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Assessing Bifacial Solar Modules

Photovoltaic modules use light energy (photons) from the Sun to generate electricity through the photovoltaic effect. Most modules use wafer-based crystalline silicon cells or thin-film cells. The structural (load carrying) member of a module can be either the top layer or the back layer. Most modules are rigid, but semi-flexible ones based on thin-film cells are also available. The cells are connected electrically in series, one to another, to a desired voltage, and then in parallel to increase amperage.

Now (2019), a team of researchers at the National Renewable Energy Laboratory (NREL) has launched a three-year study to evaluate bifacial modules that collect light on both sides of a panel as well as following the sun throughout the day. The main benefit of bifacial panels is that you can increase power production without expanding system footprints or reconfiguring the panels too much. Early results have shown a significant boost from the bifacial panels. Data from June through November 2019 revealed a 9% gain in energy production using bifacial panels compared with their one-sided cousins. The cells themselves are not much more expensive than one-sided solar cells.

To determine how much more power such panels can produce the team needs to collect data. NREL researchers believe that new data will remove barriers to advancing this cutting-edge technology by providing information and best practices that can increase installation efficiency, reduce costs, and improve durability. So far, the results have not disappointed.

During the current study the team plans to evaluate the benefits of different ground covers beneath the solar panels. Since the bifacial PV energy gain relies on reflected light, the more the ground can reflect, the more powerful the panels become.

The energy output of bifacial solar modules has been the subject of scientific research for quite some time. In 2015, a study tried to determine the annual incident irradiation on a solar field comprising bifacial photovoltaic panels deployed in multiple rows and separated by a distance between the rows. The calculation of the annual incident irradiation was compared between two types of deployments: (a) bifacial photovoltaic panels installed with an optimal tilt angle facing south, (b) bifacial photovoltaic panels installed vertically and facing the east-west direction. The study showed that bifacial photovoltaic panels installed with an optimal tilt angle could produce 32% more energy than vertical bifacial photovoltaic panels under the same environmental conditions. On the other hand, more vertical collectors could be installed in fields with the same field dimensions.

In 2016, a study was conducted on the electrical performance of N-type bifacial PV modules. The study found that the energy output of bifacial modules was significantly higher than that of regular modules for micro inverter and string inverter PV systems in different weather conditions. The monthly energy output of a bifacial module using a micro inverter PV system was on average 4.03% higher than that of a regular module in a first six-month study. The monthly energy output of bifacial modules using string inverter PV systems was on average 3.21% higher than that of the regular modules in a subsequent one-year study. This indicated the advantages of the application of a transparent backsheet on the N-type c-Si solar cells and showed good potential for large-scale application in rooftop and household photovoltaic systems.

Some of the advantages of bifacial solar modules are: Bifacial solar modules ensure that indirect light of reflected sun rays is captured from the rear of solar cells, thereby achieving higher efficiency. In addition to the pure yield increase, bifacial solar cells need a relatively small area to increase the output of the solar module. If you embed the solar photovoltaic cells in a glass composite, they are highly protected against environmental and mechanical influences, and therefore last longer. Bifacial solar modules are ideal for flat roofs and ground-mounted solar panel systems, where light is reflected from the ground. The volume of light being absorbed from the rear side depends on the ground. With white ground and an optimal elevation of the bifacial solar modules, additional yields of up to 25 percent are possible. With the use of bifacial solar modules, the direction the modules are facing is of less importance; there are therefore fewer restrictions in module applications and design. By installing PV solar panels vertically, heavy snow loads or sand will not inhibit the modules from generating electricity.

In the study's first year, the team of researchers is testing natural ground cover. They will continue by adding crushed rock in the second year and they are considering rolling out some kind of white fabric for a third comparison.