Thermodynamics

444 | ThermodynamicsThis equation can be expressed as the exergy of an isolated system during aprocess always decreases or, in the limiting case of a reversible process,remains constant. In other words, it never increases and exergy is destroyedduring an actual process. This is known as the decrease of exergy principle.For an isolated system, the decrease in exergy equals exergy destroyed.Surroundings∆ X sys = –2 kJSYSTEMX dest = 1 kJQExergy DestructionIrreversibilities such as friction, mixing, chemical reactions, heat transferthrough a finite temperature difference, unrestrained expansion, nonquasiequilibriumcompression or expansion always generate entropy, and anythingthat generates entropy always destroys exergy. The exergy destroyedis proportional to the entropy generated, as can be seen from Eq. 8–31, andis expressed asX destroyed T 0 S gen 0(8–33)Note that exergy destroyed is a positive quantity for any actual process andbecomes zero for a reversible process. Exergy destroyed represents the lostwork potential and is also called the irreversibility or lost work.Equations 8–32 and 8–33 for the decrease of exergy and the exergy destructionare applicable to any kind of system undergoing any kind of process sinceany system and its surroundings can be enclosed by a sufficiently large arbitraryboundary across which there is no heat, work, and mass transfer, andthus any system and its surroundings constitute an isolated system.No actual process is truly reversible, and thus some exergy is destroyedduring a process. Therefore, the exergy of the universe, which can be consideredto be an isolated system, is continuously decreasing. The more irreversiblea process is, the larger the exergy destruction during that process.No exergy is destroyed during a reversible process (X destroyed,rev 0).The decrease of exergy principle does not imply that the exergy of a systemcannot increase. The exergy change of a system can be positive or negativeduring a process (Fig. 8–31), but exergy destroyed cannot be negative.The decrease of exergy principle can be summarized as follows:FIGURE 8–31The exergy change of a system can benegative, but the exergy destructioncannot.INTERACTIVETUTORIALSEE TUTORIAL CH. 8, SEC. 7 ON THE DVD.X destroyed •7 0Irreversible process 0Reversible process6 0Impossible process(8–34)This relation serves as an alternative criterion to determine whether aprocess is reversible, irreversible, or impossible.8–7 ■ EXERGY BALANCE: CLOSED SYSTEMSThe nature of exergy is opposite to that of entropy in that exergy can bedestroyed, but it cannot be created. Therefore, the exergy change of a systemduring a process is less than the exergy transfer by an amount equal tothe exergy destroyed during the process within the system boundaries. Thenthe decrease of exergy principle can be expressed as (Fig. 8–32)Total Total Total Change in the° exergy ¢ ° exergy ¢ ° exergy ¢ ° total exergy ¢entering leaving destroyed of the system