Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 38

The differential dB S should be determined analogously to equation (2.3.5).
If the subject of consideration is moving in space, then the simplest exergy balance
equation is obtained by assuming, as for the energy balance, that the coordinates system
determining velocity and location is moving together with the system boundary.
An increase

B S of the exergy system , changing from its initial to final state, does
not depend on the method of change between these states and is equal to the difference
of final and initial exergy components:

∆

⎡
⎣
i

⎤

⎦ S

B S = ( B ) fin , i − ( B ) inl , j (2.3.11)

j

where the sum of initial or the sum of the final components is:

( B ) S = B k + B p + B S + B b +··· (2.3.12)

and where i and j are the successive numbers of the final and initial (respectively)
exergy components, B k is the kinetic exergy, B p is potential exergy, B S is the thermal
exergy of the system calculated with use of formula (2.2.13), and B b is the exergy of
photon gas (black radiation) calculated for example based on equation (2.2.31). Also
the other eventual components in equation (2.3.12), as shown in Figure 2.2.1, can be
added if necessary, e.g. exergy of surface tension which is equal to the energy of surface
tension, etc.
Exergy fluxes ( B in and B out ) exchanged with system can occur on different ways
described for the energy balance.
Electrical exergy is equal to electrical energy. Exergy of mechanical work is equal
to work. Exergy of substance flux is calculated with use of formula (2.2.12), however
kinetic exergy should be taken separately, (calculated as the kinetic energy for absolute
velocity), and potential exergy (equal to potential energy relative to the Earth surface
level). Exergy of heat exchanged with the system is determined by formula (2.2.4).
Exergy can be exchanged with the system also due to a diffusive substance
flux . Then, the exergies of diffusing substances is taken into account as the
exergy determined by formula (2.2.12) interpreted for the partial pressure of the
substances.
Any internal exergy loss δB F caused by friction is determined e.g., by assumption
that the friction heat Q F , equal to the friction work , is entirely absorbed by the sub-
stance at temperature T . For the heat absorption process, assuming the entropy growth
F = Q F / T , the exergy loss can be calculated from formula (2.2.10) as follows:

Q F T 0
T

δB F = (2.3.13)

The exergy loss by friction is the smaller the higher is the temperature T of absorb-
ing substance. The exergy loss δB F can be smaller or larger than the friction heat Q F
dependently on the temperature ratio T 0 / T . This observation is particularly important
for refrigerating processes where often T < T 0 .