Everything about Exergy totally explained
» "Available energy" redirects here. For the meaning of the term in particle collisions, see Available energy (particle collision).
In
thermodynamics, the
exergy of a
system is the maximum
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 then the
energy that's available to be used. After the system and surroundings reach equilibrium, the exergy is zero.
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 increases 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. For an
isothermal process, exergy and energy are interchangeable terms, and there's 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
co-property of a system and a reference state. Because of this, exergy is neither a
thermodynamic property of matter nor a
thermodynamic potential of a system. It is, however, the most useful application of these values, and is derivable from them mathematically. Determining exergy was also the first goal of
thermodynamics. Exergy and energy both have units of
joules. Both are also
state functions even though work itself is not.
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 content,
reversible work, and
ideal work.
History
Carnot
In 1824,
Sadi Carnot studied the improvements developed for
steam engines by
James Watt and others. Carnot utilized a purely theoretical perspective for these engines and developed new ideas. He wrote:
"The question has often been raised whether the
motive power of
heat is
unbounded, whether the possible improvements in steam engines have an assignable limit—a limit by which the
nature of things won't allow to be passed by any means whatever... In order to consider in the most general way the principle of the production of motion by heat, it must be considered independently of any mechanism or any particular agent. It is necessary to establish principles applicable not only to steam-engines but to all imaginable
heat-engines... The production of motion in steam-engines is always accompanied by a circumstance on which we should fix our attention. This circumstance is the re-establishing of
equilibrium...
Imagine two bodies A and B, kept each at a constant
temperature, that of A being higher than that of B. These two bodies, to which we can give or from which we can remove the heat without causing their temperatures to vary, exercise the functions of two unlimited
reservoirs..."
Carnot next described what is now called the
Carnot engine, and proved by a
thought experiment that any heat engine performing better than this engine would be a
perpetual motion machine. Even in the 1820s, there was a long history of science forbidding such devices. According to Carnot, "Such a creation is entirely contrary to ideas now accepted, to the
laws of mechanics and of sound
physics. It is inadmissible."
This description of an upper bound to the work that may be done by an engine was the earliest modern formulation of the
second law of thermodynamics. Because it involves no mathematics, it still often serves as the entry point for a modern understanding of both the second law and
entropy. Carnot's focus on
heat engines,
equilibrium, and
heat reservoirs is also the best entry point for understanding the closely related concept of exergy.
Carnot believed in the incorrect
caloric theory of heat that was popular during his time, but his thought experiment nevertheless described a fundamental limit of nature. As
kinetic theory replaced caloric theory through the early and mid-1800s (
see timeline), several scientists added mathematical precision to the first and second
laws of thermodynamics and developed the concept of
entropy. Carnot's focus on processes at the human scale (above the
thermodynamic limit) led to the most universally applicable concepts in
physics. Entropy and the second-law are applied today in fields ranging from
quantum mechanics to
physical cosmology.
Gibbs
In the 1870s,
Josiah Willard Gibbs unified a large quantity of 19th century
thermochemistry into one compact theory. Gibbs's theory incorporated the new concept of a
chemical potential to cause change when distant from a
chemical equilibrium into the older work begun by Carnot in describing thermal and
mechanical equilibrium and their potentials for change. Gibbs's unifying theory resulted in the
thermodynamic potential state functions describing differences from
thermodynamic equilibrium.
In 1873, Gibbs derived the mathematics of "available energy of the body and medium" into the form it has today. (See the equations below). The physics describing exergy has changed little since that time. The term
exergy was suggested in 1956 by
Zoran Rant (1904-1972) by using the Greek
ex and
ergon meaning "from
work."
Mathematical description
An application of the second law of thermodynamics
» See Also:
Second law of thermodynamics
Exergy uses
system boundaries in a way that's unfamiliar to many. We imagine the presence of a
Carnot engine between the system and its reference environment even though this engine doesn't exist in the real world. Its only purpose is to measure the results of a "what-if" scenario to represent the most efficient work interaction possible between the system and its surroundings.
If a real-world reference environment is chosen that behaves like an unlimited reservoir that remains unaltered by the system, then Carnot's speculation about the consequences of a system heading towards equilibrium with time is addressed by two equivalent mathematical statements.
B, the exergy or available work, will decrease with time, and
Stotal, the entropy of the system and its reference environment enclosed together in a larger
isolated system, will increase with time:
»
where
Tsource is the temperature of the heat source, and
To is the temperature of the surrounding.
Further Information
Get more info on 'Exergy'.
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