Technology & Engineering
Exergy Efficiency
Exergy efficiency is a measure of how effectively a system utilizes available energy. It takes into account the quality of energy and the potential work that can be obtained from it. Unlike traditional energy efficiency, exergy efficiency considers the specific characteristics of different energy sources and their conversion processes, providing a more comprehensive assessment of energy utilization.
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11 Key excerpts on "Exergy Efficiency"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter 6 Exergy In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir. When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its environment. Exergy is the energy that is available to be used. After the system and surroundings reach equilibrium, the exergy is zero. Dete-rmining exergy was also the first goal of thermodynamics. Energy is never destroyed during a process; it changes from one form to another. In contrast, exergy accounts for the irreversibility of a process due to increase in entropy. Exergy is always destroyed when a process involves a temperature change. This des-truction is proportional to the entropy increase of the system together with its surroundings. The destroyed exergy has been called anergy . For an isothermal process, exergy and energy are interchangeable terms, and there is no anergy. Exergy analysis is performed in the field of industrial ecology to use energy more efficiently. The term was coined by Zoran Rant in 1956, but the concept was developed by J. Willard Gibbs in 1873. Ecologists and design engineers often choose a reference state for the reservoir that may be different from the actual surroundings of the system. Exergy is a combination property of a system and its environment because unlike energy it depends on the state of both the system and environment. The exergy of a system in equilibrium with the environment is zero. Exergy is neither a thermodynamic property of matter nor a thermodynamic potential of a system. Exergy and energy both have units of joules. The Internal Energy of a system is always measured from a fixed reference state and is therefore always a state function.
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- 2014(Publication Date)
- White Word Publications(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter-3 Exergy In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir. When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its environment. Exergy is the energy that is available to be used. After the system and surroundings reach equilibrium, the exergy is zero. Deter-mining exergy was also the first goal of thermodynamics. Energy is never destroyed during a process; it changes from one form to another. In contrast, exergy accounts for the irreversibility of a process due to increase in entropy. Exergy is always destroyed when a process involves a temperature change. This destruction is proportional to the entropy increase of the system together with its surroundings. The destroyed exergy has been called anergy . For an isothermal process, exergy and energy are interchangeable terms, and there is no anergy. Exergy analysis is performed in the field of industrial ecology to use energy more efficiently. The term was coined by Zoran Rant in 1956, but the concept was developed by J. Willard Gibbs in 1873. Ecologists and design engineers often choose a reference state for the reservoir that may be different from the actual surroundings of the system. Exergy is a combination property of a system and its environment because unlike energy it depends on the state of both the system and environment. The exergy of a system in equilibrium with the environment is zero. Exergy is neither a thermodynamic property of matter nor a thermodynamic potential of a system. Exergy and energy both have units of joules. The Internal Energy of a system is always measured from a fixed reference state and is therefore always a state function.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter 8 Exergy In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir. When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its environment. Exergy is the energy that is available to be used. After the system and surroundings reach equilibrium, the exergy is zero. Determining exergy was also the first goal of thermodynamics. Energy is never destroyed during a process; it changes from one form to another. In contrast, exergy accounts for the irreversibility of a process due to increase in entropy. Exergy is always destroyed when a process involves a temperature change. This destruction is proportional to the entropy increase of the system together with its surroundings. The destroyed exergy has been called anergy . For an isothermal process, exergy and energy are interchangeable terms, and there is no anergy. Exergy analysis is performed in the field of industrial ecology to use energy more efficiently. The term was coined by Zoran Rant in 1956, but the concept was developed by J. Willard Gibbs in 1873. Ecologists and design engineers often choose a reference state for the reservoir that may be different from the actual surroundings of the system. Exergy is a combination property of a system and its environment because unlike energy it depends on the state of both the system and environment. The exergy of a system in equilibrium with the environment is zero. Exergy is neither a thermodynamic property of matter nor a thermodynamic potential of a system. Exergy and energy both have units of joules. The Internal Energy of a system is always measured from a fixed reference state and is therefore always a state function.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Orange Apple(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 2 Exergy In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir. When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its environment. Exergy is the energy that is available to be used. After the system and surroundings reach equilibrium, the exergy is zero. Determining exergy was also the first goal of thermodynamics. Energy is never destroyed during a process; it changes from one form to another. In contrast, exergy accounts for the irreversibility of a process due to increase in entropy. Exergy is always destroyed when a process involves a temperature change. This destruction is proportional to the entropy increase of the system together with its surroundings. The destroyed exergy has been called anergy . For an isothermal process, exergy and energy are interchangeable terms, and there is no anergy. Exergy analysis is performed in the field of industrial ecology to use energy more efficiently. The term was coined by Zoran Rant in 1956, but the concept was developed by J. Willard Gibbs in 1873. Ecologists and design engineers often choose a reference state for the reservoir that may be different from the actual surroundings of the system. Exergy is a combination property of a system and its environment because unlike energy it depends on the state of both the system and environment. The exergy of a system in equilibrium with the environment is zero. Exergy is neither a thermodynamic property of matter nor a thermodynamic potential of a system. Exergy and energy both have units of joules. The Internal Energy of a system is always measured from a fixed reference state and is therefore always a state function.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
