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Why water turbine have higher efficiency then steam turbine?

  1. Nov 16, 2008 #1


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    I wonder why water turbine (like in hydroelectric power plant) has, in most design, more then 90% efficiency when steam turbine around 30%.
    In both cases we converting the pressure to work, so why such a difference in efficiency?
  2. jcsd
  3. Nov 16, 2008 #2


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    Staff: Mentor

    A water turbine isn't reallly a thermodynamic cycle, it is just one piece of a thermodynamic cycle. The efficiency of a steam turbine is a lot higher than the efficiency of a steam engine. One of the main components that reduces the efficiency is the boiler feed pump.

    Have a look at a typical steam cycle: http://www.taftan.com/thermodynamics/STURBINE.HTM

    To make it analagous to a hydroelectric plant, you only deal with the turbine itself, or the flow from points 1 to 2. In reality, though, a hydroelectric dam is actually a solar powered steam engine, with a terribly low efficiency.

    Also, you're not really correct about the efficiency of the components either. Steam turbines operate in the 75% efficiency range.
  4. Nov 16, 2008 #3


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    I am electrical , specially in RF and uW, so of this might be of base.

    I never know that the boiler feed pump could have such impact on the overall efficiency.
    My assumption was that the base ranking cycle (temperature pressure differential) is the limit.
    I look around and I found that in did the boiler feed pump efficiency can vary from 20% to 85%. There is nice article in Power Engineering by Joseph Nasal about that :

    What interesting is that he calculate the optimum efficiency for boiler feed pump around 75%. If we combine that with the efficiency of steam turbine of 75% (not sure that correct approach) we should have total of 56.25% that will be still pretty high compare to real system. Such efficiency can't be achieve with out adding combine cycle right now if I am correct.
    It still look that the steam turbine efficiency (75%)is around 10-15% lower then water turbine. Is that only because of heat loses? Or it something els that play role in that?

    What I have in mind originally is: what limit the total convention of heat in to work in steam turbine it self.

    In the water turbine the pressure is a result of the gravity, so there is external force. That force have distinctive direction.
    Because water have some define temperature there is movement of the molecule in all direction with predominant vector directed to the earth.

    In the steam example there is no external force (or it do not have to be) but the energy stored with in the working fluid.
    The temperature of the steam is fare higher and the movement of the molecules are far more random, and the gravity vector have fare less impact (could be totally neglected).

    What this mean is that all current heat engines are trying to achieve is heat to work convention by random moving molecules to move object in particular direction (rotation of steam turbine or movement of piston).
    That mean that Carnot cycle indicate chase not certainty of pressure/heat convention in to mechanical power.
    The increment of temperature or pressure increases the chance of molecule collision per unit time.
    So in my understanding is that: if we can improve the molecules collision chance in particular direction with out increasing pressure or temperature , we can improve the efficiency of heat engine?
  5. Nov 16, 2008 #4


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    You're not doing the math right. The boiler feed pump is all "wasted" energy, so you don't just take the fraction of it's efficiency off the system's total efficiency. You subtract the work from the total work of the system.
    Of that, I'm not really sure, but I suspect it is.
    No, the boiler feed pump creates the pressure differential that causes the steam to flow in a particular direction. It is most certainly not random.
  6. Nov 16, 2008 #5


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    The pressure is not random, the molecules in the fluid have random movement besides the overall push of feed pump. Basically each molecule is independent entity. Each molecule “attacking” the turbine under different angle even if they have general direction of the movement. That mean not all force can be absorb by turbine, unless the molecules movement is perpendicular to the object, and it not counterbalance by other molecule.
    The whole design of contemporary steam turbine tray to improve that. It still fare from perfect.

    I think the problem is that we train to do too many thing in the same time:
    Deliver the pressure, expand , gain velocity, transfer the kinetic energy in to the tribune.
    I believe if the pressure can be converted to velocity out side of the engine , the efficiency can rise.
    The question how and, are there devices that can convert pressure to velocity efficiently?

