Thermodynamics SECOND LAW OF THERMODYNAMICSThis calculation indicates that the Carnot cycle, operating with ideal components under real worldconstraints, should convert almost 3/4 of the input heat into work. But, as will be shown, thisideal efficiency is well beyond the present capabilities of any real systems.HeatRejectionTo understand why an efficiency of 73% is not possible we must analyze the Carnot cycle, thencompare the cycle using real and ideal components. We will do this by looking at the T-sdiagrams of Carnot cycles using both real and ideal components.The energy added to a working fluid during the Carnot isothermal expansion is given by q_{s}. Notall of this energy is available for use by the heat engine since a portion of it (q_{r}) must be rejectedto the environment. This is given by:q_{r} = T_{o D}s in units of Btu/lbm, (1-37)where T_{o} is the average heat sink temperature of 520°R. The available energy (A.E.) for theCarnot cycle may be given as:A.E. = q_{s} - q_{r}. (1-38)Substituting equation 1-37 for q_{r} gives:A.E. = q_{s}- T_{o D}s in units of Btu/lbm. (1-39)and is equal to the area of the shadedFigure 28 Carnot Cycleregion labeled available energy inFigure 28 between the temperatures1962° and 520°R. From Figure 28 itcan been seen that any cycle operatingat a temperature of less than 1962°Rwill be less efficient. Note that bydeveloping materials capable ofwithstanding the stresses above1962°R, we could greatly add to theenergy available for use by the plantcycle.From equation 1-37, one can see whythe change in entropy can be definedas a measure of the energy unavailableto do work. If the temperature of theheat sink is known, then the change inentropy does correspond to a measureof the heat rejected by the engine.Rev. 0 Page 85 HT-01

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