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1、1The energy content of the universe is constant, just as its mass content is. Yet at times of crisis we are bombarded with speeches and articles on how to “conserve” energy. As engineers, we know that energy is already conserved. What is not conserved is exergy, which is the useful work potential of
2、 the energy. Once the exergy is wasted, it can never be recovered. When we use energy (to heat our homes, for example), we are not destroying any energy; we are merely converting it to a less useful form, a form of less exergy.Exergy and the Dead StateThe useful work potential of a system is the amo
3、unt of energy we extract as useful work. The useful work potential of a system at the specified state is called exergy. Exergy is a property and is associated with the state of the system and the environment. A system that is in equilibrium with its surroundings has zero exergy and is said to be at
4、the dead state. The exergy of the thermal energy of thermal reservoirs is equivalent to the work output of a Carnot heat engine operating between the reservoir and the environment. Exergy FormsNow lets determine the exergy of various forms of energy. 第1页/共27页第一页，共28页。2Exergy of kinetic energy Kineti
5、c energy is a form of mechanical energy and can be converted directly into work. Kinetic energy itself is the work potential or exergy of kinetic energy independent of the temperature and pressure of the environment. Exergy of kinetic energy:Exergy of potential energy Potential energy is a form of m
6、echanical energy and can be converted directly into work. Potential energy itself is the work potential or exergy of potential energy independent of the temperature and pressure of the environment. 第2页/共27页第二页，共28页。3Exergy of potential energy:Useful WorkThe work done by work producing devices is not
7、 always entirely in a useable form. Consider the pistoncylinder device shown in the following figure.The work done by the gas expanding in the pistoncylinder device is the boundary work and can be written asThe actual work done by the gas is第3页/共27页第三页，共28页。4The word done on the surroundings isAny u
8、seful work delivered by a pistoncylinder device is due to the pressure above the atmospheric level.Reversible WorkReversible 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 spec
9、ified initial and final states. This is the useful work output (or input) obtained when the process between the initial and final states is executed in a totally reversible manner. IrreversibilityThe difference between the reversible work Wrev and the useful work Wu is due to the irreversibilities p
10、resent during the process and is called the irreversibility I. It is equivalent to the exergy destroyed and is expressed as 第4页/共27页第四页，共28页。5where Sgen is the entropy generated during the process. For a totally reversible process, the useful and reversible work terms are identical and thus irrevers
11、ibility is zero. Irreversibility can be viewed as the wasted work potential or the lost opportunity to do work. It represents the energy that could have been converted to work but was not.Exergy destroyed represents the lost work potential and is also called the wasted work or lost work.Second-Law E
12、fficiencyThe second-law efficiency is a measure of the performance of a device relative to the performance under reversible conditions for the same end states and is given by for heat engines and other workproducing devices and第5页/共27页第五页，共28页。6for refrigerators, heat pumps, and other workconsuming
13、devices. In general, the secondlaw efficiency is expressed asExergy of change of a system Consider heat transferred to or from a closed system whenever there is a temperature difference across the system boundary. The exergy for a system may be determined by considering how much of this heat transfe
14、r is converted to work entirely. Lets take a second look at the following figure.第6页/共27页第六页，共28页。7Taking the heat transfer to be from the system to its surroundings, the conservation of energy isThe work is the boundary work and can be written asAny useful work delivered by a pistoncylinder device
15、is due to the pressure above the atmospheric level.To assure the reversibility of the process, the heat transfer occurs through a reversible heat engine.第7页/共27页第七页，共28页。8Integrating from the given state (no subscript) to the dead state (0 subscript), we haveThis is the total useful work due to a sy
16、stem undergoing a reversible process from a given state to the dead state, which is the definition of exergy. Including the kinetic energy and potential energy, the exergy of a closed system is on a unit mass basis, the closed system (or nonflow) exergy is 第8页/共27页第八页，共28页。9Here, u0, v0, and s0 are
17、the properties of the system evaluated at the dead state. Note that the exergy of the internal energy of a system is zero at the dead state is zero 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 init
18、ial exergies of the system,On a unit mass basis the exergy change of a closed system is 第9页/共27页第九页，共28页。10Exergy of flow The energy needed to force mass to flow into or out of a control volume is the flow work per unit mass given by (see Chapter 3).The exergy of flow work is the excess of flow work
19、 done against atmospheric air at P0 to displace it by volume v. According to the above figure, the useful work potential due to flow work is 第10页/共27页第十页，共28页。11Thus, the exergy of flow energy isFlow Exergy Since flow energy is the sum of nonflow energy and the flow energy, the exergy of flow is the
20、 sum of the exergies of nonflow exergy and flow exergy.The flow (or stream) exergy is given by第11页/共27页第十一页，共28页。12The exergy of flow can be negative if the pressure is lower than atmospheric pressure.The exergy change of a fluid stream as it undergoes a process from state 1 to state 2 isExergy Tran
