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Positioning of a Blocking Diode in Solar Panels

03/07/2014 5:29 PM

My shed has one side of its roof facing east, and the other west. When the sun rises and sets, this will mean that half of my panels are in sun and the other half are not.

What I have done is put a schottky rectifier between the two pairs of panels so the side in sun is not dragged down by the one in shade.

This is my shed here with 4 of the 8 panels visible. Each side has 4 100 watt panels in parallel.

All of the negatives of the panels are linked together. The 4 positives of the left side go into the left side of the diode and the right 4 positives go into the right side. The middle pin of the rectifier goes to the controller.

So, is this a good setup and is the diode needed? The panels themselves have no blocking diodes, only bypass diodes.

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Guru

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#1

Re: Positioning of a blocking diode in solar panels

03/07/2014 5:58 PM

Found this...."By-pass diodes will not be of use unless panels are connected in series to produce a higher voltage."


http://www.solar-facts.com/panels/panel-wiring.php

http://www.ibselectronics.com/pdf/ac/Diotec/applications/solardiodes.pdf

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Guru
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#2

Re: Positioning of a blocking diode in solar panels

03/08/2014 7:00 AM

A circuit diagram is worth a thousand words.

Del

(old cat proverb)

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Guru
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#3

Re: Positioning of a Blocking Diode in Solar Panels

03/08/2014 10:47 PM

Dear Hydro,

Sell your diodes on ebay.

As long as the panels have light they produce power. The voltage will be enough to participate and also deliver "some" current.

No way your panels of group A are going to feed the B's or vice versa.

Each of your panels probably has 2 diodes in it already (check in the connection box). That is how they usually come.

In your scenario 2 groups each and everything parallel, when you like the low voltage.

And let the horses worry. They have a big enough head for it. Without hydrogen in it. Regards. D

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#4

Re: Positioning of a Blocking Diode in Solar Panels

03/08/2014 11:16 PM

hydrogenhead:

You may simply waste power from the panel that has lower voltage.

Perhaps best way is to keep them in series so if any power is generated in anyone of them then it will be used up and will show in the form of dynamically changing voltage and current both. It is basically is a charge source. If solar cells in one panel do not source anything then of course there will be voltage drop due to bypass diodes. In your wiring you are introducing one diode extra so there will be that much voltage drop from working panel and other panel will remain inactive.

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Power-User

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#5

Re: Positioning of a Blocking Diode in Solar Panels

03/08/2014 11:30 PM

As long as all panels are in parallel and have internal blocking diodes - no need for external diodes. Just connect as shown below. Diodes shown here are inetrnal diodes.

Shadows due to near by trees, buildings, clouds hovering over and variations in Sun's trajectory over the year etc- can result in variations between panels - though on same side. Hence this connection is safe- a diode for each panel.

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Power-User

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#6

Re: Positioning of a Blocking Diode in Solar Panels

03/09/2014 12:19 AM
Your Schottky diodes will waste a little power but do no harm.

The simplified equivalent circuit of a silicon PV cell is a current source in parallel with a forward-biased silicon diode. On that basis the open circuit voltage of each panel will be the sum of all its diode's forward voltages, so with two similar panels in parallel the voltage will be the same and no reverse current should flow back into the darker panel.

There are two factors that modify this ideal situation. Firstly there is a shunt resistance in each cell due mainly to impurities in manufacture. However in good cells this has a high ohmic value (on the order of 1000 Ωcm2 per cell for commercial units) and can practically be ignored. Secondly, the more brightly illuminated panel is likely to be warmer and its diodes will therefore have a lower forward voltage, making it less likely for current to flow back into the darker panel.

If one panel has a shorted cell, the balance will not work, of course!

Some useful reading here: http://pveducation.org/pvcdrom/solar-cell-operation/impact-of-both-resistances

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#7
In reply to #6

Re: Positioning of a Blocking Diode in Solar Panels

03/09/2014 12:37 AM

NeilA;

That is a good one. I also like that warning signal for cost savers "If one panel has a shorted cell, the balance will not work, of course!" to remain careful.

It is a fact that bypass diode and solar cell's own diode are in parallel. This bypass diode works if solar cell becomes totally open. Now that you have brought out this short circuit idea, I find it very interesting one.

There is one theory that engineer's living handbook is in many working brains. You find endless possibilities if you keep talking, without bothering too much about garbage. You may end up with a quality diamond in coal somewhere.

Nice link.

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#8

Re: Positioning of a Blocking Diode in Solar Panels

03/09/2014 5:50 AM

Hydro-, the "cables" going from the regulator, as pictured, are way too small which will give extra resistance going into the battery bank resulting in a lower voltage at the battery terminals than what would be desired to achieve a "full charge". Larger cross-sectional cables in this use would be not so much necessary for the Amps but to give so little resistance as possible. This is important, especially since you're already introducing some voltage loss with the diodes. BTW, the largest regulator in the series you're using, I believe, is 30 Amps. On a sunny day, just 4 of your panels will produce more than that. Also it's best to place the regulator as close to the batteries as possible. (Sorry if this is a bit off topic.) Success in your project.

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#9
In reply to #8

Re: Positioning of a Blocking Diode in Solar Panels

03/09/2014 6:16 AM

kendall:

Good idea.


Even though it is a free power, why to waste it. After all it is your power once you have captured it.


Perhaps adding a voltage booster right there will also reduce the cost of the cables. That is one reason higher voltage output panels have relatively thin cables. Perhaps 48V or 96V system sounds relatively better but more than that may be too risky. One may get shock.

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#10
In reply to #9

Re: Positioning of a Blocking Diode in Solar Panels

03/09/2014 1:56 PM

SOLAR panel cables now have standard output cables or they do not fit the normalized plugs. It is the equivalent of 4 mm2. The wires have been developed for this reason.

They can be used without conduits and have a double insulation mantle. Extra diodes and voltage boosters are all junk for the project. The battery charger/ inverter is supposed to deal with these problems.

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#11
In reply to #8

Re: Positioning of a Blocking Diode in Solar Panels

03/09/2014 5:55 PM

Yes, the thin wire was getting warm. I have since replaced it with 64 amp twin and earth cooker cable. I think the conductors on it are about 8 or 10 AWG.

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#12

Re: Positioning of a Blocking Diode in Solar Panels

03/19/2014 9:18 AM

If you are using the DC current from your PV array, you solution may work. If you utilize AC, you will require an inverter. Older installations use string inverters which result in the shading problem you describe. Newer installations use micro-inverters with Maximum power point tracking (MPPT) to minimize shading problems.

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Del the cat (1); dvmdsc (2); hydrogenhead (1); kendall (1); NeilA (1); Shyam (3); SolarEagle (1); welderman (1); xyz (1)

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