Environmental Engineering Reference
In-Depth Information
Figure 3.14 Collector Efficiencies
η C at Different Irradiances E and
Temperature Differences
∆ϑ
Modifications of the loss factors can consider this. The optical efficiency
η 0
also depends on the angle of incidence and the solar spectrum. For very exact
calculations these effects must be also taken into account.
P IPES
A system for solar water heating also requires pipes. These pipes transport the
heat transfer medium from the solar collector to the heat storage unit and the
water from the storage unit to the consumer.
From an energy point of view, pipes incur undesirable losses. They do not
play a role in room heating except for a short period in winter during which
they may provide a very small contribution to the overall demand. If the pipes
are badly insulated or not insulated at all, they waste the majority of the
collected heat on its way to the consumer.
The collector flow rate, i.e. the mass of heat transfer medium that flows
through the collector per hour, is influenced by the lengths and diameters of
the pipes. On one hand, the collector flow rate should be as high as possible so
that the collector efficiency does not decrease due to high collector
temperatures. On the other hand, the flow rate should not be too high so as to
keep the pumping energy demands within an acceptable range.
The collector flow rate m
can be calculated using the collector power
output Q
out , the heat capacity c of the heat transfer medium and the desired
temperature difference
∆ϑ HTF of the heat transfer fluid between collector inlet
and outlet:
(3.19)
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