Exergy, An International Journal最新文献

This paper presents second-law based performance evaluation criteria to evaluate the performance of heat exchangers. First, the need for the systematic design of heat exchangers using a second law-based procedure is recalled and discussed. Then, a classification of second-law based performance criteria is presented:

• 1.

  criteria that use entropy as evaluation parameter, and

• 2.

  criteria that use exergy as evaluation parameter.

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.

A thermodynamic for the effect of the annualized cost of a component on the production cost in 1 000 kW gas-turbine cogeneration system was studied by utilizing the generalized exergy balance and cost-balance equations developed previously. Comparison between typical exergy-costing methodologies were also made by solving a predefined cogeneration system, CGAM problem. It was successful to identity the component which affects the unit cost of system's products decisively. It has been found that the cost of products are crucially dependent on the change in the annualized cost of the component whose primary product is the same as the system's product. On the other hand, the change in the weighted average cost of the product is proportional to the change in the annualized cost of the total system.

The exergy of an energy form or a substance is a measure of its usefulness or quality or potential to cause change. A thorough understanding of exergy and the insights it can provide into the efficiency, environmental impact and sustainability of energy systems, are required for the engineer or scientist working in the area of energy systems and the environment. Further, as energy policies play an increasingly important role in addressing sustainability issues and a broad range of local, regional and global environmental concerns, policy makers also need to appreciate the exergy concept and its ties to these concerns. During the past decade, the need to understand the connections between exergy and energy, sustainable development and environmental impact has become increasingly significant. In this paper, a study of these connections is presented in order to provide to those involved in energy and environment studies, useful insights and direction for analyzing and solving environmental problems of varying complexity using the exergy concept. The results suggest that exergy provides the basis for an effective measure of the potential of a substance or energy form to impact the environment and appears to be a critical consideration in achieving sustainable development.

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.

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.

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).

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.

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.