Construction World March 2017

ENVIRONMENT & SUSTAINABILITY

KEY to SOLAR PV industry boost By Bevan Jones, Soltra Energy technical director

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 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.

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

Bevan Jones, Soltra Energy technical director.

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. 

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

to run at its MPP by regulating the current

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

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CONSTRUCTION WORLD MARCH 2017