Why 36 solar cells in a panel?

[somemathguy]

I'm working with another guy building solar panels using the ebay cells, and we are trying to determine the number of cells to use per panel. Production panels are typically 32, 33, or 36 cells depending on physical configuration. From what I've read, 32-cell panels are more apt to be used as 'self-regulating' panels without a charge controller...ie. in most climates, the potential difference under load is small enough that that no controller is needed. Not so with 36 cells that can be up around 17-18V which will destroy any 12V battery.

In any case, is there any reason why we shouldn't build panels with 39 or 42 cells? We live in Canada at about 45" latitude, on the coast of Nova Scotia where it's cloudy or overcast much of the time. Local PV enthusiasts say that we can expect about 3Ah per 50W panel per day on the worst days in winter, and about 12Ah per 50W panel per day on the best days in summer. We're building a 12-volt system. My buddy suggested that production panels use 36 cells because it's a reasonable 'all-around' number, suitable for panels used most anywhere in the world, and there are no 42-cell panels because the only buyers would be people up north, where demand for solar is low due to 1) lower solar insolation 2) decreased population so fewer buyers!

Any thoughts?

Kevin

[ghurd]

It looks to me worst case average for a month is about 2.25 hours, or about 7ah per day.

http://www.rpco.com/products/solar/solar_uscanada.htm

I would go 36 cells. 38 or 40 would be OK, but the gain is likely to be quite small.

Do not go past 40 cells. It could ruin some solar controllers.

Don't go less than 36 or cloudy weather will almost shut them off.

If there are 42 and 6 panels, thats the same as 7 panels of 36.  7 panels would be better than 6.

G-

[picmacmillan]

when i built my panels with the e-bay solar cells, i used 28 cells per panel...now i know most use 36...i did it this way for 3 reasons.

1. the glass we have could only fit that many cells...
   
2. someone suggested using this many wouldn't matter explained below
   
3. we put 2 panels in sereis to get 30vdc(sufficient for battery charging)............each completed panel produces 15.5 volts (x2)30vdc...once this is reached we put the rest in parallel to increase the current...even if we needed more power for battery charging we could put another panel in series but ist is not the case here...i know some that cre to differ, but this is my opinion on this.......pickster





"80%" src="http://www.otherpower.com/images/scimages/1135/solar_panel_test_2.jpg">


[DanG]

Overvolting is used sometimes in hot climates, there are 72V arrays used to feed 48V systems to keep VDC output useable during solar maximum as PV cell output degrades with higher temperature. The extra 3.3VDC also might help make up for power losses from a long cable run, but might make 12V controllers unhappy.

If you bundle a charge controller the w/ panels that can handle all the 23.5VDC output without ever shutting down from overvoltage I might go for it on a few panels...

[picmacmillan]

don't know why it went to a small font?..i will try and put the photo in again?if it doesn't work ..go to my files i have some photo's...also visit...http://www.internetfred.com and click the link for solar

[Ungrounded Lightning Rod]

In any case, is there any reason why we shouldn't build panels with 39 or 42 cells? We live in Canada at about 45" latitude, on the coast of Nova Scotia where it's cloudy or overcast much of the time. Local PV enthusiasts say that we can expect about 3Ah per 50W panel per day on the worst days in winter, and about 12Ah per 50W panel per day on the best days in summer.

The extra cells are to compensate for voltage drop.  Voltage drop comes from two things:

 1) Long or thin wiring.

 2) Heating of the panel.

 3) Near-full batteries.

Dim light doesn't affect voltage much at all.  (That's set by the band-gap in the semiconductor and its operating temperature.)  What dim light affects is current.  You compensate for dim light by using more panels, not more cells per panel.

In a dim-light, short-day situation you want enough cells in your panel that you don't start dropping current before the batteries are charged.  Charging time is too precious.  So you don't use self-regulating panels - stick with the ones with a couple extra cells and a charge controller - and boost cell count if you have to run the power a long way from the panels to the batteries.

But in a northern climate heat is mostly a problem in the summer, when the charging time is actually longer than at lower latitudes.  It's in the low latitudes and deserts where boosted cell count is a major issue.  At high latitude you just use enough extra cells to insure you keep charging in your typical hot weather and you're OK.  So you can do better by spending the extra money on panels, rather than extreme cell counts.

(Caveat:  Panels absorb a lot of heat from the sun, so they need air circulation behind them to stay cool even in cool climates.  So use care in mounting them to provide the cooling, or compensate with extra cells if you can't avoid having it cooled on only one side.  And if you're building your own panels you may have more of a cooling problem than a commercial panel due to your construction technique.  So you'll want extra cells to compensate for that.)

