Shading

kevpat · April 2012

Newbie here. Shopping for PV system, and trying to educate myself. Can I please be pointed in the direction of learning about the effects of shading on cells, pannels, and arrays? I'm @ Lat 43° 26' 21" N, long 70° 46' 29" W (So. Maine). I've been reading about diodes being used in the pannels, and micro-inverters used for each pannel as a means for mitigating shadings effects. My roof is bathed in sun between aboiut 9AM and 4PM, but as the sun begins its' decent for the winter, branches causing shading come into the equation.

All help is appreciated.

Kev

Jim45D · April 2012

Re: Shading

Solar and even the slightest of shade do not mix. You will lose a significant amount of solar production. I've heard say that amorophous panels are tolerant of this, but I wouldn't bank on it.

solarix · April 2012

Re: Shading

I would say you are probably a good candidate for the SolarEdge distributed system. Check out their videos at solaredge.com

BB. · April 2012

Re: Shading

Even one branch can easily cost you 50% to 100% of the panel/array's output, no matter the controller used.

Yes--there are some things you can do with solar arrays to reduce the losses to possibly 10% of string (more or less)... But, in the end, you either need to accept the shading or remove/move the cause of the shade, or relocate the panels.

One problem with "green shade" (bushes, trees, etc.)--Is that they usually only get worse as time goes on (greenery grows out/higher) either making the problem worse or forcing you to trim/remove the source of shading.

More or less, solar panels output a (sort of) fixed voltage and variable current (related to amount of sunlight falling on array). If you shade one or more cells, it pretty much takes those out of the energy production. And reduces the output voltage (Vmp-array or Vmp-panel) by upwards of 12 volts or by 1/3 to 1/2 the output current of the panel.

Which will happen to your panels/array is very difficult to predict. It depends on the solar array and how it is wired (with bypass diodes) and the connection with other panels/charge controllers.

In some rare cases, people have "worked around" some shading issues... For one person, their panels had "one" string along the bottom of the panel and a second parallel string at the top of the panel (when mounted in "landscape" or "sideways"). This allowed the "top 1/2" of the panel to still produce while the bottom 1/2 was covered by snow... Still cost about 1/2 of the power production until the snow melted or was removed from the panel.

If you have a large MPPT GT or Solar Charge controller with several (to 10 or more) panels in series)--It is possible to wire the string so that only one panel is shaded--which still leaves enough voltage for the controller to produce power while current is "bypassed" around the shaded panel. More useful with predictable shade (chimney, vent stack, etc.).

-Bill

Ralph Day · April 2012

Re: Shading

By 4pm in the winter you're not losing much valuable energy anyway. The sun has put most of it's energy into your system between 11am and 3pm, especially in the winter. Any shading after 4pm is nothing to worry about IMO.

Ralph

kevpat · April 2012

Re: Shading

Thanx for the replies. Its eye-opening to realize the extent shading has on a system. A prospective PV installer has suggested the pictured array for us, and that, being grid-tied to CMP (Maine) it would effectively supply all our yearly electric needs, with our costs at the minimal level to maintain a service connection to the grid. It has a Solectria String inverter, 250W Canadian Solar Monosilicon PV Panels, mounted on a 60 deg pitched roof oriented at 175 degs. The shading travels from the lower left to the lower right, while rthe upper panels are unaffected. I suppose ther's a way to wire the array to minimize the shadings' effect on the array. Of course, the salesman makes it sound exciting, but I'm agreing w/many of his conclusions regarding the cost/payback factors. Shading is one of the effects that weren't discussed, so I'm trying to get a grip on this factor.
Again, thanx for the replies, and I am always open to more discussion/info on this and any other stuff.

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jagec · April 2012

Re: Shading

kevpat wrote: »

Thanx for the replies. Its eye-opening to realize the extent shading has on a system. A prospective PV installer has suggested the pictured array for us, and that, being grid-tied to CMP (Maine) it would effectively supply all our yearly electric needs, with our costs at the minimal level to maintain a service connection to the grid. It has a Solectria String inverter, 250W Canadian Solar Monosilicon PV Panels, mounted on a 60 deg pitched roof oriented at 175 degs. The shading travels from the lower left to the lower right, while rthe upper panels are unaffected. I suppose ther's a way to wire the array to minimize the shadings' effect on the array. Of course, the salesman makes it sound exciting, but I'm agreing w/many of his conclusions regarding the cost/payback factors. Shading is one of the effects that weren't discussed, so I'm trying to get a grip on this factor.
Again, thanx for the replies, and I am always open to more discussion/info on this and any other stuff.

