Thermodynamics

Chapter 8 | 469The arguments presented here are exploratory in nature, and they arehoped to initiate some interesting discussions and research that may lead intobetter understanding of performance in various aspects of daily life. The secondlaw may eventually be used to determine quantitatively the most effectiveway to improve the quality of life and performance in daily life, as it ispresently used to improve the performance of engineering systems.SUMMARYThe energy content of the universe is constant, just as itsmass content is. Yet at times of crisis we are bombarded withspeeches and articles on how to “conserve” energy. As engineers,we know that energy is already conserved. What is notconserved is exergy, which is the useful work potential of theenergy. Once the exergy is wasted, it can never be recovered.When we use energy (to heat our homes for example), we arenot destroying any energy; we are merely converting it to aless useful form, a form of less exergy.The useful work potential of a system at the specified stateis called exergy. Exergy is a property and is associated withthe state of the system and the environment. A system that isin equilibrium with its surroundings has zero exergy and issaid to be at the dead state. The exergy of heat supplied bythermal energy reservoirs is equivalent to the work output ofa Carnot heat engine operating between the reservoir and theenvironment.Reversible work W rev is defined as the maximum amountof useful work that can be produced (or the minimum workthat needs to be supplied) as a system undergoes a processbetween the specified initial and final states. This is the usefulwork output (or input) obtained when the process betweenthe initial and final states is executed in a totally reversiblemanner. The difference between the reversible work W revand the useful work W u is due to the irreversibilities presentduring the process and is called the irreversibility I. It isequivalent to the exergy destroyed and is expressed asI X destroyed T 0 S gen W rev,out W u,out W u,in W rev,inwhere S gen is the entropy generated during the process. For atotally reversible process, the useful and reversible workterms are identical and thus exergy destruction is zero.Exergy destroyed represents the lost work potential and isalso called the wasted work or lost work.The second-law efficiency is a measure of the performanceof a device relative to the performance under reversible conditionsfor the same end states and is given byh II h thh th,rev W uW revfor heat engines and other work-producing devices andfor refrigerators, heat pumps, and other work-consumingdevices. In general, the second-law efficiency is expressed asThe exergies of a fixed mass (nonflow exergy) and of a flowstream are expressed asNonflow exergy:Flow exergy: c 1h h 0 2 T 0 1s s 0 2 V22 gzThen the exergy change of a fixed mass or fluid stream as itundergoes a process from state 1 to state 2 is given byExergy can be transferred by heat, work, and mass flow, andexergy transfer accompanied by heat, work, and mass transferare given byExergytransferby heat:h II Exergy recoveredh II Exergy suppliedf 1u u 0 2 P 0 1v v 0 2 T 0 1s s 0 2 V 2¢X X 2 X 1 m 1f 2 f 1 2 1E 2 E 1 2 P 0 1V 2 V 1 2 T 0 1S 2 S 1 2 1U 2 U 1 2 P 0 1V 2 V 1 2 T 0 1S 2 S 1 2 m V 2 2 V 2 12¢c c 2 c 1 1h 2 h 1 2 T 0 1s 2 s 1 2 V 2 2 V 2 12 1e e 0 2 P 0 1v v 0 2 T 0 1s s 0 2 g 1z 2 z 1 2X heat a 1 T 0T b QCOPCOP rev W revW u 1 Exergy destroyedexergy supplied mg 1z 2 z 1 22 gz