Solar Panel Shading---Panels in Parallel or Series

[Al F]

Occasionally there is mention of the very serious affects shading has on solar panels.

 

It is easy to read a statement like "shade really decreases the amount of power a solar panel supplies" and think "yep I'm sure it does".

 

However, do you really realize just how little shade it takes to almost totally kill the output of a single solar panel, or even worse yet almost kill the output of several solar panels wired in series?

 

Bottom line: Covering one single square on one panel will almost totally end its power out put. Covering that one single square in a string of 3 panels in series will almost totally end the power output of all 3 panels. 6 panels in series?? Shade one single square and you will loose most all the power from all 6 panels.

 

Here is a great article and video by "Gone With The Wynns" showing the effects of shading on two solar panels. The first part is the panels in parallel and the second is with the panels in series.

 

I am not trying to say don't wire your panels in series. Just be aware of how many panels you have in series and exactly were they are on your roof. There are very good and valid reasons for wiring panels in series. Increased voltage, lower wire size from panels to controller, probably less power loss from a long wire run.

 

 

[dewilso]

I had watched that video, its the reason I decided to do parallel on mine. But after EVERYTHING is saucered and cooled, it just comes down to .... Ford, or Chevy?

[hemsteadc]

Bottom line: Covering one single square on one panel will almost totally end its power out put.

As a 6-panel series guy I can disagree with that.

 

It depends on what kind of bypass diodes are put in the panels circuitry. If a panel has zero bypass diodes, yes, that extreme a power loss would be expected. Bypass diodes are supposed to disable a portion of a panel when that portion is shaded.

 

I can shade up to 1/3 of any one of my panels and still retain a good portion of my power. i measured it once but I can't recall the data. I'm not measuring it again because someone on here will be sure to tell me my data is bad.

[skp51443]

Agree, shading can't be evaluated without first understand your panel's bypass diode setup or lack of one. Depending on that you can drop the output of one cell, a bank of cells, one panel or the whole string.

 

We originally went parallel because we had shading issues and blocking diodes but no bypass diodes in our panels, we'd have lost a lot of potential power to shade. Doing it again I'd want series hookups and make the needed changes to the system design, panel selection/mounting and modify the existing roof mounted stuff to make it work. Short term pain but long term better results.

[Yarome]

Agree, shading can't be evaluated without first understand your panel's bypass diode setup or lack of one.

 

X3.

 

There is much to consider. Also thrown into the equation are the two "camps" of covering every square inch possible with panels vs. strategic placement/planning as the priority. More is not necessarily "more".

 

It often seems that folks concentrate too much on $/watt and total wattage.. however... actual production/$ is the only benchmark. Planned properly it's not uncommon to see a 500watt array (@ $2/watt) outproducing a poorly planned 1000watt array (@ $1/watt) within the same real estate.

 

Ya gotta "know" your panels and plan/wire accordingly.

[Jack Mayer]

 

 

Ya gotta "know" your panels and plan/wire accordingly.

This is why the DESIGN STAGE is so important. You have to PLAN what you are doing. Not just throw things together. And as a general statement, cheaping out generally produces a compromised system.

 

As to covering the roof.....it is something I DO recommend for all but the smallest systems. At the price of panels, as a percent of total system cost, it is generally prudent to "over produce" on the solar.

 

As to shading of panels....let me provide an "alt" view. Having shaded panels - panels that are shaded "sometimes" but not "all the time" can be a good design choice. Assuming they are in parallel. Just because a panel does not get 6 hrs of unshaded sun does not mean it is not a valuable asset. If you got 3 hours out of it it may meet your design goals. "It just depends...."

[Al F]

Agree, shading can't be evaluated without first understand your panel's bypass diode setup or lack of one. Depending on that you can drop the output of one cell, a bank of cells, one panel or the whole string.

 

We originally went parallel because we had shading issues and blocking diodes but no bypass diodes in our panels, we'd have lost a lot of potential power to shade. Doing it again I'd want series hookups and make the needed changes to the system design, panel selection/mounting and modify the existing roof mounted stuff to make it work. Short term pain but long term better results.

 

 

X3.

 

There is much to consider. Also thrown into the equation are the two "camps" of covering every square inch possible with panels vs. strategic placement/planning as the priority. More is not necessarily "more".

 

It often seems that folks concentrate too much on $/watt and total wattage.. however... actual production/$ is the only benchmark. Planned properly it's not uncommon to see a 500watt array (@ $2/watt) outproducing a poorly planned 1000watt array (@ $1/watt) within the same real estate.

