Author Topic: Why 36 Cells per panel?  (Read 35025 times)

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Subman

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Why 36 Cells per panel?
« on: May 19, 2010, 03:11:32 PM »
Hi I am going to try my first DIY solar panel. I am having some confusion thou. This maybe a stupid question but it bugs me.  Why do most so called 12V solar panels have 36 cells. Each cell is .5V therefore producing 18V. A 12V battery only requires 13.8V to charge, so why 18V???

ghurd

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Re: Why 36 Cells per panel?
« Reply #1 on: May 19, 2010, 03:35:04 PM »
Most people expect a FLA battery needs 14.4V to get 'full'.
More than 15V to equalize.

Blocking diode uses up 0.7V.  (edited typo)
Wire losses use some volts, as do physical connections (like terminal connection on the battery termial, both for the wire and battery post).
Hot panels make less volts, relative to peak power.
Less than perfect sun (1000W/m^2) moves the peak power graph to the left (toward 0) and down.

And the peak power is usually closer to 0.47V per cell, which in itself is a whole volt.

Might Google 'self regulating solar panels' which usually used 30, maybe 32 or 33 cells.
The idea was if they could not make amps at enough voltage, then they did not need a charge controller.
Problem was they could barely charge a battery, making them about useless.

Use 36 cells.
G-
« Last Edit: May 21, 2010, 08:28:52 AM by ghurd »
www.ghurd.info<<<-----Information on my Controller

Rover

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Re: Why 36 Cells per panel?
« Reply #2 on: May 19, 2010, 03:44:04 PM »
Hi I am going to try my first DIY solar panel. I am having some confusion thou. This maybe a stupid question but it bugs me.  Why do most so called 12V solar panels have 36 cells. Each cell is .5V therefore producing 18V. A 12V battery only requires 13.8V to charge, so why 18V???

Better off having a controller regulate something to regulate , as opposed to having not enough to worry about regulating.
Rover
<Where did I bury that microcontroller?>

Subman

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Re: Why 36 Cells per panel?
« Reply #3 on: May 19, 2010, 05:44:39 PM »
Thank you so much. Finally a straight answer that makes sense.

DamonHD

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Re: Why 36 Cells per panel?
« Reply #4 on: May 19, 2010, 11:53:31 PM »
I suspect that many people wonder about this so for now I've made the thread sticky, but maybe we can edit a link to it into a library thread / FAQ at some point...

Rgds

Damon

SparWeb

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Re: Why 36 Cells per panel?
« Reply #5 on: May 20, 2010, 10:36:23 AM »
...maybe we can edit a link to it into a library thread / FAQ at some point...

-And perhaps include a simple graph of the current dropping off as the voltage gets higher than 16V or so, too.  Such graphs are included with most large commercial solar panels, now, to help NEC code installations and setting the MPPT, but the info is also good for answering this kind of basic question and explaining how they work.
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

Nothing40

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Re: Why 36 Cells per panel?
« Reply #6 on: May 20, 2010, 11:20:43 PM »
"Blocking diode uses up 0.7A."  -I think ghurd meant 0.7V    ;)

==
(Yes!  Thanks!
G-)
« Last Edit: May 21, 2010, 08:29:44 AM by ghurd »

DanG

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Re: Why 36 Cells per panel?
« Reply #7 on: May 21, 2010, 06:33:50 AM »
You can make identical sub-panels of less cells, say 6 to 10 cells each, and place them in series to build useful voltages - no law says you have to put 36 or 40 cells on one sheet of glass. I think I would rather repair or refurbish smaller panels, especially if it involves a complete do-over.

DanG

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Re: Why 36 Cells per panel?
« Reply #8 on: May 21, 2010, 05:44:18 PM »
And a quick note - custom voltage home made panels would only play nice with like panels unless specs were hashed out before hand. A six cell panel would make a nice heating element cludged in to the wrong circuit.

tecker

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Re: Why 36 Cells per panel?
« Reply #9 on: May 22, 2010, 04:26:04 AM »
 Most commercial panels pump out 19 to 21 volts to get the higher amperage . The panel itself has to conduct as well as produce power . So you have to think in terms of the panel has to conduct 12 and produce power over that to get the cells to exchange ions in the solution. The added  voltage (or pressure )get's translated into amperage to your work . One of the fun things to to with solar panels in  charging circuits is to add inductive loads in line . Put some 12 volt fans in series (with some by pass switches in there as well )on the charge line and watch the sun play with the circuit . You'll find the charging is not that much different on sunny days.
« Last Edit: May 22, 2010, 04:29:39 AM by tecker »

Ungrounded Lightning Rod

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Re: Why 36 Cells per panel?
« Reply #10 on: June 03, 2010, 03:44:14 PM »
In addition to the already mentioned issues (voltage drop from charge current in panel (and other) resistance, voltage reduction from hot panels, voltage drop in the diode), having extra cells doesn't hurt anything (except your pocket book B-)  ).

