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The MPP is the maximum power point of a solar panel and a PV inverter will typically try to find this MPP to yield the maximum energy ouput. But sometimes it might be neccessary to operate the panel away from the MPP to reduce power output (because the grid operator needs to reduce feed-in or because the inverter can't handle that much power, or whatever). Starting from the MPP, the inverter can go two ways to reduce power:

  1. It can increase the current drawn from the panel to force a lower voltage output from the panel
  2. It can decrease the current drawn from the panel. Power will go down, because the voltage will not increase by the same factor. I assume this is the preferred implementation to reduce wear of the inverter.

So the power the inverter can feed to the grid reduces in both cases, but the solar irradition to the panel is not changing. Where is the excess energy going?

In case 1) it feels quite intuitive to say the higher current results in higher conduction losses in the panel and increases panel temperature. Is that correct?

In case 2) I'm a little lost. When we draw less current than the panel is capable of delivering with a given solar irradiation we probably have some surplus free charge carriers? Are they somehow reducing efficiency of the panel, so that less photons are converted to electric energy but just directly heat up the panel?

PS: There is already this question, but there is not really a good answer for my question.

Qmechanic
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jusaca
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1 Answers1

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The answer is easier if you only consider a single cell, and apart from some details the same answer holds for an entire string of cells and then for an entire panel that consists of several strings.

The extreme cases of the two modes that you describe are the open clamp and the short circuit conditions. Under short circuit conditions, the voltage across the cell is 0 and no power is dissipated in any load (hence short circuit!). Your interpretation is correct: the absorbed power gets dissipated in the internal resistance of the cell.

In the other extreme there is no net current flow. The voltage builds up and drives the electrons and holes over the depletion region of the pn junction. This means that they will meet and recombine. The power is dissipated as heat also in this mechanism.

These two mechanisms always happen in any power condition. In the case of the MPP, we have the optimum condition. where the sum of these two losses is minimum. Any voltage value in between these conditions only changes the balance between these loss mechanisms.

W_vH
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