Environmental Engineering Reference
In-Depth Information
and is based on detailed hourly computer simulation. Consequently, the net beam radiation on
the absorber plate can be expressed by the equation:
'
I
=
I
×
(
R
+
0
×
R
)
(41)
b
b
4
3
The net total radiation on the absorber plate is obtained by adding the diffuse component
to the beam component
'
'
I
=
I
+
I
(42)
t
b
d
3.
Ambient temperature model
When hourly ambient temperature data is not available, hourly data can obtained from
knowledge of the daily mean, maximum and minimum values using a model developed for
this purpose.
4.
Effect of dust on transmittance of glass tubes
The effect of dust on the transmittance of the glass tubes varies with the season. The heat
collection amount drops sharply especially when there is a sandstorm and dust accumulates
rapidly. This causes the transmittance of the glass tubes to drop. When it rains, on the other
hand, the transmittance is restored because the rainfall washes away the dust. Climatic
conditions in the UAE are such that there is little rainfall, but nevertheless, the cleaning effect
of rainfall appears in the collector measurements. In the UAE, moreover, the temperature
differences between day and night are relatively large; in the mornings, dew accumulates on
glass tube surface and they become damp, so mucu so that water droplets sometimes fall from
them. If the level of dew accumulation is slight, dust can easily adhere to the tube surface, but
if the extent of dew is great enough, it will serve to wash away dust from the tube.
The relationship between the cumulative level of dust affecting the transmittance of the
glass tube and the cleansing effect of various natural climatic conditions is extremely
complex. In order to develop a model of the effect of dust accumulation on the transmittance
of the glass tube, it was assumed that the clean glass tube transmittance of 98% is restored
after each tube cleaning and that following a tube cleaning the transmittance drops
exponentially due to dust effect according to the equation (see Sayigh et al. [..]):
τ
−
τ
m
=
exp(
−
0
.
055
×
N
)
(43)
τ
−
τ
0
m
where τ is the transmittance after
N
days has elapsed since cleaning, τ
m
is the transmittance
after one month has elapsed since cleaning, τ
0
is the transmittance immediately measured
after cleaning (= 98%). The monthly drop in transmittance (τ
0
-τ
m
) is dependent on the month
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