Environmental Engineering Reference
In-Depth Information
7.1. Heat Collecting Subsystem
7.1.1. Heat Collector Efficiency
Instantaneous Heat Collection Efficiency
Figure 12 shows the measured collector efficiency of the whole collector bank for a
typical month (June 1985) and the ideal efficiency of a single collector measured under
controlled conditions at the manufacturer's laboratory. The ideal efficiency can be correlated
to the x-parameter by the following polynomial equation:
0
2
η
=
0
913
−
2
46
x
−
1
92
x
(26)
c
The measured collector efficiency is seen to be lower than the ideal efficiency due to heat
losses from the piping system as well as losses due to attenuation of solar radiation received
by the absorber plates because of dust deposition on the glass tubes of the collectors. In order
to exclude the data during the warm-up and cool down periods in the early morning and
before sunset, only the data for the period 10:00 am to 5:00 pm were plotted. The amount of
heat collected was estimated from the measured inlet and outlet water temperature to the
collector field and the flow rate of water. Therefore, all the heat loss from the internal and
external piping system was included in the instantaneous efficiency shown in figure 12.
Figure 12. Measured efficiency of collector bank and the ideal efficiency of a single collector for a
typical day.
Figure 13 shows the instantaneous efficiency of the collector bank at mid-day during the
months of January and June 1985. In a clear day, the efficiency at mid-day (12:00 noon) is
usually close to the highest value for that day. It can be seen that, for the month of January
where some days are usually overcast, the mid-day efficiency drops for those overcast days.
June is normally a sunny month with rare overcast periods, the mid-day efficiency fluctuates
only slightly. The mid-day efficiency can drop slightly during periods of sand storms where
the air is laden with small dust particles that reduce the solar radiation falling on the absorber
plates of the collectors.
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