String Inverters: Ideal Choice for Partially Shaded PV Systems
String Inverters: Ideal Choice for Partially Shaded PV Systems, Says IEA-PVPS
The International Energy Agency Photovoltaic Power Systems Programme (IEA-PVPS) has released a comprehensive report highlighting advancements and challenges in enhancing the performance of photovoltaic (PV) systems affected by partial shading.
According to the report, titled conventional string inverters (SINV) outperform module-level power electronics (MLPE) in managing energy output in partially shaded systems.
Efficiency of SINV and MLPE in Shaded Systems Since the early days of PV technology, simple bypass diodes have been an effective solution to manage different operating points in partially shaded systems. These diodes remain the preferred choice for applications with slight to medium shading, especially when paired with conventional SINV.
Outdoor tests, such as those conducted in Denmark, show that SINVs outperform MLPEs in terms of annual AC yield under partial shading, with differences typically below 3%. This advantage is particularly evident when shading, caused by objects like chimneys or dormers, affects less than 10% of a module. However, for systems experiencing significant shading, such as multiple obstructions or neighboring buildings, MLPEs or independent MLPE (indMLPE) systems can improve performance by enhancing the DC/DC power of heavily shaded modules.
Despite this, the additional energy losses incurred during the conversion process in MLPE systems often reduce their efficiency in moderately shaded conditions. These losses, coupled with the higher costs of MLPEs, make SINVs a more cost-effective solution for most PV applications.
Shade-Tolerant Modules and New Innovations The report also highlights advancements in shade-tolerant modules, often referred to as hotspot-free modules. These modules feature designs with fewer solar cells per bypass diode, such as half-cell configurations, which reduce the risk of hotspots and enhance overall efficiency. This makes them an ideal companion to SINVs, offering a balance of high performance and reduced risks associated with complex power electronics.
In the future, the industry is likely to see an increase in smaller PV roof systems equipped with new multi-string SINVs capable of operating at lower DC voltages. These inverters bridge the gap between module inverters and traditional SINVs, providing outputs above one kW per string while maintaining high efficiency.
Challenges and Opportunities for MLPE Manufacturers Manufacturers of MLPE components face the challenge of improving efficiency and providing accurate performance data across various operating conditions. Current efficiency differences, often underestimated in data sheets, can significantly impact annual yields. SINV manufacturers have already addressed this issue in the past, setting a benchmark for transparency and reliability.
Moreover, MLPEs should incorporate features that allow them to disable maximum power point tracking (MPPT) when operating with half-cell modules, preventing hotspot conditions and enabling maximum power generation over time without risks.
Conclusion As the report indicates, SINVs remain the performance leader for slightly to moderately shaded PV systems, offering a reliable and cost-effective solution. Innovations in shade-tolerant modules and new inverter designs will continue to push the boundaries of efficiency, ensuring optimal energy performance for diverse PV installations. Meanwhile, MLPEs will remain a valuable option for specialized applications with heavy shading or unique module configurations, provided their design evolves to meet emerging market needs.
By embracing these advancements and addressing current challenges, the PV industry is well-positioned to optimize energy output under various shading conditions while driving down costs and improving system reliability.
