Bi-facial Solar Modules
Bifacial modules are those which produces solar power from both sides of the panel in contrast to the conventional mono-facial modules, which produces power from only one side where solar radiation is received. In the case of Bi-facial, power is produced from both the sides of the panel using direct radiation falling on the module as well as indirect radiation coming through reflection from ground or diffusion. Fig 1  below illustrates the working of the bi-facial module:
The power generation from the rear side highly depends upon the reflectivity of the ground surface and in certain cases, manufacturers claim that the increase in generation can be as high as 30% from the rear side of the modules.
Bifacial modules can be framed or frameless. Some use glass-on-both-side and others use clear back-sheets. Most of the bifacial solar cells use mono-crystalline design. One point to be noted here is that there are frameless, dual-glass modules with exposed rear side available for BIPV applications (Building integrated photovoltaics) but are not considered bi-facial.
An important parameter to judge the performance of bi-facial modules is bi-faciality (ϕ). It is defined as the ratio of the front and rear power. In general, ϕ is less than 1 as the metal patterns are often optimized for the front efficiency. The current standards for testing of bi-facial modules provide information regarding higher current and power seen on the field due to bi-faciality. They do not talk about a way to ascertain the performance of qualify a module.
Some of the advantages of using bi-facial modules are:
- Higher power produced from both sides of the module
- Higher durability as both sides are UV resistant
- PID (Potential induced degradation) concerns are reduced when the modules are frameless
- BoS (Balance of System) costs are reduced because more power can be generated in a smaller array footprint
Challenges in deploying bi-facial modules
One of the major challenges in bi-facial module deployment is the unpredictability of the extent of power generation from the rear side as it mainly depends upon the reflectivity of the ground surface which can be highly variable. The process for indoor testing of current-voltage characteristics is still not standardized for bi-facial modules further contributing to the unpredictability of the output.
For free-standing bifacial modules, the optimum orientation for maximum power generation is a trade-off between the front and rear-side outputs, and the efficiency is dependent on factors such as the ground reflectance, tilt angle, and installation height. In extended arrays, additional factors, such as direct shading and reduced ground albedo due to adjacent rows also need to be considered. Thus, predicting yield and optimizing the design for a bi-facial array is very difficult and complicated.
Due to the above challenges, the installed capacity of bi-facial systems is miniscule. Since the additional “bi-facial” yield is very difficult to estimate it is normally considered as an add-on and the modules are sold at rated mono-facial power only.
Things to keep in mind for a developer/EPC investing in bi-facial modules
- No holistic simulation models exist at the moment to ascertain the generation from bi-facial modules
- Since the generation from the rear side can be highly unpredictable at the moment, it should be considered a bonus and should not have financial implications
- Degradation mechanism of bi-facial modules is not considered fully which must be kept in mind while assuring back to back warranty
- Structures must be designed keeping in mind that the tendency of glass to glass modules to break is very high, especially in hot climatic conditions
Things to keep in mind for O&M of bi-facial
- Most of the bi-facial modules are glass to glass and so the possibility of breakage is high and thus due precautions must be taken during handling of the modules
- Current that flows in a bifacial module is very high. So it is important to regularly keep track of by-pass diode status and possible development of hot spots
- Since generation from the back-side depends on the condition of the ground, it is important to maintain leveling and vegetation levels to keep the ground reflectivity constant and uniform
With the improvement measurement standards, simulation tools, the falling price of mono-wafers and advances in technology, bi-facial module technology will be contributing higher and higher to mainstream technology. ITRPV roadmap 2017 predicts an increase to around 30% by 2027 while Bloomberg even anticipates 40% by as early as 2020.
1. Bi-facial solar cell-A brief Overview by Ingrid Romijn, bifi workshop, Konstanz, Germany, Oct 25,26th 2017