An energy efficiency or first law efficiency will determine the most efficient process based on wasting as little energy as possible relative to energy inputs. An Exergy Efficiency or second-law efficiency will determine the most efficient process based on wasting and destroying as little available work as possible from a given input of available work. Design engineers have recognized that a higher Exergy Efficiency involves building a more expensive plant, and a balance between capital investment and operating efficiency must be determined in the context of economic competition. Applications in natural resource utilization In recent decades, utilization of exergy has spread outside of physics and engineering to the fields of industrial ecology, ecological economics, systems ecology, and energetics. Defining where one field ends and the next begins is a matter of semantics, but applications of exergy can be placed into rigid categories. Researchers in ecological economics and environmental accounting perform exergy-cost analyses in order to evaluate the impact of human activity on the current natural environment. As with ambient air, this often requires the unrealistic substitution of properties from a natural environment in place of the reference state environment of Carnot. For example, ecologists and others have developed reference conditions for the ocean and for the Earth's crust. Exergy values for human activity using this information can be useful for comparing policy alternatives based on the efficiency of utilizing natural resources to perform work.
eBook - PDFTowards an Environment Research Agenda
A Third Selection of Papers
- A. Winnett(Author)
- 2004(Publication Date)
- Palgrave Macmillan(Publisher)
Exergy is a measure of the maximum theoretical useful work obtainable from a thermal system (as it is brought into equilibrium with its surrounding environment), and this may be not be the only or relevant criterion in a particular situation. An innovative attempt to analyse different societies in terms of energy and exergy flow diagrams has been made by Sciubba (1995). He examined the sustainability of a variety of social structures ranging from ‘primitive’ tribal groups, via industrial (and ‘post-industrial’) societies, to a future envisaged as being dominated by a highly ‘robotised’ or cybernetically controlled social organization. In essence, energy and exergy were employed here as ‘technology level indica- tors’. Sciubba recognized that his model was an oversimplification of the complex interactions that arise between human societies and the natural world. Nevertheless, he believes this energy–exergy approach can adequately represent these various interactions. Consequently, the model could be used to examine alternative societal arrangements that might be ‘leaner’ in terms of resource extraction, while being as ‘comfortable’ as present industrialized societies. Not all forms of soci- etal organization were found to be self-sustaining, with certain size and technology-related restrictions applying to most societies. Sciubba (1995) argues that neither resource scarcity nor biosphere capacity appears to constrain human development, although many energy analysts and environmentalists (for example, Goldemberg, 1996; Lovins, 1977; Parkin, 2000; Porritt, 2000) would suggest that the contrary is the case. Geoffrey P. Hammond 203 Ecology and ‘free energy’ In the field of ecology, strictly the branch of the natural sciences that deals with the relation between biological organisms and their physical surrounding, the concept of Gibbs free energy or function (G) is used in preference to exergy (Haynie, 2001).
- Efstathios Michaelides(Author)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
The term exergy (exergie in German) was introduced by Rant [48] in the 1950s as a new word in the terminology of thermodynamics – roughly translated from Greek as “extracted work.” This term was adopted by several authors in Europe. Among those, Baehr differentiated between energy and exergy, and also introduced the word anergy (anergie) for the work dissipated in irreversibilities, the lost work [49]. Evans introduced the term essergy (from the essence of energy) but this term is not currently in use [50]. Following the “energy crisis” of the 1970’s and the worldwide realization that the human society must engage in conservation measures for the preservation of the global energy resources and a sustainable future, the concept of maximum work – derived from either the availability or the exergy concept – was widely adopted by the scientific community; it became part of the undergraduate curricula and was introduced in textbooks; and it became the tool in thousands of optimization studies for thermal and chemical systems. In the early part of the 21st century, the concept of maximum mechanical work is widely used globally under the term exergy, which has persevered in the scientific and engineering literatures. The numerous applications of the exergy concept in the optimization of energy conversion engines; the efforts for energy conser- vation; the conservation of natural resources; and environmental protection have made the exergy method one of the key engineering tools for the optimization of energy conversion systems and processes. An indication of the wide acceptance of the term exergy and the adoption of exergy analyses is the unanimous opinion paper issued by the Physical, Chemical and Mathematical Sciences Committee of Science Europe that exergy and the exergy destruction footprint (the lost work) be used exclusively in the policy making arena [51] with the slogan “Forget Energy, Think Exergy.” Problems 1.
