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Concept
Exergy
Summary
From a scientific and engineering perspective, second-law based exergy analysis is valuable because it provides a number of benefits over energy analysis alone. These benefits include the basis for determining energy quality (or exergy content), enhancing the understanding of fundamental physical phenomena, and improving design, performance evaluation and optimization efforts. In thermodynamics, the exergy of a system is the maximum useful work that can be produced as the system is brought into equilibrium with its environment by an ideal process. The specification of an 'ideal process' allows the determination of 'maximum work' production. From a conceptual perspective, exergy is the 'ideal' potential of a system to do work or cause a change as it achieves equilibrium with its environment. Exergy is also known as 'availability'. Exergy is non-zero when there is dis-equilibrium between the system and its environment, and exergy is zero when equilibrium is established (the state of maximum entropy for the system plus its environment).
Determining exergy was one of the original goals of thermodynamics. The term "exergy" was coined in 1956 by Zoran Rant (1904–1972) by using the Greek ex and ergon meaning "from work", but the concept had been earlier developed by J Willard Gibbs (the namesake of Gibbs free energy) in 1873.
Energy is neither created nor destroyed, but is simply converted from one form to another (see First Law of Thermodynamics). In contrast to energy, exergy is always destroyed when a process is non-ideal or irreversible (see Second Law of Thermodynamics). To illustrate, when someone states that "I used a lot of energy running up that hill", the statement contradicts the first law. Although the energy is not consumed, intuitively we perceive that something is. The key point is that energy has quality or measures of usefulness, and this energy quality (or exergy content) is what is consumed or destroyed.
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