Shading

[kevpat]

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]

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]

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

[BB.]

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]

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]

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.

[jagec]

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.

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]

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]

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]

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.