Shade and 1 panel or 2

westbranch · August 2009

I am about to wire up my second installation with 2 - 100 A 12V batteries and use one 120W panel with the SB 2000e MPPT CC .

Now that the sun is dropping I was looking at the shade patterns , and I do have to do some chainsawing. Problem is that there will still be a shadow line that will cross any panel, no matter where I put it... result is a loss of PV output as we all know...

However I am thinking that I could use 2 or 3 smaller panels (40 - 60 W +-) in parallel and slightly further apart than 1 panel would cover. This might be better as panel #1 would be in full sunlight when the shade starts to affect the second ( & third) panel and later on it flips around when the sun comes out behind the tree...

I know there are a lot of variables here, money included, but does anyone think this is a good way to deal with a moving target? or just KISS?

for comparison here are 2 spec pages , 120w PV are at the bottom of table
http://global.mitsubishielectric.com/bu/solar/products/pvmodule/spec02.html
http://lib.store.yahoo.net/lib/wind-sun/KC65T.pdf

cheers

Eric

Cariboocoot · August 2009

Re: Shade and 1 panel or 2

So what you're wondering is: with one 120 W panel, when the shadow falls on it will the output drop below 50%? If so, then (2) 60 W panels in parallel set apart so that the shadow hits only one at a time would be better.
I think this would be highly dependent on the size of the shadow: how many cells would actually be blocked.
That said, it has been my experience that even a small shadow falling on one panel in series knocks the string down significantly.

Certainly the dual panels would work, but would there be enough of an advantage to justify the extra cost/hassle. Hard to say without a practical experiment.

And gosh, wasn't that completely unhelpful?

niel · August 2009

Re: Shade and 1 panel or 2

as you had said there will be a shadow no matter what meaning there is no gain to separate smaller pvs so imho kiss.

icarus · August 2009

Re: Shade and 1 panel or 2

I disagree with Neil on this one. If you are wiring in parallel, then keeping the shadow restricted to just one panel will yield a bigger harvest than a shadow on one bigger panel.

I have two panels that get shaded, by separating them, I get full output from one or the other all the time. If it was just one higher wattage panel, I would get close to nil output for a lot of the day,

Tony

westbranch · September 2009

Re: Shade and 1 panel or 2

well the chainsaw has been busy, but more to remove. Fire season is almost over but the open fire ban is still in place, we have had a SoB of a fire season here with the closest fire to us exceeding 60.000 Ha ( ~ 150,000 acres). It has been burning for almost 2 months and jumped the guard again this weekend...

Just read Tony's post about 'ever growing loads' and thought over my system design, and I am now thinking about using 3 - 12 v batteries to make sure there will be expansion capacity. 2 batteries will allow me to stay just below 30 % DoD so 3 will move me down to a planned 20% or so DoD.

I have decided to go with a separate 12V, 50+ W, 3+ A panel for each battery so now I will have to go with 3. I don't see any major problems hooking up the charger(s) at this point, one to each battery,

but I do have a ?? in the back of the mind as to how to put 3 - 10 cell - 12 V batteries together to make sure of a balanced supply to the distribution center.

Looked at Mikes favourite link http://www.smartgauge.co.uk/batt_con.html and am thinking (ahead too) that example 3 might be the easiest version for 3 batteries as well as expansion to 4 if ever needed.

Comments?

Eric

Cariboocoot · September 2009

Re: Shade and 1 panel or 2

I agree, Eric; it has been a horrible wildfire season. The lake next to ours is still burning, but under control.

I also agree that for more than 2 batteries in parallel method #3 is best. Keep those wires equal length and equal size - it's the best way to keep the current flow even.

So for your set-up you'll have a separate panel and controller per battery? Sounds like extra work. Why not wire the panels together in parallel and feed them through one MPPT to charge all batteries simultaneously? There'd be fewer pieces and less wiring so the end result should be less $ spent and marginally more equalized charging.

And always be sure you've got enough charge current available to handle the cumulative Amp/hrs.

dwh · September 2009

Re: Shade and 1 panel or 2

Aye, bad fires this year. I live in Sunland, which was threatened by the Los Angeles "Station Fire". We weren't evacuated though we were on alert for possible evacuation.

If you look at the lower left edge of the containment zone on this map, you'll see Sunland (just above the 210 highway icon):

http://latimesblogs.latimes.com/lanow/2009/09/significant-progress-reported-in-station-fire.html

mike95490 · September 2009

Re: Shade and 1 panel or 2

I agree with Tony, space panels apart, and in parallel, so only one gets nailed.

