Structure of Conventional Photovoltaic Modules
As is well known, common silicon-based photovoltaic modules are primarily composed of glass, solar cells, back sheets, encapsulants, and frames. Based on their structural composition, photovoltaic modules can be further divided into bifacial and monofacial modules. The difference between the two lies in the fact that bifacial modules can generate electricity from both sides, which gives them a significant advantage in power generation compared to traditional monofacial modules. Additionally, as the manufacturing cost gap between bifacial and monofacial modules gradually narrows, bifacial modules have become the mainstream choice in the market. According to CPIA data, the market share of bifacial modules reached 67% in 2023 and is expected to continue increasing in the future.
Structure of Conventional Bifacial Modules (Glass + Back Sheet)
Bifacial Modules - Higher Overall Power Generation Efficiency
Unlike conventional monofacial photovoltaic modules, bifacial modules are encapsulated with transparent materials (glass or transparent back sheets) on the back side. In addition to generating power from the front side, the back side can also receive scattered and reflected light from the environment for power generation. The photovoltaic conversion efficiency on the back side of bifacial modules is 60%-90% of that on the front side, resulting in a system power generation gain of approximately 4%-30% compared to traditional monofacial module systems. Furthermore, photovoltaic systems using bifacial modules can also reduce their Balance of System (BOS) costs accordingly.
Differences Between Full Tempered and Semi-Tempered Modules
Based on the degree of physical tempering, glass can be classified into full tempered glass and semi-tempered glass. The strength of full tempered glass is increased by three to five times compared to ordinary glass, while the strength of semi-tempered glass is increased by about twice that of ordinary glass. Additionally, there are certain differences in the thickness of the two materials. During the processing and production process, glass with a thickness of less than 2mm can only be processed into semi-tempered glass, while full tempered glass requires a higher thickness.
For photovoltaic modules, the front side is most susceptible to external impact, so the material of the front glass determines the module's sturdiness and impact resistance. Modules can be classified as tempered or semi-tempered based on the material of the front glass. Semi-tempered modules consist of two pieces of semi-tempered glass (both 2.0mm) laminated with encapsulant and solar cells. In contrast, full tempered power generation modules use full tempered glass (2.5mm/3.2mm/2.8mm) on the front side and an organic transparent back sheet on the back side.
So, the question arises: how should one choose between full tempered and semi-tempered photovoltaic modules?
Semi-tempered modules present unavoidable risks in application - explosion. Explosions can be categorized into external explosions and spontaneous explosions, with the latter primarily caused by uneven stress, including:
Lamination stress: Many semi-tempered module glasses tend to self-explode along the busbar due to uneven lamination stress during the lamination process. Over time, this can easily lead to spontaneous explosions.
Mechanical stress: Frameless semi-tempered modules installed with clamps experience significant mechanical stress during high wind or snow loads. Due to the poor ductility of glass, mechanical stress cannot be evenly transmitted, leading to concentrated stress at the edges of the glass in contact with the clamps, resulting in breakage.
Thermal stress: The thermal conductivity of glass is inferior to that of ordinary back sheets. Photovoltaic modules are subjected to strong light over long periods. In areas with large temperature differences between day and night, semi-tempered modules experience significant thermal stress, leading to spontaneous explosions.
Impurities: It is inevitable for glass to contain tiny impurities. If defects are located within the tensile stress layer of the glass, they can easily cause stress concentration. Once the accumulated stress exceeds the intrinsic strength of the glass, it can lead to breakage.
Instances of bending deformation in semi-tempered modules, causing micro-cracks in solar cells and glass breakage, have been observed both in China and abroad.
In western China, semi-tempered modules installed using block methods showed 10-20% bending deformation within just 1-3 years, with 1.5% of semi-tempered modules experiencing breakage.
In Arizona, USA, semi-tempered modules installed for 10 years exhibited yellowing, delamination, and cracking. In severe cases, the front side of the modules darkened and changed color, while the back side showed large-scale delamination, and some back sheet glasses broke.
In contrast, full tempered modules have a lower risk of explosion compared to semi-tempered ones. According to the American standard ASTM C1048-1997b, the surface compressive stress range for various glasses is as follows: tempered glass > 69 MPa (10,000 psi), while semi-tempered glass ranges from 24 MPa (3,500 psi) to 52 MPa (7,500 psi). The national standard for tempered and semi-tempered glass for curtain walls in China also specifies stress requirements, with semi-tempered glass at 24-60 MPa and tempered glass above 90 MPa. Full tempered modules use 2.5mm/2.8mm/3.2mm full tempered glass on the front and a transparent back sheet on the back, effectively addressing thermal stress issues and providing exceptional load-bearing capacity. In contrast, semi-tempered modules use 2.0mm semi-tempered glass on both sides, resulting in lower load-bearing capacity and a higher likelihood of spontaneous explosions under long-term thermal stress.
At a forum on enhancing the quality and safety of photovoltaic systems held on June 15, 2023, organized by the China Photovoltaic Industry Association and the China Electronic Technology Standardization Institute, it was suggested that for projects with complex installation conditions and weaker load-bearing capacities, full tempered transparent back sheet modules should be prioritized, especially in distributed scenarios.
Reducing costs and increasing efficiency is an eternal theme in the photovoltaic industry. Photovoltaic power generation has demonstrated strong cost competitiveness and has become a major growth form of renewable energy. Considering safety, efficiency, and quality, bifacial modules tend to adopt encapsulation solutions that are safer, lower in cost, and of higher quality. It is believed that full tempered solutions may become the mainstream choice for future bifacial modules.
