Environmental Engineering Reference
In-Depth Information
Analogously, the energy emission
e
ω
arriving from the Sun at the absorbing surface
within the solid angle
ω
, is:
e
R
2
L
2
e
ω
=
(b)
where the Sun density emission
e
and the exergy
b
of the black radiation density emitted
by the Sun are:
σT
4
e
=
(c)
σ
3
(3
T
4
T
0
−
4
T
0
T
3
)
=
+
b
(d)
The heat
q
, absorbed by the surface at temperature
T
a
, is extracted in the amount
determined from the following energy conservation equation for the absorbing surface:
q
=
ε
a
e
ω
−
e
a
+
e
0
(e)
and the exergy
b
q
of this heat, absorbed by the heat source temperature
T
a
, is:
q
T
a
−
T
0
b
q
=
(f)
T
a
Determination of
e
a
and
e
0
in equation (e) is required. The absorbing surface of
emissivity
ε
a
and temperature
T
a
, radiates its own emission
e
a
to the whole hemisphere:
ε
a
σT
a
e
a
=
(g)
and the respective exergy
b
a
which is:
ε
a
σ
3
(3
T
a
+
T
0
−
4
T
0
T
a
)
b
a
=
(h)
The considered absorbing surface, beside emission from the Sun, obtains from the
remaining part of environmental hemisphere the black (
ε
0
=
1) radiation energy
e
0
,
(
e
0
e
0,(2
πω
)
), at temperature
T
0
, which is absorbed in amount determined by emissivity
ε
a
of the adsorbing surface:
≡
ε
a
σT
0
2
L
2
R
2
e
0
=
−
(i)
However, regarding exergy, according to the definition, the exergy of environment
radiation is zero:
b
0
=
0
(j)
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