Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 40

Figure 2.3.3 Bands diagram of exergy balance at varying environment with positive compensation term
( B e > 0), (from Petela, 2010).

the environment parameters could then be helpful. Fluctuation of environment param-
eters is potentially one of the natural low-value resources, like e.g., a waste heat at
very low temperature. A particular problem is a variation of the effective sky temper-
ature , discussed by Duffie and Beckman (1974), which determines the radiant heat
lost from the Earth's surface. The fluctuation of environment parameters has relatively
insignificant influence on the high-values natural resources, e.g. natural fuels.
It is also possible to consider the variation of environment parameters with altitude
and this effect is taken into account in consideration of the concept of mechanical
exergy, discussed in paragraph 2.2.3.2. However, application of mechanical exergy
(eZergy) leads to the exergy balance with gravity input term. Petela (2009a) proposed
to insert an appropriate term, called gravity input G , as an additional exergy input
in the left hand side of the exergy balance equation. Thus equation (2.3.9) for the
constant environment parameters becomes:

B in + G = B S + B out + δB (2.3.15)

The gravity input G can be positive, zero or negative. The bands diagram for the
exergy balance with included positive gravity input is shown in Figure 2.3.4. The value
of G is calculated from exergy balance equation.
The gravity input G can appear only when a substance is considered in the exergy
balance and if eZergy is applied to the substance. The interpretation of the algebraic
sign of G from exergy viewpoint could be proposed as follows.
In the case G < 0; due to the effect of a gravity field on the considered process, the
process product expressed by the total exergy value of the right hand side of the exergy
balance equation diminishes and has to be balanced by the negative gravity input G
added to the left hand side of the equation. The considered process can be recognized
as opposing the effect of the gravity field.
In the case of G > 0; the presence of a gravity field during the considered process
generates a certain “surplus'' of exergy disclosed by the right hand side of the exergy
balance equation. This surplus has to be balanced by a positive gravity input G added
to the left hand side of the equation. The gravity field favors the process by contributing
with some exergy input.