- Cell to module power ratio can be improved through light management and reducing resistance losses
- White EVA and reflective ribbons helps in improving the light management, like LONGi’s triangular shaped ribbon
- MBB and slicing cells into 2 or more pieces are main methods to reduce resistance losses
After we listed the different mechanisms that are responsible for gains and losses in a module impacting the Cell-to-Module (CTM) power ratio in an earlier article (see Gain & Losses At Module Level), here we discuss how in practice it can be improved through two complementing mechanisms — light management and reducing resistance losses.
Improving Light Management:
- Reducing the reflectivity of the glass by means of antireflective coatings
- Optimization of cell spacing
- Increasing the reflectance of inactive areas by using reflective backsheets or white EVA on the rear
- Reducing the optical shadowing of metal components is possible, for example, by making ribbons reflective, by engraving several grooves, special coating, painting or gluing reflective polymer films onto the ribbons
Reducing Resistance Losses:
- Increasing number of busbars (and MBB)
- Using half-cut cells or strip cells for shingling
- Optimizing ribbon cross-section according to the width
- Optimized junction box and cabling design
As can be seen above, improving light management primarily involves changing the BOM. Glass applied with antireflective coatings is already a standard. The increasing interest in white EVA is a clear sign of efforts in this direction. White EVA is used as the bottom encapsulation layer, which in a finished module increases the light reflection from the cell gaps, resulting in power gains of up to 5 W. White EVA’s market share has increased to 17%, as shown in our TaiyangNews Market Survey on Backsheet and Encapsulation Materials 2021.
Using reflective ribbons is yet another approach for enhanced light management. While light capturing ribbons have been known for several years, LONGi’s triangular shaped ribbon is a recent example of commercial activity in this area.
The effort to reduce resistance losses requires considerable optimization/change in module making, and these approaches have evolved as advanced module technologies. TaiyangNews has been extensively covering these technologies, essentially larger wafers, half cell, MBB, bifacial and reduced/no gap technologies. Any of our previous publications, the latest being TaiyangNews Advanced Module Technologies 2021, can be referred to for the fundamentals of these technologies and previous developments. However, this chapter provides a brief summary along with the recent developments.
MBB: Reducing electrical losses mainly involves changes to the interconnection process. A first step in this direction was to increase the number of busbars. The PV industry quickly adapted to 5 busbars a couple of years ago. However, instead of following the incremental path of going to 6 busbars, which was adapted only by Q Cells, the industry took a big leap to MBB where the number of busbars ranges from 9 to 12. Employing circular copper wires instead of flat ribbons was part and parcel of MBB. Now, however, a few companies are employing flat but narrow ribbons also in the MBB configuration. MBB requires special combined tabber and stringing tools, which are freely available in the market. The Smart Wire Connection Technology (SWCT) from Meyer Burger is also a high end variant of the MBB approach but not available in the market anymore after the company decided to move from equipment sales into cell/module manufacturing. In addition to power gain, the MBB approach enables the reduction of finger width to a greater extent. The benefit of reducing the finger width is twofold — it cuts shading losses and lowers paste consumption.
Slicing cells: While it sounds counterintuitive, carefully slicing processed cells has its benefits. The half-cell approach, where a cell is sliced into two pieces, has nearly become the standard in today’s context. A few companies, however, have also launched products based on 1/3 cell strips and are evaluating further options. The cell’s current, which greatly influences resistance losses, gets reduced proportionately to the number of slices in a cell, thus reducing the losses. The approach requires doubling the stringer capacity to match the module production capacity at the fab level and also needs a laser tool to slice the cell. With increasing wafer sizes, which correspondingly increases cell currents, the half cell is more or less becoming inevitable.
There is an industry-wide belief that the full-cell configuration does not make much sense starting from the M6 wafer format. On the flip side, the half-cell configuration causes edge losses, which are more evident with high efficiency cell architectures such as HJT. However, the industry along with larger wafers has been predominantly adopting non-destructive laser cutting. Some companies are also planning to make half wafers, meaning the slicing of silicon substrates is done in wafer fabs, especially with G12 formats used for HJT.
Reduced/no gap technologies: Ideally, the solar cells in a PV module must be packed as densely as possible to save on materials. The practice traditionally has been to space the cells in a string to provide cushion for mechanical stress during operation. However, developments in materials, production equipment and manufacturing technologies have enabled manufacturers to reduce this gap and gain on active module area, and even eliminate this gap altogether. In fact it was the latter, called shingling, that hit the commercial space first. Here, the cells are sliced into several strips and connected to each other in a similar fashion to a shingle structure of tiles placed on roofs using conductive adhesives. This concept has gained a lot of interest and garnered several followers. But, the technology is strongly protected with patents with Maxeon and Solaria claiming ownership of the IP.
JinkoSolar developed another zero-gap technology called Tiling Ribbon (TR), as a workaround for the IP situation with shingling. Here, the stringer is equipped with a special functionality to press the round wire to make it flat exactly to a length where it would bend in order to connect the top of the next cell. Instead of placing the cells side by side, the cells slightly overlap at the edges. Compared to shingling, TR technology uses an interconnection media and avoids laser stripping of cells into several pieces. In order to cushion the region of cell overlap during the lamination process, TR uses structured EVA that would compensate for the inconsistencies due to overlapping.
Following the TR technology template, several companies have successfully reduced cell gaps. That means the interconnection media is still flat and slightly bent, but instead of overlapping, the cells are placed very close. While the cell spacing is typically 2 mm in the traditional module design, the latest product generations of leading companies are able to reduce this gap to between 0.6 mm to 0.9 mm. On top, companies like LONGi use a so-called segmented ribbon for interconnection, which contains parts of triangular shape and flat sections. The triangular side is soldered on the sunny side of the cell to enhance optical gains.