Tabbing Ribbons: Design and Purpose
Tabbing ribbons employed in the photovoltaic panels are a part of the arrangement for collecting energy from the cell and conveying it over two line to a junction box. The junction box can feed an inverter or other loads.
PV Current Collection
A solar cell when uniformly lit will normally produce a uniform density of current all over its surface which must be collected and carried away. The cell size is, as a standard practice, 6in by 6in. We need to collect current from the entire surface. Can we put a six inch by six inch conducting pad on the surface to collect current? Obviously, not!. No light will go through because the pad will shade the entire area. We cannot even make a single point contact and connect a wire to it. Charges developing at points remote from the contact would lose a lot of energy reaching over the contact. A possibility would be to make a zig-zag metallic trace which would cover the entire surface and collect current from it. The external lead contact could be connected to it, say, at the middle. But that would mean charge which was collected at the corners and other far away points would have to travel over a large length of the conductor before it could reach the external lead. This would again cause a loss of voltage and power, reducing the net conversion efficiency. So, as a compromise, fine parallel lines or ‘fingers’ are metallized on the entire surface with precalculated spacing between them. A single, wider print at right angle to these lines could be made to touch them at their middles and collect the charge collected by each. Such a line is called a bus bar (BB) as per electrical engineering practice. And the cell would be called a 1BB cell. Even a single bus bar is not considered sufficient because charges from the extremities of the fingers would still travel large distances and lose power. Can we make two bus bars symmetrically placed to make collection more efficient without causing more shadowing due to larger metallized area? Indeed, not only two but three four and five bus bars are being applied with increasing efficiency. Increasing the number of bus bars would allow using narrower fingers thus offsetting the increased shading due to an additional bus bar.
Ok, we now have bus bars on the positive and negative faces of the cell. How much voltage would exist across the cell (between the front bus bars and the rear bus bars)? Close to about 0.5 volts or slightly more. So, the cell voltages must be connected in series to increase the voltage output to a useful level. Normally 30 or 36 cells are connected to deliver enough voltage to charge a 12 volt battery. These series connected cells are called a string, and the process is called stringing. Tabbing is the process in which the cells are connected in series.
Flatt copper conductors about 2mm wide (suitable for most panels) with pre-soldered surfaces are used to connect front bus bars of one cell to the corresponding rear bus bars of the next cell to achieve a series connection. Tabbing ribbons are especially made for this purpose. Tabbing ribbons (as well as the bus ribbon) are made from “oxygen-free high conductivity” (OFHC) copper. Normally, a solid round conductor is rolled to produce a flat conductor ribbon. The ribbon surfaces are covered with solder to prevent oxidation in storage and transit. Often the surface is silver plated to ensure better conductivity and reflectivity (See “shading” below). The solder layer (often 0.025 mm thick) also provides the solder mass which melts to make the connection. The copper used is “dead soft” so that it can be easily bent for soldering to the cells and after the connection, does not stress the cell.
Shading Due to Ribbon
The ribbon shades a small portion of the cell surface subtracting from the power output. The top surface is made highly reflective so that it reflects light back up rather than absorbing. A fraction of the reflected light is again reflected back to the cell surface due to total internal reflection at the glass-air interface above and helps reduce the shading loss due to the ribbon. The ribbon surfaces are normally flat. However, Ubrich of UK are offering a special ribbon LCR-XP™ with a grooved top surface which reflects light in such a manner that more of the light is brought back to the cell through the double reflection explained earlier. Ubrich claim 80% of the light reflected by the grooved surface is recovered.
Tabbing can be done manually, but for large scale operations automated machines are used. Automatic tabbing machinery utilizes various methods including hot air jets radiative heat to raise the temperature to 150ᵒ C or more to make a quick melt of the solder on the two joining surfaces. This is called reflow soldering where additional solder is not applied but existing solder on one or both mating surfaces melts and makes the joint. The soldering process has to be carefully controlled to prevent chances of microcracks in the cell due to thermal or physical stress. These cracks are difficult to detect but can cause a premature failure. PV modules operating at a higher temperature may often fail due to deterioration of the solder joint. During soldering, the cells and the copper ribbons get heated, which expands them, and then allowed to cool after the solidification of solder. Differing coefficients of thermal expansion of the substrate and the ribbon as well as a non-uniform temperature gradient across the cells caused during the soldering process are generally responsible for buildup of thermal stresses in the components.
Tabbing Ribbons: Where Do They Lead? The Bus!
Now that the string is complete, the ribbons on the positive face of the last cell in the string are all connected in parallel to another similar, but much wider ribbon called the bus ribbon. This becomes the positive bus wire. Similarly ribbons on the negative face of the last cell are connected to the negative bus wire. The positive and negative bus ribbons connect the string current to the junction box. This bus is so called because it receives current from multiple strings in parallel with the first bus. This paralleling arrangement of strings is done to increase the current output. The bus wires lead to the junction box. Bus wires are also ribbons flatted from similar material the tabbing ribbons, but they are generally four to six mm wide. They are generally called bussing ribbons.
Factors Critical to Tabbing Ribbons Design
The following considerations are important in tabbing ribbons design and affect its field performance.
- Base Material. Choice of metal affects the conductivity, and hence, conversion efficiency. Normally copper is used as the base material for making PV Ribbon. Popular alloys comply to Copper Development Association specs and are called CDA alloys. Some companies may use CDA 102 for tabbing ribbon and CDA 110 or better for bussing ribbon. Aluminum alloys may also be used as a base metal.
- Solder Coating. Composition of the solder and thickness of coating affect the strength and reliability of the solder connection.
- Yield Strength. Ribbons with low Yield Strength minimize chances of solar cell breakage during the stringing process.
- Ductility. More ductile materials allow easier elongation under stress and hence improve reliability.
- Camber. Camber is a measure of the wire’s straightness. Tabbing ribbons and bussing ribbons must be straight without any camber (bending in the plane of the ribbon). Control of camber is more difficult when producing low yield strength ribbons.