Exergy
Exergy is a measure of the maximum useful work that can be obtained from a system as it comes into equilibrium with its environment. It takes into account both the quantity and quality of energy, providing a more comprehensive assessment of energy resources and processes. In engineering, exergy analysis is used to optimize energy systems and improve efficiency by identifying and minimizing energy losses.
6 Key excerpts on "Exergy"
eBook - ePub- Mary K. Theodore, Louis Theodore(Authors)
- 2021(Publication Date)
- CRC Press(Publisher)
...Magnitudes of work (“| W |”) are used to avoid any confusion regarding the sign conventions associated with work. FIGURE 46.1 Illustration of the change in Exergy of a system between two states, and the associated amount of work produced, versus the theoretical maximum amount of work possible. The state of the reference environment, for which the Exergy is by definition = 0, is denoted by Ref. 5 Note that, while the discussion of Exergy changes is being framed with respect to the energy change of a system, which is usually of interest for closed-system processes (or transient open-system processes), this discussion is equally applicable to a steady state open-system process (e.g., as in a steam turbine), where X 2 − X 1 is not the Exergy change of the system (system properties do not vary at steady state), but rather the Exergy change between the outlet and inlet flow streams. A concept central to engineering and any conservation study is that of process efficiency. In particular, energy efficiency is a major consideration when analyzing any energy-producing (or consuming) process. In the simplest terms, energy efficiency is a quantification of how ideally the process operates. In direct analogy to energy efficiency, one may also define Exergy efficiency. However, unlike a typical energy efficiency which accounts for energy losses, an Exergy efficiency must account for both Exergy losses and Exergy destruction due to the irreversible nature of any real process. Hence, Exergy efficiency analysis is more comprehensive than energy efficiency analysis, because it incorporates the “usefulness” of the energy. By contrast, energy efficiencies assign the same weight to energy, irrespective of whether it is in the form of shaft work (high in quality energy), or a stream of low temperature fluid (which likely contains little quality energy, depending on reference environment of interest) [ 5 ]. Theodore et al...
eBook - ePub- Kornelis Blok, Evert Nieuwlaar(Authors)
- 2016(Publication Date)
- Routledge(Publisher)
...The picture is only valid for substances with a constant specific heat Note that in all cases the temperatures (e.g. in [7.4] to [7.8]) need to be given in absolute terms – i.e. in kelvin. So far, we have focused on the conversion of heat to work. However, the concept of Exergy has a broader use and can be utilised to determine the maximum conversion efficiency for all types of energy conversion. The Exergy content of energy carriers can be calculated using the basic thermodynamic properties of substances. See Box 7.2 for the general definition of Exergy. Box 7.2 General description of the concept of Exergy The concept of Exergy is not only used for energy in the form of heat but can also be used for all other energy flows. A thermodynamic property similar to Exergy (but not the same) is the Gibbs free energy (‘free’ means ‘free to do work’). The definition of Exergy differs from that of the Gibbs free energy with respect to the choice of the reference system: in the environmental reference system the most stable compounds occurring in nature are used, rather than the chemical elements. The Gibbs free energy is defined as: where: G = Gibbs free energy H = enthalpy of the substance S = entropy of the substance T = absolute temperature of the substance (in the definition of Exergy T = T ref) This property is often used in chemical thermodynamics to analyse chemical processes and equilibria. For a chemical reaction operating at temperature T the change in Gibbs free energy ΔG is zero when equilibrium is reached. 7.3 Exergy analysis An energy balance may be useful for tracking sources and destinations of energy flows, but for analysis of improvement options it does not give a good indication of where actual improvements need to be made...
eBook - ePub- V. Babu(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
...CHAPTER 10 Exergy It was demonstrated in Chapter 8 that, among all engines that operate in a cycle between two thermal reservoirs, the Carnot engine produces the maximum work and its efficiency is the highest possible. Although the Carnot efficiency of simple cycles may be calculated in a straightforward manner, it is not easy to evaluate for complicated and realistic cycles and so it is essential to be able to determine, in some manner, the maximum possible efficiency in such cases. In addition, for devices that execute non-cyclic processes, it is desirable to be able to define an appropriate ideal process between the same initial and final state and evaluate its performance. Although the isentropic efficiency is such a process, its applicability is restricted to adiabatic devices for which the ideal process is an isentropic process, for instance, turbines, compressors, nozzles and diffusers. It would be extremely useful to be able to quantitatively assess the performance of devices such as mixing chambers and heat exchangers, to name a few, or turbines, compressors, nozzles and diffusers when they do not operate adiabatically. With these issues in mind, in the present chapter, a new quantity termed Exergy, is defined. It is quite general and allows us to calculate a limiting value against which the actual performance of any device or cycle may be compared. 10.1 Exergy of a system We start by defining the so-called “dead state”. The dead state, corresponding to pressure P 0, temperature T 0, zero velocity and zero elevation from the datum level, is such that it is not possible to develop any work from a system that exists at the dead state. The ambient state at 25°C and 100 kPa, is taken to be the dead state. The Exergy of a system at a given state, denoted X, is defined as the maximum theoretical work that can be developed as the system goes from the given state to the dead state. By definition, the Exergy of a system that is at the dead state is zero...