Attachment not found.

If shade is only an issue after 4PM in the winter, I wouldn't worry too much about it. Very little power is contained in those high-angle rays. Mounting those panels in landscape mode, as pictured, and putting the bottom panels on a separate string from the top panels will minimize the effects.

One thing that you can do is set up a webcam to take pictures of your roof every half hour or so on a sunny day, and you can see how high the shadows get, and when they would shade your prospective array.

PV-watts says that the system you describe will probably produce around 5,000 kWh a year on average (assuming Portland, ME).

kevpat · April 2012

Re: Shading

Again, much thanx to all for the replies. I've sent a request to the prospective installer for a discussion on our shading issue, and the info gleaned from you kind folks will help my understanding.

Relating to strings: what are the determining factors for strings in an array? Isolating any shade from other panels/strings seems to be very important, but do strings have other factor; ie, do strings need to be equal in panel numbers (in our situation, 8 panels for two strings), or could we have the upper 12 panels on a string, and the other four on another string?

Appoligies for my ignorance, and thanx for any repliues. This thread is very helpful.

Kev

vtmaps · April 2012

Re: Shading

kevpat wrote: »

do strings need to be equal in panel numbers (in our situation, 8 panels for two strings), or could we have the upper 12 panels on a string, and the other four on another string?

Each parallel string should be equal in voltage to the other parallel strings.

Perhaps the best solution for you is to use microinverters. Each panel has its own inverter and shading of one panel has no effect on the others.

--vtMaps

ggunn · April 2012

Re: Shading

kevpat wrote: »

Relating to strings: what are the determining factors for strings in an array? Isolating any shade from other panels/strings seems to be very important, but do strings have other factor; ie, do strings need to be equal in panel numbers (in our situation, 8 panels for two strings), or could we have the upper 12 panels on a string, and the other four on another string?

For a string inverter, the strings must be of equal module count. The number of modules in a string determines the string voltage, and the voltage must be the same for parallel strings going into the same inverter.

NorTracNY · April 2012

Re: Shading

Are the cameras that take a 360degree shot not commonly used in your area? Both contractors that came to my place were required to take a hemispherical photo which is analyzed for shading. I got the impression the state rebate depends on them ensuring there will not be more than a 20% drop from ideal.

jagec · April 2012

Re: Shading

kevpat wrote: »

Again, much thanx to all for the replies. I've sent a request to the prospective installer for a discussion on our shading issue, and the info gleaned from you kind folks will help my understanding.

Relating to strings: what are the determining factors for strings in an array? Isolating any shade from other panels/strings seems to be very important, but do strings have other factor; ie, do strings need to be equal in panel numbers (in our situation, 8 panels for two strings), or could we have the upper 12 panels on a string, and the other four on another string?

Appoligies for my ignorance, and thanx for any repliues. This thread is very helpful.

Kev

For a string inverter with the pictured panel layout and afternoon shading that goes from left to right, this is the best way to wire up the 2 strings:

Attachment not found.

An 8-panel string seems a little low to me. Does the Solectria inverter have lower voltage requirements than a Sunny Boy?

kevpat · April 2012

Re: Shading

I’m informed that the modules, 240W Monosilicon Canadian Solar (CS6P-240M) have diodes which effectively create three, 20-cell sub-circuits, which can be bypassed independent of the other 40 cells in the module. This sounds like a good thing for the shading mitigation issue; micro-inverters were also touched upon, but didn’t appear to be cost-effective. I will discuss this in more detail when the guy comes tomorrow AM.

The inverter is a Solectria PVI 4000 has 4 fused inputs, MPPT Range of 200-550 VDC, Max input of 600 VDC, Strike (?) Voltage of 235 VDC, Max Input current of 20 A. My ignorance-level regarding this aspect of the system is quite high, so I don't know what any of these specs mean, as I havent delved inti i nverters at all, other that to understand that they convert DC to AC.

