Now that we have learned all about the physics of solar cells, let us discuss PV modules. In this video, we will learn about electrical interconnection strategies for PV cells and their effect on the module performance. Let us start with a reminder of some of the fundamental concepts of electrical circuits. Kirchoff's current voltage laws. Kirchoff's current law states; that for every point in an electrical circuit, the sum of all currents entering the junction is equal to the sum of all currents leaving the junction. Kirchoff's Voltage Law states; the sum of all voltages in a closed loop is zero. This means that the voltage generated in a loop must equal the voltage drop of the remaining components. Using these two laws, the effects of connecting cells either in series or in parallel can be explained. Before we get to that however, we should keep in mind that photovoltaic cells operate similar to diodes, as you can see in this four quadrant graph. It uses the so-called generate reference system, which generated energy is counted positive. Active operating region of the solar cells is in the first quadrant here. When reverse biased, cells can also operate in the second quadrant. If the reverse voltage is too high however, PV cells will break down. If on the other hand, the forward voltage is too high, meaning beyond the open-circuit voltage, the cell operates in the fourth quadrant. Here, the current becomes negative and power is dissipated. Also keep in mind the effects of irradiance variations, which linearly affect current but hardly impact voltage at all. Let us now consider cell interconnections in the simplest case, where all our cells are identical. In case of parallel connections, the string voltage is limited by the voltage of the individual cells. While the string current is the sum of all individual currents, the actual operating point of the string depends on the load. Connecting the cells in series leads to a string voltage corresponding to the sum of all individual cell voltages. The current on the other hand is constant throughout the string. Again, depending on the load, the operating current and voltage will vary. Comparing these two interconnection strategies, we can see that serial interconnections lead to higher voltages and lower currents compared to parallel interconnections. Since a lower current also means low ohmic losses, serial strings are the preferred interconnection strategy. As mentioned earlier, the power production not only depends on the irradiation, but also on the operating point of the PV cells which are defined but the connected load and various environmental conditions. As we've learned in earlier lectures, the current varies linearly with variation, while the voltage various logarithmically. In practice, only the change in current is significant for power generation. What happens however if the cells are not identical? For example, one of the cells may be shaded, which leads the current being reduced while the voltage stays approximately the same. In parallel connections, the power losses are relatively minor as a reduced current only affect a single cell on the string. Therefore, the string output power is only reduced by the power lost in the single cell. In serial connections, things become a little bit more complicated. The string current is limited by the current of the weakest cell, meaning that the current of the entire string will be reduced. This in turn leads to significantly larger reductions in string power output than in the case of parallel interconnections. These power losses are referred to as mismatch losses, which can not only be caused by cell shading, but also other causes which we will discuss in the next video. These mismatch losses cannot only occur between cells within a module but also between modules in a string. Coming back to the serial interconnections, if the string is bypassed using a bypass diode, the voltage drop for the whole string will be around 0.5 volts. In that case, since most of the cells will operate close to the short-circuit current, the shaded cell gets reverse biased as the current from unshaded cells gets pushed through it. Let us now look more into what happens to reverse biased cell. Due to reverse bias, the shaded cell operates in the second quadrant experiencing a negative voltage. But flipping its IV curve along the current axis, we can intersect it with the combined IV curve of the unshaded cells to obtain the operating point of the string. This means, the amount of power dissipation not only depends on the amount of shading but also on the number of cells in the string. As we can see a large amount of power is dissipated in the shaded cell which leads to significant heating referred to as hot spots. With high enough temperature, this can permanently damage the PV cell and the module. This effect can however be easily mitigated by installing bypass diodes, which will be discussed in the next video. Before we get to that, let us conclude this video by looking at the standard module design and interconnection strategy used. If we look at standard model designs, we will find that silicon cells are usually connected in series. The main reason for this lies in the electrical parameters with relatively high current and low voltage. By connecting the cells in series, the string current is kept as low as possible reducing ohmic losses. While the highest string voltage makes it easier to transform to the required voltage levels for great feed-in. However, as shown previously, this interconnection strategy is highly susceptible to cell mismatch. Most common PV modules features 60 cells per module in six rows while other common sizes include 48,54 or 72 cells. The main consideration of having these numbers of cells is that an even number of rows simplifies the layout and reduces the length of DC lines within the module. In the last years, modules using half-cut cells have also become increasingly popular usually featuring 120 cells where two times 20 half-cell strings are connected in parallel and three of these clusters in series. The advantage of the half-cell modules are low ohmic losses and six pixels instead of three for shade management. Most of today's modules reach rated output powers of more than 300 watts at STC. Currently, leading manufacturers in industry are aiming for models achieving output power of more than 500 watts. These modules are usually based on larger wafer sizes and feature 144 half-cut cells. To summarize this video, cells and typical PV modules are connected in series in order to keep ohmic losses low and achieve higher voltages. Compared to parallel interconnections, this however leads to a higher susceptibility to mismatch losses as the current of the string depends on the lowest current among all cells. In the next video, we will learn how bypass diodes help to mitigate these losses. Finally, we looked at a typical module layout consisting of 60 cells. The modules made from 120 half- cuts cells are also becoming increasingly popular. Regardless, most modules reach nominal output powers beyond 300 watts and leading manufacturers are competing for modules beyond 500 watts.
