Exergy Flow Diagrams Research Articles

Beyond thermoeconomics? The concept of Extended Exergy Accounting and its application to the analysis and design of thermal systems

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.

SECOND LAW ANALYSIS OF A SOLAR THERMAL POWER SYSTEM

This communication presents second law analysis based on exergy concept for a solar thermal power system. Basic energy and exergy analysis for the system components (viz. parabolic trough collector/receiver and Rankine heat engine etc.) are carried out for evaluating the energy and exergy losses as well as exergetic efficiency for typical solar thermal power system under given operating conditions. Relevant energy flow and exergy flow diagrams are drawn to show the various thermodynamic and thermal losses. It is found that the main energy loss takes place at the condenser of the heat engine part whereas the exergy analysis shows that the collector-receiver assembly is the part where the losses are maximum. The analysis and results can be used for evaluating the component irreversibilities which can also explain the deviation between the actual efficiency and ideal efficiency of solar thermal power system.

Thermodynamic Analysis of Air-Cooled Gas Turbine Plants

At present high temperature, internally cooled gas turbines form the basis for the development of highly efficient plants for utility and industrial markets. Minimizing irreversibility of processes in all components of a gas turbine plant leads to greater plant efficiency. Turbine cooling, like all real processes, is an irreversible process and results in lost opportunity for producing work. Traditional tools based on the first and second laws of thermodynamics enable performance parameters of a plant to be evaluated, but they give no way of separating the losses due to cooling from the overall losses. This limitation arises from the fact that the two processes, expansion and cooling, go on simultaneously in the turbine. Part of the cooling losses are conventionally attributed to the turbine losses. This study was intended for the direct determination of lost work due to cooling. To this end, a cooled gas turbine plant has been treated as a work-producing thermodynamic system consisting of two systems that exchange heat with one another. The concepts of availability and exergy have been used in the analysis of such a system. The proposed approach is applicable to gas turbines with various types of cooling: open-air, closed-steam, and open-steam cooling. The open-air cooling technology has found the most wide application in current gas turbines. Using this type of cooling as an example, the potential of the developed method is shown. Losses and destructions of exergy in the conversion of the fuel exergy into work are illustrated by the exergy flow diagram.

Second Law analysis of a Rankine heat engine with reheat and regenerative options for solar thermal power generation

SYNOPSIS This communication presents the Second Law analysis based on the exergy concept for the performance of Simple and Regenerative-Reheat Rankine heat engines generally used in Solar Thermal Power Systems. Basic energy and exergy analysis for the heat engine system components are carried out for evaluating the energy and exergy losses as well as the exergetic efficiencies for both the Simple and Regenerative-Reheat Rankine heat engines. Relevant energy and exergy flow diagrams are drawn to show the various thermodynamic and thermal losses. It is found from the results that most of the exergy losses occur in the boiler and these losses are significantly reduced by the incorporation of a regenerator and reheater, whereas the results of the energy analysis shows that the main energy loss takes place in the condenser. This is because in the boiler high quality energy is lost but in the condenser low quality energy is lost. The incorporation of the regenerator and reheater results in a reduction of the total irreversibility rate of the cycle by 28.77%. The corresponding improvement in the exergetic efficiency is 16.42%.

Exan™ Pro: Process visualization tool: Increasing your insight into energy conversion and large chemical processes

Exan Pro is a powerful tool for the visualization of process modeling results, which can be obtained using Aspen Plus™ or other flowsheeting packages. Clear Exergy, Energy, Molar, Mass, Volume, Process and Block flow diagrams can be created in an automatic way. In a flow diagram, the width of a stream schematically represents its contents. Exan Pro especially allows you to perform and visualize a complete exergy or energy analysis, which has been calculated using Aspen Plus, by Aspen Tech. Inc., and ExerCom, by Stork Engineers & Contractors b.v. In addition to the visualization using flow diagrams, the results can also be presented using total exergy or energy balance graphs and pies, showing the relative exergy losses. After loading the simulation results, the flowsheet is automatically laid out using specially developed algorithms. The main process flow path is shown in a straight line. Next, the other process paths branch from this main process path. This way the backbone of the flowsheet, is visualized in the form of a Tree. If the process is too complex for complete visualization in one step, the process can be partitioned into sub processes using the partitioning tools. Next, the utilities, which are used by the unit operations, can be assigned to the units, allowing separate modeling of the main process and the utility generation processes. After selecting the desired type of flow diagram, the flow diagram of the (partitioned) flowsheet is automatically send to Visio, a powerful technical drawing package. The flow diagrams can easily be sized, printed, edited and published in high quality, using this package. It can be concluded that clear visualization of process information through flow diagrams can be used as an efficient communication tool during process design. It increases the insight of process engineers into the processes. Weak spots in the simulated processwe quickly recognized and consequently the optimization of the process structure is accelerated. Therefore, process visualization contributes to a more rigorous design of efficient processes.

