A poorly installed mounting system can cause self-shading, especially on sloping
roof installations. However, most self-shading usually occurs with rack-mounted
systems. The rows of modules can cause shading on one another if arranged too
close together. You can minimize self-shading by increasing the spacing between
the module rows. Lowering module tilt angles also helps to reduce self-shading,
but doing that also decreases performance. Perform a shade study to determine the
shading at noon on December 21, but also across the solar window until 4 p.m.
Direct Shading
Direct shading causes great loss in energy
yield. The closer the object, the darker will
be the shadow. It is estimated that PV
module output diminishes by more than
50 percent due to direct shading. Direct
shading must be reduced, if possible. Direct
shading is the worst possible situation for
PV module output.
Shading Analysis
A shading analysis assesses the shading
resulting from the location. Shade analysis
tools make it easier to predict where shade
will occur. In order to perform a shading
analysis, the surroundings must have a
shadow outline. The shadow outline is recorded from the center point of the PV
array. It should also include points at the edges of the array.
The shadow outline can be found using:
Site plan and sun path diagram
Sun path diagram on acetate
Shading analyzer
Site Plan and Sun Path Diagram
When using a site plan and sun path diagram, you compute the dimensions and
distance of the shadow-casting objects. You can calculate the azimuth and eleva-
tion angle from the site plan and sun path as well.
The elevation angle is calculated for all obstacles in the area surrounding the PV
system. The azimuth angle is calculated for the obstacles directly from the site plan.
Sun Path Diagram on Acetate
The sun path diagram can be printed on acetate for viewing the objects and sur-
rounding the PV system. The acetate is bent in a semi-circle. The elevation and
Direct shading is evidence by a dark shadow and can be caused
by any nearby object.
Courtesy of PerfectPower, Inc.
76 ADVANCED PHOTOVOLTAIC INSTALLATIONS
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