There is a lot of talk about bifacial panels and how they can produce up to 30% more energy than monofacial modules. Bifacial technology is fascinating and has a lot of potential, but little is said about the design conditions necessary for the bifacial gain to actually be used and converted into more generation.
Generally, when talking about bifacial panels, there is a lot of talk about albedo and the need to ensure that the ground can reflect the irradiance coming from the sun. This is certainly a very important parameter in projects with bifacial panels, but in order to actually take advantage of the Gain, it is also necessary to ensure that the irradiance reflected by the ground and falling on the back of the panel is as uniform as possible.
If the irradiance reaching the panel is not uniform, such as the front side being limited by the performance of the worst cells, there will be current limitation as the cells receiving less irradiance will limit the cells receiving more irradiance. Ensuring that the back side of the panel is receiving sufficient and uniform irradiance can be challenging and make the design more complex.
I bring up some points that need to be considered in photovoltaic projects in general, but that should be evaluated in systems with bifacial panels with greater attention. I seek to share some important aspects knowing that this text in no way exhausts the complexity and discussion on this topic.
1) Ground Cover Ratio (GCR)
This parameter is the ratio between the module area and the total area available for the project, that is, the more modules placed in the same area, the higher the GCR. With the costs of photovoltaic systems falling, it has become a trend to work with increasingly higher GCRs.
But for bifacial panels, this may mean not really taking advantage of the Gain potential as this means that less radiation will reach the ground and therefore less radiation will be reflected to the back of the modules.
Because of this fact, high GCRs are generally not recommended in soil projects with bifacial panels and need to be carefully taken into consideration in these projects;
2) Pitch
Pitch is the distance between rows of modules and has a direct relationship with GCR, as high GCRs imply greater module density and, therefore, smaller distance between rows and vice versa. For the same reasons that high GCRs should be avoided, low pitch values should also be carefully analyzed so as not to considerably reduce the amount of reflected irradiation;
3) DC/AC Ratio
As the modules do not produce their maximum power at all times of the day and the STC power is rarely reached in real operating conditions, placing more panel power than the inverter power is a common and even recommended practice to increase power generation. energy.
However, on days with high irradiation, the power produced by the set of modules usually exceeds the inverter's output power at certain times of the day and so-called clipping losses occur. Clipping is when the inverter limits the generation of modules close to the time of highest irradiance.
The time with the highest incident irradiance is also the time with the highest reflected irradiance and if the project is not dimensioned correctly, the extra generation produced by Bifacial Gain may not be used as it will be limited by the inverter. Exactly for this reason, it is recommended that the overload is not so high in projects using this technology.
4) Panel height in relation to the ground
When inclined, the lower part of the panel, the closest to the ground, must be at a sufficient height above the ground (some sources say 0,5 meters, while others say 1 meter, but to have a good notion of the ideal height, only through simulations with photovoltaic software) to guarantee uniformity in the irradiation that arrives at the back.
The higher the elevation in relation to the ground, the greater the uniformity of the radiation that reaches the rear of the panel and also the greater the amount of radiation that reaches the ground and is reflected towards the rear of the panel.
5) Structures suitable for bifacial modules
Structures for bifacial panels must also have a special design. Just as it is not recommended that there be any type of shading on the front, the back should also not receive shading.
Common ground structures for single-facial panels have profiles that cross the back of the panel perpendicularly and, if used with bifacial modules, block the irradiance of some cells and contribute to current limitation at the back.
Exactly for this reason, in floor structures suitable for bifacial panels, the profiles do not block the rear part and are thinner, designed not to cause any type of shading;
6) Higher density of MPPTs per kWp
As the rear irradiance is not uniform and varies with the position of the panel in the row, assuming uniform front irradiance throughout the string, the module with the lowest gain limits all other modules. To reduce the scope of this limitation, it is recommended to use a greater number of MPPTs, aiming to prevent mismatches between strings from significantly limiting generation.
The points mentioned here demonstrate the complexity of projects with bifacial panels and the need to consider a large number of variables when designing your photovoltaic system with this technology.
Project aspects considered important such as height in relation to the ground, density of MPPTs per kWp, appropriate structures, DC/AC ratio, GCR and pitch are even more sensitive in projects with bifacial technology as they directly influence the amount of reflected irradiation, as well as the its uniformity, directly influencing Bifacial Gain and the amount of energy generated, consequently.
Remembering that after careful evaluation of these technical aspects, it is necessary to evaluate the viability of the project, its LCOE, among other important points and stages.
Answers of 4
Excellent article Lia……congratulations for sharing information that we often don’t pay attention to so often.
Good afternoon!
Congratulations on the article. I have some questions to clarify, if possible.
1- The issue of shading on the rear. I noticed that the panel has the connection box, practically covering a cell.
2 – The issue of mismatch. Couldn't it be solved with Power Optimizers?
3 – Are the rear part of the modules connected to exclusive MPPT's, i.e. separate from the front part?
Thank you.
Dear colleague, your approach to using this type of board is interesting. However, it was not suggested to measure irradiation to obtain the ideal point and also the height. However, it is known that the applicability of these modules increases the cost of the system and can discourage potential interested parties. Another point that I found interesting and was really surprised by was the fact that the use of overload was encouraged, even if it was controlled. For me, overload is something above the normal load and by logic it should be intended for use for a limited time and not as a characteristic of normal use. From the research I did, inverter manufacturers define the use of overload that is available, but its use considerably increases losses, reducing its useful life. In fact, written in English manuals, even the warranty may be denied if continued use is noted.
Thank you for spreading knowledge and I look forward to your next post
Good article! I would like to add the observation of the materials placed on the ground, see reflectivity table, the more reflective materials the more concern we should have with excess generation.