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Old 08-11-2017, 12:47 AM   #1
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Default Grid tie solar panel with PWM controller

Grid tie solar panel with PWM controller

I have the mounting feet, 10ga wire, lap sealant and a 30A PWM controller. It was all purchased/acquired some time ago and I figured I might as well use it.

This is a low budget fun project, limited to purchasing a panel. I do not want to buy a new MPPT controller now as that would increase the project cost by 40% even with a cheapo MPPT controller.

I have two panels to choose from. Both are new but the larger one is on clearance. They are now the same price.

Panel #1 is 150W monocrystalline and is what you'd call a 12V panel, 36 cells, with 8A output.

Panel #2 is 250w polycrystalline and is what you'd call a grid tie panel, 60 cells, also with 8A output.

The PWM controller I have can accommodate either. I realize the 8A output is as good as I should expect from either panel with that PWM controller as it basically just cuts the voltage down to a usable level. I know that it can't realize the full potential of either panel.

Here's the question:

Will the 60 cell panel end up far outperforming the 36 cell panel in low light conditions?


My thinking is that a drop from .5v to .22v per cell would still provide minimal charging from the 60 cell panel. The 36 cell panel could only drop from .5v to .36v per cell to produce the same result.

I realize that a grid tie solar panel with PWM controller is an odd and inefficient combo but I think the grid tie panel might still produce results in up to 20% less light.

All comments welcomed.
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Old 08-11-2017, 08:04 AM   #2
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Interesting question. A few random thoughts:

1. voltage comes up pretty quick even under crummy conditions (something like 20% insolation). Below that point I suppose the higher voltage panel would have an advantage.

2. if space is not an issue I'd get the higher voltage panel in case you do get MPPT later.

3. higher voltage panels like residential 60 or commercial 72 were not designed for mobile applications and prefer more support. A 36 can generally be supported by the ends but bigger panels like another support in the middle, for example.
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Old 08-11-2017, 08:49 AM   #3
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Most likely you will be getting similar performance with both panels using PWM controller. I agree that in low light the larger panel could perform a little better but it would depend on both panels output characteristics.

Even thou large panels are gaining popularity in DIY Van installs, I am still questioning viability of using commercial panels in mobile applications as stated in the previous post. Internet is full of opinions, including mine, which should be questions, but a good source of reasonably good info could come from folks installing panels on RV for living such as the AM Solar Co. in Springfield Oregon - Mounting solar panels on RV roof - mounting solar panels on RV - solar panel mounts for RV . Personally, I wouldn’t go with any larger panels than they use.

Another point often missed using large panels are cooling capability in mobile applications. Panels tend to be flat and larger panels will likely have less convection cooling, the distance from the panel center to the edge is longer.
The convection cooling will also vary with panel to roof air gap, small gap will decrease cooling efficiency especially on large panels. Efficiency can vary as much as 10-25% with temperature. https://www.civicsolar.com/support/i...cts-efficiency

For house roof, it is a mood point because panels are usually mounted on sloped roofs allowing for good air flow.
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Old 08-11-2017, 10:32 AM   #4
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Great points fratermus & GeorgeRa, thanks for the help.
Summary:

60 cell panel
Pros:
- could realize more output if later upgrading to MPPT
- maybe a bit better low light performance
Cons:
- some additional mounting cost for both support and cooling
- not intended for mobile use or off grid
- MPPT controller required if expanding array

36 cell panel
Pros:
- intended for mobile use & off grid
- no additional mounting hardware needed
- easy to acquire additional panels with same specs
- controller could handle 2nd panel added later

Cons:
- smaller output gain if upgrading to MPPT later

It would cost less to upgrade to a MPPT controller than to buy and install a second panel. I could reuse my old design for a tilt-able mount - http://www.classbforum.com/forums/f8....html#post2419 - and that would be a low cost solution for added support, cooling and output.
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Old 08-11-2017, 10:41 AM   #5
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I agree that cooling is important in hot areas. I disagree that convection cooling is predominantly a function of air coming up from the sides of the panel (natural convection). Forced convection, as when a breeze blows across the panel, displaces the heated boundry layer facilitating (I suspect) a much more rapid heat loss.

When retrofiitting commercial Class B RVs the smaller panels will probably be more practical because there is less contiguous roof space to use.

Mine is a DIY cargo van conversion so it has a massive flat area for panels. When I chose panels I picked 3x 72cell 190w panels for my build. Because they are mounted on a Vantech H3 rack there is with several inches of airspace underneath. I've never had any other panels mounted up there so I cannot provide comparison data.
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Old 08-11-2017, 01:27 PM   #6
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The 60 cell panel may have zones isolated by blocking diodes, and if it does, it will be better in partial shade.

The cooling thing will be argued forever, and without a complete airflow drawing and simulation problem guess is really tough. Event the little lip around the edge changes everything in airflow, and most have open bottoms. I wouldn't discount the underside airflow though, because if it is high enough you will get a convective current coming up the center and out the perimeter, and those can be quite effective.
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