Exergy

These benefits include the basis for determining energy quality (or exergy content[1][2][3]), enhancing the understanding of fundamental physical phenomena, and improving design, performance evaluation and optimization efforts.

Equation (2) is useful for processes where system volume, entropy, and the number of moles of various components change because internal energy is also a function of these variables and no others.

A multi-state system may complicate or simplify the problem because the Gibbs phase rule predicts that intensive quantities will no longer be completely independent from each other.

From equation (3): Rudolf Clausius recognized the presence of a proportionality constant in Kelvin's analysis and gave it the name entropy in 1865 from the Greek for "transformation" because it quantifies the amount of energy lost during the conversion from heat to work.

Further, exergy losses can occur if mass and energy is transferred out of the system at non-ambient or elevated temperature, pressure or chemical potential.

The entropy and exergy balance equations for a control volume (CV), re-stated to correctly apply to situations involving radiative transfer,[1][2][9][10] are expressed as,

The approaches by Petela[11] and Karlsson[13] both assume that reversible conversion of non-blackbody radiation is theoretically possible, that is, without addressing or considering the issue.

In contrast for opaque overcast skies the solar radiation can be completely diffuse with a maximum intensity in the direction of the zenith and monotonically decreasing towards the horizon.

From the second law of thermodynamics, the incoming entropy of the solar radiation cannot be destroyed and consequently reduces the maximum work output that can be obtained by a conversion device.

Chemical exergy is defined as the maximum work that can be obtained when the considered system is brought into reaction with reference substances present in the environment.

Equation 10 is similar but uses standard molar chemical exergy, which scientists have determined based on several criteria, including the ambient temperature and pressure that a system is being analyzed and the concentration of the most common components.

Utilization of the exergy concept often requires careful consideration of the choice of reference environment because, as Carnot knew, unlimited reservoirs do not exist in the real world.

Exergy has been applied in a number of design applications in order to optimize systems or identify components or subsystems with the greatest potential for improvement.

Referencing the inherent qualities of a system in place of a reference state environment[25] is the most direct way that ecologists determine the exergy of a natural resource.

[27] This determination allows for the assumption of qualities in a natural state: deviation from these levels may indicate a change in the environment caused by outside sources.

There are three kinds of reference substances that are acceptable, due to their proliferation on the planet: gases within the atmosphere, solids within the Earth's crust, and molecules or ions in seawater.

[25] By understanding these basic models, it's possible to determine the exergy of multiple earth systems interacting, like the effects of solar radiation on plant life.

[29] To understand the ramifications of these practices, exergy is utilized as a tool for determining the impact potential of emissions, fuels, and other sources of energy.

[29] Combustion of fossil fuels, for example, is examined with respect to assessing the environmental impacts of burning coal, oil, and natural gas.

The current methods for analyzing the emissions from these three products can be compared to the process of determining the exergy of the systems affected; specifically, it is useful to examine these with regard to the reference state environment of gases within the atmosphere.

This would need to be carried out mathematically backwards through time, to a presumed era when the oil and coal could be assumed to be receiving the same exergy inputs from these sources.

A common hypothesis in systems ecology is that the design engineer's observation that a greater capital investment is needed to create a process with increased exergy efficiency is actually the economic result of a fundamental law of nature.

Testing this idea in living organisms or ecosystems is impossible for all practical purposes because of the large time scales and small exergy inputs involved for changes to take place.

The exact proportion of exergy in a substance depends on the amount of entropy relative to the surrounding environment as determined by the Second Law of Thermodynamics.

Since high-exergy energy carriers can be used for more versatile purposes, due to their ability to do more work, they can be postulated to hold more economic value.

[36] This intuition confirmed by Dewulf[37] Sciubba[38] lead to exergo-economic accounting[39] and to methods specifically dedicated to LCA such as exergetic material input per unit of service (EMIPS).

The applicability of the EMIPS methodology relates specifically to the transport system and allows an effective coupling with life cycle assessment.

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 will not allow to be passed by any means whatever...

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.

In the 1880s, German scientist Hermann von Helmholtz derived the equation for the maximum work which can be reversibly obtained from a closed system.