To harvest energy from solar PV panels efficiently, it is important to ensure that the panel voltage does not collapse under load. Techniques using Maximum Power Point Tracking (MPPT) and Maximum Power Point Control (MPPC) are often used. This article discusses these terms in more detail.
As can be seen in Fig. 1, the output current of a solar panel varies nonlinearly with the panel voltage. Under short-circuit conditions the output power is zero since the output voltage is zero. Under open-circuit conditions the output power is zero since the output current is zero. Most solar panel manufacturers will specify the panel voltage at maximum power (VMP). This voltage is typically around 70 to 80% of the panel’s open circuit voltage (VOC). In Fig. 1 the maximum power is just under 140 W with VMP just under 32 V and IMP just under 4,5 A.
Ideally, any system using a solar panel would operate that panel at its maximum power output. This is particularly true where a solar panel is used to charge a battery. The goal, presumably, would be to capture and store as much solar energy as possible in as little time as possible. Put another way, since we cannot predict the availability or intensity of solar power, we need to harness as much energy as possible while energy is available.
There are many different ways to try to operate a solar panel at its maximum power point. One of the simplest is to connect a battery to the solar panel through a diode. This technique relies on matching the maximum power output voltage of the panel to the relatively narrow voltage range of the battery. When available power levels are very low, this may be the best approach.
The opposite end of the spectrum is an approach which implements a complete MPPT algorithm. There are a variety of MPPT algorithms, but most will have some ability to sweep the entire operating range of the solar panel to find where maximum power is produced. The advantage of a full MPPT algorithm is that it can differentiate a local power peak from a global power maximum. In multi-cell solar panels, it is possible to have more than one power peak during partial shading conditions (see Fig. 2).
Typically, a full MPPT algorithm is required to find the true maximum power operating point. It does so by periodically sweeping the entire output range of the solar panel and remembering the operating conditions where maximum power was achieved. When the sweep is complete, the circuitry forces the panel to return to its maximum power point. In between these periodic sweeps, the MPPT algorithm will continuously dither the operating point to ensure that it operates at the peak.
An intermediate approach is something that is known as Maximum Power Point Control (MPPC). This technique takes advantage of the fact that the maximum power voltage (VMP) of a solar panel does not, typically, vary much as the amount of incident light changes. Therefore, a simple circuit can force the panel to operate at a fixed voltage and approximate maximum power operation. A voltage divider is used to measure the panel voltage and if the input voltage falls below the programmed level, the load on the panel is reduced until it can maintain the programmed voltage level.
Contact Conrad Coetzee, Arrow Altech, Tel 011 923-9720, firstname.lastname@example.org