Exergy, An International Journal最新文献

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.

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.

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.

An innovative method for the exergy efficiency calculation of a complex energy-intensive system with arbitrary structures is described in this paper. The method is based on a novel general equation to calculate the total system exergy efficiency, and on an exergy flow graph proposed by the authors. This approach allows a user to obtain not only the exergy efficiency of the total system, but also to show the relationship between the exergy efficiency of an individual element and that of the whole system. An example employing the method to the thermodynamic exergy analysis of a power plant is provided.

The present paper deals with transpiration cooling of two concentric spherical shells. The analysis includes the calculation for the radial distribution of temperature and volumetric entropy generation, and the total rate of entropy generation in the thermal system. Standard air is considered as the cooling fluid. Results showed that the entropy generation increases with increasing temperature difference between the sphere surfaces. Variation of either mass flow rate or radius ratio affects volumetric entropy distribution and the total rate of entropy generation of the processes. The increase of mass flow rate or radius ratio increases the total rate of entropy generation. The performance of the system is analyzed by calculating irreversibility to heat transfer ratio at both inner and outer sphere surfaces. It was found that irreversibility to heat transfer ratio at the inner sphere surface increases with increasing mass flow rate, or decreasing radius ratio. The opposite is true for the outer sphere surface.

Thermodynamic analysis of the thermomechanical coupling in a tangential Couette flow is presented for the Nahme number Na range of 0.5<Na<2.5. In the analysis, the temperature and velocity gradients, obtained from the series solutions in the Brinkman number Br, have been used for Newtonian fluids whose viscosity and thermal conductivity are expressed as linear functions of temperature. The entropy generation and the irreversibility distributions due to the thermomechanical coupling are evaluated and displayed graphically across the gap with asymmetric wall temperatures for the Couette flow of ethylene glycol.

A method is described to quantify the renewability of a biofuel, namely ethanol produced from corn. In the presentation, the ideal CO₂–glucose–ethanol cycle is considered to show that exergy can be potentially produced through the harnessing of natural thermochemical cycles. Then exergy accounting is used to evaluate the departure from ideal behavior caused by non-renewable resource consumption through the concept of restoration work. This procedure leads the authors to propose a renewability indicator. The different cycles and processes involved in ethanol production from corn are described. Based on the renewability indicator calculated for the overall process, for the conditions prevailing in Quebec, Canada, ethanol production is seen to be not renewable.

The thermodynamic optimization of a mechanically driven solar heat pump is presented. A new expression to describe the optimal thermal performance under finite operating conditions considering the internal and external irreversibilities during actual operation is derived. The optimum ratio between the condenser and collector–evaporator conductances (UA) determines the coefficient of performance (COP) for the maximum heating load of the system. An experimental air-R22 heat pump was used to determine the traditional performance parameters (COP and second law efficiency) which are compared with those obtained using the expressions derived in this work. Results show that the new model very closely represents the performance of real systems.

It is shown that Van Mieghems available potential energy for the atmosphere can be derived, in a local formulation, from an extension of the concept of exergy. The available potential energy can be interpreted as exergy applied to each layer of a thermally stratified atmosphere. This interpretation allows the application of well-known exergy theorems to the atmosphere and, therefore, deepens the thermodynamic insight in atmospheric energetics.