UFC 3-440-01
14 June 2002
Tempered, low-iron glass is by far the most common glazing used because of its
excellent optical properties and durability. Clear plastics, such as acrylics and
polycarbonates, have a history of problems with clarity over time due to ultraviolet
degradation and are not recommended. Double-glazing reduces the thermal losses
from the collector, but also decreases optical efficiency and increases weight and cost.
This fact can be seen on a collector efficiency plot as a decrease in the FR value and a
decrease in the slope FRUL. (Refer to APPENDIX F for additional discussion.) For
certain higher temperature applications, the increase in efficiency at larger values of (Ti-
applications, single-glaze collectors will suffice.
4.2.1.1.5
Insulation. An insulating material is required behind the absorber plate
and on the sides of the collector to reduce conduction losses. Insulation types currently
in use include fibrous glass, mineral insulation, and insulating foams. The primary
considerations of the insulating materials are their thermal conductivity, ability to
withstand stagnation temperatures and moisture, dimensional stability, flammability,
and outgassing characteristics. Fibrous glass, closed cell polyisocyanurate foam, and
polyurethane foams are currently used in most solar systems. Polyurethane foam is
especially well suited because of its ability to retain its shape and to resist moisture that
may be present from condensation. Often, a layer of fibrous glass will be sandwiched
between polyisocyanurate insulation and the absorber plate, since this material is better
suited to withstand the high stagnation temperatures, which can exceed 350 degrees F
(177 degrees C) in that part of the collector.
4-2.1.2
Collector Selection. Required information on the chosen collector includes
the net aperture area (Ac); overall dimensions of length or height (L) and width (W); the
manufacturer's recommended collector flow rates (CFR) and the pressure drop across
the collector at that flow rate; the internal manifold tube diameter; and the collector
weight when filled. The designer should note whether the manufacturer recommends a
maximum number of collectors per bank less than seven. Of special importance are
2
the values for Ac and CFR. While collector areas range from approximately 16 to 47 ft
(1.5 to 4.4 m2), it is recommended that collectors with net areas of 28 ft2 (2.6 m2) or
more be specified whenever possible. For large commercially-sized arrays, smaller
collectors result in higher installation costs due to increased materials and labor
required to achieve a given array area. The pressure drop is often reported in units of
"ft of water". The following range of values could apply to typical flat-plate collectors: Ac
= 28 to 40 ft2 (2.6 to 3.7 m2), Length = 8 to 10 ft (2438 to 3048 mm), Width = 4 to 5 ft
(1219 to 1524 mm), CFR = 0.01 to 0.05 gals/min-ft2 (0.007 to 0.034 L/sec-m2), pressure
drop = 0.1 to 0.5 psi (690 to 3447 Pa), internal manifold diameter = 1 to 1.5 inches (25
to 38 mm), and collector filled weight = 100 to 160 lbs (45 to 73 kg). When the designer
has this information, the final array layout can be completed.
4-2.2
Collector Sub-System Piping and Layout
4-2.2.1
Layout and Terminology. Figure 4-2 provides an example of a collector
array layout with the appropriate terminology.
4-3
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