In this video, we will learn more about mismatch losses. How bypass diodes can be used to protect PV cells and how they're commonly installed in PV modules. As we've learned in the previous video, in most PV modules, cells are connected in series to achieve low current and high voltage. Which allows us to minimize ohmic losses. It however, also means that strings are highly susceptible to mismatch losses. For example, when one cell is shaded as the current of the entire string is limited by the cell with the lowest current. A single shaded cell can lead to unacceptably high power losses in the module. Even worse, models are usually also connected in series. Meaning that a single shaded cell could severely reduce the power production off the entire PV array. We also learn that when the power of a string is reduced due to shading, the current generated in the unshaded cells nevertheless has to pass through the shaded cells. This can lead to reverse bias, which in turn means that power gets dissipated. The energy of dissipation is converted into heat and can lead to hotspots, which my permanently damaged the PV cell. Luckily, this power loss can be mitigated by using bypass diodes, which also reduce the risk of hotspots. Let us first, however, have a look at the main causes of mismatch losses. The primary cause for mismatch losses is related to differences in a radiance. For example, due to shading. This is best avoided during the planning and installation of PV systems. Unavoidable, however, is a radiance reduction due to soiling. Here soiling is the umbrella term for dirt, dust, leafs, bird droppings and other materials that may cover PV modules. In turn, they can lead to shading of individual cells, parts of cells or even entire modules. Another possible cause for mismatch losses is related to inherent cell mismatch due to differences in electrical properties. To avoid this, typically, modules only use cells from single bending class. This cell mismatch may however, also because by degradation such as buy, cell cracking, encapsulant browning or corrosion of electrical interconnections. Finally, depending on the mode of installation, temperature differences may also contribute to cell mismatch. A cell temperature is directly related to both cell voltage and current. Let us now discuss the main effect of mismatch losses. Depending on the number of cells in the string, the power dissipation in reverse biased cells can lead to the generation of significant amounts of heat, leading to so called hotspots. The higher the number of cells in series, the more power gets dissipated in the mismatched or shaded cell, and the harder it becomes. In extreme cases, when the breakdown voltage of the cell is reached, things become dangerous. On the one hand, the breakthrough process leads to permanent degradation of the cells and module, which in turn allows the formation of more hotspots in the future. On the other hand, if high enough temperatures are reached, the more you could initiate a fire. In order to avoid such scenarios and to reduce the power losses caused by cell mismatch in the first place, bypass diodes have been introduced. These bypass diodes are diodes connected in parallel to the strings, providing an alternative current path in case one or more off the cells become shaded. During normal operation, when no cells are shaded, current passes through all strings in the model. In case a cell becomes shaded, the maximum current in this shaded string is reduced. Without the bypass diode, this would reduce the current off the entire module and lead to high power dissipation in the shaded cell. With the bypass diode in place, however, the majority of the current generated by the unshaded strings can bypass the shaded string by passing through the bypass diode. This allows the unshaded strings to operate at a higher current and therefore reduces power losses in the PV module. The bypass string itself will operate the full voltage off the bypass diode, which is around 0.5 waltz. And the unshaded cells will pass current through the string. The shaded cell gets reverse biased and dissipates the power leading to cell heating. As the main current bypasses the shade itself however, only the current generated in the shaded string gets dissipated. This in turn means that the cell heating is limited and the formation of dangerous hotspots is prevented. While bypass diodes provide vital protection for cells, they also have a minor drawback. In normal operations, a small leakage current is always present, leading to a small power loss. Therefore, the number of bypass diodes is usually kept as low as possible. Another thing to keep in mind is, that separating your serially connected cells into smaller strings by using bypass diodes can lead to a stepwise IV-curve. Depending on the mount of shading or mismatch within each string, there may even be multiple local minima and maxima in the power curve. Therefore, when we track the maximum power point, a more sophisticated tracking method may be required. After this video, you can experiment yourself with different shading scenarios in this lecture's plugging. Let us now consider how many bypass diodes are required in a typical PV module. Of course, we would like to keep the number as low as possible to avoid a high leakage current, but there some constraints we have to keep in mind. First of all, we have to take into account the breakdown voltage of the cells that are to be protected. The total voltage of all others sharely connected cells in the string should be below this breakdown voltage, including some safety margin. Secondly, the maximum voltage and current of the bypass diode have to be taken into account. As an estimate for the maximum number of cells per bypass diode, we can divide the breakdown voltage of the PV cells by their open circuit voltage. Experience shows that usually one bypass diode is used for 24 cells or fewer. The exact number of cells provided pass diode, however, also depends on the model layout and expected temperature variations. Finally, let us revisit the standard model layout we've already learned about. From the last video, we know that most modules use either 64 cells or 120 half cut cells. Which are mainly connected in series and divided among three strings in full solar modules and six strings in half solar modules. Usually a bypass diode is connected toe each of these strings in parallel. In case of the half cell modules, two of the peril strings are connected to each bypass diode. In both cases, this amounts to three bypass diodes per module. The bypass diodes themselves are typically located in a junction box at the back of the module. To give you some reference, a typical 60 cell module will weigh around 18 kilograms, including the glass, frame and laminate. With the frame, it reaches dimensions of approximately 1.7 times 1 meter and a thickness of sick of 47 centimeter. Let us now quickly summarize the main points of this video. At the beginning, we discussed the possible causes for mismatch losses, including shading, soiling, inherent cell mismatch and degradation. Afterwards, we learned that this cell mismatch can lead to significant power losses that are much higher than the losses from the shaded cell itself. Furthermore, the power dissipation caused by the cell mismatch can lead to the formation of dangerous hotspots, which can damage the PV cells and modules. In order to mitigate these effects, bypass diodes can be used. These are connected in parallel to the cell strings and provide alternative current paths. Finally, we also discussed proper dimensioning of bypass diodes and that we know from experience that one bypass diode is required for every 24 cells or fewer. Thus, in the most common 60 some modules, three bypass diodes are used.
