Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 58

Figure 2.4.10 Scheme of the fluxes in energy balance of the surface on earth absorbing radiation from
sun, (from Petela, 2003).

is on the Earth and is perpendicular to the Sun's direction. From the Sun, the black
( ε = 1) radiation of exergy b ω , energy e ω and entropy s ω , all within the solid angle

ω , arrives in the absorbing surface. (For simplification the subscript “ b '' for black is
omitted in this paragraph). These three fluxes are absorbed by the absorbing surface at
temperature T a and emissivity ε a . The absorbing surface, in the solid angle 2 π , emits
its own radiation fluxes of exergy b a , energy e a entropy s a and obtains, in the solid
angle 2 π ω , the radiation fluxes of exergy b 0 , energy e 0 and entropy s 0 from the envi-

ronment at temperature T 0 (assumed to be equal to the sky temperature, T sky =

−

T 0 )

and at assumed emissivity ε 0 = 1.

For the conversion of solar radiation the energy efficiency η E and the exergy
efficiency η B are:

q
e ω

=

η E (2.4.27)

b q
b ω

η B = (2.4.28)

In the present considerations the solar radiation is considered as non-polarized,
uniform and black, at temperature T 6000 K, arriving in the Earth within the solid

angle ω . Exergy b ω of such radiation can be calculated from formulas (2.2.66) and
(2.4.1) in which the double integral represents angle ω

=

R 2 /L 2 with use of the
radius of the Sun R and the mean distance L from the Sun to the Earth:

= π ·

πR 2
L 2

b R 2
L 2

b
π

b ω = = (a)