Environmental Engineering Reference
In-Depth Information
Table 2.4.3 Comparison of the exergy and energy balances of a surface absorbing solar radiation,
for ε a = 0 . 8 and ε a = 1, (from Petela, 2003).
ε α = 8.0
ε α = 1
Item
Terms
% energy
% energy
% energy
% energy
Input
Sun
100
100
100
100
Environment
23.1
0
28.88
0
Total
123.1
100
128.88
100
Output
Reflection
20
20
0
0
Emission
42.8
1.7
53.5
2.12
Heat, (efficiency)
60.3
9.23
75.38
11.54
Loss
0
69.07
0
86.35
Total
123.1
100
128.88
100
For the considered case in Table 2.4.3, the values of efficiencies can be inter-
preted as η E =
60 . 3% and η B =
9 . 23% for ε a =
0 . 8, or, respectively, η E =
75 . 38% and
η B =
1. The energetic and exergetic efficiencies values differ signifi-
cantly and, except for reflected radiation by absorbing surface, other items of the both
balances are also very different.
The temperature T a of absorbing surface can be considered practically only in the
range T 0
11 . 54% for ε a =
T a ,max . Temperature T a smaller than T 0 requires additional energy to
generate surroundings colder than the environment, whereas for T a > T a ,max the heat
q becomes negative because the radiation of absorbing surface to the environment is
larger than the heat received from solar radiation.
Some more computation results for analyzing effect of T a ,( t a C ), are shown in
Figure 2.4.12. An increase in T a ,( t a C), will decrease exergy loss δb and heat q ,
whereas η B and b q reveal maxima as results of growing of the Carnot efficiency: η C , a
T a
=
1
T 0 /T a .
2.4.2 Solar cylindrical-parabolic cooker
The most common devices for utilization of solar radiation are cookers of different
types. The simple solar cylindrical-parabolic cooker (SCPC), shown schematically in
Figure 2.4.13, is used to demonstrate the methodology of exergy analysis of the cooker
and the distribution of the exergy losses. Also explained is the general problem of
how the exergy loss at any radiating surface should be determined, if the surface
absorbs many radiation fluxes of different temperatures. Additionally a possibility
of introduction of an imagined surface to complete the cooker surfaces system is
shown.
The cylindrical cooking pot filled with water is surrounded with the cylindrical-
parabolic reflector. The considered system of exchanging energy consists of three long
surfaces of length L . The outer surface 3 of the cooking pot has an area A 3 . The inner
 
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