Page 163 - Design of Solar Thermal Power Plants

148 3. GENERAL DESIGN OF A SOLAR THERMAL POWER PLANT

Or, the engineering simplified equation can be applied:

P LOSS ¼ P REFCAV þ P RAD þ P CONV þ P COND

2 3 4 4
ε w s T T A 1
a w w g
¼ 1 5 P AP þ
4
A 1 A 2
1 ð1 a w Þ 1 1 ð1 ε w Þ 1
A 2 A 1

0:18 1:12 0:982 d AP
1 T w 2:47 d AP L
þ0:088Gr 3 cos q
T a L
2 3
l 2pkH

ðT w T a ÞA 1 þ 4 5 ðT w 80Þ (3.33)
L r AP þ d
ln
r AP
In order to facilitate the understandings of readers, a calculation
example is given as follows.
Assume

T g ¼ T a ¼ 20 C; T w ¼ 400 C; a w ¼ 0:9; ε w ¼ 0:85; k ¼ 0:048 W=ðm$ CÞ;

l ¼ 0:033 W=ðm$ CÞ; d ¼ 0:3m; d AP ¼ 5m; r AP ¼ 2:5m; L ¼ 5m;
2

H ¼ L þ d ¼ 5:3m; q ¼ 20 ; v ¼ 22:8 10 6 m s; a ¼ 32:8 10 6 m 2 s;
2
2
2
A 1 ¼ 25 m ; A 2 ¼ 100 m ; P AP ¼ 6500 kW; s ¼ 5:6686 10 8 W m $K 4
Grashof number is:
gbðT w T a ÞL 3 9:81 ð400 20Þ 5 3 12
Gr ¼ ¼ 6 6 ¼ 2:1 10
va 22:8 10 32:8 10 293
When interior of the cavity is in a turbulent state, various
parameters are substituted into Eq. (3.33)
" #
0:9
P LOSS ¼ 1 6500
25
1 ð1 0:9Þ 1
100
4

0:85 5:6686 10 8 673 293 4 25
þ þ 0:088
100
1 ð1 0:85Þ 1
25

0:18 5
1 5 1:12 0:982 5
12 400 2:47
2:1 10 3 cos 20
20 5
" #

0:033 2p 0:048 5:3
ð400 20Þ 25 þ ð400 80Þ
5 2:5 þ 0:3
ln
2:5
¼ 176 þ 183 þ 103 þ 4:5 ¼ 466:5ðkWÞ