Shade, at what price shade?

System

Looking on to winter....

Is there ever a time, when shade encroaches (seasonally), when it is better to just take a panel or panels out of a string? Taken to its extreme, will completely covering up a panel (in total darkness) shut down the entire string? If there is a point of diminishing returns, when is that point, and can it be quantified? Does it vary dependant on the type silicon you're running?

Thanks.

BB.

Re: Shade, at what price shade?

Your panel should have bypass diodes installed to allow current to bypass a shaded panel (I am not sure all panels have bypass diodes, but most should--check your panel specifications). If not, you should probably add them to both allow better operation when shaded, and to prevent possible damage to the shaded panel...

If I remember correctly, a silicon solar cell goes "high resistance" in the dark. And if you have enough series panels in the string, you could damage a a dark cell/panel with too much voltage for the others in a string.

If you panels are in parallel (low voltage system), then bypass diodes/disconnecting a panel, is not a concern. Bypass diodes are only needed for series strings (typically MPPT solar chargers and Grid Tied inverter systems).

All of the common silicon based solar panels should behave, roughly, the same with respect to shading.

-Bill

PS: I should add, that a bypass diode does not guarantee that all will be just fine with a shaded panel in a string... If the string does not produce enough voltage with one panel shaded (minimum voltage for the solar charge controller or grid tie inverter), then the string will stop producing.

Also, if you have several strings in parallel, covering one panel in one string, will generally cause that one string's production to go down to near zero (i.e., 3 strings in parallel, cover one panel, then you would have 3/2'rds of your original output).

Assuming that you have bypass diodes in your panels/setup, you could always try throwing a section of cardboard or blanket on a panel and see what happens to your setup.--but generally, solar electric panels are very prone to large drops in output with even (what seems to be otherwise) minor shadows (like a telephone line, power lines, etc.).

-BB

System2

Re: Shade, at what price shade?

BB,
I have two strings of 15, and am trying to squeeze the most out of this 3.8KwH gridtie system. The system is on the north side of the house, in the back yard (darn HOA!). As the shadows grow long, the house will start to cut into my action, and I am looking at remounting that portion that will get winter shade to a better, but much more labor intensive location. (I threw them up quickly so I could start "stickin' it to the MAN as soon possible!)

My objective, of course, is to figure out WHEN to move them... anyway...

For your, and anyone else's information, I just got through covering one panel in the string of 15 with a large pool toy. Cut the string amperage by 1/2, from 6.0A to 3.0A. (Don't know about voltage drop.) I guess it's safe to say there ain't eenee bypass diodes in them thar panels?

As far as a clinical method to determine for presence of bypass diode, can this be done with a simple voltmeter? - I popped open the back of the terminal block, and it's filled with some hardened opaque epoxy filler. Wouldn't want to bust it open just to prove something isn't there...

Thanks

Gordon in Flower Mound

BB.

Re: Shade, at what price shade?

Gordon,

No, the test you did does not indicate if there is a bypass diode or not... Cover two panels, one from string "A" and one from string "B". That will tell you if there are bypass diodes installed (other than checking the panel specs.).

If you get the full current (at reduced voltage), then the panels have diodes installed. I would suggest that you look at the panel specifications instead though--if they are designed for high voltage Grid Tied inverters, then they probably have the bypass diodes installed...

Warning: If they don't have bypass diodes installed, shading one set of panels could damage (read destroy) the shaded panels (I am just guessing here--I don't know the rating for reverse biasing for your panels--but the possibility of damage, in my humble opinion, is there if the diodes are not installed). So, I would check the data sheet for your panels and see if the diodes are installed or not.

Disconnecting a panel and checking it with a DVM on "diode check" (do all of this in the "dark") should show if the diodes are installed or not.

