Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 11

Example 2.2.2.1 Water is heated from an irradiated black surface. Cold water of
entropy S 1 disappears (minus) and in its place there appears (plus) warm water of
entropy S 2 . Entropy S r of heating radiation disappears (minus) because of absorption
on the surface and the surface emits (plus) its own radiation of entropy S e . Exergy loss
can be calculated from formula (2.2.10) in which

=− S 1 + S 2 − S r + S e (2.2.11)

The overall entropy growth does not include entropies of any work as well
as of effect of such fields like gravitational or surface tension of substance. These
magnitudes, although they contribute to the disorder measured by entropy, have no
thermodynamic parameters and act only indirectly by changing parameters of involved
matters.
The overall entropy growth has to be positive even in the smallest step ( d> 0)
in the course of the process. For any theoretical model of reversible phenomenon
d = 0; if however d< 0 then the whole phenomenon is impossible. For example,

during design of a heat exchanger, care should be taken about the so-called pinch
point for which, if locally d< 0, then the whole process of heat exchange is impos-
sible. Thus, the entropy is very useful in verification of designed new processes from
a reality viewpoint. The larger the overall entropy growth, the more irreversible is the
considered process.
If the substance remains unchanged during a process (e.g. during physical process),
then only the respective change in the substance entropy exiting and entering the system
are taken into calculation of . If a substance disappears (chemical reaction) then its
absolute entropy has to be taken with a negative algebraic sign. If a substance appears,
the positive sign of entropy should be used.
The exergy loss expressed by formula (2.2.10) is called the internal exergy loss ,
because it occurs within the considered system. This loss is totally non-recoverable.
Internal loss of exergy for a multi-component system is calculated by summing up the
internal losses of exergy occurring in the particular system components.
Each exergy loss contributes to the increase in the consumption of the energy
carrier which sustains the process or to the reduction of the useful effects of the process.
One of the main engineering tasks is operating the processes in the way at which the
exergy loss is at a minimum. However, most often, the reduction of the exergy loss is
possible only by increasing the capital costs of the process. For example the reduction
of exergy loss in a heat exchanger is reachable by costly increases of the surface area
of heat exchange. Therefore, the economics of such reduction of exergy loss can be
verified by economic calculations. The exergy analysis explains the possibilities of
improvement of thermal processes; however only the economic analysis can finally
motivate an improvement.
Usually, from a thermal process there is released one, or more, waste thermody-
namic media (e.g. combustion products), of which the parameters are still different
from the respective parameters of such a medium being in equilibrium with environ-
ment. The waste medium represents certain exergy unused in the process. Such exergy,
if released to the environment, is destroyed due to irreversible equalization of param-
eters of the waste medium with the parameters of environmental components. The