Exergy, An International Journal最新文献

This review draws attention to an emerging body of work that relies on global thermodynamic optimization in the pursuit of flow system architecture. Exergy analysis establishes the theoretical performance limit. Thermodynamic optimization (or entropy generation minimization) brings the design as closely as permissible to the theoretical limit. The design is destined to remain imperfect because of constraints (finite sizes, times, and costs). Improvements are registered by spreading the imperfection (e.g., flow resistances) through the system. Resistances compete against each other and must be optimized together. Optimal spreading means spatial distribution, geometric form, topology, and geography. System architecture springs out of constrained global optimization. The principle is illustrated by simple examples: the optimization of dimensions, spacings, and the distribution (allocation) of heat transfer surface to the two heat exchangers of a power plant. Similar opportunities for deducing flow architecture exist in more complex systems for power and refrigeration. Examples show that the complete structure of heat exchangers for environmental control systems of aircraft can be derived based on this principle.

This second part is the continuation of Wall and Gong [Exergy Internat. J. 1 (3) (2001), in press]. This part is an overview of a number of different methods based on concepts presented in the first part and applies these to real systems. A number of ecological indicators will be presented and the concept of sustainable development will be further clarified. The method of Life Cycle Exergy Analysis will be presented. Exergy will be applied to emissions into the environment by case studies in order to describe and evaluate its values and limitation as an ecological indicator. Exergy is concluded to be a suitable ecological indicator and future research in this area is strongly recommended.

The effects of conduction and heat generation due to viscous dissipation in annuli is a topic of interest to researchers, since the applications include aerospace, food processing, electric machines, etc. In the present study, temperature rise and entropy generation in a cylindrical annuli due to conduction and viscous dissipation are considered. In the mathematical formulation, the flow developed in the annuli is assumed as laminar, since Re is selected as $\text{≤50}\text{000}$. The wall temperature of the rotating cylinder is taken as higher than the wall temperature of the stationary inner cylinder. The temperature and entropy profiles are predicted for different Brinkman numbers and temperature difference across the annuli. It is found that as Br increases, the temperature in the fluid close to the rotating cylinder wall becomes higher than the wall temperature. This results in zero temperature gradient and the entropy generation reduces to zero in this region. The point of minimum entropy generation in the fluid moves away from the outer cylinder wall as Br increases. Moreover, the efficient operation and design of bearing systems can be possible with the analysis of entropy generation.

The thermodynamic interpretation of ecosystem evolution introduced in Part 1 of this series provides a basis for quantitative analysis of strategies for reducing resource depletion. In Part 2, we express resource depletion rate as a product of consumption rate and the Depletion number (Dp)—a non-dimensional indicator of depletion per unit consumption. We introduce two generalized models of resource use that incorporate a choice between virgin resource extraction and post-consumption resource recycling. These models are used as a basis for expressing Dp as a function of non-dimensional indicators for three resource conservation strategies: exergy cycling fraction (ψ) for resource recycling, exergy efficiency (φ) for process efficiency gains, and renewed exergy fraction ($\text{Ω}$) for extent of renewed resource use. We use the resulting expressions to fully characterize strategy interaction, strategy limitations, and the roles that these strategies play in allowing resource consumption to occur with decreasing levels of resource depletion. We also show how the derived framework may be incorporated into an economic analysis to identify least-cost approaches to depletion avoidance in a toluene production and cycling system.

This paper presents a novel approach to the evaluation of energy conversion processes and systems, based on an extended representation of their exergy flow diagram. This approach is a systematic attempt to integrate into a unified coherent formalism both Cumulative Exergy Consumption and Thermo-economic methods, and constitutes a generalisation of both, in that its framework allows for a direct quantitative comparison of non-energetic quantities like labour and environmental impact (hence the apposition ‘Extended’). A critical examination of the existing state-of-the-art of energy- and exergy analysis methods and paradigms indicates that an extension of the existing ‘Design and Optimisation’ procedures to include explicitly ‘non-energetic externalities’ is indeed feasible. It appears that it can indeed be successfully argued that some of the issues that are difficult to address with a purely monetary theory of value can be resolved by Extended Exergy Accounting (‘EEA’ in the following) methods without introducing arbitrary assumptions external to the theory. In this respect, EEA can be regarded as a natural development of the economic theory of production of commodities, which it extends by properly accounting for the unavoidable energy dissipation in the productive chain. While a systematic description of the EEA theory is discussed in previous work by the same author, the present paper aims at a more specific target, and presents a formal representation of the application of EEA to a cogenerating plant based on a gas turbine process. It is shown that the solution indeed leads to an ‘optimal’ design, and that its formalism embeds even extended Thermo-economic formulations.

The operation of a Carnot refrigerator is viewed as a production process with exergy as its output. The economic optimization of the endoreversible refrigerator is carried out in this paper. The Coefficient of Performance (COP) of the refrigerator is a secondary consideration of the practical engineering effort of maximizing cooling rate and exergy whose goodness is constrained by economical considerations. Therefore, the profit of the refrigerator is taken as the optimization objective. Using the method of finite-time exergoeconomic analysis, which emphasizes the compromise optimization between economics (profit) and the appropriate energy utilization factor (Coefficient of Performance, COP) for finite-time (endoreversible) thermodynamic cycles, this paper derives the relation between optimal profit and COP of an endoreversible Carnot refrigerator based on a relatively general heat transfer law q∝Δ(Tⁿ). The COP at the maximum profit is also obtained. The results obtained involve those for three common heat transfer laws: Newton's law (n=1), the linear phenomenological law in irreversible thermodynamics (n=−1), and the radiative heat transfer law (n=4).

A general endoreversible refrigeration cycle model which includes the irreversibility of heat transfer across finite temperature differences and the heat leak loss between the external heat reservoirs is used to analyze the rate of exergy output of a multi-stage combined refrigeration system. The relations between the rates of exergy output and refrigeration and between the rate of exergy output and coefficient of performance are derived. The efficiency of exergy output is calculated. The optimal problems relative to the rate of exergy output are discussed. Some characteristic curves of the refrigeration system are presented. The results obtained here are suitable for an arbitrary-stage endoreversible combined refrigeration system.

The collective role of radiation and convection modes of heat transfer in a solar driven heat engine is investigated through a finite time thermodynamics analysis. Heat transfer from hot reservoir is assumed to be radiation and/or convection dominated. The irreversibilities due to these finite rate heat transfers were considered in determining the limits of efficiency and power generation that were discussed through varying process parameters. Results were compared with Curzon–Ahlborn and Carnot analysis cases. It is found that the upper limit of efficiency is a function of both the functional temperature dependence of heat transfer and relevant system parameters.