Environmental Engineering Reference
In-Depth Information
where
ϕ
is the view factor accounting for geometrical configuration of the considered
surface in relation to an eventual surface at which the considered radiation would
arrive.
Based on equation (2.4.86) the exergy of radiation
B
x
-
y
exchanged between any
two different surfaces at different temperature
T
x
and
T
y
can be determined according
Petela (2010):
ϕ
x
-
y
A
x
ε
x
-
y
σ
3
[3(
T
x
−
T
y
)
4
T
0
(
T
x
−
T
y
)]
B
x
-
y
=
B
x
−
B
y
=
−
(2.4.87)
where
A
x
is the surface area of one of two considered surfaces,
ϕ
x
−
y
is the view factor
for configuration of surfaces
x
and
y
,
ε
x
−
y
is the effective emissivity depending on emis-
sivities
ε
x
and
ε
y
of respective surfaces and calculated as for radiation energy exchange.
The effective emissivity simplifies to
ε
x
-
y
1. Formula
(2.4.87) is used appropriately for calculations of the five radiation exergies:
B
f
-
d
,
B
d
-
sky
,
B
d
-
ch
,
B
ch
-
sky
, and
B
ch
-
gr
.
The physical exergy of air (
B
a
1
,
B
a
2
and
B
a
3
in W), is calculated for the air mass
flow rate
m
with use of formula (2.2.15) for specific exergy, (J/kg):
=
1 when the emissivities
ε
x
=
ε
y
=
m
c
p
(
T
a
−
T
0
c
p
ln
T
a
R
ln
p
p
0
B
a
=
T
0
)
−
T
0
−
(s)
where
c
p
and
R
are the specific heat and individual gas constant. Obviously, exergy of
air entering the collector is zero, (
B
a
0
=
0), because air is taken from environment.
Exergy
B
of convective heat
E
transferred from a surface at temperature
T
to air
(environmental or heated) is calculated based on formula (2.2.4):
E
1
T
0
T
B
=
−
(t)
Formula (t) is used appropriately for calculations of the four exergies
B
f
-
a
,
B
d
-
a
,
B
d
−
0
, and
B
ch
−
0
. Potential exergies of air are equal to potential energies, (
B
p
1
=
E
p
1
,
B
p
2
=
E
p
2
and
B
p
3
=
E
p
3
). Kinetic exergies of air are equal to kinetic energies, (
B
w
1
=
E
w
1
,
B
w
2
E
w
3
).
The computation results are shown in the bands diagram (Figure 2.4.17). The solar
radiation exergy arriving at the deck
B
S
=
E
w
2
and
B
w
3
=
=
32
.
41 MW, assumed as 100%, is distributed
between five SCPP components; collector air, floor, deck, turbine and chimney. In the
diagram the exergy streams
B
, W, are represented by their percentage values
b
related
to the solar radiation exergy
B
S
. Exergy considerations disclose large degradation of
solar radiation. The floor fully absorbs the received high temperature radiation exergy
and converts it to the exergy at the lower temperature
T
f
,
eff
. Part of this
T
f
,
eff
exergy
(
b
f
-
d
=
17
.
24%) radiates to the deck and another part
b
f
-
a
= 5.10% is transferred by
convection to heated air in the collector. The remaining large part (
b
f
=
72
.
16%) is
lost during irreversible processes of absorption and emission at the floor surface.
The power
B
p
performed by turbine is the same as in the energy balance;
B
P
=
E
P
=
0
.
70% represents the exergy
efficiency
η
B
of the SCPP. Exergy efficiency is slightly higher than the energy efficiency
because the same power is related to the radiation exergy which is smaller than the
0
.
23 MW. The percentage power of the turbine
b
P
=
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