    It true that feed pump create unidirectional movement of molecules , but it do not add much.

    Evan if the feed pump have 100% efficiency then the power lost in pump is gain in turbine. The question is what is the percentage of power used by feed pump comparing to the power generated by the turbine.
    I see example with boiler rated to 150HP at 50 psi with feed pump of 3hp. That a small fraction.
    (look on the 4300 Series spec).
    Last edited: Nov 16, 2008
  7. Nov 16, 2008 #6


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    No, that's really not a useful way to look at the situation. Consider a tank of compressed air. All the molecules are bouncing around randomly, but there is no energy being lost. Similarly, no energy is lost due to the static pressure in a pipe.
    In fact, the feed pump provides exactly all of the pressure rise and mass flow rate.
    Yes, it is a small amount. The feed pump is not the only source of inefficiency in a steam cycle. The heat of rejection is the bigger one. By contrast, a hydro plant is not considered to have a heat of rejection because the elevation of the turbine is arbitrarily considered to be zero. But the elevation of the river and reservoir are, in fact, critical factors in determining the power output of the plant. What if 10 years after a hydro dam is built, someone realizes they could have easily built it 10% higher? That's 10% more energy you could have harnessed: 10% of your available energy wasted.
  8. Nov 16, 2008 #7


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    A boiler feed pump needs to be able to deliver feed-water at a higher pressure than the operating pressure of the boiler. ALL of that energy needs to be subtracted from the energy output of the boiler. In addition, you have to consider losses at the mud drum (blow-downs to purge contaminants) radiative heat losses (boiler-houses are pretty hot) and the minor losses involved in de-superheating the steam so that it arrives at the turbine at an optimal temperature to be used at its highest mechanical efficiency. Well-designed turbines have many, many stages at which the latent heat of vaporization can be exploited, including extraction and re-injection, and eventually using the steam in stages that are below atmospheric pressure. There is a lot going on there.

    Hydro dams? Not so much. The sun evaporates water, the water falls as rain, and if we are clever enough to choose optimum places to erect barriers and leverage head-level differences, we can get some really efficient, cheap power for a very long time without buying fuels, etc.
    Last edited: Nov 16, 2008
  9. Nov 16, 2008 #8


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    It look like there a lot of variable that determine the total efficiency.

    I read somewhere that if you compare the energy generated in hydro power plant with particular elevation to power plant with the same elevation that contain vortex tube the last one will generate more power .
    The explanation was that the fluid in vortex tube increase the velocity, and decrease pressure and temperature. That contribute to total energy output.
    There was some experience that at first show that glass vortex tube have higher friction then copper. But that was rejected as they check the temperature of water in vortex tube, it show like to 5C (and room was like 25C). They concluded that the room heat that lick thru more conductive copper add some energy to the system. That explain sight higher velocity in copper vortex tube then glass.

    It look like the vortex tube have some ability to convert the random movement of molecules (pressure) in to unidirectional movement (velocity) with out excessive losses.
    Could we used vortex tube to “pre-process” the steam before it is apply to the turbine?
    In such case we should get much higher velocity with lower pressure and temperature. That could be beneficial to overall efficiency.
  10. Nov 16, 2008 #9


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    I should correct my self.
    It should be Vortex Cone not tube, us vortex tube have different function.
    Sorry for that.
  11. Oct 16, 2009 #10
    I found above that we have diverted from the main question what has been asked in this thread.

    Isentropic efficiency of steam turbine used in large fossil power plants are in the range of 87% to 92%.

    In the same way water turbines like Francis or Kaplan have same range of efficiency just a little bit more than steam turbine.

