Exergy, An International Journal最新文献

Sulfuric acid is one of the most important products in inorganic synthesis. Half of the manufactured amount of sulfuric acid is used for the production of mineral fertilizers. In this paper, a technological scheme for the manufacture of sulfuric acid from liquid sulfur using double contacting with intermediate absorption is studied. The production method allows the increase of the degree of oxidation of sulfur dioxide up to 99.5%, and the decrease of its content in the waste gas. For evaluation of the exergy efficiency of the technological scheme investigated, one of the modern methods of thermodynamic analysis-exergy method is used. Exergy balance of the whole manufacture and its individual steps is done. Internal and external exergy losses, exergy efficiency are calculated. The overall exergy efficiency is 55.15%. The main sources for losses are determined, and the methods for minimizing losses are described.

In this paper, a performance analysis and optimization based on the ecological criterion has been performed for an air-standard endoreversible internal combustion engine dual cycle coupled to constant temperature heat reservoirs. The ecological objective function, defined as the power output minus the loss rate of availability is taken as the optimization criterion. The optimal performances and design parameters, such as compression ratio, pressure ratio, cut-off ratio and NTU allocation ratio, which maximize the ecological objective function are investigated. The obtained results are compared with those of the maximum power performance criterion. Since the ecological optimization technique for a dual cycle consists of both power and entropy generation rate, the obtained results lead more realistic design from the point of view of preservation of natural resources.

In a heat exchange process, heat transfer and pumping power requirements are the two main considerations. Efforts made to increase heat transfer in a fluid flow usually cause increase in the pumping power requirement. In an effort to avoid inefficient utilization of energy through excessive entropy generation, a thermodynamic analysis of turbulent fluid flow through a smooth duct subjected to constant heat flux has been made in this study. The temperature dependence of the viscosity was taken into consideration in determining the heat transfer coefficient and friction factor. It was shown that the viscosity variation has a considerable effect on both the entropy generation and the pumping power. Pumping power to heat transfer ratio and the entropy generation per unit heat transfer can become very large especially for low heat flux conditions.

The reasons of the considerable decrease of COP of a real heat pump in comparison with the Carnot heat pump have been expressed by means of the component thermodynamic efficiencies which determine the influence of the particular irreversible processes on the coefficient of performance of the considered heat pump. The COP of the total installation is expressed as a product of component efficiencies. A very large deleterious impact of irreversible heat transfer processes is demonstrated. Component efficiencies indicate the possibilities of the improvement of the installation.

The author explains his views that we need to clarify better what we mean by the terms efficiency and loss, and utilize exergy-based—rather than energy-based—measures of these quantities in order to ensure that the measures are meaningful and useful.

This paper examines material emissions produced during thirteen fuel life cycles for automobiles, on mass and exergy bases. The masses of fuel life cycle emissions are compared with the chemical exergies of these emissions. For the emissions data used, the chemical exergy results suggest that compressed natural gas use in motor vehicles produces emissions that are the most out of equilibrium with the natural environment, relative to all other fuel life cycle paths considered. It is also shown that diesel use in grid-independent hybrid electric vehicles has the lowest chemical exergies of emissions of all thirteen fuel-vehicle combinations under consideration, suggesting a lower degree of potential environmental impact. The exergy methodology presented for assessing the potential for environmental impact may help in the development and design of transportation technologies that are more environmentally benign than those presently used.

On the basis of endoreversible absorption refrigeration cycle model with the sole irreversibility of heat transfer between the working fluid and the heat reservoirs, an irreversible model of absorption refrigeration cycle with heat transfer law of q∝Δ(T⁻¹), which includes the heat leak from the heat sink to the cooled space and irreversibilities due to the internal dissipation of the working fluid besides the finite-rate heat transfer between the working fluid and the external heat reservoirs, is established and used to derive the relation between the optimal coefficient of performance and the cooling load and the optimal distribution of the heat-transfer surface areas of the heat exchangers. The practical optimal regions of the cycle are determined and new bounds of the primary performance parameters are given. A numerical example is provided to illustrate the performance characteristic of endoreversible and irreversible cycles.

A second-law-based thermoeconomic model of a sensible heat-storage system with Joulean heating is derived and discussed in which the storage element is cooled by flowing stream of gases. In this analysis, unit cost values are attached to the irreversible losses caused by the finite-temperature difference heat transfer and pressure drop during the heat removal process. Important dimensionless parameters are identified and the results are presented in terms of the optimum number of heat transfer units (NTU_opt) as a function of the dimensionless unit cost per unit heat conductance (γ_UA) and dimensionless temperature difference (τ) of the storage systems. The storage systems studied are optimized by introducing a new performance criterion described as the cost rate number $\text{(Γ}{}^{\mathrm{\ast }}\text{)}$. Performance results of low- and high-temperature storage systems are also examined and the results are compared with that obtained from Krane's analysis to illustrate the usefulness of the present approach. The influence of important unit cost parameters on NTU_opt and $\text{Γ}{}_{\mathrm{<mtext>opt</mtext>}}{}^{\mathrm{\ast }}$ are also studied in somewhat more detail.

The waste exergy approach to quantitative comparison of environmental impacts is considerably improved by proposing a separate accounting of material and energetic waste exergy and the implications are discussed within the context of sustainability. The exergy of mixing of a waste stream is found to be particularly well suited to an exergetic definition of chemical pollution and a correlative relationship with environmental pollutant cost (EPC) is suggested. A comprehensive measure of chemical environmental impact called pollution potential is defined as temperature multiplied by the change in configurational entropy per mole of pollutant in the environment. The result is related to the ideal thermodynamic work of chemical separation per mole required to instantaneously revoke a chemical pollutant, thereby returning the environment to a pristine reference condition. The current pollution potentials and total exergy of revocation of several notable atmospheric pollutants are estimated. Carbon dioxide is found to have low pollution potential in comparison to most halogenated hydrocarbons, but the vast quantities of anthropogenic carbon dioxide in the atmosphere would require much more total exergy of separation to revoke.

The author explains his views that industry, the part of society to which exergy methods seem most directed, can benefit greatly by using exergy for design, efficiency improvement and related activities, but that industry needs to be helped in overcoming barriers to the successful utilization of exergy methods.