Thermodynamics

Chapter 8 | 457since KE PE 0 and U 0 (the change in internal energy of an idealgas is zero during an isothermal process), and S sys Q/T sys for isothermalprocesses in the absence of any irreversibilities. Then,W rev,out T 0 QT sys W surr a 1 T 0T Rb QTherefore, the useful work output would be 2.65 kJ instead of 1.43 kJ if theprocess were executed in a totally reversible manner.Alternative Approach The reversible work could also be determined by applyingthe basics only, without resorting to exergy balance. This is done byreplacing the irreversible portions of the process by reversible ones that createthe same effect on the system. The useful work output of this idealizedprocess (between the actual end states) is the reversible work.The only irreversibility the actual process involves is the heat transferbetween the system and the furnace through a finite temperature difference.This irreversibility can be eliminated by operating a reversible heat enginebetween the furnace at 1200 K and the surroundings at 300 K. When 2.43kJ of heat is supplied to this heat engine, it produces a work output ofThe 2.43 kJ of heat that was transferred to the system from the source isnow extracted from the surrounding air at 300 K by a reversible heat pumpthat requires a work input ofQ H2.43 kJ300 K 1300 K2 11 kJ2 a 1 b12.43 kJ2400 K 1200 K 2.65 kJW HE h rev Q H a 1 T Lb QT H a 1 300 K b12.43 kJ2 1.82 kJH 1200 KQ HW HP,in cCOP HP T H >1T H T L 2 d HPThen the net work output of this reversible process (i.e., the reversible work)becomesW rev W u W HE W HP,in 1.43 1.82 0.61 2.64 kJwhich is practically identical to the result obtained before. Also, the exergydestroyed is the difference between the reversible work and the useful work,and is determined to beX destroyed W rev,out W u,out 2.65 1.43 1.22 kJwhich is identical to the result obtained before.2.43 kJ 0.61 kJ1400 K2>3 1400 3002 K48–8 ■ EXERGY BALANCE: CONTROL VOLUMESThe exergy balance relations for control volumes differ from those for closedsystems in that they involve one more mechanism of exergy transfer: mass flowacross the boundaries. As mentioned earlier, mass possesses exergy as well asenergy and entropy, and the amounts of these three extensive properties areINTERACTIVETUTORIALSEE TUTORIAL CH. 8, SEC. 8 ON THE DVD.