Partial exposure of cells series in solar panel...

[Externet]

Hello all.

 

If a solar panel during cloudy daylight gets total shade on several cells, as from an object fallen/covering them ;

If a solar panel in full darkness at night gets some cells illuminated by a flashlight ;

 

Is there still some current passing/generation in both cases ? Or dark cells will block flow of current for the whole panel ?

 

[Carrock]

Solar panels are basically diodes optimised to produce current from incident photons.

So a dark panel in series with lit panels is basically a forward biased diode.

It will pass current with a small voltage drop ( eg about 0.6v for silicon).

[Enthalpy]

A dark panel is a reverse biased diode so it blocks the current from other cells usually in series.

 

But the solution (used on satellites) is to add a forward diode in parallel that gives an other path to the current. Can be a Schottky too, or even a well driven MOS, to reduce the losses. Cheap, efficient, should always be used on ground photovoltaics.

[Carrock]

A dark panel is a reverse biased diode so it blocks the current from other cells usually in series.

 

But the solution (used on satellites) is to add a forward diode in parallel that gives an other path to the current. Can be a Schottky too, or even a well driven MOS, to reduce the losses. Cheap, efficient, should always be used on ground photovoltaics.

From https://en.wikipedia.org/wiki/Theory_of_solar_cells

An ideal solar cell may be modeled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model.

This shows clearly that a series dark cell is a forward biased diode if the other cells are producing current.

 

It's common to include a low forward voltage (eg Schottky) diode in series to prevent cells at night eg discharging batteries charged in daytime.

 

If illumination is uneven, or to guard against a cell going inefficient, a Schottky diode could be placed in parallel, conducting only if the cell is dropping rather than raising voltage.

 

The last thing you want in a solar cell is a low forward voltage drop.

[Carrock]

A few errors in the above:

 

The main reason for a series diode is to prevent the cells being damaged in darkness by reverse voltage.

 

If current is being taken from a partly lit battery, a cell in darkness is still forward biased but the diode (in the model) only passes external input current with no additional photocurrent; it absorbs rather than produces energy.

 

It's possible to bypass a faulty or unlit cell but more than a simple diode is needed.

[Enthalpy]

And this represents several solar cells in series, here two:

 

 

where you see that if one cell gets no light and its equivalent curernt generator drops to zero, the current that the other cells would produce can't pass through the internal diode.

 

Similarly, when one cell receives less current, it limits all the series strip.

 

That's why, on satellites, we add diodes that permit the remaining cells to produce power. Apparently it isn't common in terrestrial applications but it should be.

[Carrock]

And this represents several solar cells in series, here two:

 

SolarCellsSeries.png

 

where you see that if one cell gets no light and its equivalent curernt generator drops to zero, the current that the other cells would produce can't pass through the internal diode.

 

Similarly, when one cell receives less current, it limits all the series strip.

 

That's why, on satellites, we add diodes that permit the remaining cells to produce power. Apparently it isn't common in terrestrial applications but it should be.

 

Some ordinary diodes like the glass protected OA79 produce detectable current if you shine a bright light on them.

 

They don't require light to permit external current to pass through the internal diode and nor do solar cells.

 

If you accept this diagram as accurate

 

you will note note that your diagram is essentially equivalent to two of those in series.

 

If one cell gets no light and its equivalent current generator drops to zero, the current that the other cells would produce can pass through the still forward biased internal diode.

 

The purpose of the added zener diodes in your circuit is to permit current to pass if a solar cell goes partially or completely open circuit and probably to limit the array's maximum off load voltage.

 

That's why, on satellites, we add diodes that permit the remaining cells to produce power. Apparently it isn't common in terrestrial applications but it should be.

If, say, a rooftop solar panel in a location with mains power is reliable enough that spending £3 on diodes would on average increase lifetime enough to produce an extra £1 of electricity, I wouldn't condemn a designer for failing to include those zener diodes.

[Enthalpy]

The diagrams are not equivalent. Have a look at the diagram with two cells in series: when one current source delivers no current, the current provided by the other can't pass through the diode at the other cell as it's the wrong direction.

 

Enough rooftop solar panels suffer from partial shade, say because a neighbour has grown a tree after the panels' installation, or because of some pole. This is used as an argument against solar electricity. Diodes cost is negligible as compared with the lost electricity and diodes should be built in.