2 Panels on different plane to rest of array...

[Energetic]

We have quoted a system of 14 panels that fits very nicely on a roof. To the side of the roof is a dormer with a plane of about 20Deg compared to the main roof which is about 30Deg, the customer wants to jam an extra 2 panels on the dormer.

Conventional PV thoery dictates this is a no-no but i cant think of a very good reason not to do it with such a small differance in plane. The orientation is exactly the same but it would mean 2 panels on 1 string slightly off. The voltage and current may be slightly different from these but would it be enough to cause any issues?

 

[Gavin A]

it'd be more noticeable when the sun's lower in the sky.

 

[Eco Deal]

the lower panels will drag system down possible with solar edge or micros

 

[FB.]

As Gavin says: in summer it won't matter.
But with low sun in winter (or early/late in the day at other times of year), the difference in angle will make a big difference to panel output, so the bypass diodes will be working overtime.

By my reckoning, a 5-degree difference (35 v 30 degrees) in angle affects as follows:

Mid-day, June sun: 0.5%

Mid-day, March/Sept sun: 5%
(comparable to morning/afternoon summer sun)

Mid-day, December sun: 20%
(comparable to early-morning/late afternoon spring/autumn sun)

 

[Energetic]

There will be no reverse current flow so why would diodes be working?

Why is it that two panels would limit the others? Is the power output limited by the lowest current or voltage or both?

Wouldn't the current and voltages of both planes average out and the power output from each panel be relative to irradiance; ie not affected?

 

[FB.]

There will be no reverse current flow so why would diodes be working?

The flow of electricity would be limited by the weakest link in the chain.
It's a bottleneck. The more you try to ram through a bottleneck, the hotter it gets. This sometimes shows as individual "hot spots" on frosted panels in winter - due to mis-match of cells on a panel.
The flow of electrons through the silicon cells is limited by the amount of light hitting that panel. The weak link can't work any harder, and overheats due to the other panels forcing more power through it than its light conditions allow.
Alternatively, it will use its bypass diodes to allow the excess to flow straight past/overflow (and therefore avoid overheating) which, although useful, the bypass will be at increased risk of failure the more they're used.

 

[Energetic]

Cheers guys so the current flow of the whole string is limited by the least irradiated panel regardless of the voltage in the rest of the string.

Never had a reason to look into it before and had been thinking of the panels as individual voltage and current generators wired in series combining as in thevenins theorum.

Has anyone got a full internal wiring diagram of a panel including the bypass configuration?

 

[FB.]

I made this sketch a few months ago:

 

[wstevew]

I made this sketch a few months ago:

So panel orientation should be considered where possible where there is shading?

 

[sedgy34]

we had to get a system working that a company had done and they left it not working but still commissioned it for the 43p in december there are slight different inclination but has not been a problem and its generating fine. so proof is in the pudding
but yes what a mess!!!!

 

[wstevew]

6 on one tracker and 10 on another?

 

[FB.]

6 on one tracker and 10 on another?

Bung 'em all on one tracker if you like; the bypass diodes will take care of panels not producing at the same rate - until, that is, the bypass diodes burn out due to being active too many times for excessively long periods. Bypass diodes aren't really designed to take prolonged and heavy flows of power; they're just to deal with a moving shadow (perhaps a chimney or TV aerial) across a panel - the shadow usually only affects part of the panel and only for an hour or two.

My own part-snow-covered panels during the winter showed that the reduction in power (due to some panels being covered in snow) is basically only proportional to the area covered by snow.

 

[SolarCity]

we had to get a system working that a company had done and they left it not working but still commissioned it for the 43p in december there are slight different inclination but has not been a problem and its generating fine. so proof is in the pudding
but yes what a mess!!!!

What inverter is used on that? Surely a dual MPPT inverter?