Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 36

In contrary, if the mixing is a result of acting of pump or ventilator, etc., then a forced
convection occurs.
Energy can be also exchanged with the system due to a diffusive substance flux .
Then, the enthalpy of the diffusing substance has to be taken into account. For exam-
ple, consider a system boundary demarcated over the laminar zone of a mixture of gases
of a non-uniform temperature distribution. If it is assumed to be a laminar (no con-
vection) mode of transparent gases (no radiation), then the energy E L , W, exchanged
through the boundary due to the heat conduction and enthalpy of diffusing substance,
is composed of two respective terms. The first term represents the heat conducted
according to Fourier's law and the second term expresses enthalpy of diffusing gas
according to Fick's law. Thus:

A k ∂T T c pi D Li ∂c i

∂y

E L =−

∂y +

(2.3.8)

where A ,m 2 , is the surface area, k , W/(m K), is the overall conductivity of gas mix-
ture, T , K, is the temperature of the gas at the boundary, y , m, is the space coordinate
perpendicular to the system boundary surface and perpendicular to the gas flow direc-
tion, c p , i , J/(kg K), D i ,m 2 /s, and c i , kg/m 3 , are respectively the specific heat at constant
pressure, the laminar diffusion coefficient, and the concentration of gas component,
where i is the successive number of the gas mixture component.
Real processes occur with friction on which a friction work has to be spent. The
friction work increases the energy of system due to absorption of heat in amount
equivalent to friction work. Friction causes dissipation of energy which can be only
partly recovered. The friction heat does not appear as a member of the energy balance
equation; however it affects the final system energy and the components of exiting
energy.
Chemical energy is assumed to be the same for the substance considered as the
component of the system and for the substance component separately exchanged with
the system. The enthalpy and internal energy include generally physical and chemical
components.

2.3.3 Exergy balance equations
The exergy balance equation is the basis of the exergetic part of thermodynamic anal-
ysis. Exergy analysis can be applied to diversified problems which, however, like the
energy analysis, require an appropriately well-defined system for the analysis. The
system boundary should be the same as for the matter balance.
The exergy conservation equation can be applied only to reversibly occurring pro-
cesses. For real processes the exergy conservation equation is fulfilled only when the
unavoidable exergy loss, due to irreversibility of the process, is taken into account.
Thus, corresponding to energy equation (2.3.2), the following traditional exergy
balance equation is applied:

B in = B S + B out + δB (2.3.9)