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Old 07-02-2018, 05:57   #1
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Solar - more cells = better?

Will the number of cells in similar sized solar panels make any difference?
For use in the tropics. 100w at 32 vs 36 cell....then I also found a 75w with 44 cells.

About the film material on flexible ones. Some uses ETFE. Is that a better one?
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Old 08-02-2018, 06:08   #2
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Re: Solar - more cells = better?

...if difference...significant or just a couple of percentage?
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Old 08-02-2018, 07:56   #3
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Re: Solar - more cells = better?

Watts is Watts, fewer cells means that the cells being used are more efficient.

I will say that a cheap panel with a PET topsheet won't last in the tropics. First they cloud over, then the PET becomes brittle and fractures. This isn't fracture from movement, rather from shrinkage.
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Old 08-02-2018, 08:06   #4
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Re: Solar - more cells = better?

More cells = higher Voltage. For the same Size it also less current (Amps). Same area produce usually the same power, there are more efficient and less efficient modules, but as a rule of thumb the Wattage is the same for the same area.

Is it good or bad depends on the needs and on the solar regulator / charger.

If you want to use a cheep PWM controller, you want the Voltage at maximum power point Ump around 15-16V, so 32 cell modules are optimal for 12V with PWM, the modules deliver almost Imp as charging current. The downside is, this current requires some cable thickness. The controller just switches the solar module through to the battery / switches off if the battery voltage goes to high. If you use a panel wit more cells, the voltage will be higher, but also the current is limited to maximum Isc (shortcut) and the module would be less efficient with a PWM controller, because there is no transformation to a lower voltage, its like a shortcut for the excess Volts..

If you use a MPPT controller, the 32 cell modules are "useles" when used in parallel. The controller works best if it can transform high Voltage, low current into low Voltage (14.4V) and high current. So modules with higher Voltage (more cells) are better. e.g. a 330Wp module with 96 cells 1m x 1.5m in size delivers 60V Ump and almost 5A. This is a good thing, you can use smaller cables to the controller, and after the controller your current will be 23A / 14V. You can add more modules in parallel, so with 5 of them you would reach 100A charging current, but only 30A solar current over the cable to the controller. Same module with a PWM would deliver only 6A (84 W), you would loose 246W, because of the wrong controller.

So there are different modules for a good reason. There are optimal sizes for different controller types.
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Old 08-02-2018, 10:13   #5
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Re: Solar - more cells = better?

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Originally Posted by CatNewBee View Post
More cells = higher Voltage. For the same Size it also less current (Amps). Same area produce usually the same power, there are more efficient and less efficient modules, but as a rule of thumb the Wattage is the same for the same area.

Is it good or bad depends on the needs and on the solar regulator / charger.

If you want to use a cheep PWM controller, you want the Voltage at maximum power point Ump around 15-16V, so 32 cell modules are optimal for 12V with PWM, the modules deliver almost Imp as charging current. The downside is, this current requires some cable thickness. The controller just switches the solar module through to the battery / switches off if the battery voltage goes to high. If you use a panel wit more cells, the voltage will be higher, but also the current is limited to maximum Isc (shortcut) and the module would be less efficient with a PWM controller, because there is no transformation to a lower voltage, its like a shortcut for the excess Volts..

If you use a MPPT controller, the 32 cell modules are "useles" when used in parallel. The controller works best if it can transform high Voltage, low current into low Voltage (14.4V) and high current. So modules with higher Voltage (more cells) are better. e.g. a 330Wp module with 96 cells 1m x 1.5m in size delivers 60V Ump and almost 5A. This is a good thing, you can use smaller cables to the controller, and after the controller your current will be 23A / 14V. You can add more modules in parallel, so with 5 of them you would reach 100A charging current, but only 30A solar current over the cable to the controller. Same module with a PWM would deliver only 6A (84 W), you would loose 246W, because of the wrong controller.

So there are different modules for a good reason. There are optimal sizes for different controller types.
CatNewBee, help me out here. What's an 'Imp' and what's a 'Ump'?
I couldn't make sense out of the numbers you used when trying to show MPPT controllers are better than PWM controllers.
Would you give it another try?
Thanks
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Old 08-02-2018, 10:32   #6
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Re: Solar - more cells = better?

Ump is not a thing

Voc is maximum voltage

Isc is maximum current

Vmp/Imp are the values that give Pmax-power:

Pmp = Vmp * Imp

MPPT can more efficiently find the "peak output" from the solar panel if there is a wider range of voltage to work with, i.e. a higher Voc rating.

For PWM higher voltages mean wasted power.
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Old 08-02-2018, 11:31   #7
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Re: Solar - more cells = better?

I would recommend 36 cells as normally the optimum for a 12v battery system. 32 cells were popular in the early days of solar, when good regulators were hard to find. The 32 cell panels or "self regulating panels" could supposedly be used without a regulator, but they were not very successful, and 36 celłs has proven to be better for a 12v system.

There are some exceptions and limitations:

The first, is that the best, high efficiency, panels are not generally available in a 36 cell configuration. The 12v market is so small that most solar panel manufacturers only produce high efficiency solar panels in a configuration that is optimal for grid tie or home use. It is difficult to find the best high efficiency solar panels in anything other than 60+ cells at least if you are looking at rigid panels. There are a few of manufactures that produce high efficiency flexible solar panels with a lower cell count, perhaps because flexible panels are not commonly used on buildings.