Solar cells are essentially constant-current devices (if they have enough voltage to push the current).  Each absorbed photon creates one electron-hole pair.  Up to a certain fraction of them can be separated by the internal doping, with the electron fed to the negative terminal and a hole filled by an electron from the positive terminal (releasing a little energy as heat).

Hook a panel to a load at a lower voltage than its potential output and the panel's output will drop until the current is the amount produced by the least illuminated cell.  The panel will heat up a bit more than if it were driving a higher voltage (because the electrons from the positive electrode have a bit more energy when they fall into the holes).  But it would heat up still more if it were just sitting in the sun with no load and all the electron-hole pairs were all recombining without taking a trip through the external circuit and releasing the full 0.6V electron-volt of bandgap difference energy.

Similarly it's OK to short a panel:  It heats up the same amount as if it were unhooked.  The heat is distributed a little differently (some in the wiring rather than all in the semiconductor).  But that's not an issue.  As a result of this, some solar panel controllers avoid overcharging the battery by shorting the panel(s) upstream of the blocking diode, in order to maximize power delivered to the load.  (A parallel, shorting, transistor wastes power when you don't want to use it because you have all you need.  A series, circuit-opening, transistor wastes power due to its voltage drop when you want to use as much as you can get.)

GbreadMan

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Re: Why 36 Cells per panel?
« Reply #11 on: June 03, 2010, 08:49:13 PM »
This brings up a fascinating question.  Since heat makes a solar panel less efficient (from my DIY panels up to 20% less efficient), would running copper pipe containing cold water through a solar panel help to quickly dissipate the heat?  You'd have a win-win with hot water and more efficient solar panels.

Would be quite the experiment...a hybrid plate collector / photovoltaic.  Epic mad scientist project.

Dan

DamonHD

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Re: Why 36 Cells per panel?
« Reply #12 on: June 04, 2010, 12:16:26 AM »
Such stuff already exists and is called PV/T, eg here: http://www.zen-international.com/technology/solar-thermal-energy/products/pvtwin/

Rgds

Damon


Ungrounded Lightning Rod

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Re: Why 36 Cells per panel?
« Reply #13 on: June 04, 2010, 02:56:15 PM »
This brings up a fascinating question.  Since heat makes a solar panel less efficient (from my DIY panels up to 20% less efficient), would running copper pipe containing cold water through a solar panel help to quickly dissipate the heat?  You'd have a win-win with hot water and more efficient solar panels.

Would be quite the experiment...a hybrid plate collector / photovoltaic.  Epic mad scientist project.

Dan


Thought about doing that.  Came to the conclusion that, at the temperature I'd want the water coming out for heating, the cells would be too hot for good efficiency.  So I'd have an expensive device that did two jobs poorly.

DamonHD

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Re: Why 36 Cells per panel?
« Reply #14 on: June 05, 2010, 01:27:37 AM »
The PVTwins used to come in two varieties: one designed to deliver cooler water (eg for underfloor heating or whatever) and run the PV element more efficiently, and the other designed to deliver sanitary hot water (eg for washing).

Rgds

Damon

tecker

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Re: Why 36 Cells per panel?
« Reply #15 on: June 06, 2010, 03:24:05 AM »
Ok I like this idea a lot . That leaves some Questions as to what the temp range would be .I really don't know  . However mid day heating accounts for up to 25% degradation in output so the temperature range should include sinking the mid day heat
. For tracking panels this might be 8 hrs or more for fixed panels I would say half . Adding more hardware to the tracking mech might be Too much fun .
This might work  For XY adaptation of fixed  Might and mabes to figure out anyhow.

Opera House

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Re: Why 36 Cells per panel?
« Reply #16 on: June 28, 2010, 12:09:06 PM »
Like it or not , many panels will eventually have bad cells.  Having those extra cells may get them to pass that 80% warranty point.   I have a pair of PHOTOWATT PW750 75 watt panels.  OC voltage is supposed to be 21.6 volts.  These two just about make it to 19V.    On a nearly charged battery they barely produce a couple hundred mills.  I now run them in series to get a couple amps out of them.  I'll be running them on their own power point controller to keep them useful.   Shame, these have only seen about a year of sun in the seven or so years I've had them. 

