Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 31

2.3 THERMODYNAMIC ANALYSIS
2.3.1 Significance of thermodynamic analysis
The thermodynamic analysis is the method which can be applied to the examining of
any energy conversion phenomenon. In the first step the analysis develops the con-
servation equations of mass and energy, based on the First Law of thermodynamics,
which allow for the traditional energy analysis of the process. Next, the examining can
apply the Second Law of thermodynamics, which allows for the entropic evaluation
of the process irreversibility, and then it applies the crowning of the whole thermo-
dynamic analysis with exergy analysis . Thermodynamic analysis based on developed
balance equations for mass, energy, exergy and on the entropy growth equations, pro-
vides different (energy, entropy and exergy) views of the same phenomenon in terms
of engineering quantity, probability and quality, respectively.
An energy balance, (based on the First Law of Thermodynamics), is developed to
better understand any process, to facilitate design, operation and control, to point at
the needs for process improvement, and to enable eventual optimization. The degree of
perfection of energy utilization in the process, or its particular parts, allows for com-
paring the degree of perfection, and the related process parameters, to those in other
respective processes. Comparison with the currently achievable values in the most effi-
cient systems is especially important. Also the priorities for the required optimization
attempts for the systems, or its components, can be established. Such establishing can
be carried out either based on the excessive energy consumptions or on the particularly
low degree of perfection.
Entropy analysis, (based on the Second Law of Thermodynamics), requires the
complete data obtained from mass and energy considerations to allow for develop-
ing entropy relations to verify the correctness of a mathematical model of mass and
energy results. The analysis allows for identification and location of the sources of
irreversibility contributing to the overall unavoidable degradation of energy. Entropy
can be used for process optimization by minimization of entropy generation. However
the entropy has limited application for micro systems containing a denumerable num-
ber of independent particles. The smaller the number of particles, the less precisely the
Second Law is fulfilled. For example for any microbiological system containing only
a few components the Second Law may not be fulfilled.
Exergy is the concept derived from joint application of the First and Second Laws of
Thermodynamics. Exergy balance is developed according to a similar methodology as
for energy analysis, and with the same purposes. Whereas thermodynamic probability
is expressed in units of entropy, exergy is expressed in energy units. Consequently
exergy data are more practical and realistic in comparison to the respective energy
values. Thus, the exergy analysis provides a more realistic view of process, which
sometimes dramatically differs in comparison to the standard energy analyses. Exergy
analysis can be compared to the energy analysis like the second different projection in a
technical drawing disclosing additional details of the subject seen from a different side.
The knowledge about nature is continually studied with many methods and obser-
vations. The scale of approach may be microscopic (e.g. a microscopic observation
or differential calculus) or macroscopic (phenomenological considerations or integral
calculus). Usually the studies are organized by focusing attention on the particular