In the previous lecture, we discussed about the nonlinear power characteristic of solar cells and PV panels, and that the actual output power of a PV panel is determined by the load connected at the terminals. In this lecture, we'll explore in more detail how different types of loads can be powered from PV panel, as well as what limitations and type of load is necessary to extract the maximum power from a PV device. The simplest type of load we could connect and power directly from a PV panel is a DC load or consumer that has a resistive characteristics, such as a DC bulb, LED, or heater. Such a configuration is rarely used in practice, since the DC load will only operate properly, and that nominal power if this voltage and current ratings are compatible with the panels electrical characteristic. As we learned in our last lecture, the power voltage or current characteristic of PV panels changes continuously during the day. Moreover, the load's resistance will decide operation point on the current-voltage characteristic of the panel, and it needs to be well-matched to the panel to extract near the maximum power available. In reality, the load may not even be constant, as is the case for most electronics. If several consumers can be turned on or off or a DC heater with different heat setting points. Even if the panel will match with the load, what happens when their radiants or temperature changes? The load will not extract the maximum power available from the PV bound anymore, and moreover, the current or voltage of the PV panel may no longer be compatible with the DC load characteristics. On the other hand, whatever single PV panel cannot provide sufficient power to the load. What can we do in practice? As we learned in the last lecture, several panels can be connected in series, thus increasing the power output. However, this would increase the period voltage as well, and the PV stream may no longer be compatible with the voltage rating of the load. A simpler example would be a DC motor or a battery that operate at relatively low voltages, but require high power to operate efficiently. Similarly, we could connect panels in parallel to increase the power output. This will increase the PV current as well, which will cause heating and resistive losses in the cables. To avoid this, we would need thicker and more expensive DC cables. Let's take another example of loading a PV panel. Can we charge a battery directly from a PV panel? Yes, under certain conditions, if the battery charging voltage is compatible and lower than the opposite liquid voltage of the PV panel. Otherwise, it may discharge instead of charging the battery, and the situation, it will be the battery voltage that determines the operation point on the current voltage characteristic of the panel. The battery voltage must be well-matched with a maximum power point voltage of the panel to charge the battery with near the maximum power available from the panel. Let's take a concrete example where we are charging a 24 volt battery directly from a 230 watts standard PV panel, which has its MPP voltage equal to 28 volts. We also assume that the PV panel is operating under optimal irradiance and temperature conditions. In this case, the green area represents the maximum power that can be extracted from the panel, if it is operated at a maximum power point, corresponding to 28 volts and 8.25 amps. However, the battery charging voltage is around 24 volts. This will shift the operating point and the current and will distract juristic over panel towards lower voltages, as depicted in the figure on the right. As a consequence, the PV panel will only output 210 watts, corresponding to the red area in the left figure, the 21 watts difference in power will be lost and dissipated as heat. Nonetheless, in this example, the battery and panel are relatively well-matched. Let's take a more extreme example. When the battery and panel are not well-matched, as would be the case for charging a 12 volt battery from the same panel, and this situation, the battery would force the panel to operate at an even lower voltage, depicted in the red area on the figure on the right. Consequently, the panel can only output 106 watts of the 231 watts possible. Over half of the available power, 125 watts will be lost and dissipated as heat in the panel due to the mismatch in the battery and the MPP panel voltage. Therefore, proper dimensioning of the battery or panel is necessary to make such a system work efficiently. Moreover, there is no control of the battery charging current, which is an important aspect where I'm charging batteries, and we have the same limitations as for the DC load when trying to increase the PV rated power. How can we extract power from PV panels in a safe, reliable, and flexible manner so that the panel and load voltage and current ratings are always compatible, and we extract the maximum power available from the PV panels at any given time under variable environmental and load operating conditions. Well, to achieve this, we need an active device between the PV panels and the load that is capable of converting the input voltage and current to match the electrical requirements of the load. Moreover, the conversion should occur with high efficiency and minimal losses. The active device should be capable finding and tracking the maximum power operating point of the generator independent of the irradiance and temperature conditions. As well as it needs to be able to react and adapt changes in the load. In the following lectures, we'll discuss how such an active device could find and track the MPP in practice, as well as how it could be realized and function. To summarize, the output power over PV device depends on electrical characteristics of the load connected at a terminal. Moreover, the current and voltage ratings of the load must be compatible and wall match with the PV panel in order for the load to operate safely and efficiently. Passive loads such as DC loads and batteries, are not able to track the maximum power point under changing environmental conditions, therefore, active lows are necessary for efficient loading and power extraction from the PV devices.
