Page 82 - Energize October 2022

TECHNICAL

A horizontally mounted panel produces 90% of a tilted The total distance occupied by a single row amounts to 2,71 L
panel’s energy and is independent of azimuth. The advantage of and a horizontally mounted system occupies a length of L per
horizontally mounted panels on total power produced for a given array. Therefore, if the space were completely filled it could
area comes from the fact that no spacing is required between accommodate 2,7 times the number of arrays that a tilted
panels, allowing the complete area to be covered with panels. system could. Allowing for the fact that a horizontal system
The fact that a horizontal panel does not require a specific generates 10 to 20% less energy annually than a tilted array,
orientation or azimuth also allows the shape of the panel array the horizontal array could generate:
to be optimised for the geometry of the site.
2,7 * 0,8 = 2,16 times the energy of a tilted array
Tilted panel arrays and shading
For ground-mount solar, it is usual to mount the solar panels Most horizontal or low tilt angle systems claim at least double
in rows. Tilted panels need to be spaced apart to reduce the the energy per m2 compared with tilted arrays or tracker
effect of shading. The spacing between panels depends on the systems. The other factor affecting this issue is that the cost of
latitude and the amount of shading that can be tolerated. The solar panels is reducing while the cost of mounting structures
minimum panel spacing will be determined by the amount of is increasing. Current thinking is that the cost of solar panels
shading allowed during the worst period, i.e., when the sun is at cannot be reduced much further while the cost of mounting
its lowest. This is calculated using sun path diagrams, an example structures could be reduced by using different mounting
of which for Johannesburg, is given in Figure 6. configurations. Horizontal or low tilt panels allow the array
to be mounted close to the ground, with a much simplified
support structure.

Roof mount solar arrays
For other than flat roof buildings, roof mount arrays benefit
from the existing tilt angle resulting from the pitch of the
roof, and because they are mounted flat on the roof surface,
do not suffer from inter row shading. The orientation of
the roof, however, is not always optimum, and on pitched
roofs, the orientation of rooftop arrays cannot generally be
changed. Rooftop arrays do not generally feature spacing
between rows.
Solar PV arrays mounted directly on rooftops can
generally achieve a higher energy generation capacity
Figure 6: Sun path diagram for Johannesburg (Gaisma) than tilted rows, even on flat roofs. A horizontal mount PV
system on a flat roofed building has the advantage of being
The optimum tilt angle for this latitude is 30°. Taking the June solstice independent of azimuth, and hence independent of the
as the worst case, and choosing a window of 08h00 to 16h00 for orientation of the building, which can be a restricting factor
no shading, gives a minimum sun elevation of 15°. The minimum for tilted arrays.
panel row spacing is calculated using the diagram in Figure 7. Horizontally mounted arrays on flat rooftops are becoming
common on industrial buildings with flat or slightly tilted
rooftops. The use of flat mounting on such structures gives
a great advantage compared to tilted panels, with increased
energy generation per area and simplified mounting structures
(see Figure 8). For very low angles of tilt, orientation has little
Figure 7: Row spacing for shade limiting or no effect.

If the length of the array is L, then the height of the top edge
of the array above the horizontal is L* sin 30 = 0,5L, and the
horizontal distance between the top edge of one array and the
lower edge of the next array will be:

0,5L/tan 15° = 0,5L/0,27= 1,85 L

The horizontal distance occupied by the tilted panel amounts to:

L* Cos 30° = 0,86 L Figure 8: Horizontal mount panels on flat roof (Rhino energy)

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