Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 92

Figure 2.4.21 Energy streams of solar cell (from Petela, 2010).

q k and q 0 , respectively, both transferred to the environment. Therefore, the energy
conservation equation for the considered system, defined by the system boundary is:

q S = q r + q k + q 0 + q C + E (2.4.107)

where

10 − 5 σT S q S = 2 . 16 × (i)

q r = ρ C q S (ii)

q k = k ( T C − T 0 ) (iii)

ε C σ ( T C − T 0 ) q 0 = (iv)

10 − 5 is the view (Sun-Earth) factor, σ is the Boltzmann radiation
constant of a black surface, k is the convective heat transfer coefficient and T 0 is the
environment temperature. The solar cell surface is assumed to be perfectly gray at
emissivity ε C , (reflectivity ρ C

and where 2 . 16 ×

ε C ).
The useful heat q C can be determined from equation (2.4.107) if the electrical
energy E is known, e.g., from the measurement.
The solar cell can be evaluated by the energy electrical efficiency: η E , el =

= 1 −

E/q s ,by

the energy heating efficiency: η E , q = q c /q s , or by the energy cogeneration efficiency:

η E , cog q c ) /q s

According to exergetic interpretation the exergy b S incoming to the considered
surface from the Sun is split into the exergy b r of reflected solar radiation, the exergy of
heat b 0 radiating to the environment, exergy of heat b k transferred to the environment
by convection, exergy of useful heat b C transferred from the solar cell to its interior,
electric energy E and the exergy loss δb due to the irreversibility of the considered
system. Thus, the exergy equation for the system shown in Figure 2.4.21 is:

= ( E +

b S = b r + b k + b 0 + b C + E + δb (2.4.112)

where:

10 − 5 σ

3 (3 T S +

T 0 − 4 T 0 T S ) b S = 2 . 16 × (v)