Solar Panels, bypass diodes, and block diodes

[guilio2010]

So I want to confirm my thoughts on a solar system array. Typically, solar systems have bypass diodes so that if an area gets blocked or damaged by light, current can flow around that area and still produce power.

IE 5 panels 20VDC@1A in series would produce 100W or 100VDC@1A to the Charge Controller.

To bring in more power, and ad hearing to controller specifications, I can parallel solar arrays.

IE 10 panels 20VDC@1A (5 Panels in series). 200W or 100VDC@2A to the Charge Controller.

To prevent any power back feeding into the a series of panels if it gets damaged/shaded, there are blocking diodes at the end of each series.

My thought is if one panel gets damaged/shaded, I have 1 series at 80VDC@1A and the other series at 100VDC@1A. with 2 separate power systems at different voltages, I would assume that the power from the 80VDC will not pass through to give me 180W so I get 100W as my total power. Correct? The issues I can't wrap around is that you have a voltage applied to both diodes so one panel should deliver full 100VDC@1A, but since the other panel is less, may you get less current? This is given that you are not less than the required Vf needed to make the diode positive.

 

[IRstuff]

?? That's precisely what the blocking diodes are designed to prevent.

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[VEBill]

Intersperse the panels (wire the series and parallel sections like a checkerboard) and you could minimize the imbalancing effect of partial shading of the array. Maybe...

 

[OperaHouse]

That makes little difference having a combination of series/parallel and it can be a quick way to destroy a panel. If going to higher voltages, you are likely using MPPT which decides by testing for maximum I x V. In theory this is great but is quickly being abandoned to microinverters for each panel. Blocking diodes should be used on each string to prevent back current flow which can destroy panels. Panels are basically diodes in series, rather poor diodes. In years back you could actually see the "holes" where the cells were shorted and formed hot spots. That is the reason why controllers all included a blocking diode. With multiple strings in parallel the good panels could destroy a panel that is heavily shaded. If one string is at 100V and another string is 80V a MPPT controller will settle at some voltage in between. The power point only varies with temperature and to a very minor degree IR loss. I operate my panels at power point and wouldn't own a conventional MPPT controller if you gave it to me.

 

[guilio2010]

Thanks All,

I am using MPPT for the panels. I guess I am looking at this as a bad design??? I get what they are doing, but I am questing how reliable it is. If I have 2 sets of panels that are brought into a combiner box, then to the MPPT Controller, to me the MPPT won't help if one set of panels is shaded. If the blocking diode Vf is .7V, I can't see where having two sets of panels in parallel would be that closely equal so power is delivery from both equally. IE, one at 100V, the other at 102V. The 102V would be the only functional set of panels. Am I missing something? If the above is true, why would they do this?

I guess the easier way to put it is:

If I have a panel that is Bypass due to shading, wouldn't that also be blocked at the same time?

Or

If my output voltage of the array differs more than the Vf of the diode, I only have 1 functional panel.

 

[OperaHouse]

Not at all. MPPT is a fairly good solution, just not optimal when there is considerable shading. I provide all my hot water wit PV power that is usually wasted in most solar setups. Most solar systems are not well integrated in that aspect.

Solar panels are current sources and those will add together. A poor model is a voltage source with a resistor in series. As current is drawn the voltage will lower. So panels in parallel will add together if somewhat matched in voltage. Shading will drop panel voltage in about 8V increments. IV charts show this at different sunlight levels at 25C. That is a temperature you will only see in winter conditions as black panels get quite hot in the sun. There will be a number in the data sheet that indicates how much the power point voltage will drop for each C rise in panel temperature.

 

[guilio2010]

Thanks Opera,

Let me think about it more, but I just want to make sure I am understanding correctly. Attached is my point viewpoint. To me, the blocking diode is reversed bias in slide 3 even if it's relatively close in voltage. We have paralleled diodes installed already on paralleled battery banks (downstream from the MPPT), and it operates similar where the diodes are functioning as a OR GATE. I just can't see where the controller would matter as the paralleled panels are upstream and parallel diodes would still act as an OR GATE.

Thanks,
guilio

 

[guilio2010]

From looking at this, given if the solar company supplies a complete system with equally sized connection cables with equal resistance, etc..., which I agree in our scenario is the likely case...then I do see this condition being ok. I just fail to see where if your voltages on each series of panels were mismatched, even by a few volts, then your power from those panels, where connected in parallel in a combiner box before it goes to the controller, would be more than one series of those panels. I guess it comes down to the likelihood of that happening, but if you had a system in remote location and a few leaves fell and stuck on a panel, then maybe?!?!