Best advice for solar PV installations
By Jack Ward, CEO of Soltra Energy
Rooftop solar photovoltaic (PV) solar
systems are mushrooming around the world. In South Africa they have gained
significant ground as users look to supplement Eskom power, combat load
shedding or move off the grid altogether.
Over the past year, system components
have grown more mature, capable and reliable. At the same time solar PV
systems have become more price competitive and viable, particularly when seen
in the light of spiralling electricity rates.
While small-scale solar systems are
increasingly sold in hardware stores and builders’ supply depots, users should
resist the temptation to effect a DIY installation. Here are just some of the
reasons why:
Although most roofs can support the
added weight of a solar energy system, some can’t. It takes a professional –
preferably a structural engineer - to check the condition of the rafters and
assess the capability of the roof to safely support the added dead load of the
solar array, the mounting rack and the temporary live load imposed by the
installation crew. (Unsurprisingly, the latter calculation is often omitted by
DIY’ers.)
While the type of solar panels (be wary
of thin film modules which have a lower yield) and the number to be installed
will need to be professionally selected and calculated, it’s important to
choose a manufacturer that will back its products with an optimum performance guarantee
of 80% over a 25 year period and offer panels with an expected lifetime of over
30 years.
Note that the orientation (north–facing)
and angle of inclination of the solar array are both critical for optimum
performance. So is the spacing between solar arrays to allow channels for
electrical cabling. Ideally, the array should be located near the main
electrical service board if a grid-tied system is being installed.
Grid-tied PV systems should be
interconnected by a licensed electrician while solar hot water systems (often
employed in tandem with a solar PV system) should be installed by a licensed
plumber.
Choosing a solar PV inverter also
requires expert intervention. The inverter converts solar energy to electricity
and will also have to be expertly sized and selected. Many inverters are energy
wasters. Choose one that isn’t.
In most grid-tied installations, the
building’s electrical demand may be determined and used to specify the inverter
needed for the solar PV system. In other cases, however, the estimated peak
array output is used as the basis for specifying the inverter.
In a remote or stand-alone solar PV
system installation, the average daily electric load of the building needs to
be calculated first. The building’s electric demand should include the Watt
demand of all AC loads running at the same time, plus the wattage from the
surge of starting motors, and all DC loads operating simultaneously. This
demand should be further increased by a factor of 1.2 to 1.4 in order to account
for inverter losses.
In both grid-tied and remote site
situations, the initial estimate of the inverter’s capacity may be impacted by
future plans, such as increasing the size of the PV array. This needs to be
accommodated at the planning stage by a professional PV system engineer.
Eskom produces electricity that is a
true sine wave. A modified sine wave inverter produces a slightly squared off
electrical waveform, but some computers, power tools, refrigerators and most
other powered equipment can use this generated electricity.
On the other hand, pure sine wave
inverters produce a true sine wave that is the same as Eskom’s which is needed
by high-end audio and other specialised equipment that are electrically
sensitive, such as life support systems.
Importantly, in grid-tied installations
the inverter must be able to be shut down rapidly in situations where utility
power goes down. This is called anti-islanding and is a safety function for any
Eskom personnel and electricians who may be working on the lines in the area.
An off-grid system will require storage
batteries (the number and capacity are critical) and – usually – a backup diesel
or petrol generator to take up the slack on cloudy days.
There are any number of economic studies
that reveal how this calculations should be made and more about the long-term
value of installing a solar PV system. But in the main, these studies are only
as good as the input data. Make sure you are professionally advised when
working with the numbers.
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