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.
5 Key excerpts on "Exergy Efficiency"
eBook - ePub- Mary K. Theodore, Louis Theodore(Authors)
- 2021(Publication Date)
- CRC Press(Publisher)
...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. [ 1 ] provide examples of steady state energy and exergy balances, as well as definitions of energy and exergy efficiencies. In summary, exergy serves as a measure of the maximum work potential that a system can afford due to a separation of the system from equilibrium with the chosen reference environment. 46.6 Energy Efficiency, Exergy, and Environmental Impact While the engineer strives to maximize process efficiency in order to minimize operating costs and abide by applicable regulations, environmentalists are primarily concerned with the impact that process inefficiencies could have on the surrounding environment (i.e., the ecological impact). For example, consider a typical nuclear power plant, which operates by sustaining atomic fission within a nuclear reactor in order to generate energy. The end-product of such a plant—electricity—is actually not produced directly from the nuclear fission process, but rather by a series of energy transformations, whereby the energy released by the nuclear reaction is transferred to process water (heat), which in turn operates a steam turbine in order to rotate a shaft (shaft work) which turns an electric generator (electricity). Due to the practical limitations on how much of the nuclear reaction’s heat may be converted into work, there is inevitably unused thermal energy which is removed via process cooling water taken from the outside environment...
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 - ePubEssentials of Energy Technology
Sources, Transport, Storage, Conservation
- Jochen Fricke, Walter L. Borst(Authors)
- 2013(Publication Date)
- Wiley-VCH(Publisher)
...If we take T h = 320 K for a forced-air home heating system together with T surr = 295 K in the house, then under ideal thermodynamic conditions, E x = 0.08 · Q or E x / Q = 0.12 = 8%. This is very different from “100% furnace efficiency” and reflects an exergy loss of 92%. It may have been better to use more of the exergy where it is needed, namely, for operating machinery or high temperature processing. An even worse case of exergy degradation is in electric resistive heating of a house. We first have the exergy in the form of electricity arriving at the heater. This is only about 35% of the exergy in the primary fuel used at the power plant, for example, natural gas. Then the electricity is degraded to low temperature heat in the forced-air heating system and the exergy loss is nearly complete. The foregoing suggests the definition of an Exergy Efficiency or “second law efficiency.” It takes into account the energy available for useful work from a given energy form, combined with its inevitable degradation during use. Exergy-saving processes are a prerequisite for effective energy use and for a low carbon economy. We define an Exergy Efficiency by 3.23 In the above example of a “100% efficient” gas furnace, we had η = 1 and ζ = 0.08. For a power plant with an exergy input E xIn = E prim in the form of primary chemical or nuclear energy and an exergy output E xOut = W (e.g., mechanical work or electricity), we have for the Exergy Efficiency 3.24 The energy and exergy efficiencies of a power plant are nearly the same, because the input and output energies are of high quality and essentially are pure exergies. We return to the example of home heating. An exergetically favorable situation may exist compared to a gas furnace when heating a house with an electrical HP. An HP takes anergy from the environment at T surr and delivers heat Q at temperature T h. We have Q = COP · W with a relative exergy content E xOut / Q = (T h − T surr)/ T h...
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...