Regarding your output drop when shading one string, what you are seeing is the basic effect of the equivalent of putting two strings of batteries in parallel. When you shade one of the batteries in the string, that stings voltage drops by XX volts (panel rating). The other string is still outputting its full voltage (say 200 volts vs 230 volts DC)... Depending on the MPPT tracking algorithm of the Grid Tied inverter, it simply never sees the lower 200 volt string and just maximizes the Voltage*Current=Power of the 230 volt strings output.

I believe you system is operating normally (regarding how it is affected by shading).

-Bill

Solar Guppy

Re: Shade, at what price shade?

In the other post, OP says he has evergreens

You have nothing to worry about and there is nothing you need to do. As a panel is shaded, the bypass diode kicks in causing the string to have a lower vmp/voc. The inverter will track to the best mppt point of the now imbalanced strings, nothing will ever break, at worst, if enough panels become shaded you lose the production of the entire partially shaded string. Again nothing will be damaged, nothing needs to be done

mike95490

Re: Shade, at what price shade?

FlowerMoundPV wrote: »

(darn HOA!)

In California, HOA's cannot force you to locate solar (PV or Thermal) in an un-efficient location.
that's about the only thing our governator got right.

System2

Re: Shade, at what price shade?

Guppy,

My Evergreens (Hs-180's) are NOT UL rated (got 'em at a "discount"/"not for use in USA" ?!?), and, after tearing into the silicone packing in the terminal block of one panel, found no diodes. (Will verify with MM tonight after the sun goes down)

BB,
I did the test with the other string off-line, to get a true test. Am VERY concerned about damaging shaded panels. Ordered a fistful (100) of 3A/50V Schottly diodes, to install in parallel, 3 per shaded panel, to get 9A protection). Will shut down the string that gets partial shade until they arrive and are installed. Don't want to take any chances... By the way, what would the damage manifest itself as? Other than reduced output? Curled-up crinkly-looking silicon cells? Suitable for dipping? LOL
Seriously, invested TOO much in this system to make stupid mistakes.

On the good news front... Got our first electric bill with a FULL month's PV production. Of our 3800KwH consumed, 12.7% made from solar! wooHOO! Wife is finally starting to smile again!!

Thanks for all your help~

BB.

Re: Shade, at what price shade?

Gordon,

Solar Guppy is the expert here--I don't know the industry like he does...

I went to the Evergreen website and looked at the specs... The 120 watt (cedar) panels clearly state they have diodes, the 195 watt (spruce) models did not say anything.

I am not trying to scare you--I just don't want to be the person 3,000 miles away on a keyboard that is the reason that you blew something up during testing.

I would probably just call Evergreen tech support and ask them the question directly (or the distributor). Trying to keep things dry and weather proof is very important too and adding diodes into the wiring could just cause other problems down the road (especially if they are not needed).

Regarding UL--that may or may not be an issue... If you are grid tied in the US, generally a building inspector will require UL/NRTL (Nationally Recognized Testing Laboratory), as will your home owners insurance (if there is a fire). The chances that the solar panels cause a fire--probably pretty remote.

I don't know that having or not having UL would affect whether or not a diode is installed.

-Bill

mike95490

Re: Shade, at what price shade?

Diodes in a PV panel, are not likely to be the round fat, leaded devices you saw in science kits. They are likely to be surface mount chips, about 1/4 " square, and as thick as a wooden pencil lead is. (just the lead, not the entire pencil) Of course, I've never opened one (PV panel) up, but the chip components are the modern way, and the leaded ones, old school.
see samples at top of page http://dkc3.digikey.com/PDF/T073/P1131.pdf

BB.

Re: Shade, at what price shade?

Went to the Evergreen Spruce product page and found the spruce installation manual and found:

• The module contains factory installed bypass diodes located inside the junction box.
• The junction box is not designed or certified to be field accessible or maintainable and should under no circumstances be opened. Opening the junction box may void the warranty.

So, you are fine--don't worry about bypass diodes...

Don't doubt the Guppy !

-Bill

Solar Guppy

Re: Shade, at what price shade?

Anything evergreen makes, including the offspec HS modules for sun electronics has the diodes ... Don't worry, be happy