Exergy, An International Journal最新文献

This paper deals with a thermodynamic analysis of Belousov–Zhabotinski systems and similar ones with four or five independent variables, based on some experimental results obtained in our laboratory with a Rayleigh–Bénard cell. The new variable, never before considered in similar analyses is the relative extent of the chemical potential difference between two next layers in these Belousov–Zhabotinski systems. The mathematical approach shows some differences with regards to the traditional one. These analyses can be applied both to non-living and also to several cases of living systems. It is to be remarked that up to recent years, the researchers thought that the reactions, which take place in test-tubes and in the several laboratory vessels, could not show oscillations that are common in other fields of science. Consequently the oscillations, which were found in the Belousov–Zhabotinski systems, had a great momentum from a theoretical, even if not from a practical point of view. The analysis of Glansdorff and Prigogine was one of the motivations of this study.

In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as objective for performance analysis of an irreversible closed Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite time thermodynamics (FTT) or entropy generation minimization (EGM). The analytical formulas about the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the irreversible compression and expansion losses in the compressor and turbine. The obtained results are compared with those results obtained by using the maximum power criterion. The influences of some design parameters on the maximum power density are provided by numerical examples, and the advantages and disadvantages of maximum power density design are analyzed. The power plant design with maximum power density leads to a higher efficiency and smaller size. However, the maximum power density design requires a higher pressure ratio than maximum power design. When the heat transfer is carried out ideally, the results of this paper become those obtained in recent literature.

The performance optimization of an endoreversible Braysson cycle with heat resistance losses in the hot- and cold-side heat exchangers is performed by using finite-time thermodynamics. The relations between the power output and the working fluid temperature ratio, between the power density and the working fluid temperature ratio, as well as between the efficiency and the working fluid temperature ratio of the cycle coupled to constant-temperature heat reservoirs are derived. Moreover, the optimum heat conductance distributions corresponding to the optimum dimensionless power output, the optimum dimensionless power density and the optimum thermal efficiency of the cycle, and the optimum working fluid temperature ratios corresponding to the optimum dimensionless power output and the optimum dimensionless power density are provided. The effects of various design parameters on those optimum values are studied by detailed numerical examples.

Using energy and exergy analyses, a dynamic simulation is carried out with a CaO/Ca(OH)₂ chemical heat pump system for heating and cooling applications. The system consists of hydration/dehydration reactor connected to condenser/evaporator with a control valve in between. During the dehydration process, heat is supplied at 700 K for dehydration of Ca(OH)₂ and steam is condensed at 293 K. During evaporation/hydration process, heat is supplied at 290 K for evaporation of water at 273 K and heat of hydration is supplied to a load at 353 K. Duration of one cycle takes about 12 hours. Two subsystems are used to provide for heating/cooling demands in a continuous manner. Using synthetic demands of a residential dwelling, various performance parameters have been calculated for a 24 hour period. The results showed that CaO/Ca(OH)₂ chemical heat pump system could satisfy heating and cooling demands of a typical dwelling. Its energy and exergy efficiencies were 58.7% and 61.6% for heating and 12.7% and 4.5% for cooling respectively.

We investigate analytically the First and Second Laws (of thermodynamics) characteristics of fluid flow and heat transfer inside a cylindrical annulus. Inside the annular gap, the relative rotational motion between the inner and outer cylinders induces fluid motion. The net surface heat flux is always equal at the inner and outer cylinders, but it is varied in magnitude. Simplified governing equations in cylindrical coordinates are solved to obtain analytical expressions for dimensionless entropy generation number (N_S), irreversibility distribution ratio (Φ), and Bejan number (Be) as a function of flow governing and geometric parameters. Spatial distributions of local and average entropy generation rate, and heat transfer irreversibility, are presented graphically. The effects of velocity ratio (λ), group parameter $\text{(Br/Ω)}$, and Brinkman number (Br) on the above parameters are tested.

Exergy analysis is a highly effective method of analysis for thermal processes because it provides insight that cannot be obtained from energy analysis alone. In general the field of exergy analysis is both well formulated and well understood. However, the exergy flux, or the maximum work obtainable, from thermal radiation (TR) heat transfer has not been clearly formulated. Several researchers claim that Petela's thermodynamic approach for determining the maximum work obtainable from radiation is irrelevant to the conversion of fluxes because it appears to neglect a number of fundamental issues—issues that are unusual in the context of exergy analysis. In this paper it is shown that Petela's result gives the exergy flux of blackbody radiation (BR) and represents the upper limit to the conversion of solar radiation (SR) approximated as BR. This conclusion is obtained by resolving a number of fundamental questions including that of: inherent irreversibility, definition of the environment, the effect of inherent emission and the effect of concentrating source radiation. Correctly identifying the exergy flux of TR allows the general exergy balance equation for a control volume to be re-stated so that it correctly applies to TR heat transfer. An ideal (reversible) thermal conversion process for BR fluxes is also presented. Finally, exergetic (second-law) efficiencies are presented for common solar energy conversion processes such as single-cell photovoltaics

The author explains his views that better coverage of exergy is needed to improve thermodynamics education and to make it more interesting to students, and that a basic level of “exergy literacy” is needed among engineers and scientists—particularly those involved in decision making.

This paper outlines recent thermodynamic optimization work on the geometric layout of schemes for distributing hot water and exergy over a large system. Constrained are the amount of insulation material, the volume of all the pipes, and the amount of pipe wall material. Unknown are the distribution of insulation over all the links of the network, and the configuration of the network itself. The main focus is on how the geometric configuration may be selected in the pursuit of maximized global performance, and how closely a non-optimal configuration performs to the highest level. Maximum global performance means minimum heat loss to the ambient, minimum pressure loss, and minimum exergy destruction. Three configurations are optimized:

• (a)

  an area covered by a coiled steam, where all the users are aligned on the same stream,

• (b)

  a sequence of tree-shaped flows on square areas in which each area construct is made up of four smaller area constructs, and

• (c)

  a sequence of tree-shaped flows where each area construct is made up of two smaller area constructs.

Energy and exergy analyses are conducted of the fluidized bed drying of moist materials for optimizing the operating conditions and the quality of the products. In this regard, energy and exergy models are developed to evaluate energy and exergy efficiencies, and are then verified with experimental data (for the product, wheat) taken from the literature. The effects of inlet air temperature, fluidization velocity, and initial moisture content on both energy and exergy efficiencies are studied. Furthermore, the hydrodynamic aspects, e.g., the bed hold up, are also studied. The results show that exergy efficiencies are less than energy efficiencies due to irreversibilities which are not taken into consideration in energy analysis, and that both energy and exergy efficiencies decrease with increasing drying time.