Glass-Glass module designs are an old technology that utilises a glass layer on the back of modules in place of traditional polymer backsheets. They were heavy and expensive allowing for the lighter polymer backsheets to gain the majority of the market share at the time. However, despite these disadvantages, the ITRPV[2] predict an increase in market share of about 30% by 2029. This is based on the increase in market share of bifacial modules as well as an increase in utility-scale PV installation, which prefer more durable module designs such as glass-glass.
Double-glass modules boast increased reliability, especially for utility scale PV projects. These include better resistance to higher temperatures, humidity and UV conditions and have better mechanical stability, reducing the risk of microcracks during installation and operation. These are particularly important in utility-scale PV sites and for the expected lifetime of modules. Due to an increased reliability of the double-glass module design, they are expected to only degrade 0.45% per year as opposed to the traditional polymer backsheet at 0.7% p.a. Therefore. over a 30 year lifetime it can be expected to still operate at 85% of the nameplate capacity.
The weight of glass-glass modules are still an issue, with current designs using 2 mm thick glass on each side for framed modules, the weight is about 22 kg, while 2.5 mm on each side will increase the module’s weight to 23 kg. Compared to traditional glass-foil modules, which are about 18 kg, this is a 20% increase in weight. Although there is no standard on glass thickness, in general it is a more complex and expensive process to produce very thin, tempered glass. However, 2.5 mm glass thickness does allow for frameless designs, which can reduce costs dramatically.
Furthermore, glass-glass modules lose the CTM gains associated with reflecting light that had originally passed through the spaces between the cells. Traditional backsheets are often highly reflective, allowing more light to be captured or “totalled internally reflected” if they are not first absorbed when entering the front of the module. In double-glass modules, this effect is lost due to transparency of the back glass layer.
Another major change that is also incorporated for glass-glass modules is swapping EVA for polyolefins as an encapsulant. This is due to the free radicals produced during the cross-linking lamination process of EVA. While traditional backsheets are somewhat permeable to the free radicals, the double-glass module is not. The same can be said about moisture, which can enter from the sides of the module and trapped by the double-glass design. Therefore, the non-permeability to these degrading factors is a major argument against glass-glass since the it will trap both free radicals and moisture inside the module, potentially reducing their lifetime in the field. The issue of trapped moisture is actually a major argument against double-glass modules, often used by tedlar backsheet developers. However, polyolefins are used as an encapsulant instead, solving the free radical challenge.
Despite the challenges of the glass-glass modules design, the increased reliability, subsequent 30 year warranty and transparent back enabled bifacial technology to exist. Similarly, the glass-glass design is used in conjunction with a number of higher efficiency solar cell and module designs, especially since some advanced architectures such as HJT are naturally bifacial.