 

Ya gotta "know" your panels and plan/wire accordingly.

One of the great things about the Escapees Forum is the educational information that is willing shared.

 

That was the intent of this topic I started.

 

There have been some replies stating the shading information I gave is not totally correct. The replies seem to provide good logical information, but a lack of detail for education.

 

Would those who made above 2 replies, I quoted above, please provide details!!

 

1st. Bypass diodes.

I just looked through the specification documentation for this Kyocera 340 watt panel I don't see anything in there indicating a bypass diode or lack of one. Are the bypass diodes, diodes which are added externally to the panels or do some panels come with them built in? Or do the diodes go in the junction box?

Additionally please point us to some websites, such as Jack Mayer's site, which discusses bypass diodes. Granted I may have overlooked them when I was designing and installing my system, but I don't recall seeing a discussion about bypass diodes and just how important they are if you have panels wired in series.

2nd. Panels costing $2/watt being much better than panels for $1/watt

For example this Kyocera 340 watt panel from Northern AZ Wind & Sun for less than $1/watt. What should folks be looking at to know that this panel is much inferior to panels for $2/watt? Please provide a link to a panel at $2/watt and what is in that panel which makes it better.

 

Personally this info request is not so much for myself, other than my education.

 

Hopefully information provided would help others planning or designing a future system.

 

My system consists of two 325 watt Kyocera panels wired in parallel so the bypass diodes don't affect me. Also one panel is installed up front and one at the rear of my 29' MH, so it didn't seem practical to wire them in series anyways. My set up did max out the space on my roof while still allowing space to walk around to access and clean the panels.

 

[skp51443]

If you buy a panel with no internal bypass diodes you can add external ones to bypass the whole panel. Internal bypass diodes can bypass the entire panel, a group of cells or single cells.

 

Bypass diodes aren't free either in performance or cost and you need to decide if it is worth limiting yourself to a panel that offers them.

 

http://www.electronics-tutorials.ws/diode/bypass-diodes.html

 

http://www.pveducation.org/pvcdrom/modules/bypass-diodes

 

http://www.solaredge.com/sites/default/files/se_technical_bypass_diode_effect_in_shading.pdf

 

active http://www.digikey.com/en/articles/techzone/2012/dec/active-bypass-diodes-improve-solar-panel-efficiency-and-performance

 

more https://www.google.com/search?q=solar+panel+internal+bypass+diode&oq=solar+panel+internal+bypass+diode&aqs=chrome..69i57.14752j0j4&sourceid=chrome&ie=UTF-8

[Al F]

Stanley,

 

Great info in the links. I now understand much better about the bypass diodes.

 

I guess if you test your panels wired in series and covering one solar cell area causes the current to drop for all the panels in the series string it is time to install a bypass diode for each panel.

[JRP]

How do you tell before purchase, which of the commonly available panels have these "substring bypass diodes" if its not listed in the description?

 

I'm getting ready to do a Solar installation on my ocean going sailboat, where shading is impossible to avoid as the mast & sails temporarily cover most available spots on the boat depending on their position in relation to the sun. I want to be sure to buy panels with multiple substring bypass diodes, but don't find that info in most of the sales listings.

[skp51443]

Jim, Have you looked at some of the printed - glassless panels, like Uni-Solar? It has been a while but aside from being flexible a lot of them had per-cell bypass diodes. Only drawback was watts per meter but if that design gave a lot more meters of coverage it wasn't a problem.

 

https://www.emarineinc.com/Best-Marine-Solar-Panels

 

 

I was able to download a full data sheet on my Siemens panels many years back, I'd sure hope poking around the manufacturer's site would turn something similar up today. Some sellers that know solar rather than just sell it might have data sheets available for their products too.

http://www.sciencecenter.net/eesolar/siemens/sr100p1.pdf

"Built-in bypass diodes (12V configuration) help system performance during partial shading."

 

If you look at the layout you'll see it is 2 6 volt strings.

 

 

P.S. Buying from a reputable manufacturer means you can go dig up a datasheet from 15 plus years ago! Buying from China no-names you might not be able to get any support once the mounting glue has dried.

[Al F]

I spent about 20 minutes researching my Kyocera 325GX-LFB panels and was unable to determine if there were built in bypass diodes. I pretty much assume they don't only because none of the specs I found didn't say they were included.

 

I am in heavy clouds, or I would test by shading one solar cell.

[Yarome]

Would those who made above 2 replies, I quoted above, please provide details!!

