Downloaded by 80.82.77.83 on April 5, 2017 | http://pubs.acs.org Publication Date: May 28, 1980 | doi: 10.1021/bk-1980-0122.pr001
PREFACE he usefulness of direct application of the Second Law of Thermo-1- dynamics to the planning and engineering of energy conservation is being recognized finally; for example, see the 1978 National Energy Conservation Act, Section 683. The utility of the Second Law was amply shown at a recent workshop at George Washington University which occurred on August 14H6, 1979 and was sponsored by the Department of Energy. Over the years prominent thermodynamicists have advocated Second Law analyses for properly evaluating energy-conversion processes on the basis of available energy (exergy). Available energy goes back to Maxwell and Gibbs. Unfortunately, it has not yet taken hold in engineering practice or in managerial decision making. However, it is becoming increasingly clear in the energy-conversion literature that traditional gauges of energy efficiency are unsatisfactory. The reason is that the scientific concept of energy is assumed to be the commodity of value. (In science, the word "energy" is associated with the First Law of Thermodynamics, which says that no energy is consumed (used up) by processes.) Whereas, the true resource of value is the lay concept of energy—known as available energy (or exergy)—in the scientific and engineering literature. The Second Law of Thermodynamics says that exergy is the fuel that drives processes, and that it is consumed in doing so. Various inconsistencies arise as a consequence of viewing (scientific) energy as the resource. Because energy cannot be consumed, whatever energy is supplied with fuel must end up somewhere—if not in the desired product, then in some waste. Consequently, effluent wastes are grossly overestimated in value while consumptions within processes—the major inefficiencies— are overlooked completely. For example, the usual homeheating furnace (or electric power plant boiler) appears to be very efficient (ca. 70%). For every 100 units of energy supplied with fuel, about 70 units go into the heated air and 30 units are lost with combustion gases discharged via the chimney. In actuality, such a furnace is only about 15% efficient; 30% of the fuel's exergy is consumed by the combustion process, which converts chemical exergy into thermal exergy. About 45% is consumed in the transfer of heat from the very hot products of combustion to the warm air; 10% is lost with the exhausted combustion gases. Thus, a total of 75%, not zero, is consumed, while about 10%, not 30%, is lost with the exhaust. ix Gaggioli; Thermodynamics: Second Law Analysis ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Downloaded by 80.82.77.83 on April 5, 2017 | http://pubs.acs.org Publication Date: May 28, 1980 | doi: 10.1021/bk-1980-0122.pr001
Frustrated by the inconsistencies associated with energy efficiencies, practitioners continue with new proposals of alternative definitions. Ironically, the vast majority persist in using energy as the measure of "potential to cause change." Consequently, the frustrations are destined to be perpetuated. The key to resolving this dilemma simply is to recognize that exergy is the proper measure. With exergy analysis, which involves the same calculational procedures as energy analysis, the true inefficiencies and losses can be determined. The concept of exergy is crucial not only to efficiency studies but also to cost accounting and economic analyses. Costs should reflect value; since the value is not in energy but in exergy, assignment of cost to energy leads to misappropriations, which are common and often gross. Using exergy content as a basis for cost accounting is important to management for pricing products and for their evaluation of profits. It is also useful to engineering for operating and design decisions, including design optimization. Thus, exergy is the only rational basis for evaluating: fuels and re sources; process, device, and system efficiencies; dissipations and their costs; and the value and cost of system outputs. The chapters in this symposium volume illustrate the usefulness and develop the methodology of such Second Law analyses, now made much more comprehensible as a result of recent progress in Thermo dynamics; survey the results of efficiency analyses of a variety of proc esses, devices, systems, and economic sectors; and teach the methods of engineering application of exergy to efficiency analysis and costing. While baring many misconceptions resulting from energy analyses, the results of the efficiency analyses show great potential for alleviating the energy problem via conservation—even moreso over the intermediate and long term than over the short term—and pinpoint where the oppor tunities are. In turn, the cost analyses show how economic analysis decisions regarding energy systems can be facilitated greatly, while avoiding the misappropriations, which are often gross, that result from energy analyses. The symposium volume will be valuable to energy and process engineers involved in design and in operating decisions, to managers in the private and government sectors who are involved with energy use and development, and to public service commissions. Marquette University 1515 W. Wisconsin Ave. Milwaukee, W I 53233
RICHARD A . GAGGIOLI
October 17, 1979 χ Gaggioli; Thermodynamics: Second Law Analysis ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
