Page 188 - Design of Solar Thermal Power Plants

3.3 THERMAL PERFORMANCE OF PARABOLIC TROUGH COLLECTOR 173

factor g, specular reflectance r, transmittance s, absorptance a, cosine
factor, end loss correction factor and IAM in optical mechanism analysis.
Based on K sa defined in the equation, influence of variation of trun-
cation factor g along with the incidence angle shall also be integrated
in IAM. Thus incidence angle correction factor K gsa shall be expressed as
Eq. (3.57), in which transmittance-absorptance-absorptance product (gsa)
is a collective property of parabolic trough collector, and varies along with
the incidence angle q. In addition, when the incidence angle equals zero,
vertical incidence of solar beam onto the aperture of parabolic trough
collector occurs; then, in this case, the product is (gsa) n .

ðgsaÞ
K gsa ¼ (3.57)
ðgsaÞ n
An incidence angle comprehensive correction coefficient K(q)is
defined as the product of cosine factor F cos , end loss correction factor F end
and incidence angle correction factor K gsa , which can be expressed as

KðqÞ¼ F cos F end K gsa (3.58)
K gsa is calculated by applying the following empirical equation
q q 2
K gsa ¼ 1 þ a 1 þ a 2 (3.59)
cosðqÞ cosðqÞ
in which both a 1 and a 2 are constants to be determined through
experiments.
Thus S can be expressed as

S ¼ G DN rðgsaÞ KðqÞ¼ G DN rðgsaÞ F cos F end K gsa (3.60)
n
n
Then the calculation equation of cosine factor F cos , calculation equation
of end loss correction factor F end and calculation Eq. (3.59) of incidence
angle correction factor K gsa are substituted into Eq. (3.60), and thus S can
be expressed as
2
q q f
S ¼ G DN rðgsaÞ cosðqÞ 1 þ a 1 þ a 2 1 tanðqÞ
n
cosðqÞ cosðqÞ L

f f
8 9
> >
n
n
> rðgsaÞ cosðqÞ 1 tanðqÞ þ rðgsaÞ a 1 q 1 tanðqÞ >
> L L >
> >
< =
¼ G DN
> >
> f >
2
> >
: þrðgsaÞ a 2 q 1 tanðqÞ ;
> >
n L
(3.61)
in which rðgsaÞ refers to the maximum optical efficiency of parabolic
n
trough collector.