More cels question

System · March 2011

I am assembling my first panel from 36-3x6 cells. Since I'm in the cloudy Northwest, is there any advantage to adding MORE than 36 since I probably won't get full voltage most of the time? What are pros and cons?

Cariboocoot · March 2011

Re: More cels question

If you build a panel whose Vmp is outside of the "typical" levels it will not work well with the less expensive PWM type controller. Normally a "12 Volt" solar panel has a Vmp of 17.5 Volts, which works out right for charging 12 Volt batteries. A "24 Volt" panel is 2X that. Some panels, however, have a Vmp of around 26 Volts which is too high for 12 Volt systems (loss of potential power) and too low to charge 24 Volt systems. Such "outside the typical" panels should be used with MPPT type controllers to achieve maximum power potential. You can't count on cloudy days always, so any time you do get full sun you'd be wasting potential power. This is one of the advantages of the MPPT system: you can put together an array with higher Voltage and it will still output above system charge levels even in lower light conditions, without any power loss in full sun.

So you sort of need to plan the whole system out first.

BTW, homemade solar panels have limitations. Don't count on them for a 25 year run.

System2 · March 2011

Re: More cels question

What is it that limits DIY longevity? Any data on how long the do last?

Cariboocoot · March 2011

Re: More cels question

CVINTON wrote: »

What is it that limits DIY longevity? Any data on how long the do last?

It would be impossible to say because of the number of factors involved. This includes not only how well they're made and of what, but also exactly what kind of environment they're subjected to. A well made panel in a cool, dry environment will last longest. But panels are usually out there in heat and rain, both of which cause damage. Heat because of the thermal cycling of the materials - literally tears things apart from expansion & contraction. And rain, or other ambient moisture, always manages to seep inside and fog glass, corrode connections, et cetera.

The must luck people have had with homemade panels have been with those used on an "as needed" basis. Not out in the weather all the time; just supplying some recharging for the occasional camping trip.

You may find building them to be quite a challenge. The cells you can buy are typically "seconds" and often defective in some way. They are easily broken when handled. Soldering connections is difficult without a temp controlled iron. You can't readily get the same glass used by commercial manufacturers which has the right properties. And sealing them up against the elements is probably the biggest challenge.

On the whole, a learning experience but not really a good choice if you are going to be dependent on their output for your power. Also, they are not legal to be used in a grid-tie application. Even off grid insurance companies will not appreciate them; any fire may be blamed on non-UL listed panels and then they don't pay the claim.

Just some things to think about.

BB. · March 2011

Re: More cels question

As Marc typed, it is hard to say how long the panels will last... If you build them out of plastic and plywood--probably on the order of a couple months.

If you are building them on glass with EVA sheets, vacuum forming, baking in oven, who knows--perhaps a few years?

Here is a nice thread with several links on how to build/assemble your own solar PV panels:

mike90045 wrote: »

panel building:
http://fieldlines.com/board/index.php/topic=145005.0

In case the above link "breaks":

Oztules' series:

http://fieldlines.com/board/index.php/topic,144982.0.html
http://fieldlines.com/board/index.php/topic,144995.0.html
http://fieldlines.com/board/index.php/topic,145004.0.html

Still don't recommend people build their own to save money--But some good information for those that want to try.

My factory panels had failed ~5 years (overheated junction box?)--It took a while, but my supplier did eventually replace all 20 panels with new (and hopefully improved) production panels.

Solar panels are exposed to full sun, extreme weather, and temperature extremes--They may look easy to build, but it is not easy to build something that will hold up to full environmental assault for 20-40+ years.

-Bill

mike95490 · March 2011

Re: More cels question

Panels produce their full voltage, with just a little light. The brighter the light, the more AMPS they put out, so putting more cells in series, for higher voltage, won't accomplish much, you need brighter light (mirrors, reflection off water or snow) to get more usable amps.

RCinFLA · March 2011

Re: More cels question

CVINTON wrote: »

I am assembling my first panel from 36-3x6 cells. Since I'm in the cloudy Northwest, is there any advantage to adding MORE than 36 since I probably won't get full voltage most of the time? What are pros and cons?

PV cells are illumination based current source, not a voltage source. The maximum voltage of a cell is clamped to the equivalent of a forward biased silicon diode. This inherent diode has approximately a -2 mV per deg C temperature coefficient just like a regular silicon diode. Voc (V open circuit), is where all the illumination generated current is shunted down the cell's inherent diode and is about 0.65 vdc at full sun and 25 deg C.

Good quality cells produce about 35 mA per square centimeter in full sun (1000 W/m^2).

The maximum power point is where the load is adjusted to a point where just a small amount of the illumination generated current is wasted down the cell's inherent diode, delivering most of the current to the output load. This is at about 0.52 vdc per cell at 25 degs C. It doesn't matter if the cells are small or large sized as far as the voltage is concerned, only that a larger cell generates more current due to its larger surface area.

The reason for 36 cells in a panel meant for a 12 v lead acid battery is to allow for summer high temp of panel (about 0.44 vdc per cell at 60 deg C) and still accomplish an absorb phase charge voltage of 14.5 vdc with some charge controller and wire run voltage loss.

Cell illumination generated current is pretty flat from just below the MPP voltage down to short circuit load on cell.

Adding more series cells with PWM (on-off pulsing) controller will gain nothing. You can put 50 cells in series but you will get the same charging current into the battery. An MPPT controller will convert the greater cell stack voltage to the battery voltage and yield a higher battery charge current. The extra cost of an MPPT charger controller is usually not justified unless the PV power level is 1000 watts or greater. The maximum power point voltage for an overcast day is at almost the same panel voltage as a sunny day. There are actually two counteracting effects. To lose no more then 3% of illumination generated current down the inherent diode you operate at a slightly lower diode conduction point (voltage) but due to the overcast illumination the cells run cooler so the diode clamping conduction point will be at a slightly higher voltage. You just get less current output for the similar panel voltage.

Most homebrew panel 'death' is caused by humidity electro-migration and corrosion. You must get all the air out, either by vacuum sealing or total encapsulation in resin.

More cels question

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