Thermodynamics

424 | ThermodynamicsAIR25°C101 kPaV = 0z = 0INTERACTIVETUTORIALSEE TUTORIAL CH. 8, SEC. 1 ON THE DVD.T 0 = 25°CP 0 = 101 kPaFIGURE 8–1A system that is in equilibrium with itsenvironment is said to be at the deadstate.FIGURE 8–2At the dead state, the useful workpotential (exergy) of a system is zero.© Reprinted with special permission of KingFeatures Syndicate.8–1 ■ EXERGY: WORK POTENTIAL OF ENERGYWhen a new energy source, such as a geothermal well, is discovered, thefirst thing the explorers do is estimate the amount of energy contained in thesource. This information alone, however, is of little value in decidingwhether to build a power plant on that site. What we really need to know isthe work potential of the source—that is, the amount of energy we canextract as useful work. The rest of the energy is eventually discarded aswaste energy and is not worthy of our consideration. Thus, it would be verydesirable to have a property to enable us to determine the useful workpotential of a given amount of energy at some specified state. This propertyis exergy, which is also called the availability or available energy.The work potential of the energy contained in a system at a specified stateis simply the maximum useful work that can be obtained from the system.You will recall that the work done during a process depends on the initialstate, the final state, and the process path. That is,Work f 1initial state, process path, final state2In an exergy analysis, the initial state is specified, and thus it is not a variable.The work output is maximized when the process between two specifiedstates is executed in a reversible manner, as shown in Chap. 7. Therefore, allthe irreversibilities are disregarded in determining the work potential.Finally, the system must be in the dead state at the end of the process tomaximize the work output.A system is said to be in the dead state when it is in thermodynamic equilibriumwith the environment it is in (Fig. 8–1). At the dead state, a system isat the temperature and pressure of its environment (in thermal and mechanicalequilibrium); it has no kinetic or potential energy relative to the environment(zero velocity and zero elevation above a reference level); and it does notreact with the environment (chemically inert). Also, there are no unbalancedmagnetic, electrical, and surface tension effects between the system and itssurroundings, if these are relevant to the situation at hand. The properties ofa system at the dead state are denoted by subscript zero, for example, P 0 , T 0 ,h 0 , u 0 , and s 0 . Unless specified otherwise, the dead-state temperature andpressure are taken to be T 0 25°C (77°F) and P 0 1 atm (101.325 kPa or14.7 psia). A system has zero exergy at the dead state (Fig. 8–2).Distinction should be made between the surroundings, immediate surroundings,and the environment. By definition, surroundings are everythingoutside the system boundaries. The immediate surroundings refer to theportion of the surroundings that is affected by the process, and environmentrefers to the region beyond the immediate surroundings whose propertiesare not affected by the process at any point. Therefore, any irreversibilitiesduring a process occur within the system and its immediate surroundings,and the environment is free of any irreversibilities. When analyzing thecooling of a hot baked potato in a room at 25°C, for example, the warm airthat surrounds the potato is the immediate surroundings, and the remainingpart of the room air at 25°C is the environment. Note that the temperature ofthe immediate surroundings changes from the temperature of the potato atthe boundary to the environment temperature of 25°C (Fig. 8–3).