 

Sorry, Al. I wasn't ignoring you... just away from the house for a few days. I'll get on that.

[Chief 62]

How do you tell before purchase, which of the commonly available panels have these "substring bypass diodes" if its not listed in the description?

 

I'm getting ready to do a Solar installation on my ocean going sailboat, where shading is impossible to avoid as the mast & sails temporarily cover most available spots on the boat depending on their position in relation to the sun. I want to be sure to buy panels with multiple substring bypass diodes, but don't find that info in most of the sales listings.

 

Best bet would be to call the companies tech support and get the straight scoop from someone about your questions.

[Yarome]

Cost differences. Probably the most obvious cost difference would be poly-crystalline (or multicrystal) vs mono-crystalline (or singlecrystal) cells. Mono-crystalline cells will, in general, have a higher efficiency rating (Mono's @15%-22% vs Poly's @9%-16%). They will also have a much lower degradation rate.. they will maintain higher efficiency ratings longer than a poly panel. Individual cells within a panel will also have more consistent output between individual cells and matched panels in an array. More uniform current transmission is going to translate into more efficient transmission along your wiring and improved performance if using an MPPT controller. Mono's are also more heat resistant/tolerant. They will maintain higher energy production in higher temperatures. Either ambient or internally generated. Individual cells can also handle higher feedback voltage which will prolong the life of individual cells during shading.

Less visible would be the inter cell wiring quality/materials and whether or not they have some type of backflow/bypass/cell optimization installed.

Even within mono-crystalline panels though there are various levels of purity/quality. You have to check all of the rating values when comparing.

On the flip side.. poly/multi crystalline are geared more toward bottom line pricing. They are more likely to have cheaper components and are likely to have no additional energy management technologies. They will also vary in quality/purity of the cells themselves so, again, it's important to check all the rating values when comparing.

Less visible would be the frame construction/materials and glass clarity.

That's not to say that there are not companies out there that are using mono cells w/substandard materials or poly panels that aren't leaps and bounds above their competition.

Will $.70-$.80/watt panels get the job done? Sure! Are you 'really' saving money with a lower per watt panel price? That's debatable. It will require more real estate/panels to produce an equal amount of energy production than a higher quality panel. Occupying more real estate.. are you increasing shading issues by other roof top "appliances"? Are you better off with a small footprint high efficiency panel with no shading issues?

To cover or not to cover? If you are using high efficiency high quality mono panels and covering every square inch.. I say go for it. Most folks won't make that kind of investment though knowing that it's a given that there will be constant shading issues. It's a $ to return thing (a $2.50/watt panel turns into a $5/watt panel when only operating at 50%) If your choice is to cover every square inch with cheaper poly panels.. which is generally what folks do when "blanketing" their roof tops.. it's just possible that you might have saved more money and would have higher production with only a few high efficiency panels strategically placed. Does that make sense?

Not saying I'm right or wrong on what strategy is the best in all situations.. just my own take. The cost to production breaking point is going to vary with each unique installation.

Al, for the Kyocera's you linked to earlier.. those are poly-crystalline. They do not have any type of diode or power management installed. (Maybe I'll write up something on diodes and such later) They ARE though a "mainstream" mfg and produce some fairly decent panels.. for what they are. You'll notice too though that they have different series of panels that will vary in quality within their product lineup. MFG's further down the food chain are the ones you really want to be leery of. Even at $.70/watt they may not be worth it.

[Yarome]

Shading and diodes. I'm no expert... but my take on it...

Hmmm.. how to keep this simple? So.. what's going on when a single cell in a panel array is shaded and why does the voltage drop so dramatically? The shaded cell is not only not producing voltage (a very small percentage at that).. it is now a "sink hole" for all the other cells to back-feed into. Basically becoming a heat sink.. converting a good percentage of energy production of all the other cells into heat.

Considering that a single standard cell is capable of absorbing tremendous amounts of voltage (in the 100's of volts range) without damage.. and considering that a standard 12v solar panel is not capable of producing that amount.. you'll likely not find many panels with any type of back-feed, bypass or active bypass diodes installed... in a 12v panel that is. The "standard" at the moment.. regardless of how "high end" a panel might be is to use bypass diodes in 12v configurations (aka.. 1 per panel) or at the very most.. a 6v configuration (1 per 6v string of cells). Bypass diodes CAN be installed to provide a path around panels, groups of cells or individual cells, but I'm not aware of any MFG of standard frame type panels that goes beyond a 12v configuration.

Moving into 24v or higher voltage panels.. a bypass diode is more common (in 12v strings).

