Yunus A. Çengel,
University of Nevada, Reno
Michael A. Boles,
North Carolina State University
| Dead state | is a state a system is said to be in when it is in thermodynamic equilibrium with its environment.
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| Decrease of exergy principle | can be expressed as the exergy of an isolated system during a process always decreases or, in the limiting case of a reversible process, remains constant. In other words, it never increases and exergy is destroyed during an actual process. For an isolated system, the decrease in exergy equals exergy destroyed.
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| Environment | refers to the region beyond the immediate surroundings whose properties are not affected by the process at any point.
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| Exergy (availability or available energy) | is property used to determine the useful work potential of a given amount of energy at some specified state. It is important to realize that exergy does not represent the amount of work that a work-producing device will actually deliver upon installation. Rather, it represents the upper limit on the amount of work a device can deliver without violating any thermodynamic laws.
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| Exergy balance | can be stated as the exergy change of a system during a process is equal to the difference between the net exergy transfer through the system boundary and the exergy destroyed within the system boundaries as a result of irreversibilities (or entropy generation).
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| Exergy balance for a control volume | is stated as the rate of exergy change within the control volume during a process is equal to the rate of net exergy transfer through the control volume boundary by heat, work, and mass flow minus the rate of exergy destruction within the boundaries of the control volume as a result of irreversibilities.
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| Exergy destroyed | is proportional to the entropy generated and is expressed as Xdestroyed = T0Sgen³ 0. Irreversibilities such as friction, mixing, chemical reactions, heat transfer through a finite temperature difference, unrestrained expansion, non-quasi-equilibrium compression, or expansion always generate entropy, and anything that generates entropy always destroys exergy.
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| Exergy of a closed system (or nonflow system) | of mass m is X = (U - U0) + P0(V - V0) - T0(S - S0) + m <![CDATA[<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::Image 107::/sites/dl/free/0072383321/22362/Image107.gif','popWin', 'width=37,height=38,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif">Image 107 (0.0K)</a>]]>Image 107 /2 + mgz.On a unit mass basis, the exergy of a closed system is expressed as f= (u - u0) + P0(v - v0) - T0(s - s0) + <![CDATA[<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::Image 107::/sites/dl/free/0072383321/22362/Image107.gif','popWin', 'width=37,height=38,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif">Image 107 (0.0K)</a>]]>Image 107 /2 + gz where u0, v0, and s0 are the properties of the systemevaluated at the dead state. Note that the exergy of a system is zero at the dead state since u = u0, v = v0, and s = s0 at that state. The exergy change of a closed system during a process is simply the difference between the final and initial exergies of the system.
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| Exergy of the kinetic energy | (work potential) of a system is equal to the kinetic energy itself regardless of the temperature and pressure of the environment.
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| Exergy of the potential energy | (work potential) of a system is equal to the potential energy itself regardless of the temperature and pressure of the environment.
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| Exergy transfer by heat | Xheat is the exergy as the result of heat transfer Q at a location at absolute temperature T in the amount of Xheat = (1-T0/T)Q.
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| Exergy transfer by work | is the useful work potential expressed as Xwork = W - Wsurr for closed systems experiencing boundary work where Wsurr = P0(v2 - v1) and P0 is atmospheric pressure, and V1 and V2 are the initial and final volumes of the system, and Xwork = W for other forms of work.
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| Exergy transport by mass | results from mass in the amount of m entering or leaving a system and carries exergy in the amount of my, where y = (h - h0) - T0(s - s0) + <![CDATA[<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::Image 107::/sites/dl/free/0072383321/22362/Image107.gif','popWin', 'width=37,height=38,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif">Image 107 (0.0K)</a>]]>Image 107 /2 + gz, accompanies it. Therefore, the exergy of a system increases by my when mass in the amount of m enters, and decreases by the same amount when the same amount of mass at the same state leaves the system.
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| Immediate surroundings | refer to the portion of the surroundings that is affected by the process.
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| Irreversibility | I is any difference between the reversible work Wrev and the useful work Wu due to the irreversibilities present during the process. Irreversibility can be viewed as the wasted work potential or the lost opportunity to do work.
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| Reversible work | Wrev is defined as the maximum amount of useful work that can be produced (or the minimum work that needs to be supplied) as a system undergoes a process between the specified initial and final states. Reversible work is determined from the exergy balance relations by setting the exergy destroyed equal to zero. The work W in that case becomes the reversible work.
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| Second-law efficiency hII | is the ratio of the actual thermal efficiency to the maximum possible (reversible) thermal efficiency under the same conditions. The second-law efficiency of various steady-flow devices can be determined from its general definition, hII = (exergy recovered)/(exergy supplied).
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| Surroundings work | is the work done by or against the surroundings during a process.
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| Useful work | Wuis the difference between the actual work W and the surroundings work Wsurr.
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| Useful work potential | is the maximum possible work that a system will deliver as it undergoes a reversible process from the specified initial state to the state of its environment, that is, the dead state.
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2002 McGraw-Hill Higher Education