MagicValleyHPV

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Re: Why 36 Cells per panel?
« Reply #17 on: July 02, 2010, 10:31:55 AM »
This brings up a fascinating question.  Since heat makes a solar panel less efficient (from my DIY panels up to 20% less efficient), would running copper pipe containing cold water through a solar panel help to quickly dissipate the heat?  You'd have a win-win with hot water and more efficient solar panels.
These were purchased from a large experimental project in the early '80s, near the Solar One project just NE of Barstow, Calif. Five individual cells anchored to a stick of 1/2" copper tubing. The front of this aluminum 'wedge' shaped container is capped with plastic Fresnel lens - one lens for each cell. I don't remember what the cell specs are. I have about 75 or so of these littering my back yard. The problem of course, is that they need to track the sun to be effective.




solarpanelDIY

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Re: Why 36 Cells per panel?
« Reply #18 on: September 19, 2011, 08:17:12 PM »
this is an array.you have to stick with that.It was calculated by practical exprience.

Bruce S

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Re: Why 36 Cells per panel?
« Reply #19 on: September 20, 2011, 07:16:17 AM »
this is an array.you have to stick with that.It was calculated by practical exprience.
Last post was well over a year ago.
Might look at dates of the posts :-)
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Bruce S
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roosaw

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Re: Why 36 Cells per panel?
« Reply #20 on: February 29, 2012, 08:07:33 AM »
There is of course the boost converter.
With this device you could, in theory, wire all the cells in parallel (no bypass diodes) at 0.5ish volts and boost the voltage (and lower the amps of course) to whatever you wished to make it play well with other panels.
A boost converter is an inductor, a transistor (PWM driven duty cycle control of the transistor) and a diode (wiki has a good discussion).  It uses the panel to "charge" the inductor with current then opens the switch and the inductor discharges to the load circuit via magnetic induction.  By changing the duty cycle on the transistor you control the amount of voltage boost.
The same circuit can be run to control the the panel output voltage (MPP) and if yo put two back to back you can MPP the panel and boost the voltage to an acceptable level.
The advantage is that in the morning when either the panel is at a high angle or insolation is low the panel can still a) operate at its MPP and b) produce a voltage that can be used.
Course active tracking is still the best "watt improver" but that is not always an option.
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Ungrounded Lightning Rod

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Re: Why 36 Cells per panel?
« Reply #21 on: March 04, 2012, 08:46:38 PM »
Remember that the voltage from a single solar cell is about the same as a diode voltage drop - because it's essentially the same mechanism.  So the output of the first cell of your series string is essentially wasted pushing current through diode drops in your semiconductors.  Putting all the cells in parallel won't get you ANY output unless you're using all FETs and Schottky diodes or the like.

This is one of the reasons that higher voltages is desirable:  Lower percentages of loss in semiconductors in the associated electronics.  (Another is lower current in the wiring.)

rain1224

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Re: Why 36 Cells per panel?
« Reply #22 on: March 31, 2012, 06:57:17 PM »
I agree with DanG's answer.

Yyrkoon

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Re: Why 36 Cells per panel?
« Reply #23 on: August 20, 2012, 12:04:54 AM »
Here is another way to look at it. The higher voltage you have( and smaller current ), the smaller size of wire you can use to transport it to where you need the power converted. This means, you can use smaller wire to transport the same amount of power, and save money on copper.

Take for example one type of panel we have here. 30v @ 6A. If you drop that voltage down to say 15v, you have to double the current in order to maintain the same power. Which means you need more copper to keep loses at a minimum. On top of that ( someone correct me if I am wrong here). I believe that the more current your transport, the more line loses you will have anyhow. So on the other end of the coin. Some panels like the one I described above can have a high series rating. The ones I described above can be run in series on up to 300V. Smaller wire yet, for delivering the same amount of power.

There can be downsides to this approach however. Such as regulating voltage down to working nominal voltage for your given system. Then when converting DC to DC you will have some efficiency loses as well.

Anyhow, there may not be such a thing as a free lunch. But sometimes, you can get a discount. All a matter of perspective.

Xan

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Re: Why 36 Cells per panel?
« Reply #24 on: January 23, 2015, 02:49:28 PM »
I think putting all cells in parallel would not be able to run a boost converter as you would not have enough voltage to overcome the characteristic voltages in the semiconductors. And it would also require pretty heavy wiring as you would be mainly getting your watts mainly from current, if it could work at all.

However, I do agree boost converters can be useful. I have half a folding panel set that has 24 cells and they are wired series/parallel for a 6 volt nominal output. Pretty much useless for anything, really don't know why it was made that way!? So, I made up a boost converter circuit and that works really well & means I can now feed the output of that panel into my 12 volt system and get some useful charging amps.  :D

Bruce S

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Re: Why 36 Cells per panel?
« Reply #25 on: January 26, 2015, 07:31:14 AM »
Actually , those 6V folding panels are very useful to me.
I use one most everyday to recharge my cell phones. The output is perfect for a USB to micro-usb cable to charge put my personal and work phones.
Once they are charged, I then plug in a freebie-stick ( 2500ma LiFe) that is used for overcast days.
I have not had to use the walwart since acquiring this little gem  ;D.
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Bruce S   
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