What is a bifacial module? What performance gains does it allow?

In the case of biglass solar panels with bifacial cells as the FLASH 425 Half-Cut Glass-Glass TOPCon, energy production takes place on both sides of the module (front and rear).

This type of module is referred to as "bifacial", in contrast to a "monofacial" module where only the front face of the cells generates energy.

Bifaciality factor

This additional performance gain is characterized by the bifaciality factor (or coefficient) which quantifies the power produced by the rear face relative to the front face.

This coefficient is around 80%± 10% for TOPCon modules, this means that under the same test conditions, the power produced by the rear face of the module corresponds to 80% of that of the front face.

For example, if we subject the rear side of our DualSun FLASH 425 TOPCon panel to radiation of 1000W/m² (STC condition), it will produce up to 80% of 425 W equal 340 W.

It is important to note that this gain is purely theoretical and corresponds to identical exposure to light from the front and rear faces, which in practice does not exist. To have an estimate of the gain on the rear panel closer to reality, specific test conditions have been defined (see paragraph below).

For reference, this factor is of the order of:

• 70% (±10%) for a PERC type module
• 90% (±5%) for heterojunction type modules.

Calculation of the power gain due to bifaciality:

The norm IEC TS 60904-1-2:2019 describes the measurement protocol to quantify this potential gain. The test conditions are called BNPI (Bifacial NamePlate Irradiance).

To calculate the power under BNPI condition, two maximum power measurements (Pmax) are carried out on a solar simulator:

• A measurement of the power on the front panel according to STC conditions (at 1000 W/m2) 
• A measurement on the rear side with an irradiance of 135 W/m2, in order to take into account less exposure to light than for the front face in real conditions.

These two measured power values are added to obtain the nominal power of the module under BNPI condition which is of the order of 10-11% higher than the power measured on the front face only under STC conditions for TOPCon type modules, as described in the example in the table below:

Example of FLASH 425 Half-Cut Glass-Glass TOPCon :

Even if it comes close to reality, the power under the BNPI condition also remains a theoretical value, depending on local climatic conditions, the type of installation and the albedo effect of the surface behind the modules.

Albedo Effect

The albedo effect is a physical property that characterizes the ability of a surface to reflect sunlight. It is generally expressed as a coefficient between 0 and 1.

• 0 represents a totally absorbing surface (all light is absorbed)
• 1 represents a totally reflective surface (all light is reflected)

Surfaces with a high albedo reflect much of the incident light, while those with a low albedo absorb more.

In practice, it is complex to evaluate the power gain due to bifaciality.

• For transparent bi-glass modules installed on residential roofs, where exposure of the rear surface to light is low, it will be around 5%

• For transparent bi-glass modules installed with an optimized slope on a white-coated, reflective flat roof, it can be as high as 30%.

The general rule for maximizing rear panel production is simply to move the rear panel as far away as possible from its support and to install the panels above the most reflective background possible.

> To find out more: What are the advantages of DualSun FLASH TOPCon panels?