The highly efficient PV module technology that is widely used in the industry is a bifacial module. These efficient PV modules need to be used with devices such as inverters to maximize value. Recently, many inverters that match bifacial modules have appeared in the industry. Which inverter is the best match for bifacial modules? Based on a large amount of empirical data, this article describes the inverters needed by bifacial modules.
1 Bifacial Module
The solar cell technologies used by bifacial solar modules which are currently on the market include the PERC technology based on the p-type silicon wafer, the PERT technology based on the n-type silicon wafer, and the HIT technology of heterogeneous structures.
- Standard PV module and bifacial module
As shown in Figure 1-1, in addition to receiving solar radiation from the front, the rear of the bifacial module can also receive scattered light from the air, reflection light of the ground, and direct solar light coming from the rear during the morning and evening. Therefore, the power generated by the bifacial module is greater compared with the standard PV module designed for the same PV plant.
- Energy yield gain from the rear of the bifacial module
We have tested standard and bifacial modules with the same structure for a long time. As shown in the figure, the energy yield gain from the rear of the bifacial module varies depending on the scenario, and the energy yield increases by 5%–39%. In addition, the bifacial module can further increase the energy yield by 2%–6% based on its excellent performance of good response to low light and low power loss under the working temperature.
Generally, the energy yield gain of a bifacial module compared to a standard PV module is about 7%–45% in the scenarios listed in Figure 1-2.
2 What Inverters Does a PV Plant with Bifacial Modules Need?
2.1 Higher Input Current and Higher Efficiency Inverter
The following table lists some parameters of the bifacial module with the power of 300 W on the front side from a well-known vendor. As the bifacial module gain increases, the open-circuit voltage and peak power voltage remain unchanged, while the peak power and peak power current of the PV module increase. In this case, designers need to select a more appropriate inverter with a larger DC input current based on the actual gain.
- Parameters of a bifacial module
- SUN2000-70KTL-INM1 parameters
2.2 Finer MPPT Granularity
- Rear gain of a bifacial module varies greatly depending on the position
As shown in Figure 2-1, the rear radiation of the bifacial module is uneven. As a result, the overall output power of the PV module is different, and the current discrete rate of the PV module reaches more than 5%. In this case, the MPPT granularity of inverters should be finer. In addition, the mismatch loss caused by inconsistency should be avoided when the string is designed and when it connects to inverters.
Every two strings connected to Huawei SUN2000-70KTL-INM1 inverter dedicated for bifacial modules form one MPPT circuit, which means that the inverter has the finest MPPT granularity in the industry. This minimizes the mismatch caused by bifacial modules. Based on PVSYST simulation, it is found that the mismatch loss caused by inverters which form one MPPT circuit by every two strings is 1.1% lower than that caused by common inverters in the bifacial module system.
2.3 Highly Adaptive and the Most Accurate and Efficient MPPT Algorithm in the Industry
- I-V curves of different PV modules
As shown in Figure 2-2, since the mismatch of the bifacial module is high, its I-V curve is more complex than that of the standard PV module, and its power-voltage curve will generate multiple peak values. This poses higher requirements on the detection precision and MPPT of inverters.
Huawei string inverters have multiple MPPT units, which can greatly avoid energy yield loss caused by string mismatch. The detection precision of a string reaches 0.5%. In addition, Huawei inverter uses the most efficient MPP intelligent tracking algorithm in the industry. The inverter adopts the adaptive MPPT technology. When the irradiance is stable, the maximum power point of the PV module can almost be reached. When the irradiance rapidly changes in cloudy weather, the inverter can quickly respond and track the maximum power point in real time, so it can adapt to the bifacial module properly.
In addition, as the bifacial module has multiple peak values, the inverter can intelligently identify whether the maximum power point has been reached. The high-speed multi-peak scanning algorithm is enabled to ensure that the inverter is always operating at the maximum power point of the PV module, thereby effectively improving the energy yield of the bifacial module.
2.4 Secure and Reliable Protection Design
- The fuse failure rate increases as the current increases.
The current of the PV module is affected by radiation and temperature, so it cannot be controlled. When the fuse has a low-current overload, the fusing time becomes long. When the fuse is almost blown, it is in a high-temperature heat balance state, or the insulation between the cable and the fuse box is damaged. As a result, fire accidents may occur. The output current of a bifacial module is even larger, which is more likely to cause low-current overload. The fuse can then be blown or even result in a fire due to such high temperatures.
- Fuse faults caused by high temperatures
- The fuse of a single specification cannot adapt to the current mainstream PV modules.
Currently, the maximum reverse withstand current capabilities of bifacial modules from mainstream vendors are 15 A and 20 A, as listed in the following tables. In this case, the DC combiner box or the string inverter with built-in fuses cannot adapt to the PV modules of another specification regardless of the fuse specifications. That is, the built-in 20 A fuse cannot protect the 15 A PV module and the built-in 15 A fuse is blown frequently due to the large operating current.
- Maximum rated current of fuses from two mainstream bifacial module vendors
Note: Do not connect two strings or more PV modules in parallel to the same fuse in the combiner box.
