Quantcast Collector efficiency and heat losses -Cont.

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MIL-HDBK-1003/13A
In general, the NBS standard will give higher collector efficiencies,
possibly 5%-10%, but the accepted consensus standard at this time is ASHRAE
Standard 93-77.  The Department of Energy (DOE) is using the ASHRAE standard
in developing its program for national certification and rating of solar
collectors.  Therefore, all data given in this report and future reports will
conform to the ASHRAE standard.
Figure 2-7 shows many contemporary solar collectors as of the writing of this
report.  Data is from ASHRAE 93-77 tests.  In some cases the Naval Civil
Engineering Laboratory (NCEL) has conducted the test itself.  A typical NCEL
test report of a solar collector is given in Durlak (1979a) which is the
latest report.  It summarizes other reports available.
A large amount of test data on solar collectors is becoming available through
the national certification program run by SEIA, the NCEL tests, and
individual laboratories testing for the manufacturers.  The National
Certification Program managed by SEIA (See Section 1.2) is now the primary
source of solar collector test data.
Table 2-6 represents a random sampling of the many solar collectors
available.  It is not a comprehensive list nor is it an endorsement of any
particular collector.  These data were excerpted from the Solar Rating and
Certification Program of SEIA, July 1983 Edition.  The main criteria for each
collector in Table 2-6 is that it have an accepted ASHRAE 93-77 performance
test.  Other than that, collectors were chosen to provide a variety of types,
materials, construction techniques, geographical locations, and cost
information.  A few cautions are advisable.  Prices may be up to one year old
from the publication date and should be checked if a purchase is anticipated.
Manufacturers may have other models available.  For example, Table 2-6 may
give details for a single glazed collector and chances are the manufacturer
would also have a double glazed model with valid ASHRAE 93-77 test data.  The
user may know of other collectors with test data available.  These could be
readily compared to similar models in Table 2-6.
To select a collector from Table 2-6, first note that collectors constructed
of similar materials (copper, aluminum, etc.) are grouped together.  Then, it
is necessary to pay attention to the y-intercept (called efficiency intercept
in Table 2-6) which gives the highest efficiency of a collector, and the
slope which gives a measure of the rate at which the collector efficiency
decreases.  These parameters will be used later in estimating the solar
collector performance.  In general the more negative the slope, the less
efficient the collector.  However, this must be balanced with the value of
the efficiency intercept.  For example, in Figure 2-7 and Table 2-6 note that
double glazed collectors start out at a lower instantaneous efficiency (y-
intercept) but do not lose efficiency very fast (less negative slope) so that
when comparing with single glazed collectors the operating temperature (Ti)
will ultimately determine which is best (see Table 2-4 also).  When the cost
of the collector is also considered, it becomes very difficult to
"intuitively" pick a best collector in Table 2-6.  The user should consider
several options of collectors when using the worksheets in the later
sections.  In choosing a collector, Figure 2-7 should be used only for
qualitative judgments, while Table 2-6 should be used for typical slope and
intercept values.  This avoids the errors associated with trying to "read
off" numbers on Figure 2-7.
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