21、sfer by Heat, Work, and MassExergy can be transferred by heat, work, and mass flow, and exergy transfer accompanied by heat, work, and mass transfer are given by the following.Exergy transfer by heat transferBy the second law we know that only a portion of heat transfer at a temperature above the en
22、vironment temperature can be converted into work. The maximum useful work is produced from it by passing this heat transfer through a reversible heat engine. The exergy transfer by heat is Exergy transfer by heat:第12页/共27页第十二页，共28页。13Note in the above figure that entropy generation is always by exer
23、gy destruction and that heat transfer Q at a location at temperature T is always accompanied by entropy transfer in the amount of Q/T and exergy transfer in the amount of (1T0/T)Q. Note that exergy transfer by heat is zero for adiabatic systems.第13页/共27页第十三页，共28页。14Exergy transfer by workExergy is t
24、he useful work potential, and the exergy transfer by work can simply be expressed as Exergy transfer by work: where , P0 is atmospheric pressure, and V1 and V2 are the initial and final volumes of the system. The exergy transfer for shaft work and electrical work is equal to the work W itself. Note
25、that exergy transfer by work is zero for systems that have no work.Exergy transfer by massMass flow is a mechanism to transport exergy, entropy, and energy into or out of a system. As mass in the amount m enters or leaves a system the exergy transfer is given by Exergy transfer by mass: 第14页/共27页第十四
26、页，共28页。15Note that exergy transfer by mass is zero for systems that involve no flow.The Decrease of Exergy Principle and Exergy DestructionThe exergy of an isolated system during a process always decreases or, in the limiting case of a reversible process, remains constant. This is known as the decre
27、ase of exergy principle and is expressed as Exergy DestructionIrreversibilities such as friction, mixing, chemical reactions, heat transfer through finite temperature difference, unrestrained expansion, nonquasiequilibrium compression, or expansion always generate entropy, and anything that generate
28、s entropy always destroys exergy. The exergy destroyed is proportional to the entropy generated as expressed as 第15页/共27页第十五页，共28页。16The decrease of exergy principle does not imply that the exergy of a system cannot increase. The exergy change of a system can be positive or negative during a process
29、, but exergy destroyed cannot be negative. The decrease of exergy principle can be summarized as follows:Exergy BalancesExergy balance for any system undergoing any process can be expressed asGeneral:General, rate form:第16页/共27页第十六页，共28页。17General, unit-mass basis:whereFor a reversible process, the
30、exergy destruction term, Xdestroyed, is zero. Considering the system to be a general control volume and taking the positive direction of heat transfer to be to the system and the positive direction of work transfer to be from the system, the general exergy balance relations can be expressed more exp
31、licitly as 第17页/共27页第十七页，共28页。18where the subscripts are i = inlet, e = exit, 1 = initial state, and 2 = final state of the system. For closed systems, no mass crosses the boundaries and we omit the terms containing the sum over the inlets and exits.Example 8-1Oxygen gas is compressed in a pistoncyl
32、inder device from an initial state of 0.8 m3/kg and 25oC to a final state of 0.1 m3/kg and 287oC. Determine the reversible work input and the increase in the exergy of the oxygen during this process. Assume the surroundings to be at 25oC and 100 kPa.We assume that oxygen is an ideal gas with constan
33、t specific heats. From Table A2, R = 0.2598 kJ/kgK. The specific heat is determined at the average temperature Table A2(b) gives Cv, ave = 0.690 kJ/kgK.第18页/共27页第十八页，共28页。19The entropy change of oxygen is We calculate the reversible work input, which represents the minimum work input Wrev,in in this
34、 case, from the exergy balance by setting the exergy destruction equal to zero.第19页/共27页第十九页，共28页。20Therefore, the change in exergy and the reversible work are identical in this case. Substituting the closed system exergy relation, the reversible work input during this process is determined to beThe
35、 increase in exergy of the oxygen is 第20页/共27页第二十页，共28页。21Example 8-2Steam enters an adiabatic turbine at 6 MPa, 600C, and 80 m/s and leaves at 50 kPa, 100C, and 140 m/s. The surroundings to the turbine are at 25C. If the power output of the turbine is 5MW, determine(a)the power potential of the ste
36、am at its inlet conditions, in MW.(b) the reversible power, in MW.(c)the second law efficiency.We assume steadyflow and neglect changes in potential energy. 第21页/共27页第二十一页，共28页。22The mass flow rate of the steam is determined from the steadyflow energy equation applied to the actual process,0 (steady
37、)Conservation of mass for the steady flow givesThe work done by the turbine and the mass flow rate are第22页/共27页第二十二页，共28页。23whereFrom the steam tables: 第23页/共27页第二十三页，共28页。24The power potential of the steam at the inlet conditions is equivalent to its exergy at the inlet state. Recall that we neglec
38、t the potential energy of the flow.0 第24页/共27页第二十四页，共28页。25The power output of the turbine if there are no irreversibilities is the reversible power and is determined from the rate form of the exergy balance applied on the turbine and setting the exergy destruction term equal to zero.0 0 (steady)0 第25页/共27页第二十五页，共28页。26The secondlaw efficiency is determined from第26页/共27页第二十六页，共28页。27感谢您的观赏(gunshng)第27页/共27页第二十七页，共28页。内容(nirng)总结1。General, unit-mass basis:。感谢您的观赏(gunshng)第二十八页，共28页。