eBook - ePubSustainable Development Indicators
An Exergy-Based Approach
- Søren Nors Nielsen(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...Unfortunately, in many cases, the difference in viewpoints serves only to blur the scientific debate, amongst other things disturbing discussions about issues such as what sustainability is about. From the beginning of the 20th century, we may identify “hot spots” where interest in the message from the second law—the imperfect conversion of energy and the fact that part of it was inevitably transformed and lost—led to an increasing awareness of the fact that only a certain part of the energy is available to us. Several terms have been applied to this part, such as essergie, arbeitsfähigkeit, availability and Exergy. As all these terms deal with the same aspect, namely the capacity of a given amount of energy to do work, we will here use work energy to designate this capacity and use it in a manner that makes it synonymous and exchangeable with Exergy. 2.2 Introducing Energy Forms to Everyday Life The most fundamental thing about work energy is the need to see energy not just as energy but as energy in various forms and capabilities of doing work. This first point of shifting forms might seem obvious, but what is less obvious is that the different forms of energy are not equally good at doing work. This is what the work energy (Exergy) concept is about. Thus, work energy adds a quality aspect to our view. In the way we talk and think about energy in our everyday life, we may implicitly know most of this already, but often it seems that this knowledge has not taken root in our minds—at least not enough to be put into practice. In fact, the implicit aim of initiating a project like the one presented here was to shed new light on the role of the various activities we are undertaking and to lay bare all (ir)rationalities. During the energy crisis in the early 1970s, we became aware of the important role played by oil and coal as the primary energy carriers used to run our societies...
eBook - ePubEssentials of Energy Technology
Sources, Transport, Storage, Conservation
- Jochen Fricke, Walter L. Borst(Authors)
- 2013(Publication Date)
- Wiley-VCH(Publisher)
...By using the Carnot efficiency in Eq. (3.1), the Exergy E x, measured in joules, is related to the heat Q at temperature T h as follows: 3.6 where T surr is the temperature of the surroundings in which the device works, for instance, T surr ≈ 300 K. The fractional Exergy content in Q is 3.7 This fraction is always less than one, but increases with the temperature T h. The Exergy E x is the part of Q that can at best be converted into useful work W. The remainder of Q is called the anergy A x. We can write 3.8 For instance, electric energy is pure Exergy because it can be completely converted into work. In contrast, the heat in a hypothetical completely mixed ocean at a temperature T surr is pure anergy because T h = T surr. With no temperature difference to the environment, no useful work can be extracted in spite of the enormous energy (here anergy) in the oceans. Actually, there are some modest temperature differences in the oceans at different depths, which can be used for extracting work in so-called ocean thermal energy converters (OTEC s). Generally, the Exergy is calculated with respect to the temperature T surr of the surroundings, according to Eq. (3.6). As there is some choice regarding the value for the temperature T surr of the surroundings, the Exergy determination may be somewhat imprecise. This holds especially for temperatures T h not much different from T surr. Overall, however, the Exergy concept is an excellent means to assess the “quality of a heat source” and its usefulness. Problem 3.4 Find the Exergy fraction C x = E x / Q in a heat reservoir at (a) T h = 330 K, (b) 400 K, and (c) 6000 K (the highest temperature theoretically obtainable from direct solar radiation). In all cases, use an environmental temperature T surr = 300 K. In the example of heat transfer through a wall (Figure 3.8), not only is entropy produced but Exergy is converted into anergy as well...
eBook - ePubRoutledge Handbook of Ecological Economics
Nature and Society
- Clive L. Spash, Clive L. Spash(Authors)
- 2017(Publication Date)
- Routledge(Publisher)
...Thermodynamics can say nothing about the associated political barriers, monetary costs or toxicological impacts. Thermodynamics is also of very limited use for mathematical models in economics. On the other hand, thermodynamics plays a vital role in the critique of mainstream economics, its lack of realism and the related framing of the obstacles to sustainability. At the policy level, thermodynamics can inform us about the opportunities for energy transformation improvements that can be used for overall energy policy, and it raises some basic issues about the unsustainability of current societal practices both in terms of energy and materials use. Notes 1 In Kelvin's definition, the phrase “with no other change” is essential, since a well-known toy, a drinking bird, can continue a cyclic movement as long as water is supplied. Furthermore, a heat source at one temperature can be transformed completely into work—the isothermal expansion phase in the Carnot cycle. Practically, an infinite length of a piston expanding beyond any limit is never available to humans. So “the cyclic process” is indispensable for Kelvin's statement. 2 Editor's Note: This proved theoretically problematic, as mentioned later in the chapter, and so of no use. However, the role of materials [see Chapter 10 ] is too easily forgotten in an over emphasis on energy and this was really the point and the importance of his contribution here. 3 Editor's Note: also found in Boulding (1966). 4 Exergy is the maximum amount of mechanical work obtainable from a system as it reversibly approaches thermodynamic equilibrium with its environment. Key further readings cited Baumgärtner, S. (1996). Use of the entropy concept in ecological economics. In: Faber, M., Manstetten, R. and Proops, J. (Eds.). Ecological Economics: Concepts and Methods. Cheltenham, England: Edward Elgar, 115–135. Georgescu-Roegen, N. (1971). The Entropy Law and the Economic Process...