The second exception is that some MPPT solar controllers require more than 36 cell panels (for a 12v battery bank). This only applies to very small number of controllers. Unfortunately, included in this category is the very popular Victron range. If you do want to use 36 cell panels with these controllers it is probably optimum to wire a couple of 36 cell panels in series, but this is less than ideal.

So given a choice of otherwise equal panels (this is unlikely), 36 cells are best for a 12v system unless you have one of the Victron controllers when around 40+ cells is optimal.

However, cell count (providing it enough) is not usually the dominant factor effecting purchase. Hence, I have just installed some 96 cell rigid panels (on a 24v system). These panels do not have the optimum cell count, but other factors such as the high efficiency are more important.
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Old 08-02-2018, 13:25   #8
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Re: Solar - more cells = better?

Quote:
Originally Posted by guyrj33 View Post
CatNewBee, help me out here. What's an 'Imp' and what's a 'Ump'?
I couldn't make sense out of the numbers you used when trying to show MPPT controllers are better than PWM controllers.
Would you give it another try?
Thanks
I stands for current,
U for voltage and
P for Power.

*mp stands for the maximum power, so Ump is the Voltage at max. power and Imp is current at maximum power

*sc stands for short circuit, makes only sense for I, Isc is the current, that the module produces if you short + and - of the connector. The voltage then is obviously U=0

*oc stands for open circuit, makes only sense for the Voltage, so Voc is the Voltage at the wires when they are not connected and no current is flowing, obviously I=0.

Imp is the current at maximum power point along the U/I curve of a panel.

This are the most important parameters of a solar panel. Uoc gives you an idea, what maximum input Voltage your regulator should be able to handle, Ump and Imp allows you to dimension your wires and calculate Pmax of a panel. Imp is also important for the selection of your controller (maximum Input current) According to your wiring (parallel panels / serial panels, strings of serial connected panels etc.) you can calculate the resulting array voltage and current.

There are some more parameters, like module efficiency and temperature coefficient, isolation resistance, maximum string voltage, maximum string current. This are important if you consider to create a larger array of serial or parallel modules.

Some use V instead of U. in their data sheets. Ohms law: R = U / I where R stands for Resistance, U for Voltage, I for Current. I guess, it depends on the country, what symbols are used.
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Old 08-02-2018, 13:48   #9
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Re: Solar - more cells = better?

I have two 96 cell panels and love them. Made a Bimini cover over the cockpit with them and it works great. Of course a MPPT is essential for that much power.
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Old 08-02-2018, 15:07   #10
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Re: Solar - more cells = better?

I am planning to make a aft roof with 5 of them (BenQ Solar Sunforte PM096B00 330Wp) with a Victron smart solar MPPT 150/100...

Guess I could charge with up to 100A
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Old 08-02-2018, 15:14   #11
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Re: Solar - more cells = better?

Isn’t a bigger factor for efficiency shading? I thought there were newer panels that did a better job of not losing power when a portion was shaded, no?
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Old 08-02-2018, 15:19   #12
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Re: Solar - more cells = better?

Most modules have bypass diodes for partial shading, in full shade no module will work either. I connect all modules in parallel with string-diodes. So I get most power out of them.
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Old 08-02-2018, 15:37   #13
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Re: Solar - more cells = better?

For a simple, small system with PWM controller you want 36 cell panels. The reason is that you want to be operating well below the maximum power point (MPP), after consideration of voltage drops in the wiring, diodes, and transistors (inside the regulator), usually a volt or two (this should be calculated for each installation). A solar panel is nearly a constant current source, independent of voltage, up to a few volts below the MPP where it starts to drop off - so it is best to avoid the knee in the curve around the MPP. The 32 cell panels have an MPP at a lower voltage, so when delivering power to a nearly charged battery the current falls off - hence "self-regulating" (which is not enough to safely regulate but does slow charging).

For smaller systems - say less than 200W (YMMV) - MPPT controllers don't usually make sense as the extra cost could be better used to simply buy another, or larger, panel. With larger installations the panels can be arranged serially to provide a higher voltage, and thus enable the use of smaller cables and make good use of an MPPT controller.

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Old 08-02-2018, 15:39   #14
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Re: Solar - more cells = better?

Quote:
Originally Posted by john61ct View Post
Ump is not a thing

Voc is maximum voltage

Isc is maximum current

Vmp/Imp are the values that give Pmax-power:
Depends where you went to school.

Both U and V are used as the symbol for "electrical potential energy" aka voltage. So both Vmp and Ump are equally acceptable.

William Rankine used U in his seminal 1853 paper where he introduced the term "potential energy".
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Old 08-02-2018, 15:51   #15
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Re: Solar - more cells = better?

I have 60 cell 345 watt LG grid tie pannels each controlled by 100/30 mppt victrons . I did most of the wiring myself . DO NOT under estimate the advantage of using the 10 guage wire with the factory connectors . I got 4 through deck gland ,into the rear beam and into the engine room of our Outremer in less tha an hour . Lareger cables would be a nightmare and way more expensive cables off ebay ,just cut off the end you dont need . If you can one controller per pannel
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