Understand bifacial photovoltaic modules

Do you know what bifacial solar modules are? As the name suggests: they are modules that have two faces.

But what does it mean? What is the difference between bifacial and conventional modules?

With the aim of increasing energy generation in photovoltaic plants, several global manufacturers have recently started to offer solar modules that can receive light from both sides.

This seems strange, since sunlight always shines through the top of the module.

However, it is a fact that there is a certain amount of diffused and reflected light from the ground that can be used by the rear part of the module, as long as it is prepared to generate energy with its rear part.

Bifacial modules can offer efficiency increases of up to 30% over conventional modules.

However, this increase depends on the installation method (ground height, inclination angle) and also on the type of soil. A poorly reflective soil will provide little additional yield. A soil that reflects a lot of light could offer a significant increase in the generation of solar plants with bifacial modules.

Bifacial modules are only suitable for solar plants. In roof top installations, the rear part of the module is glued to or very close to the roof, receiving little or no light. In this case, the performance of a bifacial module would be comparable to that of a conventional module.

Bifacial solar cells

To manufacture bifacial solar modules, the solar cells must be bifacial. Let's understand this.

Conventional solar cells receive light only in their upper part, where the silicon is exposed. On the back, conventional cells are metallized, that is, they are covered with a metallic layer (silver paste) along their entire length, which improves the electrical characteristics (reduces the ohmic resistance of the cell) but prevents the entry of light from the rear. This is neither good nor bad. Solar cells have simply always been like this.

In the quest to increase the efficiency of photovoltaic systems (a very commendable effort), cell manufacturers have developed devices that can receive light from both sides. The figure below compares the structures of conventional and bifacial photovoltaic cells.

Construction of bifacial modules

If you already understand what a bifacial crystalline cell is, you must be imagining that the construction of the solar module must be a little different. Conventional modules have a backsheet made of white, opaque plastic.

This way, the back does not receive light. Differently, bifacial modules must be constructed with a glass backsheet in place of the backsheet. Apart from the type of cell used, this is the only constructive difference between a conventional module and a bifacial module.

Characteristics of bifacial modules

Bifacial modules have electrical characteristics and physical dimensions very similar (or even identical) to those of conventional modules. The open circuit voltage (VOC), which depends on the number of cells, is similar for both types of modules. The same can be said about other characteristics such as short-circuit current (ISC), maximum power (PMPP) and others. What changes then?

The solar modules have their peak power and other characteristics specified for a standard test condition (STC – Standard Test Condition) that corresponds to a light intensity of 1000 W/m2 and a test temperature of 25 oC.

In bifacial modules, in addition to the standardized conditions in STC, which are obtained through a flash test, the manufacturer informs what the characteristics of the same module may be (approximately) in different light reflection conditions.

Below, we see some characteristics of the bifacial modules of the CS3U-370|375|380|385MB-AG family from Canadian Solar, which are manufactured in Brazil.

In the table we find the traditional characteristics of the modules, specified in the STC conditions. The novelty is the presence of power, voltage and current values that correspond to the capture of solar irradiation, which can be up to 30% higher in bifacial modules when compared to traditional modules. However, as the manufacturer informs, superior performance depends on the type of mounting (type of structure, height, angle, etc.) and the ground reflection coefficient (albedo).

Conclusion

Bifacial modules are a good bet for solar plants, as they capture light on both sides, and their performance has already been proven. The cost of a bifacial module is not much higher than that of a conventional module and the additional performance can provide superior performance and economic return in solar plants. For photovoltaic systems on roofs, conventional modules are still the best option.