Solar array
The output of a solar array depends on the type of panels
used, the amount of sunshine, temperature, orientation and
atmospheric conditions. As a guide, a 1kW [1000W] solar
array installed in an unobstructed sunny position in Cairns
will produce an average daily output as follows:
· polycrystalline solar array 4.0kWh/day
· monocrystalline solar array 4.2kWh/day
· amorphous solar array 4.9kWh/day
Not all the electricity produced by the array is available
to the grid, as there are small cabling losses and also
losses in the inverter. The figures quoted include an allowance
of 10% to cover these losses.
An array is sized according to the amount of electricity
required, subject to constraints such as budget and available
roof space. There may also be some technical and administrative
constraints applicable and these are mentioned below.
The array comprises a number of interconnected and usually
identical solar panels. The panels typically have a nameplate
rating of between 64W and 160W and have a nominal voltage
of either 12V or 24V. The operating voltage of each panel
is determined by the inverter, which continuously adjusts
the voltage of the array so as to yield maximum output.
The peak power voltage depends on the intensity of the sunlight
and also on temperature. For a nominal 12V panel it typically
lies between 15V and 17V, for a nominal 24V panel the values
are 30V to 34V.
The solar panels are connected together in series to form
one or more strings. The voltage of each string is the sum
of its individual panel voltages. The number of panels in
each string is chosen so that the peak power voltage of
the entire string suits the inverter. This value is usually
between 85V and 180V.
Some solar panels designated specifically for grid-connect
applications have much higher peak power voltages, being
about 70V. They offer an advantage of requiring fewer panels
per string, but in other respects have similar properties
to nominal 12V or 24V panels
Mounting and Orientation
The solar array needs to be securely installed, able to
withstand wind and other loads and as far as possible face
the sun. The three methods described here are roof frames,
solar roof tiles and ground frames.
The most common method is to mount the solar panels on frames
attached to the roof of a building. If the roof faces north,
the panels can lie almost flat on the roof. In fact, the
panels are raised a few centimetres off the roof to allow
ventilation and not obstruct the flow of water.
A more common method is to provide the frame with legs
so that the array is mounted at an angle to the roof. This
is particularly effective on a gently sloping south facing
roof as it allows winter sun to be reflected off the roof
onto the panels in addition to direct radiation. As far
as possible north or south facing roofs are chosen, this
being consistent with the design principle of aligning dwellings
so that the long side of the building runs east-west.
Solar tiles allow the array to become part of the roof,
being fixed directly to battens in place of conventional
tiles. Monocrystalline 75W and 85W solar tiles are commercially
available for this purpose.
Ground mounted frames are used where the roof is too steep,
shaded or otherwise unsuitable. Ground mounted arrays have
the disadvantage of being more expensive than roof frames
and taking up garden space. Any ground frame needs to be
certified by a qualified engineer.
Usually solar arrays are mounted in a fixed position and
are not adjusted to allow for the daily or seasonal movement
of the sun. This is especially so for roof mounted arrays.
Nevertheless tracking systems are available, however this
topic lies outside the scope of these notes.
For grid connected solar systems, the array is oriented
so as to maximise the expected average yearly output. For
Cairns and the Tablelands the optimum orientation is facing
north, tilted at an angle of between 14 deg. and 17 deg.
from horizontal. This is different from arrays for stand-
alone systems where the tilt is about 30 deg. in order to
improve winter performance.
Electrical Installation
A solar array is a generator of electricity that can produce
a potentially lethal voltage and which cannot be readily
switched off. The electrical protection and fire safety
issues involved have motivated the development of a comprehensive
Australian standard for solar arrays. A draft of this document
is currently being circulated within the industry and stakeholder
organizations for comment. It is likely to be promulgated
as a standard in 2004.
Whilst at this stage the cost of compliance is not clear,
for grid-connected arrays it is likely to be significant.
Shading
Shading of solar panels, for example that from tree branches,
accumulated dust on panel surfaces or ash deposited from
bushfires will reduce output. For amorphous panels the reduction
in output is in proportion to the amount of shading. However
for monocrystalline and polycrystalline the reduction is
much greater.
Solar panels should be cleaned periodically with a damp
cloth. This is especially so after bushfires or after lengthy
periods without rain.
