What are bifacial solar panels

In classic photovoltaic panels, efficiency can be improved using PERC technology   . PERC increases light absorption in a photovoltaic cell by using reflected light from the bottom of the cell to produce electricity. Learn more about PERC technology  here  .

In contrast, double-sided photovoltaic panels have  two active sides  that absorb light. As a result, its efficiency is much higher than that of traditional monofacial panels. Electricity is produced simultaneously from the front and rear sides of the photovoltaic panel, which results in a higher performance of said module.

Bifacial panel construction

The bi-face panels are made of tempered glass on both sides. What sets them apart is  the active layer on the front and back of the panel  . Allows light to be absorbed from both sides. In bi-facial modules, the back layer of the protective film, the so-called “Back Foil” and was replaced by a panel of tempered glass or other light transmitting material.

Cell structure of bifacial cells.

Two-phase panels have three possible electron flow configurations. It depends on the structure of the binary cells of the cell itself.

We distinguish the following types of cells:

• p-PERC- based on cell structure p + – p – n + pio
• n-PERC- based on the n-type structure of p + – n – n + cells
• Silicon heterojunction (Si): a cell made of thin inner layers of amorphous silicon.

How much more efficient are double-sided solar panels?

In 2016, SolarWorld conducted double-sided cell efficiency tests under the prevailing climatic conditions in Central Europe. The research covered double-sided panel installations at a photovoltaic farm near Dresden. Studies have shown that stationary double sided panels achieve  maximum efficiency of 25% in  comparison with the photovoltaic modules  one  face. Therefore, they lead to lower electricity production costs.

What does it come from? First, from the parameters of the panel itself. Below are the properties of several popular double-sided modules:

By adding the maximum gain, we get the  theoretical maximum power  , and for a panel with a power of 330Wp, it can be even  412.5Wp!

It is obvious, of course, that the maximum profit is not achieved throughout the day. So how does it work in practice?

According to research conducted by NREL, double-sided panels achieve  6-9% higher average efficiency  compared to standard photovoltaic panels. However, this study compared modules installed in a uniaxial tracker. We estimate that for south-facing stationary systems, the  average efficiency will  increase by  10-15%  .

Why do some panels have a higher maximum gain than others? The efficiency of the two-phase module is determined by the so-called two-sided index (bifaciality) [Ø]. The bilateral index expresses the relationship between the nominal power of the rear part of the module and the nominal power of its front part.

Ø double-sided index = Prear / Pfront

Pre-rated power from the rear of the module
Pfront- rated power from the front of the module

The bifacial index [Ø] for individual bifacial cell junctions is:

Research conducted by the NREL institute shows that the best bifacial index is shown by two-phase modules based on a heterojunction. These panels, made of a thin layer of amorphous silicon, have demonstrated in laboratory tests the ability to produce energy from both sides at a comparably high level. The rear of the panel under laboratory conditions reached about 85% -95% of the power of the front of the module. It is as if the efficiency of the back is only 15% lower than the front surface of the module.

We have already mentioned that in addition to the power of the module and its bifacial index, the arrangement of these panels also plays an important role. What variants should I consider?

The shelter is made of double-sided panels. Effective and efficient.

The performance of the panels and the positioning in relation to the sun

Of course, these types of panels are not suitable for ceiling mounting. Only ground installation is possible.

There are three basic ways to place double-sided panels facing the sun:

• East-west vertical
• Standard  (southbound)
• In a uniaxial tracking system  (tracker)

SolarWorld’s research showed that devices placed vertically from east to west were ineffective and, at the same time, had to be 15 m apart from each other. This greatly reduces the number of modules that can be installed in the available space.

Much better results were obtained when the  panels  were  positioned in the traditional way  , that is, at a 35 degree angle and the front of the module facing south. This is the optimal configuration, both in terms of performance and cost of installation. In this position, the rear can additionally generate up to 25% energy efficiency. The height at which the panels are installed is also important. As can be seen from the tests, the height at which the bi-face panels are mounted is of great importance for performance. Mounting them on a higher frame gave better results in producing electricity.

By far the most efficient, but also the most expensive way, is to mount the panels on a uniaxial tracking system.

The light-reflecting surface also plays an important role in the efficiency of double-sided panels.

How does the surface of the substrate affect the performance of the bi-facial module?

The surface on which the photovoltaic panels are placed plays an important role in the efficiency of their work. An important element is the shape of the surface itself and its ability to reflect sunlight. The flatter the floor, the more light will return to the modules.

An important element for the effectiveness of the two-sided modules was also the so-called albedo coefficient.

The albedo coefficients for different surfaces are presented in the following table:

As can be seen from the data above, white surfaces reflect light better. In the case of lighter soil, the albedo coefficient reaches 80%, and in the case of snow even 95%.

Therefore, the place of installation of the double-sided panels is not without importance. Different performances will be of the modules that are installed, for example, in the vicinity of the lawn, and different ones on a concrete surface, additionally painted white.

How much do double-sided panels cost?

The following table shows the production costs for single-sided panels and three types of double-sided modules.

As can be seen from the calculations carried out by the NREL Institute, the production costs of double-sided panels are not much higher than those of traditional panels, the so-called Monofacial. For 1 kW of bi-facial panels produced, we will pay 5 to 6 cents more. So for a 300W module we will pay around $ 18 more.

At the same time, from a module with a nominal power of 300 W on one hand, we can obtain a double-sided power of approximately 425 W. At the same time, by installing it correctly, the investment costs can be amortized more quickly. than in the case of traditional photovoltaic installations mounted on the roofs of buildings.