Thermodynamics

Chapter 8 | 425The notion that a system must go to the dead state at the end of theprocess to maximize the work output can be explained as follows: If thesystem temperature at the final state is greater than (or less than) the temperatureof the environment it is in, we can always produce additional workHOTby running a heat engine between these two temperature levels. If the finalPOTATO70°Cpressure is greater than (or less than) the pressure of the environment, wecan still obtain work by letting the system expand to the pressure of the25°Cenvironment. If the final velocity of the system is not zero, we can catchImmediatethat extra kinetic energy by a turbine and convert it to rotating shaft work, surroundings25°Cand so on. No work can be produced from a system that is initially at theEnvironmentdead state. The atmosphere around us contains a tremendous amount ofFIGURE 8–3energy. However, the atmosphere is in the dead state, and the energy it containshas no work potential (Fig. 8–4).The immediate surroundings of a hotpotato are simply the temperatureTherefore, we conclude that a system delivers the maximum possible workgradient zone of the air next to theas it undergoes a reversible process from the specified initial state to thepotato.state of its environment, that is, the dead state. This represents the usefulwork potential of the system at the specified state and is called exergy. It isimportant to realize that exergy does not represent the amount of work thata work-producing device will actually deliver upon installation. Rather, itrepresents the upper limit on the amount of work a device can deliver withoutviolating any thermodynamic laws. There will always be a difference,large or small, between exergy and the actual work delivered by a device.This difference represents the room engineers have for improvement.Note that the exergy of a system at a specified state depends on the conditionsof the environment (the dead state) as well as the properties of the system.Therefore, exergy is a property of the system–environment combinationand not of the system alone. Altering the environment is another way ofincreasing exergy, but it is definitely not an easy alternative.The term availability was made popular in the United States by the M.I.T.School of Engineering in the 1940s. Today, an equivalent term, exergy,introduced in Europe in the 1950s, has found global acceptance partlybecause it is shorter, it rhymes with energy and entropy, and it can beadapted without requiring translation. In this text the preferred term is FIGURE 8–4exergy.The atmosphere contains atremendous amount of energy, butno exergy.Exergy (Work Potential) Associated© Vol. 74/PhotoDiscwith Kinetic and Potential EnergyKinetic energy is a form of mechanical energy, and thus it can be convertedto work entirely. Therefore, the work potential or exergy of the kinetic energyof a system is equal to the kinetic energy itself regardless of the temperatureand pressure of the environment. That is,Exergy of kinetic energy: x ke ke V 2(8–1)2 1kJ>kg2where V is the velocity of the system relative to the environment.