A naive assumption when considering integrating a solar PV panel into a project is that you simply point the panel in the general direction of the Sun and the pure DC power comes out ready to be used to charge the battery. . To some extent this assumption is true, but running the solar panel with something like a conventional PWM charge or boost controller probably won't get you close to a full panel specification.
The key words here are "maximum power point" (MPP), which refers to the sweet spot on the IV curve of a solar cell. This is a feature that is important not only for photovoltaics, but also for wind turbines and other highly variable power sources. This maximum power point tracking is commonly known as "MPPT", but this acronym encompasses many different algorithms, each with their own advantages and disadvantages. In this article, we will see what these MPPT algorithms are and when you can choose a particular one.
strong links
Solar cell specifications usually indicate open-circuit voltage and short-circuit current. Between these extremes, the voltage of a solar cell depends on the amount of current it produces. As the current drawn increases, the voltage it can produce decreases.
Think of a nozzle at the end of a hose. the more water you let out, the less pressure there is in the nozzle. If you miss it, you get peak flow, but it spills over your feet. If you close the cap, a little less water comes out, but it swells more.
Power is the product of voltage and current, P = I * V, and that's what we're interested in. In the specifications, one of I or V is zero, so no power is produced. Among these cases is the maximum power of the given lighting.
The purpose of the feed point follower is to resist current flow from the solar cell so that it operates at an intermediate current and voltage that maximizes its output; open the valve to push the water through the water wheel as fast as possible. . .
It is this simplicity that hides the complexity of MPPT; With a photovoltaic solar panel, your MPP will constantly change as the sun's radiation changes due to passing clouds, the changing angle of the sun, and many other factors. This means that the MPP must be constantly updated, which means that the optimal voltage is determined with under or over voltage. Over the years, a number of different approaches have been developed to solve this problem.
Although there are several variations and specific MPPT algorithms, the following are the most common algorithms.
constant voltage
One of the first and fundamental MPPT algorithms, constant voltage (CV) tracking changes the output current to maintain a constant reference voltage. This approach uses a fraction of the open circuit voltage, typically around 80%, which means that it technically does not detect the MPP at all.
This is still a useful approach due to its simplicity, but it offers low efficiency and suffers in situations where the solar panel is poorly positioned and does not see full sunlight. The low requirements make it an ideal target for single-chip solutions such as Consonance-Elec's ubiquitous MPPT ICs such as the CN3722 and CN3791.
A basic 5 A CV MPPT board is available in a variety of configurations, often for less than $10. This makes them ideal for hobbyists and cost-sensitive applications where performance is not critical and a slightly coarse MPP will suffice.
sit back and watch
This MPPT algorithm is a fairly simple detection algorithm. As the name suggests, this algorithm is based on a slight pressure on the MPP current set point, measures the current and voltage, increases or decreases the output power and repeats accordingly.
Although quite efficient in its basic form, the main problem with the perturbation and observation algorithm is that it tends to oscillate around the MPP, which will reduce overall efficiency.
In order for this algorithm to determine whether the MPP setpoint needs to be adjusted, you need to change the MPP setpoint so that you can observe the effect of this change on the output power. Although it will follow the optimal MPP, it will constantly fluctuate around it. In this case, making the adjustment step size as small as possible may seem like a reasonable optimization, but it makes the system react too slowly when there is a rapid change, such as a cloud covering the PV panel.
This approach is a good compromise between a simple constant voltage approach and something that is still amateurish.
increased permeability
Incremental conductance measures changes in current and voltage to predict the effect of a change in voltage using the system conductance (ΔI / ΔV) - slope of the power curve. When you mix and look at the fixed size steps and see if they are in the right direction, the increased conductivity changes the step size depending on how far it is from optimum. This causes that when it is already in MPP, the step size is reduced to zero and it just stays there.
Calculating slope, the relationship between changes in voltage and current, is more erroneous than measuring levels. Although the higher bandwidth therefore makes use of a better optimization strategy, it also means stricter hardware requirements, including precision components and a controller that can continuously perform the necessary calculations. This makes it a good choice for an application where performance is important and the budget is generous enough to cover overhead.
The beginning
The first is just a minimal introduction to MPPT and the three most common algorithms, and the devil is in the details. For example, in both optimization methods, what happens if a bird flies by while you are stepping on a fixed point or taking a stress step? You may mistakenly conclude that the change you made caused the power drop. Temperature and other real world conditions also play a role, so what works on paper may not work in practice, which helps explain the consistency of the constant voltage method.
But none of these are so complicated that you don't need to implement your own MPPT controller. One such example can be found in Application Note 5324 (AN5324) for the STM32F334 microcontroller, which is the software required to use the high-resolution timer on this MCU and other peripherals such as the ADC and on-board opamp to implement, which essentially relates and outlines details. Perceptual Control MMC Switch Mode Converter and Algorithm.
While a mounted MPPT controller is probably the best option for any project that needs Just Work™ done, it's one of those great devices that is very simple and as complex as you want it to be. be a great hobby project idea.