    Steam is a compressible fluid but water is not compressible. This is also a reason for higher efficiency of water turbines.
  12. Oct 16, 2009 #11
    I have to wonder if the density difference between steam and water contribute, also
    it seems to me leakage would be less with higher density fluid for a given "leak"

  13. Oct 16, 2009 #12
    Yes, for the turbine itself, but if you have to burn coal to get the steam, the overall efficiency is limited by the Rankine cycle thermodynamic efficiency. See
    Typical coal-fired power plants in operation require about 10,000 Btu per kilowatt-hour output(about 34%). Natural gas-fired power plants are 40% - 43% I think.
    Bob S
  14. Oct 16, 2009 #13
    Hi everyone.:smile:
    1st of all i apologize 4 postin this over here. i'm a new user n couldn't figure out how 2 post a NEW tread.

    i needed help with these Qs below.:confused:

    Y would u use uv spectrophotometry 2 analyse the content of a particular drug e.g. nitrazepam tablets. British Pharmacopeia uses this method 4 this drug. they use 20 tablets 2 start with. but we only used 2 accordin 2 our practical manual.

    What would be the advantage of usin 20 tablets instead of 2? is it because by usin 20 u get a bigger sample.

    can some1 plz help me out?
  15. Nov 28, 2011 #14
    The one I quoted is the only meaningful answer.
    The question is badly conceived. We are trying to compare the efficiency of a THERMODYNAMIC CYCLE which converts HEAT to MECHANICAL ENERGY, to the efficiency of machinery which converts MECHANICAL ENERGY (potential energy, in particular) to another form of MECHANICAL ENERGY (shaft kinetic energy).
    It would be very surprising to me to acknowledge that converting something (energy) you can find in a poor state (heat is an "inferior" form of energy, according to the second law of thermodynamics) into something precious (like kinetic energy) can be easier than converting something valuable into something poor.
    In hydro power plants, the sun makes the job, but you don't pay for its conversion procedure (governed by sun heat). In fossil fuel fired power plants, boilers do the job but you pay for the heat you release, which cannot be turned completely into kinetic energy. Hence, 30% (up to 60%, actually, for combined cycle) overall efficiency of the thermodynamic cycle.
    It would be very interesting to evaluate the efficiency of the sun-driven thermal cycle which "secretly" governs hydro power plants..

    PS: no matter how much power is used by pumps. The real problem is the second law of thermodynamics, pumps are a minor issue...
  16. Nov 29, 2011 #15
    Don't you get back the energy that the boiler feed pump puts in (with losses)? It compresses the steam, but doesn't that energy get returned when it decompresses through the turbine?
  17. Nov 29, 2011 #16
    I think the only way to compare the efficiency's of a hydro to a steam system, would be if both energy sources were free. Hydro to Geo, otherwise we really are talking apples to oranges. If the steam or water head are free, then the efficiency's could be measured.
    Steam in this measure might be more efficient, as it expands and can have multiple layers of delta pressure zones. (BTW not my area, just trying to look at it objectively.)
  18. Nov 29, 2011 #17
    In thermo class, when looking at a refrigeration cycle, my teacher said it was harder for a compresser to push a vapor than a liquid because of its compressability factor. Therefore it was less efficient to move a vapor then a liquid.
  19. Nov 29, 2011 #18


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    Looking only at the energy of the steam or water going into a turbine, how is it possible for the efficiency to exceed the 59.3% limit from Betz law?
  20. Nov 29, 2011 #19


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    This is the whole point, isn't it? Whatever you do to improve the efficiency of the mechanical bits, you can't get over the thermodynamic efficiency limits of a Heat Engine. The confusion is in 'inappropriately' comparing two different forms of turbine. A falling 'solid' mass could give you nearly 100% efficiency if you use it to drive a machine. Falling water is not so good because there is turbulence and other practicalities involved. Steam is fundamentally different and there is a whole new layer of inefficiency.
  21. Nov 29, 2011 #20


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    Popular thread.

    Sure it's 3 years old, and has only seen exactly 4 days of activity in all that time (once in 08, once in 09, then starting yesterday).

    But with 16,000 views, it has had 4000 hits for every day it has had activity, or the equivalent of 14 times a day every single day in those 3 years.
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