UFC 3-440-01
14 June 2002
seven banks of four collectors each. The length required for the collector banks is the
width of the collectors plus connective piping. It is conservative to estimate 6 inches
(152 mm) of connective piping between collectors, 3 ft (914 mm) between banks in the
lateral dimension, and 4 ft (1219 mm) around the banks for personnel clearance. The
bank widths are then estimated to be 31 ft (9449 mm) for the seven-collector bank and
17.5 ft (5334 mm) for the four-collector bank. The distance required between collector
rows can be found from Figure 3-5. For example, an 8 ft (2438 mm) collector at 40
degrees N latitude requires row spacing of about 2.5 times 8 ft (2438 mm), or 20 ft
(6096 mm). The array layout should be determined by keeping in mind that the piping
length should be minimized while geometric symmetry is maintained. This guidance
results in a tendency for the banks to contain as many collectors as possible, and for
the array layout to be rectangular in area with an even number of banks installed in
multiple rows. Therefore, the case of four banks with seven collectors each is the most
preferred. A number of roof area dimensions should be proposed so the architect has
some flexibility in determining the building orientation and dimensions. Figure 3-6
shows three possible collector array layouts for the 28-collector array. Similar
consideration can be given to the use of a 4 by 10 ft (1219 by 3048 mm) collector. The
result would be 21 collectors (possibly rounded to 24 or 20), 25 ft (7620 mm) row
spacing (if needed), and banks of seven, six, or five collectors respectively.
3-4.1.7
Array Support Structure. The support structure must transmit the various
loads incident upon the array to the building roof structure without overstressing it. The
design must meet all code requirements and should be coordinated with, or reviewed
by, a qualified structural engineer. At the system layout stage, the structural engineer
or architect should have an idea about the building and roof type before the support
structure is planned. Although steel has often been used for array structures, all
systems designed under this guidance will be made from aluminum, to avoid the cost of
applying and maintaining a protective finish. Although it is difficult to generalize,
experience has yielded some useful estimates about the weight and cost of large
collector support structures. As a rough guideline for rack-type structures, the weight of
the structure should be less than 5 lbs/ ft2 (239 Pa) of collector area. The cost of the
support structure typically represents less than 15 to 20 percent of the total solar
system cost. Any support structures falling outside of these guidelines could be
considered inefficient from a cost versus performance view. It is expected that the
support structure may be heavier and more costly in areas where design loads are
higher or where stepped collector rows are required. Further, stepped arrays require
elevated walkways for maintenance a personnel, which results in higher material and
design costs.
3-9
Previous Page
