Exergy, An International Journal最新文献

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.

In general the field of exergy analysis is both well formulated and well understood. However, the exergy flux, or maximum work obtainable, from thermal radiation (TR) heat transfer has not been clearly formulated. In a previous article it was shown that Petela's result, from his thermodynamic approach, does in fact represent the exergy flux of blackbody radiation (BR) and the upper limit to the conversion of solar radiation (SR) fluxes approximated as BR. This conclusion was obtained by resolving a number of fundamental issues including questions relating to: inherent irreversibility, definition of the environment, the effect of inherent emission and the effect of concentrating source radiation. In this paper, a new expression based on inherent irreversibility is presented for the exergy flux of TR with an arbitrary spectrum. It is shown that previous approaches by Petela and Karlsson are equivalent and assume that reversible conversion of non-blackbody radiation (NBR) is theoretically possible. However, evidence is presented indicating that the conversion of NBR is inherently irreversible. The analysis in this paper emphasizes the proper formulation for TR exergy by re-stating the general exergy balance equation for thermodynamic systems so that it correctly applies to NBR heat transfer. Finally, it is shown that the exergy flux of NBR, or the maximum work obtainable from NBR conversion, can be a small fraction of the energy flux.

The building of an industrial society can be viewed as a process of self-organisation with a decrease in entropy in society and a corresponding increase of entropy through dissipation of energy into the environment. The process is driven by the “degradation” of high quality energy to low-quality heat as energy flows down potential gradients at the same time creating a favourable potential gradient driving the reaction. The post-industrial society is characterised by an increase in complexity, which can be monitored by the exergy consumption. Here a first attempt is made to relate the complexity of a number of products, as represented by the number of their functional parts, to their actual economic value.

A computational model based on the exergy analysis is presented for the investigation of the effects of the evaporating and condensing temperatures on the pressure losses, the exergy losses, the second law of efficiency, and the coefficient of performance (COP) of a vapor compression refrigeration cycle. It is found that the evaporating and condensing temperatures have strong effects on the exergy losses in the evaporator and condenser, and on the second law of efficiency and COP of the cycle but little effects on the exergy losses in the compressor and the expansion valve. The second law efficiency and the COP increases, and the total exergy loss decreases with decreasing temperature difference between the evaporator and refrigerated space and between the condenser and outside air.

The author explains his views that the public is often confused when it discusses energy, and needs to be better educated about exergy if energy issues and problems are to be addressed appropriately.

The present study describes details of exergy-based performance characteristics of a heavy-duty gas turbine, 150MW-class GE 7F model. Results have shown that a chemical reaction in the combustor of which the exergy destruction ratio is 28.3% at full-load is one of the major sources of exergy destructions in the gas turbine. It was found that, in spite of its usefulness to the performance enhancement of the combined cycle plant in part-load operations, the variable inlet guide vane located in front of the multi-stage compressor caused the increase of exergy destruction in the first stage (about 10 times lager than that of other stages below 80% load) and decreased the overall compressor efficiency. Also, it was discovered that the magnitude of exergy destruction by the cooling air in turbine stages is large enough to influence the overall turbine efficiency. The exergy destruction by the cooling air is more than half of the total exergy destruction of each cooled turbine stage.

Classical approaches to the formulation of the defining equation for entropy are presented in this paper that eliminate dependence on the arbitrary treatment of the Carnot function f(θ) that has long existed and is featured in modern thermodynamic textbooks.

The author explains his views that aspects of exergy relate to government policies in a variety of fields, including natural resources, energy, environment and industrial development, and that our governments need to use—or be encouraged to use—exergy in establishing public policies, to increase the benefits they bring.

When two thermodynamic systems at different states are mixed, the exergy contend of the combined “bigger” system may actually be smaller than the exergy content of either of the two systems. Therefore, from the second-law point of view, mixing of systems should be avoided unless the systems being mixed are nearly at the same state. In this paper, we examine the merging and breaking up of families, companies, and states using the entropy generation and exergy destruction associated with various mixing processes of thermodynamic systems as a guide.

In analogy to thermodynamic systems, we present arguments that the more dissimilar are the items being merged, the larger the destruction of the figure of merit or exergy. Therefore, forcing very dissimilar things into a unity may create highly destructive situations. Also, things that are similar in some aspects and dissimilar in other aspects should be combined only partially, involving the similar aspects only. The individual items should maintain their individuality in regard to the dissimilar aspects to avoid destruction. It is also pointed out that breaking up of countries, companies, and even families with irreconcilable differences may sometimes be the best thing to do, and each part of the whole may be much better off after the break-up.