Page 139 - Solar Power in Building Design The Engineer's Complete Design Resource

SITE EVALUATION 109

be a safe derating factor. Other losses such as shading, PV degradation due to aging,
and sun tracking are in general not taken into account.
With reference to the preceding, the overall calculated dc-to-ac losses amount to 0.77:

DC-to-ac loss = 0.95 × 0.92 × 0.98 × 0.996 × 0.98 × 0.99 × 0.95 × 0.98 = 0.77
Array tilt angle loss The optimum tilt angle for PV module performance is the lat-
itude angle of the particular terrain. As discussed earlier in this chapter, irradiance at
the latitude angle is perpendicular to the solar PV module. At this angle the annual
solar power energy output from the PV module is at its optimum. An increased tilt
angle above the latitude will increase power output production in wintertime; however,
it will decrease in summertime. Likewise, decreasing the tilt angle from the latitude
will increase power production in summertime.
The following table relates the tilt angle and roof pitch, which is the ratio of the ver-
tical rise of the roof to its horizontal run.

ROOF PITCH TILT ANGLE (DEGREES)
4/12 18.4
5/12 22.6
6/12 26.6
7/12 30.3
8/12 33.7
9/12 36.9
10/12 39.8
11/12 42.5
12/12 45.0

Photovoltaic array azimuth angle (0 to 360 degrees) The azimuth angle is
the angle measured clockwise from the true north of the direction facing the PV array.
For fixed PV arrays, facing south, the azimuth angle is therefore 180 degrees clock-
wise from the north.
PV arrays that are mounted on sun-tracking platforms can move in either one-axis
or two-axis rotation. In one-axis rotation the azimuth angle is rotated clockwise from
the true north. In PV modules installed on platforms with two-axis rotation the
azimuth angle does not come into play.
As a rule, for optimum energy output PV arrays in the northern hemisphere are
mounted or secured in an azimuth angle of 180 degrees or tilted at an angle in a posi-
tion north facing south direction and in southern hemisphere the azimuth angle is
reversed installed in a tilted angle in a position south facing north.