The International Electrotechnical Commission (IEC) stands as one of the most authoritative international standardization bodies within the electrotechnical field globally. Photovoltaic module testing standards released by the IEC, such as IEC 61215, are widely adopted within the solar industry. As the industry rapidly progresses, with various technological breakthroughs continually emerging, there has been much discussion on how to accurately assess the power generation performance of bifacial modules. Today, we turn to the scientific essence in conjunction with the authoritative IEC standards to interpret the power generation performance of bifacial modules.
What is Bifaciality?
The 2021 version of IEC 61215 makes a clear definition of the relevant parameters for bifacial modules. Bifaciality, or the bifacial factor, is the ratio of the electrical parameters of the front and rear sides of a bifacial module under standard test conditions (STC), which include the short-circuit current bifacial factor, the open-circuit voltage bifacial factor, and the maximum power bifacial factor. The commonly referenced "bifaciality" pertains specifically to the maximum power bifacial factor.
The bifacial coefficient determined in a laboratory setting is a comparison of the rear and front generation efficiencies – the higher the coefficient, the closer the rear-side generation capability is to the front. This means that comparing the figures for bifaciality alone is not particularly meaningful without considering the baseline power of the front side.
Earlier BC ("Bifacial Cell") technology was limited by the characteristics of the cell structure and immature manufacturing processes, resulting in bifaciality of less than 50%. Thanks to years of R&D efforts by companies like LONGi, the bifaciality of the latest generation BC modules has been raised to over 70%, with even further technical improvements on the horizon, closely approaching the bifaciality figures of TOPCon modules. Additionally, LONGi's second-generation BC modules have made significant breakthroughs in front-side power and efficiency, exceeding those of TOPCon modules by more than 30W.
How Can the Rear-Side Gain of Bifacial Modules Be Scientifically Assessed?
To evaluate the comprehensive power generation capacity of bifacial modules, one must consider not only the bifaciality but also the irradiance. IEC 61215 provides a reference irradiance condition for the nameplate rating of bifacial modules, known as "Bifacial Nameplate Irradiance (BNPI)" – which is when the module front receives an irradiance of 1000W/m² and the rear concurrently receives 135W/m², with the other testing conditions consistent with STC. The results obtained under BNPI reflect the average rear irradiance in typical environments, ensuring that nameplate parameters align with true operating conditions of the modules.
In a word, the IEC standard considers the BNPI condition to cover most ground-level photovoltaic module applications, and this parameter is widely accepted within the industry. Below, specifications of a certain TOPCon module on the market and data from TÜV SÜD testing of LONGi's second-generation BC modules are shown, both having been rated by authoritative institutions, clearly listing the maximum power when the module front and rear receive concurrent irradiance under BNPI conditions.
Comparing the modules of both technological approaches at their highest power levels, a TOPCon module rated at 630W achieves a combined power of 696W under BNPI conditions, whereas a 660W LONGi BC second-generation module reaches a combined power of 722W. This means that, under identical irradiance conditions, the combined front and rear power of BC is significantly ahead of TOPCon by 26W.
Furthermore, the IEC standard also defines "Bifacial Standard Irradiance (BSI)" for assessing bifacial reliability. Here, the module front continues to receive 1000W/m², but the rear is enhanced to 300W/m². This nearly encompasses all photovoltaic module application scenarios. Based on this metric, the combined power for TOPCon modules is 781W, while for BC second-generation modules, it's 798W. In other words, even under severe back-irradiance conditions of 30%, BC modules still outperform TOPCon by 17W in combined power, fully validating BC's overall lead in generation capability.
The comprehensive power generation capacity of bifacial modules is a critical concept, the accurate interpretation of which is vital for the healthy progress of the solar industry. It is necessary to uphold the scientific spirit, follow authoritative standards, and conduct comprehensive evaluations based on real-world operating conditions. Only by doing so can we truly reflect the real performance of solar modules, providing a reliable reference for the industry's development.
About LONGi
Founded in 2000, LONGi is committed to being the world’s leading solar technology company, focusing on customer-driven value creation for full scenario energy transformation.
Under its mission of 'making the best of solar energy to build a green world', LONGi has dedicated itself to technology innovation and established five business sectors, covering mono silicon wafers cells and modules, commercial & industrial distributed solar solutions, green energy solutions and hydrogen equipment. The company has honed its capabilities to provide green energy and has more recently, also embraced green hydrogen products and solutions to support global zero carbon development. www.longi.com/