Environmental Engineering Reference
In-Depth Information
exergy loss of the system, caused by such a way, is called
external exergy loss
and its
numerical value is equal to the exergy of the waste medium released by the system.
External exergy loss is recoverable, at least in part, e.g., by utilization in other systems.
The irreversibility of radiation processes occurs due to such basic phenomena as
emission and adsorption. The irreversibility mechanism of many radiation processes,
e.g., diluting or attenuation of propagating radiation, can be explained based on the
irreversibility of emission and absorption. Obviously, in the combined processes, in
which substance and radiation take place, all the sources of irreversibility should be
considered; those for substance together with those for radiation. The radiation enter-
ing or absorbed in the considered system has a negative sign of radiation entropy,
whereas radiation leaving the system, or emitted, has positive entropy. The radiation
entropy is recognized as absolute.
The problem of radiation irreversibility was considered for the first time by Petela
(1961b) and later edited in the topic (Petela 2010). Based on the overall entropy growth
for considered processes it was proved that the emission alone (not accompanied by
any adsorption) is possible (
>
0), whereas the absorption alone is irreversible, and
without accompanying emission of the considered surface is impossible (
≤
0). The
simultaneous emission and absorption is always possible (
>
0).
The exergy of radiation reaching any surface can be reflected (re-radiated) and the
reflected radiation has its exergy at the temperature of the original radiation, which
was not utilized by the absorbing surface. If the reflection process does not change the
radiation temperature then this process is reversible, and not generating any exergy
loss. However, the radiation emitted by the absorbing surface has its own exergy deter-
mined by the emissivity and temperature of the absorbing surface. This is the problem
of the efficiency of the absorbing surface, or any other device utilizing the radiation
somehow, in how much of the whole incident exergy
b
the surface, or the other device,
can be grasped and utilized. The efficiency of the absorbing device or surface is an
entirely different thing and does not depend on the theoretical potential represented
by
b
. Acceptance of such interpretation is very important in correct reasoning on the
theory of radiation exergy, because if not noticed by some researchers, it can mislead
to strange conclusions.
2.2.3 Exergy of substance
2.2.3.1 Traditional exergy
A total exergy of substance is composed of some components as shown schematically in
Figure 2.2.1. Usually, only these components are used, which vary during the analysis.
Most often is used the thermal exergy, which is the sum of physical and chemical
exergies. The physical exergy results from different temperature and pressure of the
considered substance in comparison to its temperature and pressure in equilibrium with
the environment (dead state). The chemical exergy results from the different chemical
composition of the considered substance in comparison to the common substance
components of the environment.
If the considered substance has significant velocity, then the kinetic exergy can
be recognized as equal to the kinetic energy calculated for the velocity relative to the
environment. Potential exergy is equal to the potential energy if it is calculated for the
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