IMPACT OF SHADOWS ON PHOTOVOLTAIC MODULES

The great increase in knowledge and experience that the photovoltaic sector has gained in recent years is evident. However, there are still conceptual doubts about the behavior of photovoltaic modules in the existing digital literature.

For example, what we could call the “myth of total shading” still persists. That is, the belief that partial shading on a photovoltaic module limits the energy production of the module itself and the rest of the non-shaded modules of the string to which it is connected.

Thus, we can see, in countless publications on the web, illustrations where partial shading reduces the power of a photovoltaic module to, for example, 80% and the rest of the modules in the string (despite not being shaded!) also reduce their power. power at 80% due to current limitation. However, this is at least inaccurate.

If the modules were not equipped with bypass diodes, the above behavior would be real. Fortunately, thanks to diodes, limiting current in one module does not necessarily have to affect the rest of the modules in the chain. Bypass diodes function as an alternate path alternative to the passage of current.

Let's consider a set of 4 photovoltaic modules, where the first of them is partially shaded. Consequently, the intensity of this module will be reduced compared to that of its companions in the string. In this scenario, the inverter to which it is connected is faced with the decision of adjusting the set's working point to the low intensity determined by the shaded module, or, on the contrary, maintaining a higher intensity to activate the bypass diodes. of the affected panel, which would force it to stop contributing to the total power of the chain and its current intensity would not be limited.

What happens in practice is that the inverter tries to obtain the maximum power of the string so that it runs along the PV curve until it reaches a maximum. As can be seen in the figure on the left, this maximum is reached at a point very close to the peak power of the generator; Therefore, the losses will not be as large as the “myth of total shading” would lead us to think.

Let's now imagine that 2 of the 4 modules are partially shaded. In that case, you can see in the figure on the right that two maxima are formed. One of them local (LMPP) and another absolute or global (GMPP).

Now it could happen that the MPPT of the inverter would get “trapped” in the local maximum and make the entire string work at the intensity of the “worst” module. In that case, the MYTH would be fulfilled.

Fortunately, for some years now, the most important inverter manufacturers have implemented modern monitoring algorithms in their equipment capable of reaching the GMMP quickly, so the power losses would be much lower than those predicted by the MITO.

Source: Marco Mariño – Linkedin

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