We’re going to some inclinometer surveys tomorrow to estimate shading from the trees. Camera surveys aren’t required for rebates, but it sounds like a really cool idea. In Maine, we need to show that the estimated payback period is less than 20 years. I think that an NREL type of estimate is acceptable for the rebate...not sure, tho.

You guys are showing patience and it is welcomed...Thanx...kv

BB. · April 2012

Re: Shading

The bypass diodes are really there to protect the cells from reverse voltage (reverse bias the solar cell "diode" by much more than 12 volts, it will blow the cell).

There is some advantage in series strings -- But remember those 20 cells + diode drop reduces the overall string voltage (about 12 volts in this case)--So the Vmp-array-shaded still needs to be higher than the Vmin of the inverter/solar charge controller for proper operation/to generate useful power.

-Bill

kevpat · April 2012

Re: Shading

Update:

So my guy came as promised with a solar pathfinder. After ascertaining all shading-on-the-modules probabilities, it was estimated that a loss of about 12% generation from one panel during the 4 months of shading would result if we kept our beloved elm tree. Otheer trees are not a factor, although a really nice Maple will have some effect in about 5 years, or so. This loss could be further mitigated by using Enphase 215 micro-inverters, at an added cost of about 10%. This sounds ok with us, as the string inverter was to be located on the exterior wall adjacent to a future extra bedroom, and the hum that I understand comes from these inverters could become an annoyance.

So, looks like we’re going with that system; it’s estimated to supply all our yearly electricity requirements w/o battery back-up, while we’ll maintain our grid tie-in at the minimal cost of < $10/mo.

Thanks to the forum. If anybody needs info on testing submarine nuclear piping systems...I’m your guy!!!

kev

Jacob_Michals · April 2012

Re: Shading

Hello Kevpat,

I guess your primary question is what you would expect for power production vs the cost of installing an Enphase Microinverter system plus the ability to keep your beloved trees.

Shading would have an effect on your overall system, but the ability of the units to be produce power at a maximum independently is key. Even without shading, the long term effects will offset your costs and give you an ROI in a shorter timespan. You can let the data plus members give feedback. What are your utility rates like?

I think it's fantastic for so many folks to be able to be able to finally install solar. Please share your thoughts.

Jacob
Enphase Energy

vtmaps · April 2012

Re: Shading

Jacob_Michals wrote: »

I think it's fantastic for so many folks to be able to be able to finally install solar. Please share your thoughts.

Jacob
Enphase Energy

Jacob, welcome to the forum. Please keep posting and get that 'New Guy' label converted to 'not so new guy'. When are you guys at Enphase going to come up with something for those of us off the grid? Many of us have long runs between panels and controllers and would appreciate the ability to transmit power by AC. Of course we would need to convert it to DC for charging batteries. And there are many other advantages to microinverters that we might also like to appreciate.

If you feel like responding to this, please start another thread so that we don't hijack this one too far of course.
--vtMaps

kevpat · April 2012

Re: Shading

Jacob

Central Maine Power gets $8.91, for the first 0-100 kWh, then 0.060333 per kWh. The $8.91 is for the 1st 100 kWh, and to maintain our connection to the grid. Our historical usage is a bit less than 5000 kWh/year, so our proposed system should come pretty close to a $8.91 monthly bill, until that particular charge is increased.

Yes, independent max production seems to be the way to go for optimum system efficiency, and micro-inverter usage seems to be the ticket towards that goal.

Regarding your model 215 product: what maintenance is required, if any, and have you solved the salty air issue. I couldn’t find a wiring diagram of the 215, and was wondering if there are internal diodes, fuses, etc., that may need replacing due to long-term degradation/usage. Access to these items for repair on my roof could be difficult. I understand that most, if not all, of the issues w/these mico-inverters relate to the degradation effects of salty air...perhaps the use of o-ring seals could help?

Thanx to all for the feedback.

Kev

kevpat · April 2012

Re: Shading

To address our shading issue, and based upon updated NREL calculations, using the micro-inverters will increase overall production by almost 5%, at about a 10% cost increase. So the micro-inverters will add efficiency, and we won't have the string line inverter hanging on our porch next to the electric meters.

OK, off to play w/the grandkids!!!

kev

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