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What Is Exergy?

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  • Written By: Ray Hawk
  • Edited By: E. E. Hubbard
  • Last Modified Date: 21 November 2016
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Exergy is a concept in the thermodynamics field of physics that has come to replace entropy as a more accurate representation of what happens to energy when it is used in an open system, such as in industrial or biological processes on Earth. Simply stated, exergy represents the amount of energy present in a system that is available for useful work. As the system performs its function, exergy is used in the process and can never be recovered.

In this sense, the use of exergy contributes to the overall entropy of the universe itself. This is because entropy represents a trend towards a maximum distribution of energy to the point where it is no longer possible for change to take place, which is more succinctly referred to as heat death. The concept of heat death represents a period in the far future for the entire universe, which is considered a closed system where outside energy never enters. Ultimately, energy in the fundamental form of heat is projected to be equally distributed throughout space, creating an entirely homogenous environment where life itself would be impossible. An analysis of exergy energy is, therefore, a method of taking the natural, widespread process of entropy and looking at it on a smaller, local scale for machines or any ordered process or creature that consumes energy to function or live in a higher state of order.

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Measuring chemical exergy can be important as it is a way to represent the consumption of natural resources and the inevitable loss of a part of the mass of these resources on Earth as they are converted to heat, which is lost to space. A automobile engine is a good example of a system that consumes exergy to work. As the energy in the fuel that the engine burns is converted to heat and pressure in the engine, this energy is vented to the outside environment in waste heat and mechanical motion, which eventually is carried into space and is no longer recoverable for useful work.

Exergy in this way complies with the second law of thermodynamics, which states that different energy levels between systems or bodies have a tendency to equalize over time. The first law of thermodynamics is also consistent with the principle, where it states that matter can neither be created or destroyed. Since exergy analysis is a method of describing the available energy in a system that can perform work, it does not claim that energy has been destroyed, but instead merely that it has reached a state where the system can no longer make use of it.

Calculating the exergy development for a system can be done with a variety of mathematical formulas. They are dependent on to what use energy is put in the system, whether to produce exergy heat, pressure, sound, light, or other forms of useful energy-based work. One formula used in an attempt to represent all factors is:

Ε = U – Ueq + po(V -Veq) – Τo(S – Seq) - Σμo(ni – nieq)i.

In the formula, U, V, S, and ni all represent factors within an ordered system like a machine or life form that include its internal energy level, mass by volume, and inherent entropy. Its size as measured by its molecular weight. The values represented by po, To, and uo are placeholders for the external environment, including pressure, temperature, and chemical interactions. The value of eq stands for thermodynamic equilibrium, which all systems trend towards with their surrounding environment.

While the increase in exergy and entropy are both considered to be an ultimate example of how any increased order brought to matter will eventually be undone, such a concept is based on the idea that the universe is a closed system. Like a clock that is gradually winding down, all ordered processes are considered to be headed for a state of maximum entropy and randomness. If the universe, however, is an open system where outside energy enters as continually occurs on Earth by the energy of the Sun, then increasing order can be prolonged indefinitely. Both physics theories that postulate the presence of a multiverse of multiple parallel, interacting universes, or that of a higher power overseeing the cosmos suggest the universe could be an open system where energy enters from outside and exergy is replenished.

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