Performance of Partially Shaded PV Generators Operated by Optimized Power Electronics 2024
Authors: Baumgartner Franz , Allenspach Cyril , Özkalay Ebrar , Berwind Matthew , Heimsath Anna , Bucher Christof , Joss David , Golroodbar Sara Mirbagheri , van Sark Wilfried , Granlund Alexander , Valencia Arroyave Felipe , Bründlinger Roland , Herrmann Werner , Herteleer Bert 
Published: December 2024

Inhomogeneous shading on the PV generator leads to disproportionately high losses. As the potential
of PV generation on roofs or façades is to be increasingly utilised in the coming decades, these cases will occur more frequently. The aim here is to provide an overview of the challenges and state-of-the-art technical solutions for partial shading. Current developments in PV engineering show that maximum
performance lies in the combination between optimised module placement, the use of modules that are
tolerant of shading and optimised power electronics.
Shortly after the discovery of the solar cell, blocking or bypass diodes were used to solve the inhomogeneous currents of groups of solar cells arranged in series or parallel wiring. Even today, they are still the most efficient and robust solution for the majority of common shading PV applications.
Due to the very high rated outputs of the solar modules and the presence of only three bypass diodes, high temperatures can occur on a locally shaded solar cell. This forces heat outputs of up to 200W or 100W in the butterfly module connection through the associated activated bypass diode, which must be
dissipated by the most shaded cell. If additional small-area defects occur in this affected solar cell, hotspot peak temperatures can occur, which can lead to permanent damage to the module or the risk
of fire.
However, in order to prevent a third of the module output being lost in this case, four or more bypass diodes are now used in so-called shadow-tolerant PV modules. With a higher number of bypass diodes
per module area, it is also possible to selectively bypass smaller, less efficient areas of the module, which leads to an increase in the module yield. The hotspot effects can also be comprehensively and
robustly prevented by the small number of solar cells per bypass diode, provided the bypass diode is
properly designed. The first manufacturers are beginning to place these shade-tolerant PV modules on
the markets.
Today, planners can also select different power electronics systems for the next step in system integration towards grid feed-in, i.e. the connection of the individual modules in the string. This is the classic series connection of all modules in the string to the input of the DC/AC string inverter (SINV), which leads to the highest yields for weak and medium shading. This applies, for example, to light shading
with a chimney or a ventilation pipe, where no more than one tenth of the modules in the string are
reached by the shade at the same time during the six hours around midday, even when using standard
modules with only three bypass diodes. (see Table 1)
With medium to heavy shading, the widely used DC/DC converters directly on the PV module (MLPE),
often also called power optimisers, can be used profitably. However, the combination of shade-tolerant PV modules with conventional SINVs can often deliver comparable annual yields. However, if the optimisers are also used behind each module even with weak shading (allMLPE), they deliver less yield in total than the simple SINV, as their own DC/DC losses then have a negative impact compared to simple
connectors. This only becomes apparent if the MLPE manufacturers' data sheet claims of 99% efficiency are not viable. The published measurements carried out in independent laboratories over the last four years are listed in this report, which suggest that losses are around 2% higher.
As the differences in yield between the power electronics variants SINV and MLPE are usually less than four per cent in annual yield for light to medium shading, the above-mentioned real MLPE efficiency at the specific operating points plays the decisive role in planning the most efficient system. However, as the commercial PV software planning tools currently use these MLPE manufacturer specifications which
are over estimated, no meaningful system comparison can be expected for these shading categories.
In this report the results of annual simulations performed by some sophisticated simulation tools that take these real MLPE losses into account are discussed.