Not that they aren't out there.. as Kirk pointed out.. but the main purpose.. from the industries point of view.. is in protecting cells (ie., a person running multiple panels in series back-feeding into a single shaded cell) .. not as a panel "feature" for consumers wanting to deal with partial shading issues. Back-feed diodes (so that panels are not back-fed during non producing hours.. aka "night") are not that common anymore either in most classes of panels because most solar controllers already handle that issue, but they are still out there.

If a panel DOES have a bypass diode/cell optimizers installed it will most certainly be listed in the general specs along with all of the other ratings. It's not at all a "hidden" feature. If it's listed and doesn't specify.. assume a 12v configuration. If it ain't listed.. it don't got it.

The same goes for bypass diodes (not to be confused with back-feed diodes). That technology is continually emerging. Newer technologies have been lagging a bit behind cell efficiency improvements so there really hasn't been a wide spread (aka cost effective) implementation. The current tech you'll find is what they call an active bypass.

[for you sparkies... a two terminal FET based circuit consisting of a charge pump, control logic and FET driver, MOSFET and a capacitor. I know.. cool, right!]

That's not to say that there are not plenty of stand-alone active bypass components/kits available that can certainly be installed in-line of any panel.

Integrated diodes.. or "cell optimizers" are really what we, as users, are really looking toward having more widely implemented.. but I don't know how long it will be before we start seeing many in the typical 12v panels used by RV'rs. Those are actually laminated right into the wiring of each individual cell. No more "sink holes"! Needless to say.. they don't run for $1/watt.

Just a tip: When doing searches for panels try terms like "cell optimizer" or "power optimizer".

 

Again.. I'm no expert. I don't follow the industry on a weekly basis or browse industry wide panel offerings for fun, but it might give a little insight when shopping.

[hemsteadc]

From this site

 

"The diodes ACROSS each panel are the bypass diodes. Most medium and larger panels come with these already installed. The purpose of bypass diodes is to shunt the current around a shaded, weak, or damaged panel. If the full current passes through a shaded or weak panel, overheating and damage may occur. Bypass diodes are not needed on 12 volt systems, optional on 24 volt, and should always be used on 36 volt or higher systems. See also our tip below on shading."

[Al F]

Yarome,

 

Thanks for coming back and supplying the additional info.

[Al F]

From this site

 

"The diodes ACROSS each panel are the bypass diodes. Most medium and larger panels come with these already installed. The purpose of bypass diodes is to shunt the current around a shaded, weak, or damaged panel. If the full current passes through a shaded or weak panel, overheating and damage may occur. Bypass diodes are not needed on 12 volt systems, optional on 24 volt, and should always be used on 36 volt or higher systems. See also our tip below on shading."

Great info from Norther Arizona Wind and Sun.

 

Thanks.

[skp51443]

You do not want blocking diodes if they are not needed, they reduce power production a bit. Some panels come with them but also provide for bypassing them if they aren't necessary. Better to get a quality controller that has a built-in disconnect to prevent nighttime battery discharge through the panels.

[Yarome]

You do not want blocking diodes if they are not needed, they reduce power production a bit.

 

Very true. (blocking diodes not to be confused with active bypass diodes). In an all parallel or small series/parallel array you'll likely loose more juice to an in-line blocking/back-feed diode than you will to a single cell shading situation. The loss is constant across the board vs. temporary shading losses. Let you're controller do it's job.

 

Good catch, Stan!

[hemsteadc]

How much is a bit?

[skp51443]

How much is variable, depends on the voltage your panels are operating at and the type of diode used. A silicon diode has a typical forward voltage of 600–700 mV, while the Schottky's forward voltage is 150 – 450 mV so you can get a rough idea of the lost power at your panel's working voltage. Making it more complicated is the heat loss in the diode that causes heat related issues if it isn't properly cooled in high current situations.

 

What should be apparent is that you only want, at most, one blocking diode per series string of panels as the diode losses add when used in series. A 4 panel series string could drop .6 to 2.8 volts in the diodes alone if each panel had a blocking diode installed.

 

All of this doesn't really matter because in RV use you don't want a blocking diode or diodes, you want a power relay or other low loss circuit in your controller that keeps the panels from being a drain on your batteries in no sun conditions.

[hemsteadc]

Don't all controllers block power to the panels from the batteries?

[skp51443]

No, but probably today most of them have a blocking relay or don't back-feed due to their design but there are both older controllers and cheap new ones that either don't block or have an internal blocking diode. The controller data sheets should cover this, if not assume the worst and buy something else.