ماذا تعرف عن ال exergy

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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.[1] 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 (see First Law of Thermodynamics). In contrast, exergy accounts for the irreversibility of a process due to increase in entropy (see Second Law of Thermodynamics). 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.[1] 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,[2] but the concept was developed by J. Willard Gibbs in 1873.[3] 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[2] 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. Some authors define the exergy of the system to be changed when the environment changes, in which case it is not a state function. Other writers prefer[citation needed] a slightly alternate definition of the available energy or exergy of a system where the environment is firmly defined, as an unchangeable absolute reference state, and in this alternate definition exergy becomes a property of the state of the system alone.
The term exergy is also used, by analogy with its physical definition, in information theory related to reversible computing. Exergy is also synonymous with: availability, available energy, exergic energy, essergy (considered archaic), utilizable energy, available useful work, maximum (or minimum) work, maximum (or minimum) work *******, reversible work, and ideal work.

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argmoaath

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Exergy is the maximum amount of work that can be done by a subsystem as it approaches thermodynamic equilibrium with its surroundings by a sequence of reversible processes. The term was coined by Zoran Rant (1904-1972) in 1953 from the Greek words ex (external) and ergos (work). Exergy measurements are made relative to an equilibrium state in which there are no gradients of any kind. This implies uniformity of temperature, pressure, density, chemical composition as well as uniform gravitational and electro-magnetic fields. Thus, the exergy of a subsystem is a measure of its 'distance' from equilibrium. Mechanical exergy is known as kinetic energy, while thermal exergy is more familiarly known as heat. These concepts are important in the design of energy-efficient machines. Chemical exergy is used in chemical engineering and thermoeconomics for process optimization, but also in economics and environmental science.
250px-GlobalExergy.png

Figure 1. Global exergy flows. (Source: Exergy.org)



Fuel combustion is the spontaneous recombination of hydrocarbons or carbohydrates with atmospheric oxygen, resulting in their mutual chemical equilibrium state. Thus, the heat of combustion (enthalpy) of a fuel is roughly equivalent to its exergy *******. For non-fuels, chemical exergy is a measure of distinguishability from the surroundings. A high-grade ore has more embodied exergy than a low-grade ore, and thus needs more energy to be upgraded. Exergy also is an important concept in understanding life processes. Seemingly dead structures in space convert into organized, self-reproducing structures as life and life forms by means of converting and partly destroying exergy. Nature creates a state far from thermodynamic equilibrium on Earth by an everlasting redesign of the environment mainly powered by the exergy of the sunlight.
There is a substantial amount of exergy that flows into an within the Earth in addition to the non-renewable exergy reservoirs on which our energy system currently depends. Figure 1 details these fluxes and reservoirs. Exergy flux is represented by paths ending in shaded areas of natural exergy destruction or arrows representing anthropogenic destruction for energy services. Unlabeled dashed circles along these paths represent natural exergy destruction processes such as precipitation and plant decay that are small compared to the natural destruction modes of solar exergy. The ovals illustrate accumulations of exergy.​
 

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