New IEA PVPS Technical Report on the Performance of Partially Shaded Photovoltaic Systems Equipped with Power Optimizers or String Inverters
How to Improve Photovoltaic Performance in the Shade?
Or, more precisely, which modules and technologies should you rely on to ensure optimal system operation under shading conditions?
The new technical report “Performance of Partial Shaded PV Generators Operated by Optimized Power Electronics”, published by the IEA PVPS (International Energy Agency’s Photovoltaic Power Systems Program), provides answers.
The report examines the performance of various power electronics systems, including string inverters (SINV) and module-level power electronics (MLPE), a category that includes both photovoltaic optimizers and microinverters. It also offers a comparative table highlighting the best solutions based on the shading level of the PV system.
Photovoltaics in the Shade: Technical Solutions
Non-uniform shading of a photovoltaic system can lead to disproportionately high losses. This issue is particularly common in urban installations (on rooftops and building facades), where solar systems are more likely to face unavoidable physical obstacles. These include shadows cast by chimneys, antennas, adjacent buildings, or surrounding trees.
As urban photovoltaics play a major role in the energy transition, the IEA PVPS working group emphasizes the importance of understanding the challenges and the most efficient technical solutions in this area.
“Current developments in PV engineering show that maximum performance lies in the combination of optimized panel placement, shading-tolerant modules, and optimized power electronics.”
Of course, site selection and system placement are crucial steps, but in built environments, there are often limited options.
On the panel side, the market offers shading-tolerant photovoltaic modules with bypass diodes. Installed in the junction box behind the module, these diodes literally bypass shaded cells, providing an alternative electrical path for the current flow.
Even today, these remain the most efficient and robust solution for most shaded solar panels. However, due to the increasing nominal power ratings, the presence of only three bypass diodes per module (the “classic” formula) cannot fully eliminate hot-spot effects. “With a greater number of bypass diodes per module area, it is possible to selectively bypass smaller and less efficient areas […] which leads to an increase in yield,” the authors note.
But power electronics systems also come to the rescue.
String Inverters vs Power Optimizers
The string inverter (DC/AC), even when using standard solar panels with only three bypass diodes, provides higher yields for light to moderate shading. This is the case with shading from a chimney or a ventilation pipe, where no more than one-tenth of the modules in a string are simultaneously affected during peak hours.
For medium to heavy shading (from 10% to 40%), power optimizers are the more appropriate choice, although the combination of shading-tolerant PV modules with conventional string inverters often provides comparable annual yields.
However, in systems with very intense shading, where more than 40% of the PV modules are shaded at the same time, or when there are solar modules with different orientations and the strings are too short to use multi-string SINVs, power optimizers used behind each module (MLPE) remain the most efficient system variant.
On the other hand, with weak shading, power optimizers provide lower total yield compared to simple string inverters, “because their DC/DC losses negatively impact compared to simple connectors.”
Which Solution to Choose for Shaded Photovoltaics?
To simplify, the authors have produced a table showing the increase (+) or decrease (-) in performance for PV systems with a SINV configuration, indMLPE (partially equipped with independent power optimizers), and allMLPE (fully equipped with power optimizers).
Recommendations
The report also provides a series of recommendations for system designers and stakeholders. According to the authors, MLPEs (Module-Level Power Electronics) should include a parametric option to disable hot-spot operation point prevention, allowing the generation of maximum photovoltaic power throughout the year without the risk of harmful hot-spot effects, such as those seen in half-cell modules. Additionally, commercial software tools for photovoltaic systems must improve their products, instead of relying on simple, unrealistic weighted efficiency data for the optimizer.
“It is also worth keeping an eye on future developments in power electronics, which may offer string inverters with more multi-string inputs. It could be beneficial if shadow-tolerant PV modules with higher DC voltage than today’s standard modules were introduced, making it possible to eliminate the internal DC/DC boost converter in the string inverter and increase efficiency.”
The long-term stability of the power electronics themselves is also a very relevant parameter to avoid high labor costs in the case of maintenance. Photovoltaic system designers can increase annual yield by increasing the distance between the photovoltaic module and the object creating the shadow when using a string inverter (SINV), without the need to resort to MLPEs.
