Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 37

Figure 2.3.1 Bands diagram of energy balance.

Figure 2.3.2 Bands diagram of the traditional exergy balance.

where B in and B out are the respective sum of exergy delivered and released from the
system, B S is the change in the exergy of the system and δB is the exergy loss due to
the process irreversibility, calculated from the Guoy-Stodola law, equation (2.2.10).
The bands diagram for exergy balance is shown in Figure 2.3.2. In comparison
with the respective diagram for energy balance (Fig. 2.3.1), the exergy diagram shows
the exergy δB disappearing within the system.
Like the energy balance, the exergy balance can be differently tailored depending
on the considered problem and actual conditions. For example, some components of
exergy can be neglected either due to relatively small changes, or because they are
unchanged at all. The balance equation can be written for steady or transient systems,
for system considered on the macro scale or micro scale using differential equations,
etc. Obviously, for calculation of exergies, there is no freedom in defining the reference
state, which is only the environment, as determined by exergy definition.
For any elemental process lasting an infinitely short time, the exergy balance
equation can take the form:

B in dt

+ B out dt

= dB S + δB (2.3.10)

where B in and B out are the respective fluxes of exergy delivered and extracted from the
system and dB S is the total differential exergy growth of the system.