Performance of bifacial modules using optimizers in a PV system

Bifacial panels take advantage of sunlight on both sides of the module

5 min 12 sec read

Bifacial modules have already made it clear that they are here to stay. Data from VDMA [1], which conducts studies on the evolution of the global photovoltaic industry, already indicate the growth in the manufacture of bifacial cells in relation to conventional cells (Figure 1).

We also pack any experts say factories should only produce bifacial cells in the future. For those who are not entirely familiar with bifacial module technology, it is recommended to read the article Understand bifacial solar modules.

Figure 1: Global market share of bifacial technology. Source: — Figure 1 – Global market share of bifacial technology

Bifacial modules take advantage of the incidence of sunlight on both sides of the module. From the front of the module, the main source of energy is global radiation (the sum of direct and diffuse radiation), in which the sun's rays fall directly on the module without undergoing any deviation or reflection.

From the back of the module, the energy source is reflected radiation (albedo), in which the sun's rays are reflected mainly from the ground and fall on the module in a non-uniform manner. The performance of the back of the bifacial module depends on a series of external factors such as:

Distance of the module in relation to the surface (ground, roof or slab, for example);
Tilt of modules;
Degree of surface reflectivity.

Therefore, it is common to find data sheets (data sheets) of bifacial modules (Figure 2), a variation of the main points of the IV and PV curves according to the energy gain from the back of the module.

Generally, this gain varies from 5% to 30%. Therefore, although the bifacial module always produces more energy than a conventional one, the energy gain varies due to several external factors.

Figure 2: Datasheet of a family of bifacial modules highlighting the variation of the main points of the IV and PV curves [2] — Figure 2 – Datasheet of a family of bifacial modules highlighting the variation of the main points of the IV and PV curves [2]

With so many factors that interfere with bifacial gain, it is important to note that during the operation of a system with bifacial modules, it is likely that throughout the day the bifacial gain will be different between modules.

Modules closer to the ends of the lines (as shown in Figure 3) should receive greater irradiance reflected from the surface. Thus, in a string, we will have modules with different irradiance values. The IV and PV curves of this string should follow a partial shading pattern.

Figure 3 System with bifacial modules. Modules closer to the edges of the table receive greater reflected irradiance — Figure 3 – System with bifacial modules. The modules closest to the ends of the table receive greater reflected irradiance

It is interesting to note that even without any shadow in the system, it is possible that it is under partial shade conditions. Thus, with the increased use of bifacial modules in systems, an increase in the use of solutions for strings with different levels of solar irradiation is expected, among which we can highlight power optimizers.

Optimizers are devices connected directly to photovoltaic modules, fixed to their frame and rail, which have an MPPT tracker for each module. For a better understanding of how optimizer technology works, we recommend reading the article Understand optimizers for photovoltaic systems.

In addition to important benefits such as safety and individual monitoring, optimizers are capable of making all modules work at their maximum power individually. Therefore, in a string with bifacial modules without the use of optimizers, all modules will be limited by the one that receives the lowest irradiance at the rear.

However, if this same string has bifacial modules with optimizers, each module will generate maximum energy throughout the day, and no module will be limited by the other. A simulation study tested some photovoltaic systems in scenarios with different values for the external factors [4].

The aim of the study was to analyze the generation difference from the back of the installed bifacial modules. One example is that of 7 tracker tables (1 axis) with 20 modules.

The simulated tables shown in Figure 4 feature modules in portrait position, 20 meters long, 2 meters high, 6 meters pitch, 1,7 meters high in relation to the ground, and 0,2 ground albedo.

Figure 4: Simulation of 7 tables with 20 bifacial modules each in a system with a 1-axis tracker [4] — Figure 4 – Simulation of 7 tables with 20 bifacial modules each in a system with a 1-axis tracker [4]

The ground albedo level of 0,2 is a level close to that of a ground covered by grass or low vegetation. The study analyzed how much energy each location on the tracker table receives annually, as can be seen in Figure 5.

Figure 5 – Simulation to calculate the energy received annually by the back of a tracker with bifacial modules [4]

In Figure 5, it can be seen that the yellow region received the highest amount of energy in the year: 2025 kWh/m². While the purple region received 1875 kWh/m². A difference of 8% from the smallest generation to the largest generation. It is interesting to note that the largest generation is found at the ends of the table.

Conclusion

It is important to understand that the application of bifacial modules increases the generation of the photovoltaic system, providing more energy generated per square meter. However, with the application of this new technology, a generation difference problem arises between the modules of a string. Therefore, with the growth in the use of bifacial module technology, we will witness the joint growth of power optimizer technology to solve the problem.

References

[1] International Technology Roadmap for Photovoltaic (ITRPV) – 2019 Results. VDMA. 11th Edition, April, 2020.
[2] Risen Solar. Jager Plus RSM-6-395BMDG-420BMDG, 2020.
[3] https://soltec.com/pt-br/single-axis-solar-tracker. Accessed on 30/09/2020.
[4] Deline, C, Ayala Pelaez, S, MacAlpine, S, Olalla, C. Estimating and parameterizing mismatch power loss in bifacial photovoltaic systems. Prog Photovolt Res Appl. 2020; 28: 691–703. https://doi.org/10.1002/pip.3259

Hugo Soeiro Moreira

Technical Manager at DAH Solar. He is a specialist in photovoltaic systems and a PhD and Master's student in electrical engineering at Unicamp. He is dedicated to studying the following topics: module shading, MPPT techniques and power optimizers for photovoltaic generation systems.