UFC 3-440-01
14 June 2002
plus or minus 10 degrees from the site latitude. It should be noted that as the tilt angle
increases, the minimum spacing between rows due to shading increases and larger
roof area is required.
3-4.1.4
Array Azimuth Angle. The array azimuth angle is defined to be the angle
between the projection of the normal to the surface on a horizontal plane and the local
meridian (north-south line). Zero degrees is defined as due south, a due west facing
array is defined as plus 90 degrees, and a due east facing array is defined as minus 90
degrees (in the northern hemisphere). The optimal orientation requires the azimuth
angle to be 0 degrees (due south) whenever possible, although deviations of plus or
minus 20 degrees off of due south have a minimal effect on flat-plate system
performance.
3-4.1.5
Collector Grouping. Internal-manifold collectors should be grouped into
banks ranging from four to seven collectors each, with each bank containing the same
number of collectors. Proper sizing of the collector banks is essential to maintaining
uniform flow throughout the collector array. The maximum number of collectors that
can be banked together is a function of the maximum flow rate allowed in the plumbing,
internal manifold and riser diameters, thermal expansion characteristics of the collector
piping and absorber plate assembly, and the recommended flow rate of the particular
collector chosen (usually given in gallons per minute (liters per second) per collector or
gallons per minute per square feet (liters per second per square meter) of collector
area). Thermal expansion problems are minimized by keeping the bank size less than
eight collectors.
3-4.1.6
Minimum Array Row Spacing. The minimum row spacing must be
calculated for multi-row arrays. A general routine for north-south spacing of collector
banks can be devised, based on a "no shading" criterion for a particular time of year.
The guidance presented assumes no shading of the array on the "worst" solar day of
the year (21 December, when the sun is lowest in the sky in the northern hemisphere)
for the designated time period of 10 a.m. to 2 p.m. solar time. Most large-scale military
solar systems are installed on low-slope flat roofs, and there are two possible cases to
consider. The first is for a flat roof with enough space to locate the collector array at
one elevation. The second case is for a flat roof with too little space for the collector
array. This requires the collector banks to be "stepped", that is, each succeeding row of
collectors must be elevated. This arrangement is necessary if the collector roof area
required is larger than that available or if roof area costs are more expensive than
elevated rack costs. The equations developed for minimum collector row spacing are
presented graphically in Figure 3-5.
3-7
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