eBook - PDF- Nazim Z. Muradov, T. Veziroğlu, Nazim Z. Muradov, T. Veziroglu(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
efficiency. (effec-tiveness,.or.rational.efficiency).for.various.energy.systems.are.given.in.detail.elsewhere. (Kotas,.1995) . .In.a.similar.way,.exergy.efficiency.may.be.defined.as.the.ratio.of.total.exergy. output.to.total.exergy.input: . ε = uni239B uni239D uni239C uni239C uni239E uni23A0 uni239F uni239F dotnosp dotnosp Ex Ex output input . (5 .12) where.“output”.refers.to.“net.output”.or.“product”.or.“desired.value,”.and.“input”.refers. to.“given”.or.“used .” 5.2.5 Some Exergetic Parameters Thermodynamics.analysis.of.renewable.energy.systems.may.also.be.performed.using.the. following.parameters.(Xiang.et.al ., .2004): Fuel depletion ratio . δ i dest i T Ex F = uni239B uni239D uni239C uni239E uni23A0 uni239F dotnosp dotnosp , . (5 .13) Relative irreversibility (exergy destruction) . χ i dest i dest T Ex Ex = uni239B uni239D uni239C uni239E uni23A0 uni239F dotnosp dotnosp , , . (5 .14) Productivity lack . ξ i dest i T Ex P = uni239B uni239D uni239C uni239E uni23A0 uni239F dotnosp dotnosp , . (5 .15) Exergetic factor . f F F i T i = uni239B uni239D uni239C uni239E uni23A0 uni239F dotnosp dotnosp . (5 .16) 5.2.6 Exergetic Improvement Potential Rate Van.Gool.(1997).has.also.proposed.that.maximum.improvement.in.the.exergy.efficiency. for. a. process. or. system. is. obviously. achieved. when. the. exergy. loss. or. irreversibility. ( E ˙ x in .−. E ˙ x out ).is.minimized . .Consequently,.he.suggested.that.it.is.useful.to.employ.the.con-cept.of.an.exergetic. improvement potential .when.analyzing.different.processes.or.sectors.of. the.economy . .This.improvement.potential.in.the.rate.form,.denoted. IP ˙ ,.is.given.by . IP Ex Ex in out dotnosp dotnosp dotnosp = --( ) 1 ε )( . (5 .17) Efficient Utilization of Solar, Wind, and Geothermal Energy Sources 333 5.2.7 Sustainability Index A.sustainable.development.requires.that.the.resources.should.be.used.efficiently . .Exergy. analysis.method.is.a.very.useful.tool.for.maximizing.the.benefits.and.using.the.resources.
- Lucien Borel, Daniel Favrat(Authors)
- 2010(Publication Date)
- PPUR(Publisher)
Energy and exergy analyses (thermomechanical processes) 407 pressure P a of the atmosphere outside of the system itself. Since then, the implica- tions and consequences of that idea have been studied and developed in several countries by a certain number of scientists under the general expression: exergy theory . The exergy theory is now widely recognized as being extremely fertile since it leads to a so-called exergy accounting, which takes into consideration the First and the Second Laws of thermodynamics. Only this exergy accounting renders it possi- ble to quantitatively evaluate what is qualitatively called energy degradation, i.e., to calculate with precision the consequences of the different phenomena of thermo- dynamic irreversibility and, thus to correctly calculate the thermodynamic losses of a system. As a consequence, exergy accounting is the only possible way to correctly define a thermodynamic efficiency that expresses the degree of perfection of a sys- tem, i.e., its thermodynamic quality . We shall now discuss our approach to the exergy theory . It is a very general approach, valid for any system, whether closed, open under steady-state conditions or open under non steady-state conditions. Our aim is two-fold. On the one hand, we present a scientific development, in an as general and systematic way as possi- ble. On the other hand, with applications in mind, we propose a formalism as clear and simple as possible. In particular, we think that it is important to clearly distin- guish • the state function character of the quantities which defines the thermodyna- mic state of the system (e.g., internal energy, enthalpy, coenergy, coen- thalpy); • and the path quantity character of the quantities, which represents the energy conversions (e.g., work-energy, heat-energy, transformation energy, heat exergy, transformation exergy, exergy loss, etc.).
