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Homework Help: Engine efficiency - using heat (kelvin)

  1. Apr 27, 2009 #1
    1. The problem statement, all variables and given/known data
    A power plant taps steam superheated by geothermal energy to 505 K (the temperature of the hot reservoir) and uses the steam to do the work in turning the turbine of an electric generator. The steam is then converted back into water in a condenser at 323 K (the temperature of the cold reservoir), after which the water is pumped back down into the ground where it is heated again. The output power of the generator is 84 MW. Calculate:

    1) The maximum efficiency at which this plant can operate
    2) The minimum amount of rejected heat (in MJ) that must be removed from the condenser every 24 hours
    3) Express your answer to (2) in TJ and in PJ.

    (Quick question - what would TJ and PJ mean? Terrajoules and Petajoules or something?

    2. Relevant equations

    3. The attempt at a solution

    For (1) - Can I calculate the ratio of heat going in, to heat going out.. and then use that ratio to the 84 MW to get the maximum efficiency?

    I'm thinking for number (2) - Since 84 MW is power - meaning there's a time.. you can calculate the energy (without the time) and then apply that over the 24 hours (still trying to find the right equation) - I'm really stuck on number 2.

    Number (3) - Have no idea as we have not been taught that - If it is Terrajoules and Petajoules, then it must be a simple case of moving the decimal point.
  2. jcsd
  3. Apr 27, 2009 #2


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    Science Advisor
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    Gold Member

    Hi MikeNZ, welcome to PF. (Please don't post the same question in multiple places.)

    The maximum efficiency will be that of a Carnot cycle, and this efficiency depends only on the temperature of the hot and cold reservoirs.

    Your approach to #2 looks fine. TJ and PJ are indeed terajoules and petajoules.
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