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Solar with a LOT of Shade and PWM vs MPPT
#1
Hey Guys!

I found a good deal on solar panels, which made me think about my future solar setup when my powerwall is done. This is going to be a longer post, I'm happy about anyone who cares to read it and gives me some input. sorry in advance for my medium english.

First, the Deal:
I could get 5 x used 240Wp Mono Panels for about 500 - 600 euro. So 1200Wp, I dont know how old they are but my guess is max. 10 years, closer to 5

My current setup:
12v setup, 2x 50wp poly and 2x 100wp mono panels with 2x 12v lead acid batteries(which are pretty much done)



My future setup:
7s120p used 18650s, so I'll "upgrade" to 24v

Problems at the location:
Its a completely off-grid "shed", I only have a small generator there for those times when you need a bit more juice...
The shed and the whole location is "in the middle" of the woods, no elevation and no place without shadow. I'm not allowed to build anything there because its a "water reserve" location.
The place with the least amount of shade through the day is actually the roof of the shed, but there are trees all around, some of them I cannot cut down because I don't own them, and some of them which I will not because they are really old. I get about 2 - 3 hours of full sunlight on my panels atm, the rest of the day at least  1 - 2 panels are in partial or full shade.
Fun fact: I earn about the same solar power in march and april as in the summer months, just because the leaves are not fully grown at this time.

Because of the shading problem I go with a parallel setup, because in my opinion it makes no sense upping the voltage when most of the time at least one panel is shaded.

So the Problem is this:
Data of the new panels:
Pmax: 240W
Optimum operating voltage Vmp: 32.2V
Optimum operating current Imp: 7.95A
Open Curcuit Voltage Voc: 37.3V

With 7s120p I want to stay in the 4.05 - 3.5V range, so maximum Voltage is 28,35V
The new panels need to be all parallel, because I have no other choice.
Because of the bigger space the panels use, I can almost definitely say that where will be almost zero time/day where all panels are in direct sunlight.

If you are still with me at this point, yay! :-)

Is the 30.2V optimum voltage the voltage at pmax, or can it still drop? I'm a bit worried I'm not able to reach the 28.35V. Cables will be about 15m(thick solar cable, don't know the dimension)
The theoretical max will be about 40A, the real usage max around 30A with all panels parallel. Due to shading, it will be 5A to 20A most of the time.

And I guess with a voltage range of 30.2v to 28.35v a MPPT charger doesn't really make sense?(will go for a MPP Solar charger/inverter, either PWM or MPPT). Most MPPT have a minimum voltage of 30V

Should I take the deal or wait for another one with panels in the 40V to 50V range and use MPPT? I'm just not sure if it's worth it...
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#2
VMPP is Voltage at Maximum Power Point, so yes at Pmax. But the VMPP can sink with the lack of sunlight.

About the MPPT, I don't know how many options for VMPP=30V are available out there, but I think for your situation MPPT would be better because of the efficiency.

Hope this helps...
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#3
Regardless of the conditions, MPPT will always outperform PWM. It's just the way MPPT works over PWM. There's a reason why it's called Maximum Power Point Tracking. PWM will just take the highest voltage it can take and pulse modulate it to the cells/batteries.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

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#4
thx for your opinion guys, I will buy the mppt controller then and see how it does with the parallel arrays...
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#5
Can you post the link to the mppt you plan on getting? There are many claimed mppt, but in reality end up being pwm
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
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#6
Look at the Victron units.... eg these
https://www.victronenergy.com/solar-charge-controllers
Sean likes this post
Running off solar, DIY & electronics fan :-)
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#7
the victron units are waay out of my budget, I'll get a PIP3024 like this here, model either MSE, MSXE or the new GK one
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#8
(02-19-2019, 09:12 AM)runsnbunsn Wrote: the victron units are waay out of my budget 

Granted, they aren't cheap, but when you go blue you'll never go back.

Also, I dont understand why you appear reluctant to invest in a quality inverter/charger but you appear to be considering spending 600 euros on not many, 10 year old, second hand PV panels.

Buy cheap, buy twice.
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#9
If I buy a victron the whole system costs easily 2 - 3k more...
Then the "cheap" solar panels wouldnt make much sense with such a good inverter...
This amount of money just doesnt make sense to me on a system that is mostly in shade and it mainly only used on the weekends, for non crucial things like coffee machine, fridge oder water pump(hand pump atm)

If I would setup my living house with solar... that would be a different story
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#10
(02-19-2019, 11:20 AM)runsnbunsn Wrote:  ...... a system that is mostly in shade and it mainly only used on the weekends, for non crucial things like coffee machine, fridge oder water pump(hand pump atm)

Given the shade, and the infrequent use, would a small genny/battery not be more effective/productive/cost effective ?

How about calculating how much it will cost you to put a kWh into your battery using PV (remembering to include any new purchase costs) and compare that to the cost of putting a kWh into your battery using your existing generator.
Korishan likes this post
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