Exergy, An International Journal最新文献

The author explains his views that the understanding we have developed of the relations between exergy and economics—and the tools that have correspondingly been created—are great successes, but that these areas need to be further developed and somewhat simplified to permit industry to apply them more widely and beneficially.

The author explains his views that, to better understand and address environmental concerns, we need to focus on the linkages between exergy and the environment, and that much more research is needed in this area if the benefits—which are potentially immense—are to be fully tapped.

Mixing, in general, is an irreversible process, and some entropy is generated and thus some exergy is destroyed during such a process. Therefore, combining two systems thermodynamically that are at different states may yield a system that is larger in size, but much smaller in exergy content or “usefulness”. In this paper we consider some mixing processes, and show that getting bigger is not necessarily better by examining the effect of mixing on exergy destruction.

The question of how much work is lost in an adiabatic turbine due to its irreversibilities finds different answers when discussed on basis of the isentropic efficiency, or with the exergy method. In this contribution, we seek to clarify why the two viewpoints lead to quite distinct results for the lost work. In particular, we discuss how the “reversible work” of the exergy method could be realized and how to recover the “recoverable work of friction”. The difference between both approaches is explained.

The author explains his views that we need to avoid the confusion and misleadingness of the term energy crisis when describing certain energy-related problems, perhaps through use of exergy crisis as an alternative, if we are to address such crises properly and effectively when they arise in the future.

Laser pulse heating offers considerable advantages over the conventional heating methods. This is due to their operational precision and local treatment. In order to improve the energy efficient processing, entropy analysis of the laser heating is essential. In the present study, laser pulse heating with evaporation at the surface is presented analytically. The entropy due to laser step input pulse intensity is formulated and entropy generation number inside the substrate material is computed for different heating periods. It is found that the entropy generation increases in the early heating period and this case is more pronounced as the depth below the surface increases towards the solid bulk. The behavior of the entropy generation number is similar to that corresponding to the entropy generation rate.

The non-reversible heat transfer between two fluid streams is a complex problem requiring many data and becomes more complicated if the two streams involved in the process include two-phase and two-component fluids.This paper is presented to make some thermodynamic remarks and, in particular, to show that along a heat exchanger, in whatever section normal to the flow rate, every non-equilibrium fluid state can be represented by its corresponding equilibrium state and a nonequilibrium–equilibrium deviation measured by the corresponding entropy difference or essergy difference. Within this general statement, somewhat different results are obtained in the cases of single-phase fluids, two-phase one-component fluids, two-phase two-component fluids, and mixtures of a single-phase fluid and a two-phase fluid. It is necessary to point out that the concepts of “maximum obtainable work” and of “distance from equilibrium” have been often associated, directly or implicitly, to the concept of exergy, also in good books, that have considered exergy as the basic argument. The analysis developed by Evans and by others showed that not always the two concepts can be represented by a unique parameter. In the presence of non-equilibrium states in the system, the hypothesis of a reversible way cannot be assumed, not even as a limit. Thus, it was suitable the definition of essergy as a potential which never increases in the system time evolution and which represents the distance of the system state from the environment state. In addition, it is to be remarked that, if one determine the essergy ε for a system and F is a whatever strictly increasing function, also F∘ε is an essergy parameter with the same properties of the parameter ε.

The maximum and minimum temperatures available limit the usable fraction (or Carnot efficiency) of a power cycle. The construction of LNG terminals and the need to vaporize LNG offers a thermal sink at a very much lower temperature than seawater. By using this thermal sink in a combined plant, it is possible to recover power from the vaporization of LNG.

To this purpose, in this paper combined systems using LNG vaporization as low-temperature thermal sink are considered and their pros and cons are presented. A system utilizing waste energy as heat source and with a single working fluid is analyzed in detail. However, the use of a single fluid is not the best solution from a thermodynamic point of view. Thus, a series of cascading cycles is also outlined. In these systems, both the thermal source and the thermal sink are exploited as exergy sources.

A performance optimization of a two-stage irreversible combined heat-pump system has been carried out. The irreversibility of heat transfer across finite temperature differences, the heat-leak loss between the external heat reservoirs and the internal dissipation of the working fluids are considered. The heating load per unit total cost is taken as an objective function for the optimization. The maximum of the objective function and the corresponding optimal performance and design parameters have been derived analytically. The global and the optimal performance characteristics curves are presented in terms of technical and economical parameters. The irreversibility effects and economical aspects on the general and optimal performances have been investigated.

An analytical work has been performed to study the First and Second Laws (of thermodynamics) characteristics of flow and heat transfer inside a vertical channel made of two parallel plates embedded in a porous medium and under the action of transverse magnetic field. Combined free and forced convection inside the channel is considered. Flow is assumed to be steady, laminar, fully developed of electrically conducting and heat-generating/absorbing fluid. Both vertical walls are kept isothermal at the same or different temperatures. Governing equations in Cartesian coordinate are simplified and solved analytically to develop expressions for velocity and temperature, entropy generation number and irreversibility distribution ratio. Velocity, temperature and entropy generation profiles are presented graphically.