Introduction to Bifacial Modules
How to design a solar system to maxime mppt with bifacial panels. First, let's understand what bifacial modules are. The most intuitive feature of bifacial modules is that the back side of the module can also generate electricity. This is mainly achieved through advancements in cell technology, where the previously opaque back electrode is transformed into a transparent grid-like structure similar to the front side.Through certain doping techniques, a PN junction is also formed on the back side, ensuring that reflected and scattered light can be effectively captured.
Bifacial modules typically use a double-glass design, although transparent backsheets are also used to reduce the weight of the module. Depending on customer requirements, customization options include whether to add a frame and the length of the junction box cables.
The back side of bifacial modules can also generate electricity, with the gain varying depending on the reflective environment. The backsheet is usually made of glass (double-glass), but transparent backsheets are also available to reduce the overall weight of the module. Special requirements are needed for the mounting structure and the inter-module connecting cables.
2. Configuration Relationship Between Bifacial Modules and Inverters
Module Gain
Let's look at the electrical parameters of bifacial modules. The table above shows the basic performance parameters under standard test conditions, which are similar to those of conventional modules. However, the gain table below reveals that the backside gain of this module ranges between 5% and 25%. Currently, the cost of bifacial modules is about 5 cents higher per watt compared to front-side power, which translates to an increase of about 2% to 3%, making it quite cost-effective.
The backside gain of bifacial modules also imposes specific requirements on the installation environment. As shown in the table below, white-painted ground offers the best gain compared to grass, concrete, or sand.
The gain of bifacial modules mainly comes from the backside current gain. Currently, the peak current on the front side of modules generally does not exceed 11A. When the backside power gain reaches 20%, the peak current usually does not exceed 12.5A, which places higher demands on the current-handling capability of inverters.
3. Inverter Selection
Bifacial modules significantly increase power generation compared to conventional products, so system design should moderately reduce the DC/AC ratio, and the inverter should have strong short-term overload capability.
The power gain of bifacial modules primarily comes from the current gain due to backside illumination, with Imp potentially exceeding 11A. The maximum current design of the combiner box and inverter should be increased accordingly.
Considering the backside gain, we need to slightly reduce the DC oversizing of the system, and the inverter should have reliable short-term overload capability. GoodWe's residential models, such as the SDT G2 and SMT series, come standard with a 12.5A DC current rating, making them compatible with most bifacial modules.
For commercial and industrial applications, we have also made corresponding changes. The MT series 50kW and 80kW models have introduced the BF series to accommodate high-power bifacial modules currently on the market. The 80kW model has an MPPT maximum input current of 44A, with each MPPT supporting up to 3 strings, and each string input current up to 14.6A. Similarly, the 50kW model supports up to 15A per string input current.
For ground-mounted projects, our upcoming HT series with capacities above 100kW has also been adapted. For example, the 100kW model supports a maximum current of 28A per MPPT, with each string input supporting up to 14A.
4. How to design a solar system to maxime mppt with bifacial panels
Key Points for Bifacial Module Installation
1. Junction box position, cable length, and string connection method
2.Clamp position and size
3. Mounting structure should not block the backside cells
4.Module height and tilt angle
Based on our ongoing rooftop project at the new factory, we will discuss five aspects of installation considerations.
5. String connection optimization
Junction Box Position, Cable Length, and String Connection Method
For bifacial modules with the junction box in the center, which are mostly half-cut modules, the layout can be horizontal double-row, horizontal single-row, vertical single-row, or vertical double-row.
Here, we mainly list three of these. Note that in horizontal layouts, the upper and lower modules should face the same direction, while adjacent modules should face opposite directions. This requires the module cables to be about 1.4 to 1.5 meters long.
Vertical single-row modules can have shorter cables, while vertical double-row layouts will require an additional jumper between the upper and lower modules, which should not be overlooked during initial planning.
This is the bifacial module installation at our GoodWe new factory. The inter-module connections should avoid blocking the backside cells.
For standard bifacial modules with the junction box at the end, single-row layouts are generally straightforward. In double-row layouts, horizontal double-row modules should face the same direction, with cable lengths around 1.5 meters. Vertical double-row and single-row layouts have shorter cables, but double-row layouts need to ensure that the upper and lower rows are placed head-to-head.
Some bifacial modules have the junction box at the top and bottom. The installation form is similar to the previous ones, but attention should be paid to the module orientation, which we won't elaborate on here.
6.Clamp Position and Size
Clamps have long been used in photovoltaic projects. For framed bifacial modules, installation can follow conventional methods. Here, we focus on frameless double-glass module installation.
From the diagram, it's clear that 60-cell modules generally require clamps at four points, while 72-cell modules require clamps at six points. The exact positions may vary slightly depending on the module size, and the clamp size is generally longer, around 150mm.
For tracking mounts, the rotation axis should be between modules to minimize backside shading.
7. Mounting Structure Should Not Block Backside Cells
When designing the mounting structure, components should not cross the cell area on the backside. Only edge-mounted, purlins, and connecting accessories should be used. Additionally, the inverter should not be installed behind the string but on the side to avoid blocking reflected light on the backside.
For projects using bifacial modules, the mounting structure design should consider backside shading. Structural components should not cross the backside cell area and should be placed along the edges. The inverter should also not be mounted behind the string to avoid shading.
8. Module Height and Tilt Angle
Simulations show that the higher the reflectivity and the higher the module is from the ground, the more pronounced the backside gain. When the module is more than 1 meter above the ground, the backside irradiance tends to stabilize. It is recommended to set the module height between 0.3 and 1 meter based on the actual environment.
For conventional installations, the local optimal tilt angle is recommended.
In addition to the ground material affecting backside gain, different heights also result in varying gain effects. The graph below shows the relationship between bifacial module backside gain and module height. As height increases, the gain increase slows down.
Considering foundation costs, it is generally recommended to keep the module height below 1 meter, with a range of 0.3 to 1 meters for optimal selection.
9. String Connection Optimization
Finally, let's look at string optimization. For standard arrays, the typical connection methods are shown in the two diagrams below: the first is serial connection from top to bottom, and the second is separate serial connections for top and bottom rows.
For a typical double-row photovoltaic array, the height from the ground of the bottom row is significantly different from that of the top row (especially in vertical layouts), so the reflected radiation they receive is also different.
Visually, it's clear that the backside space of the bottom row is darker. Therefore, the current of the bottom row modules will be slightly lower than that of the top row.
In this case, if method 1 is used for string connection, the current of the top row modules will also be reduced due to the lower current of the bottom row, resulting in an overall reduction in output power.
If method 2 is used, the top and bottom rows form separate strings, unaffected by each other, fully utilizing the performance of bifacial modules and increasing power generation.
Observant readers will notice that, based on the earlier conclusion that higher module height results in greater gain, the gain effects of the top and bottom rows are different. The top row has a relatively higher gain, while the bottom row has a relatively lower gain. To reduce string mismatch, it is recommended to use method 2 for string connection whenever possible, with the top and bottom strings connected to different MPPTs of the inverter. This is currently the optimal connection method for bifacial modules.