Paving is a module technology that enables PV cells to cover more of the module’s surface. Similar to shingling, a variant of the paving technique also involves some overlapping of the cell. The different approaches are characterised by the type of ribbon used when interconnecting cells.
For example the company JoyTech already offers a complete package for paving. Originally developing stringing tools to enable the use of triangular ribbons for interconnection, they eventually managed to produce a production tool that could handle 2 different types of ribbons to be used on the same module which allowed for paving of the solar cells. The front interconnection of cells uses a triangular ribbon, increasing optical cell-to-module (CTM) gains, whereas the back interconnection uses an ultra-soft flat ribbon, which enables the cells to be placed much closer together. JoyTech’s stringing tool has a throughput of about 3,000 substrates per hour and is adapted to half-cells and 7 busbar technology. The spacing between cells on JoyTech’s 350 W panel is about 0.3 – 0.5mm, which is about 10 times smaller than a traditional module.
Another method to paving is the use of round ribbons. Ribbons with a diameter of 3.5 mm are used for the front interconnection, obtaining optical CTM gains, and the length of ribbon used for the back interconnection is flattened to a thickness less than 1 mm. This thin ribbon is then bent to contact the next cell. Furthermore, the cells are actually placed such that they overlap the adjacent cell by about 1 mm, in that way paving is quite similar to shingling.
Both shingling and paving aim to reduce the gap between solar cells. However, paving uses an interconnection medium (i.e. ribbons), therefore cells do not have to be sliced in order to form interconnections, reducing any possible damage that may occur when laser cutting for shingles. Since paving still uses ribbons, the reduction in resistive losses is much lower compared to shingles. Therefore, paving may be seen as an intermediate technology before shingling.
Paving also has its own challenges. During lamination, the process involves pressing the module configuration against the encapsulant before the EVA is molten. The increased pressure at the overlap of the paved cells could lead to bending and hence cracks. A possible solution provided by the Institute for Solar Energy Research GmbH (ISFH) shows that using structured EVA would compensate for the non-uniformities formed by the overlapping cells being pressed.
Like other PV module advancements, paving can be combined with a number of other advancements. Larger wafer formats, mutli busbar, half-cells, glass-glass and bifacial modules are all possible with paving, resulting in many variants of modules with high CTM gains. ISFH obtained a 21.8% efficient prototype HJT half-cell module with the structured EVA encapsulant approach. JinkoSolar on the other hand refers to the technique as ’tiling ribbons’, featured in their 460W Tiger Panel[2].