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Old 11-01-2013, 06:48   #1
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Solar Tropics Some Info & A Question

Hi Folks

First sailing vessel for me and we're putting new solar panel(s). I've been doing my due diligence and have read all (a bunch) previous posts and have spent hours on the internet and some reading from a book a good friend gave me. Our boat is currently in the Philippines and will be home ported in Palau 7.5 degrees north.

Here's what I've gathered and want to share this one point I haven't seen in other posts:
There is a voltage/efficiency drop with ambient temperature. Panels come in configurations of different cells per panel like 30, 33 and 36 cells/panel. From: BOAT OWNER'S MECHANICAL & ELECTRICAL MANUAL, 2ND EDITION a 30 cell/panel will be inefficient due to high ambient temp inefficiency and voltage drop near or below what the battery bank requires for charging and a 36 cell/panel will perform well in high ambient temps and maintain a good charging voltage.
Conclusion: Tropics = 36 cell/panel

Question: Mono vs Poly Crystalline cells: I've read contrary info on the internet one says mono is better with high ambient temp the other says poly is better. Any info?



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Old 11-01-2013, 08:02   #2
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Re: Solar Tropics Some Info & A Question

Mono is better because its the whole crystal. Polly is broken up crystal fragments.

My pannels have 54 cells per panel so dunno what that 30/36 is referring to?.

In Palau you will have abundant solar power so to worry about differences in panel temp is not going to affect the outcome in any significant way.

My advice is to get Mono panels as many and as large as you can afford.
I have 2x 120 watts and that's good, but I could always do with another!

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Old 11-01-2013, 08:33   #3
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Re: Solar Tropics Some Info & A Question

Nominal 12v panels are nearly always 36cells. There are a spattering of "self regulating" 32 cell panels, but they are very rare these days.
If you go for a MPPT regulator more cells are ok.
There is very little difference between mono and poly cells, although the highest efficiency (smallest size for the watts) panels are mono. The temperature stats are published by all the reputable manufacturers, but they do list their specifications in different ways which means you need to do a bit of maths.

You won't find a big variation, its more important to get panels that give you the most watts for your space.
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Old 11-01-2013, 08:37   #4
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Re: Solar Tropics Some Info & A Question

I wouldn't get hung up on the mono/poly question too much, instead I'd look at the specs for specific panels you are considering. The two key factors you are looking at are NOCT (normal operating cell temperature) and temperature coefficient of Pmax.

NOCT tells you how much a manufacturer expects their panel to heat up under operating conditions (that is, when the sun is 800 W/m2 and the air temperature is 20C). For most panels this will be around 45C. If you are comparing two panels, particularly for the tropics, generally choose the lower NOCT, as the manufacturer (after testing) expects that panel to retain less heat.

Then you can also look at T-Pmax. This should be on any data sheet. Around -0.4%/degK is typical. That means for every degree (C) above 25C you will lose 0.4% power output (note that power coefficients are generally based on STC conditions of 25C, not NOCT conditions of 20C). If your panel temperature is 45C then you're 20 degrees hotter than the rated temperature, and will lose 20 * 0.4 = 8% output from your panel.

Lastly, on the cell count, if you are charging with a PWM regulator then you need to stick to panels that will operate at the maximum battery charging voltage when they are hot. You can get the temperature coefficient for voltage on your panels as well, and calculate if a 30 or 36-cell panel is needed. If you use an MPPT controller that is capable of voltage conversion then you can go with larger 54, 60, or 72-cell modules and not have to worry about that aspect.

Here is a table from poly cells:

Name:   poly.PNG
Views: 148
Size:  10.9 KB

Here is one from the same manufacturer, same power, mono cells:

Name:   mono.PNG
Views: 153
Size:  10.9 KB

They both have a NOCT of 45C (indicating they will typically operate about 25C over ambient temperature when in full sun). For this manufacturer the poly cell loses 0.43% per degree in power output, while the mono cell loses 0.45% per degree. Therefore, at higher temperatures the poly will provide slightly more power. Other cells/manufacturers will have slightly different coefficients. The best cells/modules for hot conditions have the lowest NOCT and the lowest Pmax temperature coefficient.
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Old 12-01-2013, 03:42   #5
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Re: Solar Tropics Some Info & A Question

Many thanks for all the great info. I'm learning.
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Old 12-01-2013, 04:43   #6
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Re: Solar Tropics Some Info & A Question

To me, with modern panels I think there are really only a couple of things to consider.

1 - cost/watt
2 - ease of installation

And I feel #2 outweighs #1 unless there is a huge difference. The cost per watt part will always result in larger higher voltage panels that will almost force a MPPT controller. But the savings normally pays for the better controller.

I feel spending a lot of time on efficiencies etc. etc. is just leading you down so many "it depends" holes that you will go crazy!

But I ain't no expert!

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