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In the rankine cycle, water is first pressurized and then heated to produce steam before it is used to turn a turbine. As far as i know, this is what is considered the best way to extract mechanical work on the turbine, but i want to understand why this is so. How is it that we can extract more work on a turbine from steam than from regular unheated, un pressurized water?

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A good way to understand why the phase change is so important is by looking at the Pressure vs. Volume diagram for the Rankine cycle (path 1-2-3-4-1):

enter image description here

Remember that the work output of any thermodynamic cycle is the integral of its P-V diagram (area under the curve). If the water in the Rankine cycle were not heated to steam, its change in volume would be very small, both in the pumping/heating processes and in the expansion/cooling processes. And, if its volume didn't change significantly, its P-V curve would have almost no area. Thus, the cycle would produce much less work output than if the volume changed significantly and produced a P-V diagram with a large cyclic integral. The dashed line (path 1-2-2'-3'-1) represents what the path might look like if plain water were used.

Many early heat engines used a gas-phase-only working fluid; it was not until Rankine or DeLaval came along (can't remember who), and discovered the benefits of using a phase change in the cycle.

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  • $\begingroup$ Interesting. Is this also the reason why hydroelectric water turbines and wind turbines are so much less efficient? $\endgroup$
    – Paul
    Sep 5, 2015 at 14:41
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    $\begingroup$ It's hard to compare the two, since wind and water turbines don't use thermodynamic cycles. There also may not be much incentive to make them more efficient because the energy is "free" to start with. $\endgroup$
    – Carlton
    Sep 7, 2015 at 1:57
  • $\begingroup$ Turbines themselves, converting fluid pressure and velocity into rotation, can be very efficient, >90%. It's other things in the system that reduce the efficiency. Or for wind turbines a problem is that the wind isn't forced to flow through the turbine like in a hydro or steam turbine but can flow around it. $\endgroup$
    – JanKanis
    Jan 16 at 14:19

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