Exergy
"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.[1] When the surroundings are
the reservoir, exergy is the potential of a sys-
tem to cause a change as it achieves equilib-
rium with its environment. Exergy is then 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 pro-
cess; it changes from one form to another
(see First Law of Thermodynamics). In con-
trast, 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 pro-
portional to the entropy increase of the sys-
tem together with its surroundings. The des-
troyed exergy has been called anergy. For an
isothermal process, exergy and energy are in-
terchangeable terms, and there is no anergy.
Exergy analysis is performed in the field
of industrial ecology to use energy more effi-
ciently. 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[1] of a
system and its environment because unlike
energy it depends on the state of both the
system and environment. The exergy of a sys-
tem in equilibrium with the environment is
zero. Exergy is neither a thermodynamic
property of matter nor a thermodynamic po-
tential of a system. Exergy and energy both
have units of joules. The Internal Energy of a
system is always measured from a fixed ref-
erence state and is therefore always a state
function. Some