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    Marine power plants do not need to produce as much power as a large central power station. A nuclear-powered aircraft carrier puts out about 225 MW, but this power is generated using four sets of steam turbines, each set driving its own propeller shaft. Nuclear power generates relatively low pressure steam compared with a conventional boiler, but a reactor can supply a greater amount of steam for years without refueling. The turbines are quite compact. There is usually a high pressure and intermediate pressure turbine on one shaft, and a larger low pressure turbine on its own shaft. Both sets of these turbines drive a reduction gear which is connected to a propeller shaft.

    This Kawasaki website shows a typical layout and gives some powers and weights of turbines:
    http://www.khi.co.jp/english/machinery/product/ship/ship.html

    Notice that merchant ship propulsion uses relatively low power turbines, since most merchant ships cannot afford to operate at the high speeds of naval vessels.

    Here is some information on gas turbines used in industry and aboard ship:
    http://www.dieselduck.net/machine/01 prime movers/gas_turbine/gas_turbine.htm
     
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