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مفاهیم ترمودینامیکی

مرجع:کتاب اگزرژی Dincer

Energy analysis is the traditional method of assessing the way energy is used in an operation involving the physical or chemical processing of materials and the transfer and/or conversion of energy. This usually entails performing energy balances, which are based on the FLT, and evaluating energy efficiencies. This balance is employed to determine and reduce waste exergy emissions like heat losses and sometimes to enhance waste and heat recovery.

 

However, an energy balance provides no information on the degradation of energy or esources during a process and does not quantify the usefulness or quality of the various energy and material streams flowing through a system and exiting as products and wastes.

 

FLT=First Low of Thermodynamic

SLT=Second Low of Thermodynamic

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The exergy method of analysis overcomes the limitations of the FLT. The concept of exergy is based on both the FLT and the SLT. Exergy analysis clearly indicates the locations of energy degradation in a process and can therefore lead to improved operation or technology. Exergy analysis can also quantify the quality of heat in a waste stream. A main aim of exergy analysis is to identify meaningful (exergy) efficiencies and the causes and true magnitudes of exergy losses.

 

Table 1.1 presents a general comparison of energy and exergy

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The exergy efficiency is an efficiency based on the SLT.

Engineers make frequent use of efficiencies to gauge the performance of devices and processes. Many of these expressions are based on energy, and are thus FLT-based. Also useful are measures of performance that take into account limitations imposed by the second law. Efficiencies of this type are SLT-based efficiencies.

 

To illustrate the idea of a performance parameter based on the SLT and to contrast it with an analogous energy-based efficiency, consider a control volume at steady-state for which energy and exergy balances can be written, respectively, as

 

(Energy in) = (Energy output in product) + (Energy emitted with waste)

(Exergy in) = (Exergy output in product) + (Exergy emitted with waste) + (Exergy destruction)

 

In these equations, the term product might refer to shaftwork, electricity, a certain heat transfer, one or more particular exit streams, or some combination of these. The latter two terms in the exergy balance combine to constitute the exergy losses. Losses include such emissions to the surroundings aswaste heat and stack gases.

The exergy destruction term in the exergy balance is caused by internal irreversibilities

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From energy orexergy viewpoints, a gauge of how effectively the input is converted to theproduct is the ratio of product to input. That is, the energy efficiency η canbe written as

η = Energyoutput in product/Energy input = 1 [Energyloss/Energy input]

and the exergyefficiency ψ as

ψ = Exergyoutput in product/Exergy input = 1 [Exergyloss/Exergy input]

= 1 [(Exergywaste emission + Exergy destruction)/Exergy input]

The exergyefficiencyψ frequently gives a finer understanding of performance thanthe energy efficiency η. In evaluating η, the same weight isassigned to energy whether it is shaft work or a stream of low-temperaturefluid. Also, the energy efficiency centers attention on reducing energyemissions to improve efficiency. The parameter ψ weights energy flows

by accountingfor each in terms of exergy. It stresses that both waste emissions (or externalirreversibilities) and internal irreversibilities need to be dealt with toimprove performance. In many cases it is the irreversibilities that are more significantand more difficult to address.

Efficiencyexpressions each define a class of efficiencies because judgment has to be madeabout what is the product, what is counted as a loss and what is the input.

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