eBook - PDFThermodynamics
Fundamentals and Engineering Applications
- William C. Reynolds, Piero Colonna(Authors)
- 2018(Publication Date)
- Cambridge University Press(Publisher)
9.7 Concluding Remarks Now that you know about exergy analysis you can answer correctly if someone tells you that a com-bined cycle power plant is way more efficient than a geothermal power plant, because its thermal or I-law efficiency is way higher than that of the geothermal power plant. From the point of view of thermody-namic quality and a fair comparison, the level at which energy is available is of the utmost importance, therefore the two technologies might well display a similar level of exergetic efficiency. Exergy analysis can also be combined with costing, and such a combination takes the name of exergoeco-nomics . If the values of exergy are given an economic value, such a tool can be used in order to make important design decisions related to the realization of an energy system based on sound thermodynamic reasoning. Exergy analysis can even be used to take into account the impact on the environment, if the scope of the analysis is enlarged. Note, though, that the correct selection of economic data and data related to the so-called life-cycle of industrial installations are far more arbitrary than what has been illus-trated in this chapter. Nonetheless the results of these broad-scope analyses are important tools for the correct evaluation of technologies and industrial installations with large economic and environmental impact. Last but not least, observe that also here we made several choices with regard to the defini-tions of quantities related to the exergy concept, and these choices are not unique. For example, the calcu-lation of the exergetic efficiency can lead to different results depending on what is considered as provided exergy in η II ≡ Rate of exergy obtained Rate of exergy provided . Exergy can be supplied to a certain system in var-ious forms (chemical, thermal, mechanical, kinetic, potential energy), and also the definition of the sys-tem is somewhat arbitrary.
- S.E. Jorgensen, Y.M. Svirezhev(Authors)
- 2004(Publication Date)
- Pergamon(Publisher)
All real processes are irreversible which implies that exergy is inevitably lost. Exergy is not conserved ; while energy, of course, is conserved by all processes according to the First Law. It is therefore wrong (as already mentioned briefly) to speak of the energy efficiency of an energy transfer because it will always be 100%; rather, the Exergy Efficiency is of interest because it expresses the ratio of useful energy to total energy, which is always less than 100% for real processes. All transfers of energy imply that exergy is lost because energy is transformed into heat at the temperature of the environment.It is, therefore, of interest for all environmental systems to set up an exergy balance in addition to an energy balance. Our concern is loss of exergy, because here “first class energy” which can do work is lost and replaced by “second class energy” (heat at the temperature of the environment) which cannot do work. So, as presented in Chapter 3 , the particular properties of heat and of temperature are a measure of the movement of molecules, and give limitations in our possibilities to utilise energy to do work. Due to these limitations, we have to distinguish between exergy, which can do work, and energy, which cannot do work. The latter may be called anergy (see, for instance, Cerbe and Hoffmann, 1996 ). Therefore, the energy can be represented as a sum of two items:Energy = exergy + anergy .(2.1)In accordance with the Second Law, anergy is always positive for any process.It seems more useful to apply exergy than entropy to describe the irreversibility of real processes as it has the same unit as energy and is an energy form, while the definition of entropy is more difficult to relate to concepts associated with our usual description of reality. In addition, entropy is not clearly defined for systems “far from thermodynamic equilibrium”, particularly for living systems (see, for instance, Tiezzi, 2003 ). Moreover, it should be mentioned that the self-organising abilities of systems are strongly dependent on temperature, as discussed in Jørgensen et al. (1999) . Exergy takes the temperature into consideration as the definition shows, while entropy does not. The negative entropy is as discussed in Chapters 2 and 3 , i.e. it does not express the ability of the system to do work (we may call it “the creativity” of the system as creativity requires work), but exergy becomes a good measure of “the creativity”, which is increasingly proportional with the temperature. Furthermore, exergy facilitates the differentiation between low-entropy energy and high-entropy energy, as exergy is entropy-free energy. These expressions were not properly defined in Chapter 3
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