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Brayton cycle efficiency

  1. Apr 4, 2013 #1

    In this document it is shown that the efficiency of a gas turbine is determined by the Brayton cycle and it depends on the pressure ratio ε.

    ε is the compression ratio before fossil fuel burning invests further energy into the gas.

    I want to be sure that my thinking is correct -

    If I build a reservoir and perform a very high compression (high ε), efficiency will be high ONLY if the high-pressure gas is supplied to the turbine, which will now be extracting work from the high pressure gas, correct?

    But if I am to build a reservoir and perform a very high compression (high ε), but use a pressure regulator to drop the gas pressure before the gas enters the turbine, the efficiency should drop, correct?

    It seems to me that the high ε and high efficiency has to do with the area inside the Brayton cycle curve, e.g. high pressure drops utilize better the internal energy of the supplies gas, correct?
  2. jcsd
  3. Apr 4, 2013 #2
    The document details how the efficiency of a turbine can be calculated when the Brayton cycle is employed. The efficiency of all gas turbines is not determined by the Brayton cycle.

    Engineering very high pressure components is not practical and leads to thick walled vessels, very expensive turbine casings, very expensive pumps, etc ... The other ways to increase the Brayton cycle efficiency is to increase the turbine inlet temperature to the maximum possible that the blade materials can safely operate at. Adding a regenerator and using intercooling between the compressor stages and reheating between the turbine stages can also increase the cycle efficiency.

    The second law must be considered because the cycle is only internally reversible. External irreversibility has to be considered.
  4. Apr 5, 2013 #3
    Could you elaborate on this point? Thank you for your reply.
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