Every two strings of Huawei SUN2000-70KTL-INM1 inverters dedicated for bifacial modules form one MPPT circuit and adopt a fuseless security protection solution. The design ensures that no overcurrent will occur, protects PV modules, and improves system reliability. In addition, security risks, frequent fuse replacement, and energy yield loss caused by fuse faults are avoided.
2.5 Unique, Accurate, and the Most Refined Design Tool for Bifacial module Plants in the Industry
As described above, the comprehensive power of the bifacial module is affected by many factors such as the project site radiation resource and ground reflectivity. As a result, the actual output power of the bifacial module differs greatly in different projects. Therefore, designers must not use the same serial/parallel connection of PV modules and inverter configuration for all projects and must perform refined design based on specific projects. Even in the same place, refined design is required for different scenarios. Therefore, the bifacial module system solution is more variable than standard PV modules. If all factors need to be considered, the number of design schemes of the bifacial module system will be more than 10,000. In this case, the optimal system design cannot be obtained accurately and quickly based on experience and standard design. Therefore, a more professional bifacial module design tool is required.
Generally, a physical model needs to be set up for evaluating the energy yield of bifacial modules. Research personnel from National Renewable Energy Laboratory (NREL), Sandia National Laboratories, and Fraunhofer Institute for Solar Energy Systems (ISE) in Germany have conducted a lot of research. They focus on the ray-tracing and view-factor models which can accurately describe the gain of the bifacial module from the rear. The two models are based on 3D modeling. Although more details can be displayed, algorithms are complex and computing is time-consuming, which does not meet the actual requirements of engineering applications. Huawei has simplified and optimized the two models, and launched an industry-leading intelligent design tool for bifacial module systems based on the 2D physical model (as shown in Figure 2-4). The tool can find the balance point between the calculation speed and design details, and accurately and quickly calculate the optimal configuration of the bifacial module system.
- 2D model of a bifacial module whose rear receives radiation
The intelligent design tool for bifacial module systems integrates full-scenario, adaptive, and self-learning intelligent control algorithms to accurately output the optimal design solution. This increases the energy yield by more than 3% compared with solutions provided by other standard design methods. Currently, this tool is the only accurate design tool for bifacial module plants in the industry and has been verified by a large amount of data.
In addition, the complexity of the I-V curve of the bifacial module makes the intelligent diagnosis of string faults easy to misjudge, which causes inconvenience to operation and maintenance (O&M). Huawei's latest Smart I-V Curve Diagnosis function 2.0 uses a new intelligent string diagnosis algorithm. Based on big data analysis and AI algorithms, it can automatically learn and evolve. Based on the built-in database, it can quickly master the input and output feature curves of various PV modules and automatically filter out the noise that causes misjudgment. It supports bifacial modules and is the best choice for O&M of bifacial module plants.
To sum this up, we compared the Smart PV Solution with the current mainstream inverter solutions, as described in the following table.
- Comparison of solutions for bifacial module scenarios
3 Application Cases of Bifacial Modules and Optimal Inverters
Huawei inverters have the following features:
- Higher input current and highest efficiency
- Finer MPPT Granularity
- Highly adaptive, the most accurate and efficient MPPT
- Secure and reliable protection design
- Unique, accurate and the most refined design tool for bifacial module plants in the industry
These five smart tools make Huawei inverters the best match for bifacial modules.
In fact, the solutions composed of Huawei inverters and bifacial modules have been widely applied to bifacial module plants in various scenarios. The following table lists some cases.
- Cases of Huawei string inverters used in bifacial module plants
Case 1: Gonghe Bifacial module plant
COD: June 2016
Capacity: 1 MW of fixed mounts and 1.3 MW of horizontal single axis trackers
Inverter: Huawei SUN2000-50KTL-C1
PV module: 360 W HIT bifacial module
Application scenario: grassland and sand
Energy yield gain (compared with standard PV modules): 10.5%
Case 2: Golmud bifacial module plant
COD: gradually connected to the grid since August 2017
Capacity: 60 MW of horizontal single axis trackers
Inverter: Huawei SUN2000-50KTL-C1
PV module: 345 W and 350 W PV modules
Application scenario: desert
Energy yield gain (compared with standard PV modules): 13%
Case 3: Xintai solar-agricultural project
COD: November 2017
Capacity: 100 MW of single axis trackers
Inverter: Huawei SUN2000-50KTL-C1
PV module: 310 W PV module
Application scenario: solar-agricultural scenario
Energy yield gain (compared with standard PV modules): 22%
Case 4: Lianghuai floating PV plant
COD: December 2017
Capacity: 10 MW
Inverter: Huawei SUN2000-50KTL-C1
PV module: 285 W PV module
Application scenario: white floats on the water surface
Energy yield gain (compared with standard PV modules): 15%
The bifacial module has started a new round of technology replacement. The application of new technologies requires the development of other new technologies, such as the higher inverter input current, finer MPPT granularity, more accurate MPPT algorithms, and smarter design tools for bifacial module plants. Based on the preceding analysis, we are proud to be the provider of the most adaptive PV inverter and solution design in the industry.
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