Environmental Engineering Reference
In-Depth Information
be obtained, e.g., for heat
Q
2,
c
respectively is
β
2,
c
=
Q
2,
c
/
I
, however the term with
Q
3,
u
determines the energetic efficiency
Q
3,
u
I
η
E
=
(2.4.45)
Therefore, equation (2.4.44) can be also written as:
β
+
η
E
=
1
(2.4.46)
The
exergy analysis
enables the additional quality interpretation of the cooker.
For the considered SCPC, the exergy of radiating fluxes, overall exergy efficiency of
the SCPC process and the exergy losses during irreversible component phenomena
occurring in the SCPC are calculated.
Sometimes, it is convenient to determine an exergy
B
of radiation emission at
temperature
T
by multiplying its emission energy
E
by the characteristic exergy/energy
ratio
ψ
, defined by formula (2.2.45), e.g.,
B
ψ
. Thus, the exergy efficiency
η
B
of
the SCPC is the ratio of the exergy of the useful heat
Q
3,
u
, at temperature
T
3
, and
of the exergy of solar emission at temperature
T
S
:
=
E
·
Q
3,
u
1
T
3
T
0
−
η
B
=
(2.4.47)
Iψ
S
where
ψ
S
is the exergy/energy ratio for the solar emission of temperature
T
S
, which
for
T
S
0
.
9348.
Reflection and transmission of radiation are reversible so the exergy losses in the
SCPC are considered only for the following component phenomena:
=
6000 K and
T
0
=
293 K is
ψ
S
=
-
Simultaneous emission and absorption of radiation at surfaces 2, (
δB
2
) and 3,
(
δB
3
). There is no exergy loss at the imagined surface 1, (
δB
1
=
0), because nei-
ther absorption nor emission occurs but only transmission of radiation which is
reversible. Other surfaces, 2 and 3, are solid and thus produce the irreversible
effects of radiation.
- Irreversible transfer of convection heat
Q
2
c
from the both sides of surface 2 to the
environment, (
δB
Q
2
c
).
- Irreversible transfer of radiation heat
Q
2
r
from the outer side of surface 2 to the
environment, (
δB
Q
2
r
).
-
Irreversible transfer of heat
Q
3
u
from surface 3 to water,
(
δB
Q
3
u
),
due to
temperature difference
T
3
−
T
w
,
-
Irreversible transfer of convection heat
Q
3
c
from surface 3 to environment, (
δB
Q
3
c
).
The exergy
δB
1
−
0
escaping through surface 1, results from reflections from the
SCPC surfaces to the environment. This loss is sensed only by the SCPC and is not
irreversible because theoretically it can be used elsewhere. This loss consists of the
radiation exergies
B
1
−
1
,
B
2
−
1
,
B
3
−
1
at the three different temperatures (
T
S
,
T
2
and
T
3
)
δB
1
−
0
=
B
1
−
1
+
B
2
−
1
+
B
3
−
1
(2.4.48)
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