Hello, in this lecture module, we'll be discussing how the energy generated by the solar panels can be extracted and converted into usable electricity that powers our everyday electronic devices and appliances. First, we'll start by reviewing the power characteristic of solar cells and panels. Discuss how it is influenced by the ambient operating conditions, as well as how we can scale up the power output of a PV system. As was discussed in the previous lectures, solar cells and PV panels have a non linear power characteristic due to the nature and properties of the semiconductor materials making up the solar cells. What this means is that the power output of a solar cell or a PV panel, which we generally denote as a PV device or PV generator, will depend on where on this power characteristic the PV device is being operated. Or to put it in another way, the operating points on this curve and the consequent PV power output depends on the type of load we connect to the terminals of the PV device. Operating point on this curve is also called the loading of the panel. In practice, we control the operating point by controlling the voltage or the current loading of the PV panel. And consequently, we can control how much power is extracted from the PV panel at any given point in time. Moreover, we can observe that the power characteristic has a pig point, which represents the maximum power we can extract for the current irradiance and temperature conditions. This maximum power point is characterized by a specific voltage and current which are commonly denoted as Vmp and Imp. The power characteristic and the maximum power that can be extracted at any given point in time depends on the ambient conditions, namely on the incident light and the solar cell temperature. Increased incident irradiance levels will cause an almost proportional increase in the maximum power available from the panel, as well as an increase in the current and the maximum power and short circuit conditions. For most source of technologies, this relation is approximately linear, whereas high irradiance will lead to only a slight increase in the voltage at the maximum power point and open circuit conditions. Following a logarithmic relationship. On the other hand, increased solar cell temperature will cause a decrease in the maximum power with approximately 0.35 to 0.5% per degree Celsius for crystalline silicon solar cells. This means that every 10 degrees Celsius in excess will result in a decrease in power of the module ranging between 3.5 and 5%. Similarly, higher solar cell temperature leads to an normal linear decrease in the voltages at the maximum power point and open circuit conditions. Whereas, only slightly increase in the currents of the MPP and short circuit conditions. In reality, the incident solar irradiance and cell temperature change continuously during the day, and therefore available power of the MPP devices will change as well. In sunny, clear sky days, the solar irradiance and cell temperature will change slowly in a deterministic way and can be relatively easy estimated from the sun's position and average ambient temperature. Consequently, the power characteristic allocation of the maximum power point of the PV devices changes slowly as well. PV systems located in regions with low solar variability, experience more clear sky days. Therefore, the location of the maximum power point and maximum power available at any given time is more predictable. However, fast moving clouds will cause large variations in the direct sunlight, and consequently in the power characteristic of the PV panels. Generally, PV systems located in regions with high solar variability, experience high fluctuation in in their PV power output. This means that the location of the MPP and the power available from the PV panels is less predictable, and is one of the main challenges for extracting the maximum power available from PV panels. Until now, we have discussed about the power characteristic of solar cells and panel. A typical crystalline silicon solar cell produces up to 4 to 5 watts, whereas typical solar panels produce up to 250 to 300 watts under standard test conditions. The highest wattage PV solar panels on the market today can generate over 400 watts under standard test conditions. And PV panels rated at over 800 watts are coming to market in 2021. Nonetheless, the power output of a single panel is relatively small compared to the power requirements of household, commercial, industrial buildings, and is negligible compared to the power output of congressional power plants. So how can we increase the generated PV power even further? As we do solar cells, PV panels can be connected in series within a PV string to increase the PV power output. The output power and voltage of the string will increase proportionally with a number of panels connected in series, here denoted as M. Whereas the output current of the string will remain the same. In practice, a limited number of PV panels can be connected in series, depending on a maximum voltage rating allowed, which depends on the country's regulation and system size. For example, in residential PV systems in the United States, PV strings of up to 600 volts dc are allowed. Whereas PV system in Europe and other parts of the world allow up to 1000 volts dc. Europe PV panels and regulations for utility scale PV plants, allowed connection of PV strings up to 1500 volt dc, which have the advantage of less dc cable costs and losses and increase the overall system efficiency. The PV power output can be further increased by connecting PV strings in parallel into so called PV arrays. The output power and current of array will increase proportionally with a number of strings connected in parallel denoted by N. Whereas the output voltage of the array will remain the same as the PV string. In general, the maximum power of the array is limited only by the power and current rating of the other system components. To summarize, the power output of a PV panel depends on where the device is operated on its power voltage characteristic. Namely, on the loading of the PV panel. Moreover, the PV power characteristic of a PV panel is not constant, and changes continuously during the day with the incident solar irradiance and cell temperature. This makes the maximum power available that can be extracted from a PV panel at any given point in time less predictable. And lastly, the PV power output can be increased by series connection of panels into PV strings, and parallel connection of the strings into PV arrays.