[Paulm]

Hey ! I'm in Halifax area of NS also building panels with ebay cells (36cells). Where are you. Maybe we can get together? email me meshach@allstream.net

[juiced]

Hi easters :p

    MTL here.

   There is a company just in your vicinity that produces solar water heaters if the highest quality (from what i've heard).

  Now my guy tells me not to go to the bank with solar water heaters in Canada but for the person who sees the energy situation for what it is; I recommend them.

    Im I think the company (in fact there is 2) is Solaire or something similar. If you dont find it, let me know and ill go dig.

[pyrocasto]

I use 30 cells, and I get a little more than 16.5v opened up, so I dont know how much the heat will make it drop.

[troy]

36 is a good number.  Less than that and you will lose some efficiency.  More than that and you risk cooking components that are primarily designed for 36 cell panels.  I'd be cautious about less cells unless you can add a mini "make-up" panel. Voltage is not determined by solar intensity, but by the design of the cell material and panel.  I have measured 17+ V (open) lit only by moonlight.  The hitch is, unmeasureably small current.

And don't forget, panel output only goes down with age...

Best,

troy

[iFred]

Here is what I stated regarding this from a prior post..

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The more panels in parallel the higher the current, the more in series the higher the voltage. The cell size= amps - any cell regardless of size produces a given voltage and for most cells that happens to be around .500 vdc. or ½ a volt.  

And the size of the cut cell indicates to current. All the cells I have are rated for .500 vdc or ½ volt @ 3 amps. This means that any cell hooked in series is doubled in voltage. Any cell hooked in parallel with another cell is doubled in current or amps. Example; 2 in series=.5+.5=1vdc -> 2 cells in parallel will produce 1/2vdc and 6 amps.

 It does not really matter how you hooked them up provided that, no matter what you must meet the charging voltage or your batteries and the controller.

 This is why when you look at most installations, you will see that 4 panels are hooked in series and then all the rest are generally hooked in parallel to increase the amps only after the charging voltage has been met.

 Most installations use both series and parallel hookups. As far as matching is concerned. Again this is a simple case of the size.

 It is best to match the main panels to produce a charging voltage and then you can use mismatched panels to cover the current in parallel. The smallest size panel in any series will be the net result of all the output (even if the other panels are larger, the smaller panel will dominate. However in parallel it does not care and just adds the current. This is why the matched panels are used for compliance for charging voltage and the -don't care about size dimensions panels are placed in parallel. Makes sense?

  Now as far as how many cells you put into a panel in series is as far as I am concerned totally flexible based on this above information. So, here is the criteria to help you decide quickly how to make a panel.

 First and foremost is the controller.  

 What does the controller require to keep the batteries charged no matter what? I like to be a bit above this by a factor or 3 or more. That way the charging voltage is always met even in a cloudy day. A rough estimate would be about 28-33vdc or slightly higher. This would also allow for compensation for losses. (wire resistance, controller heat and internal losses for conversion etc).

 In my panels I have 28 cells hooked in series, this produces .500 x 28 = 14 vdc. And I hook 2 in series this us 28vdc. The controller voltage is met. The battery charging is met. And produces 3 amps. The controller will later convert the voltage and current to produce a full charge voltage of about 13.8 vdc and increase the amps from voltage not used during conversion. It's hinged on what the controller requires and can put out.

 Now here is the interesting part. Since the charging voltage and controller voltage has been met the rest of the panels (10-20) are put into parallel. Increasing the amps way up. In this case, 3ampsx10 panels=30+ amps or 3ampsx20panels=60+amps. @ 13.8 x 30amps= 414 watts or 828 watts. The neat part about my panels is that for some reason for which I can't explain is that my panels are putting out 16.5vdc @ 3 amps each giving me a total of 49.5 watts each panel.  

 In my case I have already produced 12 panels and am continuing to create more as we speak. I figure I have enough to make about 20 or so panels.

  Each panel produces You can also opt for hooking a large majority of panels in series to produce 128vdc @ 3 amps and then let the controller sort out the amps. It is more efficient and produces less loss. But the expense of the controller is the question.

 Hope some of this makes sense and clears it up.. I wish you good luck!

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Visit my web site - click on making solar.

http://www.internetfred.com

. Good Luck!

[ghurd]

What controller are you using now?

G-

[somemathguy]

A Morningstar SS-10 PWM solar controller. Can take up to 25V input.