
382 Principles of Sustainable Energy Systems
© 2008 Taylor & Francis Group, LLC
α
s
can be measured independently, a thermal performance curve of a at-plate collector
allows us to establish the value of F
R
and U
c
also. Figure 7.19 shows typical performance
curves of various glazed and unglazed at-plate solar collectors.
In Equation 7.61, the product τ
s
α
s
will change with the angle of incidence. Since at-plate col-
lectors are normally xed, the angle of incidence changes throughout the day. A relationship
can be written between the actual or effective (τ
s
α
s
) and (τ
s
α
s
)
n
for normal incidence as
ταss ssn
(7.62)
where
(τ
s
α
s
)
n
is the value ...