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3 6
E L E CT R I C AL CONNE CT I ON
S P R I NG 2 01 5
MOUNTING SYSTEMS
Right on the roof
O
ne of the major risks when you
install a photovoltaic system –
besides falling off the roof – is an
installation malfunction.
Your reputation as an installer is also
in jeopardy.
Immediate thoughts go to the electrical
side of such a project, but the longevity of
an installation also relies on the mechanical
workmanship in putting the system together.
A rooftop photovoltaic (PV) system
is subject to weather that can severely
restrict its useful life. Installation costs are
considerable in a typical domestic system,
and competition means there’s a temptation
to cut corners in areas that don’t relate
directly to electrical performance.
However, compromising the structural
integrity of a rooftop installation is a bad
idea – even a dangerous one.
PV panels are not something to just ‘stick’
on the roof. They are part of the structure,
and the expertise usually associated with
construction comes into play.
In addition to the mounting system there
are other crucial aspects:
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Resistance to weathering (particularly
corrosion).
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Secure fixing to the roof structure.
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The condition of rafters and lintels in older
homes, especially in relation to wind,
which sets up conical vortices at the
edges and corners of roofs. Under wind
velocity of more than 60m/s, vortices can
exert great forces on panels.
So installers should at least be aware of
the Australian Standards applying to rooftop
installations. They should also carefully
scour the technical literature from reputable
suppliers of rooftop installation equipment.
Working at height among potentials of
hundreds of volts is dangerous. A single
panel typically has open circuit voltage of
30V DC or more, and the string voltage of 12
panels will be about 400V.
To avoid any chance of electric shock
when panels are being installed, no
electrical connections should be made.
Once the panels are in place, and before
wiring is commenced, it’s a good additional
precaution to cover them with blankets, etc,
until wiring is complete.
Malfunctioning of the PV panels at
installation time is unlikely other than
through obvious damage sustained
in transport. However, performance
degradation takes place over time. It’s a
matter of degree, but bear in mind that the
householder pays for this.
Rooftop panels should be facing as close
to due north as possible. Otherwise, they
should face north-east. If sufficient power
is not available on one side of the roof,
another set of panels (e.g. west facing) can
be installed.
The inverter for this arrangement requires
two maximum power point trackers (MPPTs).
Panels that are unequally illuminated should
not be paralleled other than via two MPPTs.
If connected in series their efficiency will be
badly affected.
Orientation can be a problem. A 30º
roof slope is generally adequate, but roof
mounting hardware to elevate the angle of
the panels is available.
Without going into theory, the orientation
is best explained with a panel facing due
north (Figure 1). A 30º roof facing the
midday sun at an elevation of 60º gets the
sun’s rays at 90º to its surface. This is ideal.
In winter, the sun might be at 35º elevation.
The effective illumination of the panel is
equal to the cosine function of 25º (check the
diagram for the definition of angles) or 90.6%
of the maximum value.
If the roof is flat the illumination will be
57%, which is well down. A minimum tilt of
10º allows for self-cleaning.
If the panels are tilted on the roof,
ensure that they don’t cast shadows
on each other (the standard tilt angles
available are 5 and 15º).
Slate, tile and steel roofs require different
mounting methods. In all cases the panel-
securing hardware will be attached to
Panel installers must study
location and roof type before
quoting, writes
Phil Kreveld.
Mounting materials are costly,
labour time may increase, and
an unhappy customer is bad
for business.
Figure 1.
A cos 25°
A
SUMMER
WINTER
60°
90°
25°