Exergy analysis: An efficient tool for process optimization and understanding. Demonstrated on the vinyl-chloride plant of Akzo Nobel

Exergy analysis has shown to be an efficient tool for process optimization and understanding. An exergy analysis has been made of the Akzo Nobel vinyl-chloride plant at Rotterdam, the Netherlands using Aspen plus ™. The processed using specially developed routines. The results of this analysis are shown in an Exergy Flow Diagram. The exergy flow diagram gives a clear picture on how the exergy is lost through the process. To improve the clarity of the diagram, only the net chemical exergy changes over the process units are shown. The exergy flow diagram shows those units which are best suited for optimization. Several optimizations have been compared on the basis of exergy loss reduction and operating costs reduction. The implementation of the optimizations of the current process is often economically unattractive, because the investment cost for the placing of new units in the current plant is high, compared to implementing it in a new plant. Therefore, exergy analysis for optimization is best applied during the development of a new process. However, exergy analysis of an existing process has shown to be an efficient tool to critically examine the process energy use and to test for possible savings in primary energy consumption.

Modeling the Energetic and Exergetic Self-Sustainability of Societies With Different Structures

The paper examines global energy and exergy flows in various models of organized human societies: from primitive tribal organizations to teocratic/aristocratic societies, to the present industrial (and post-industrial) society, to possible future highly “robotized” or “central control” social organizations. The analysis focuses on the very general chain of technological processes connected to the extraction, conversion, distribution and final use of the real energetic content of natural resources (i.e., their exergy): the biological food chain is also considered, albeit in a very simplified and “humankind” sense. It is argued that, to sustain this chain of processes, it is necessary to use a substantial portion of the final-use energy flow, and to employ a large portion of the total work force sustained by this end-use energy. It is shown that if these quantities can be related to the total exergy flow rate (from the source) and to the total available work force, then this functional relationship takes different forms in different types of society. The procedure is very general: each type of societal organization is reduced to a simple model for which energy and exergy flow diagrams are calculated, under certain well-defined assumptions, which restrain both the exchanges among the functional “groups” which constitute the model, and the exchanges with the environment. It is argued that not all societies are unconditionally self-sustained, and that certain size and technology-related restrictions apply to virtually all types of societal organizations examined here. These restrictions limit in general the distribution of the active workforce among different productive sectors; this distribution cannot be arbitrarily assigned, but depends quantitatively on the technological level of the chain of processes connected with energy extraction, transformation, distribution, and use. The results can be quantified using some assumptions/projections about energy consumption levels for different stages of technological development which are available in the literature; the procedure is applied to some models of primitive and pre-industrial societies, to the present industrial/post-industrial society, and to a hypothetical model of a future, high-technology society. No attempt has been made to study transient behavior (“evolution” or “decay” of a certain type of society), nor to relate quantitatively the steady-state case to resource conservation and environmental protection. For most of the cases examined here, neither resource scarcity nor finite biosphere capacity were considered as constraints.

Exergonomics of an EOR (OCDOPUS) project

The combined production of power, carbon dioxide, process steam, hot water, and compressed nitrogen for EOR is considered. Exergy-flow diagrams are used to evaluate plant efficiency. The exergy efficiency of the plant is defined as the sum of the exergy outputs divided by the exergy of the consumed fuel. Estimations of the invested exergy, net exergy coefficient and the sum of the specific exergy consumption are presented.

Exergonomics of the OCDOPUS project

The project of combined production of power, carbon dioxide, process steam, hot water and compressed nitrogen for oil recovery enhancement is considered. The exergy analysis to evaluate effectiveness of the plant is used. As the main tool the exergy flow diagrams are developed. Exergy efficiency of the plant is defined as a relation of the sum of delivered exergy flows to the exergy of consumed fuel. At attempt to guess the invested exergy, net-exergy coefficient and the sum of specific exergy consumption (exergonomic criterion) is presented.

Heat transformer cycles—II. Thermodynamic analysis and optimization of a single-stage absorption heat transformer

In the present paper the results of a detailed analysis of a real one-stage absorption heat transformer working with ammonia-water are discussed. Irreversibilities due to temperature differences, pressure drops and efficiencies for the absorber and the liquid pumps are taken into account and also the performance of the rectification column is considered. Heat fluxes are delivered or removed by fluids like water or humid air. The working temperatures of the process are varied. From the analysis losses of exergy are detected and an exergy flow diagram of the whole process is presented.