
372 Principles of Sustainable Energy Systems
© 2008 Taylor & Francis Group, LLC
where L = 40° δ
s
= −21.1 on January 15, and β = 60°. The hour angle h
s
equals 15° for each
hour away from noon.
The n efciency is obtained from Equation 7.32:
η
f
=
′
−
where
m
U
c
=
=
××
=
−
12
4
12
8
.
and
η
f
=
−
=
.
0 938
The collector efciency factor F′ is, from Equation 7.37,
′
=
′
++
=
+
F
U
lUDw hD
c
cfci
1
121
180
/
//
/
[( ())( )]
.
,
ηπ
092
.
×+ ×
=
π
Then we obtain the heat-removal factor from Equation 7.43:
F
Gc
U
UF
Gc
R
p
c
c
p
=−−
′
1exp
Time (h) I
h
(Wm
2
) R
b
I
d,c
(W/m
2
) I
b,c