Environmental Engineering Reference
In-Depth Information
e
ge
=
Geometry error (mirror alignment), 0.98
ρ
cl
=
Clean mirror reflectance, 0.935
e
dm
=
Dirt on mirrors (reflectivity/
ρ
cl
) [reflectivity is an input parameter, usual
value: 0.88-0.93]
e
da
=
Dirt on receiver, (1
+
e
dm
)/2
e
un
=
Unaccounted, 0.96.
It should be noted that these parameters are valid only for normal solar incidence
irradiation. To account for incident angle losses, the incident angle modifier is used,
which accounts for end shading of the trough, reflection and refraction loses, and
selective coating incident angle effects. The terms, e
sh
,e
tr
,e
ge
, and e
un
, shown above
are estimates. The clean mirror reflectance
ρ
cl
is a known value, and the two dirt effects
e
dm
and e
da
are obtained from recommendations by Duffie and Beckman (1991).
The above list of parameters account for collector geometric effects (shadow-
ing, tracking, alignment), mirror and glass envelope transmittance effects (mirror
reflectance and dirt), and a parameter for unexplained differences between field test
data and modelled data. All these values can be altered by the user if in the future
better and more accurate values become available.
Generally, the incident angle modifier is used to account for cases when the solar
irradiation is not normal to the collector aperture (Kalogirou 2004, 2009). This is a
function of the solar incidence angle (
) to the normal of the collector aperture. The
equation determined from a collector testing carried out at Sandia National Laboratory
(SNL) is given by (Dudley et al., 1994):
θ
0
.
00005369
θ
2
K
θ
=
cos (
θ
)
+
0
.
000884
θ
−
(6.2.30)
Other optical properties required include the selective coating absorptance and emit-
tance, and the glass envelope transmittance, absorptance and emittance. The glass
envelope absorptance and emissittance are constant (independent of temperature) and
independent of selective coating type. The values used in the model are
α =
0.02 and
ε
0.86 and can be changed by the user if it is required. The glass envelope trans-
mittance and the selective coating absorptance and emittance depend on the type of
selective coating. Both the envelope transmittance and the coating absorptance are
constants; whereas the coating emittance is a function of temperature. The properties
of the Luz cermet selective coating type used in the model are as follow (Forristall,
2003):
=
•
Envelope transmittance
=
0.935
•
Coating absorptance
=
0.92
0.06 at 100
◦
C and 0.15 at 400
◦
C.
•
Coating emittance
=
The emittance equation used for the selective coating considered, which coincide
with the emittance values given above (Forristall, 2003):
Coating Emittance,
ε
po
=
0
.
000327(T
+
273
.
15)
−
0
.
065971
(6.2.31)
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