14 / 64
CONSTRUCTION WORLD
MARCH
2017
12
ENVIRONMENT & SUSTAINABILITY
Keeping pace with the popularity and
growth of decentralised system
architectures, MPLEs will offer new benefits
to many market segments, one of which
is also in a rapid growth phase; the solar
photovoltaic (PV) market.
Two examples of MPLE technologies
geared to play key roles in the solar PV
industry are power optimisers and micro-
inverters. Responsible for optimising the
yield of solar panels (or modules) these
MPLEs are becoming vital components of
modern solar PV installations.
In understanding the advantageous and
strategic functions of these MPLEs,
it might be useful to cover some of the
basics. For instance, solar PV panels
generate DC electrical power according to
the amount of sunlight falling on them.
The voltage across each panel and the
current that flows through it are interrelated.
At any given level of irradiance, there is a
point at which the power generated is at
its highest. This is referred to as the
‘maximum power point’ or MPP.
A panel can be encouraged
to run at its MPP by
regulating the current
flowing through it. However, panels
(like fingerprints) have slightly differing
characteristics, so when a number of panels
are connected in series, the MPP for the
entire string is a compromise between the
MPPs of all the individual panels; the lowest
common denominator, in other words.
Within the solar PV arena, one of the
most relevant MLPEs is the micro-inverter.
In new-generation solar PV installations,
small format, grid-tie inverters – generating
240 volts AC – are connected directly
to each solar panel in the array. The AC
output of all of the micro-inverters is then
channelled into a single supply which is fed
to the load.
The other MPLE set to boost the solar
PV industry is the power optimiser which
employs DC-to-DC converter technology
to maximise the energy harvested from
solar panels connected in strings. This is
achieved by tuning the output performance
of the panels to match the performance of
the micro-inverter.
At first glance, the fitment of optimisers
seems to be an ‘overkill’, particularly when
it adds to the overall cost of a solar PV
system. However, as any experienced
solar PV practitioner will confirm, there are
many potential pitfalls associated with the
layout of solar PV panels which are reduced
or eliminated completely with the use of
power optimisers.
Major pitfalls
Shading is one of the biggest pitfalls.
When more than a third of the
power producing cells in
a solar PV panel are
in the shade, the
performance of
the entire panel
is compromised.
This has a choking
effect on all of the panels
in the same string, causing a
significant loss of overall solar PV
yield. When optimisers are used, the
shaded panel performs to the best of
its ability, and none of the other panels
is affected.
Another pitfall is represented by
power mismatches. Although all solar
panels in a string should theoretically
perform at the same level – all things
being equal – there are nevertheless
performance variations which can cause
an overall degradation of the output of the
string. Power optimisers eliminate the
negative effects of these mismatches,
allowing each panel to perform at its best.
Challenges
One of the challenges faced by many solar
PV system designers is the inevitability
that not all roof or mounting surfaces face
north – the optimal facing in the southern
hemisphere. Varying orientations have to be
accepted and accommodated.
When using conventional inverters, it is
not advisable (nor is it common practice)
to mix panels of different orientations in
the same string – because of the problems
associated with variance in individual
performance as outlined above.
This constraint creates difficulties for
architects, engineers and installers when it
comes to positioning the string. With power
optimisers in place, they have complete
freedom in this regard, with panels at
different orientations working efficiently, in
harmony, side-by-side.
Panel failures and degradation are not
uncommon in aging solar PV installations.
Identifying the offending panels is often
difficult in a large array where the only
indication of the problem is an under-
performing string.
The solution is panel-level performance
monitoring and reporting, functions that
will help to quickly identify a suspect panel.
These features are now integrated in certain
leading-edge power optimisers.
Finally, sourcing an exact replacement
panel in terms of brand or specification
to replace a damaged or degraded unit
may be difficult – if not impossible – in
the case of an aging installation. With a
conventional string, mixing new and old
panels will inevitably affect the combined
MPP and result in lost power. With power
optimisers in place, however, the new panel
will perform at its maximum – as will the
entire string.
KEY
to
SOLAR PV
industry boost
By Bevan Jones, Soltra Energy technical director
The module-level power
electronics (MLPE) market is set
to boom, according to reports,
with compound annual growth
figures touching the 20% mark,
breaching one billion US dollars
annually in 2019.
Bevan Jones, Soltra Energy
technical director.