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Is Steam Really that Great? Consider This

  1. Nov 20, 2011 #1
    All the time I hear how steam is an excellent way of transferring and storing energy, but I'm questioning it at this point. Consider a Coal-powered electric generating station. Enormous amounts of heat are produced by the coal to heat water in pipes that is transformed into steam. This steam then rushes through the pipes and eventually collides with the impellers of a turbine. But the steam has a large amount of kinetic energy only because there is enormous pressure on the inlet of the turbine and much lower pressure at the outlet of the turbine right?? If so, pressure energy is being converted into kinetic energy which is then converted to rotating mechanical energy for the generator. I believe thermal energy in the steam is also derived from this expansion, further complementing the kinetic energy. The thing is, much of the thermal energy really isn't being utilized because the steam remains in vapor form after being exhausted from the turbin, and is then condensed by evaporative cooling towers so that it may be passed back through the boiler. Obviously it would be harmful for liquid water to hit or condense on the impeller, so I'm sure that's why they let plenty of thermal energy leave the turbine exhaust, but wouldn't it be smarter to use something with a much lower boiling point so condensation on the impeller is never a hazard (ie Refrigerant Gases)? That way thermal energy isn't trashed so much? What do you guys think?
     
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  3. Nov 21, 2011 #2

    Simon Bridge

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    When someone say "this is an excellent way to" ... something, you should automatically think: "in what way?" What is the problem to be solved, and what are the constraints?

    Look at Stirling Engine for example.
     
  4. Nov 21, 2011 #3
    Also, I would think that the engineers who design and build power plants that use steam turbines might just know a little bit more about what's efficient and what isn't than someone who isn't an engineer.
     
  5. Nov 21, 2011 #4
    Well instead of being snippy, you should ask the OP some questions to figure out what level of understanding might be appropriate, and then recommend a good thermodynamics book, or even provide a simple explanation of what 'efficiency' really is. Or why we use water instead of freon or something in our thermal power plants.
     
  6. Nov 21, 2011 #5

    Astronuc

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    Certainly folks have been looking for more efficient systems. Some newer coal plants use higher superheat and they can approach 40% thermal efficiency. However, the trade off is an increase in corrosion. The Rankine (steam-water) cycle works pretty well over the range of temperatures and pressures involved. An improvement might be to add another cycle (with a different working fluid) on the backside or bottom of the Rankine cycle, but the capital costs may not make that economical. There isn't a lot of energy left.

    The Kalina cycle has been developed in order to pull out more energy from steam cycles.

    For more efficient systems, folks have develop combined cycle plants that use aeroderviative gas-fired turbines which produce about 40-45% efficiency. The exhaust is passed to a Rankine cycle, which grabs another 30% or so, and the combined efficiency is around 60% or so. Combined-cycle plants are a bit more complex than unicycle plants.


    Higher temperatures challenge materials in terms of corrosion and creep of structural elements.
     
  7. Nov 21, 2011 #6

    turbo

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    Steam really is that great. Water is widely available, and it takes a LOT of work to to vaporize it. Boilers are designed to superheat the steam, so that it is nice and dry. After the superheater section, the steam exits the boiler and its temperature is moderated with a de-superheater so that it can be routed to the turbine(s) with a relatively constant temperature. That technology is old, tried, and true.

    Nice clean condensate with feedwater chemicals is not routed through cooling stacks. The water that goes to the cooling stacks is water that has been used in the condensers to further cool the condensate and improve the pressure drop along the turbine. It is very expensive to scrub and treat boiler feedwater, so condensate losses are minimized to save money.
     
  8. Nov 21, 2011 #7

    AlephZero

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    Plus, in the right environment, you can use the "low-grade" residual heat for communal air conditioning and water heating in buildings. Small CHP (combined heat and power) systems (say up to about 50 MW) can be installed in skyscrapers or shopping malls without most people even knowing they are there.
     
  9. Nov 27, 2011 #8
    Read "fundamentals of engineering thermodynamics." The whole book pretty much describes why we use steam in turbines. Also, most of the energy derived from a turbine is through enthalpy, not KE. You have to realize that the heating and cooling processes that are taking place are really just changing the enthalpy of the water. Its true, your probably losing more energy than you need to through the phase change, but depending on the system, it might not be cost effective to change the working fluid.
     
  10. Feb 7, 2012 #9
    Thanks for all the useful replies! Astronuc, that is exactly what I was looking for.
     
  11. Feb 7, 2012 #10

    jim hardy

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    ""An improvement might be to add another cycle (with a different working fluid) on the backside or bottom of the Rankine cycle, but the capital costs may not make that economical. There isn't a lot of energy left.""

    in 1930's the industry hadn't figured out water chemistry for high pressure/temperature steam.
    So some some plants were built around Boston that used mercury as working fluid producing mercury vapor around 1,000 degF. The mercury turbine exhausted to a steam boiler at around 450 degF , running a steam turbine of the day...

    Can you imagine permitting that thing today?
    It'd make environmentalists mad as hatters.
     
  12. Feb 7, 2012 #11
    Haha wow! Unbelievable.
     
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