The lake next to ours is still burning, but under control.

That is a bad fire

Pyrocumulus Clouds in LA's Station fire (image attached)

tallgirl · September 2009

Re: Shade and 1 panel or 2

westbranch wrote: »

but I do have a ?? in the back of the mind as to how to put 3 - 10 cell - 12 V batteries together to make sure of a balanced supply to the distribution center.

Looked at Mikes favourite link http://www.smartgauge.co.uk/batt_con.html and am thinking (ahead too) that example 3 might be the easiest version for 3 batteries as well as expansion to 4 if ever needed.

Comments?

Eric

Just remember that as you increase storage capacity, the power consumption during the day needed float your batteries will increase. Losses into my battery bank are my #1 daytime load.

westbranch · September 2009

Re: Shade and 1 panel or 2

TG, if I read you right you are saying that if ie 10 VA are needed for one battery @ float, with 3 batteries I will need in excess of 30VA.

I think this is what you are saying, if so what causes this non linear power need? Wire resistance?

Eric

niel · September 2009

Re: Shade and 1 panel or 2

if i read her rightly, she is telling you that due to the increase in battery capacity that the % needed to float charge it will stay the same and that would represent an increase in float current needed. examples; 100ah battery at 1% is 1a and a 200ah battery at 1% would need 2a so there was an increase needed to float the 200ah battery over the 100ah battery.

westbranch · September 2009

Re: Shade and 1 panel or 2

Ah ha. Yes, that is why I have to gone to 3 panels for 3 batteries.

cheers

Eric

Cariboocoot · September 2009

Re: Shade and 1 panel or 2

Float current would be dependent on the load draw, which will be the same regardless of the # of batteries, and the self-discharge rate of the batteries themselves. So it is not automatically "one battery rate times # of batteries" unless the rate has zero load. In any case, a PV array capable of putting out enough current to properly charge a battery bank has enough power to keep them floating - until the sun starts going down at least.

I've probably just started another argument.

westbranch · September 2009

Re: Shade and 1 panel or 2

Marc, at this time I am sticking with 12 v to avoid any parasitic inverter loads , at least until I have been able to monitor a year of PV input, especially over the next 6 months. Not expecting any guests in the Hytte (16' x 18') as there is no heat as yet.. won't be until we dismantle the old place... when the zero clearance wall heater is removed.

Eric

System2 · September 2009

Re: Shade and 1 panel or 2

tallgirl wrote: »

Just remember that as you increase storage capacity, the power consumption during the day needed float your batteries will increase. Losses into my battery bank are my #1 daytime load.

Do you need to float them all day every day after they are fully charged? Seems like you could reduce some of that float time if you have somewhere else to use the power. If battery float is your biggest daytime load, what other loads do you have?

Not challenging - asking how and why decisions are made - so I learn something!

Marc Kurth
Bastrop, TX

Cariboocoot · September 2009

Re: Shade and 1 panel or 2

Marc wrote: »

Do you need to float them all day every day after they are fully charged? Seems like you could reduce some of that float time if you have somewhere else to use the power. If battery float is your biggest daytime load, what other loads do you have?

Not challenging - asking how and why decisions are made - so I learn something!

Marc Kurth
Bastrop, TX

"Float" is a state of charge: it comes after the Bulk and Absorb stages wherein the batteries get 'refilled'. In the Float stage the charge controller tries to supply sufficient current to keep the system at a set voltage regardless of loads. In essence, the batteries are contributing nothing to power production: they are "floating". This is solar electric Nirvana, where your panels are powering your loads. This is also where people start tearing their hair out over the thought that the panels are capable of harvesting more energy than this and so they're "wasting" sunlight.

westbranch · September 2009

Re: Shade and 1 panel or 2

Tx Marc, I just was rethinking your question, and as 'Coot said Float is Nirvana, so no you do not need to get there day after day, but good if you do and then you know that the batteries are recharged, ie through the absorb phase, when they are being used daily.

When you have them sitting unused for a while they will degrade (daily) and this small amount needs to be replaced so the Bulk stage should be quite short and then you hit Absorb. Depending on the CC you should also get to Float quickly when there has been no draw down (use).

For Example, my system does not hit Float every time I use it but I do get well into Absorb...

The 'Advanced " section discusses this 'free juice use' and balancing loads quite well right now.http://forum.solar-electric.com/showthread.php?t=5843

HTH
Eric

Shade and 1 panel or 2

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