14 June 2002
d. Array Support Structure
Purpose. A support system is required for the following reasons.
(a) Secure the collectors in the correct orientation for maximum solar
(b) Withstand the various structural and thermal loads imposed upon
(d) Be as lightweight and inexpensive as possible.
(2) Types. There are two basic types of support structures: roof-mounted
and ground-mounted. Roof-mounted structures are the most common and are
preferred over ground-mounted structures, to avoid vandalism and aesthetic problems.
Ground mounting may be necessary where there is insufficient solar access at the roof
level and in retrofit situations where the roof cannot support the array or proper access
to the roof for piping and sensor wiring is not available. Flat roofs require rack-type
structures that are heavier and more costly than the type of structure normally used to
mount collectors on sloped roofs. However, rack-mounted collectors on flat roofs are
usually easier to service.
(3) Structural Considerations. One of the most important issues addressed
by structural codes is the design load. Many loads are imposed on a collector array,
including dead and live loads; those imposed by the environment, such as wind, snow
and seismic loads; and thermal loads caused by the effects of temperature extremes
and changes. Wind loads (along with snow loads at some locations) have, by far, the
most significant effect on the structure. Dead loads are defined as those attached
permanently to the array structure. Live loads are those applied to the array structure
temporarily, other than wind, seismic and dead loads (a maintenance worker, for
example). The combination of these loads at any instant must be accommodated by
the structural design. Local building codes usually prescribe the design load
combination to be used. The design and construction of support structures is usually
governed by local building and structural codes that are often adapted from nationally
recognized U.S. codes. These codes establish the design criteria to insure structural
safety and integrity over the expected life of the system.
(4) Material Considerations. The materials chosen for the array structure
must also be able to withstand environmental degradation. Oxidation, caused by
humidity and precipitation, affects all metallic surfaces to varying degrees. Aluminum is
required for the array support structure because the oxide layer that forms on the
surface when it is exposed to moisture protects it from further degradation. Often,
aluminum is anodized to provide a controlled layer of oxidation. The use of steel would
require a coating system to be applied and maintained, which adds to the system life-
cycle cost. The effect of temperature changes must also be taken into account for
lengthy structures, especially the difference in thermal expansion between the various