The growing market for bifacial panels
The traditional concept is that of monofacial photovoltaic modules . These only have the capacity to produce electricity through the front face with the incident solar irradiation. However, in recent years and thanks to technological advances in the solar sector, bifacial modules have been developed, which can produce electricity on both sides of the module thanks to incident solar irradiation.
Thanks to this use of both sides, compared to monofacial modules, the bifacial technology makes it possible to produce extra energy with the irradiation received on the back side.
While it is true that the cost of bifacial PV modules is slightly higher, and there are other additional expenses in a bifacial plant that slightly increase the total cost, the extra energy production offsets the increased expense.
The market share of bifacial modules is expected to increase significantly in the coming years to 35% by 2030. Large-scale bifacial plants are already starting to become available, which is often the first step before the technology starts to be extended to smaller installations.
Different types of cells can be found in the bifacial market, the most widely used being the p-PERC cell, which dominates the market for this type of module, followed by modules with n-PERT cells as a percentage to be taken into account.
PERC cells, the most common, have a bifaciality of 70%-80%.
Then there are PERT cells, also common although less common than PERCs. These have a bifacility of 80%-90%, and are slightly more expensive than PERCs.
Finally, we have the HJT/SHJ cells, high efficiency cells with a bifacility between 90% and 100%, and with low temperature losses (less than 0.3%). These cells are the most expensive but the most efficient.
When calculating the amount of energy that bifacial panels can produce, the bifacial ratio must be taken into account.
This ratio is the percentage of radiation received by the back side relative to the front side over a period of time, be it an hour, a month, a year, etcetera.
There are several factors that will determine this ratio, both the arrangement of the modules, the soil albeldo and the irradiance conditions.
Among the factors affecting the ratio are the following:
Albedo. This is the percentage of irradiance reflected by the ground. This can be between 10% and 40% of the total. The higher the albedo, the higher the Bifacial Ratio.
Pitch. It is the distance between module rows. The Bifacil Ratio increases with increasing pitch up to a certain limit. It is related to the GCR (Groung Coverage Ratio), the covered surface that prevents the incidence of irradiation. The lower the GCR, the more effective irradiation we will have.
Clearance Height. It is the distance between the bottom of the module and the ground. The Bifacial Ratio increases as the height increases up to a certain limit, when this limit is exceeded it does not continue to increase.
Inclination of the modules. This also affects the Bifacial Ratio and is not always the same as that of the monofacial panels, since both sides must be taken into account when installing them.
It should also be noted that the back irradiance at the edges of the module is higher than in the center of a row, this generates a missmatch in the electrical circuit which is normally assumed to be losses.
Due to the series connection of the modules, it is usually the central module that limits the power output.
When calculating the bifaciality, it must be taken into account that the rear face of a bifacial module is not as efficient as the front face, since it does not receive direct solar irradiation.
The bifaciality of the module is the power ratio of the back side to the front side and depends mainly on the cell technology. This ratio can usually be found in the technical specifications of the PV module.
The bifacial gain reflects the increase in energy produced by the rear face compared to the front face.
Typical bifacial gain values are between 5% and 10%. This means that a bifacial system produces between 5% and 10% more energy than an equivalent system with monofacial panels.
This extra output is reflected in an increase in current, similar to what would be an increase in irradiance received on the front face of the module. These higher current values must be taken into account in bifacial systems, especially when choosing the inverter.
The irradiation at the back should be as homogeneous as possible, so that structural systems are needed that do not shade the back of the modules and that some areas receive more irradiation than others.
BIFACIAL MODULES: FRAMED OR FRAMELESS?
Bifacial modules can be framed or frameless, both have their advantages and disadvantages, although in general terms framed modules are more secure and therefore last longer than frameless ones.
Modules with a glass-backsheet frame have a lower weight, which facilitates both installation and transport, often reducing shipping costs. In addition, the protection of the frame protects against breakage and invisible micro-cracks during transport and installation, which could lead to a loss of production and effectiveness, something that can occur with frameless bifacials. The installation of framed bifacial modules can be done with traditional clips, which leads to cost savings and the possibility to more easily upgrade installations where existing panels are to be replaced.
Frameless glass-glass modules tend to have a slightly better reception of irradiation on the rear face due to the absence of a frame that causes small shadows on the edges, although not in very high numbers. They must be installed without the traditional clips, and with greater care, since they are much more fragile and more prone to breakage and micro-cracking because they have no frame.