Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 43

The concept of perfection degree can include also exergy change due to the varying
of environment parameters and the specific terms (e.g. gravity input). Thus, in the
modified version it can be proposed that the denominator of the degree of perfection
represents the feeding terms, gravity input and exergy change due to the environment
variation, whereas the numerator expresses the useful products. For example, for the
steady process in which numerous fluxes of energy are exchanged the exergy degree
η B of perfection could be proposed as follows:

B use , i + B Q , use , k + W use

i

k

η B = (2.3.18)

B feed , j + B Q , feed , m + W feed + G − B e

m

j

where
i is the number of useful exergy fluxes B use obtained from the process, including
substance and radiation,
k is the number of useful exergy fluxes B Q , use of heat
j is the number of entering exergy fluxes B feed , including substance and radiation,
m is the number of entering exergy fluxes B Q , feed of heat,
W use is the total work produced,
W feed is the total work consumed,
G is the gravity input, considered if eZergy is applied,
B e is the exergy gain in case of variation of environment parameters.

Formula (2.3.18) can be applied also for combined processes in which more than
one intended product is obtained (e.g. the combined generation of heat and power).
A particular example of application of the energy and exergy perfection degrees, with
no work, G and B e , is discussed in paragraph 2.4.4 for photosynthesis.
The exergy balance should be developed possibly with most detailed distribution
of the internal losses in order to obtain the most exact information on the possibility of
perfection improvement of the considered system. For example the internal exergy loss
could be divided into the components corresponding to friction, heat transfer at finite
temperature difference, radiation emissions and absorptions, etc. If in any part of the
considered system there occur several irreversible phenomena then, in principle, it is
possible to calculate only the overall internal exergy loss caused by the phenomena.
The splitting of the effects of these irreversible phenomena, occurring simultaneously
at the same place and time is impossible because these phenomena interact mutu-
ally. The splitting of the exergy loss in such case can be based only on the assumed
agreement. For example for combustion process the radiative heat exchange occurs
between the flame and surrounding wall. In order to split the effects of irreversible
chemical reactions of combustion from the irreversible radiation exchange, it can be
assumed that first the combustion occurs and then the heat exchange takes place. How-
ever, with such an assumption the temperature differences in heat exchange are larger
than the real.
Therefore it is better to split exergy losses according to time and location of occur-
rence, instead of according to the causes, unless the examined causes occur in different
locations and different instants.