Solar with a LOT of Shade and PWM vs MPPT

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runsnbunsn

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Aug 29, 2018
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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 voltageVmp: 32.2V
Optimum operating current Imp: 7.95A
Open Curcuit VoltageVoc: 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...
 
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...
 
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.
 
thx for your opinion guys, I will buy the mppt controller then and see how it does with the parallel arrays...
 
Can you post the link to the mppt you plan on getting? There are many claimed mppt, but in reality end up being pwm
 
runsnbunsn said:
the victron units are waay out of my budget
Click to expand...

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 spending600 euros on not many, 10 year old, second hand PV panels.

Buy cheap, buy twice.
 
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
 
runsnbunsn said:
......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)
Click to expand...

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 akWhinto your battery using your existing generator.
 
runsnbunsn said:
water pump(hand pump atm)
Click to expand...

This sounds like you are either not using the electric one, or haven't bought one yet. If you haven't gotten an electric pump yet, I would suggest going with a 48VDC pump. They cost close to the same as a standard AC pump (unless the prices have drastically changed over the past couple years) and would allow you to go with a smaller inverter. You could also go with a DC powered coffee machine (unless you are going with those expensive latte, cappuccino, etc type machines) to save there as well. There are DC powered fridges, but also propane ones, too. The coffee machine and fridge can be had from RV's.
The propane fridge actually lasts along time because it doesn't take much heat to keep the system running.
 
I'm using a MPP Solar PIP3024MSE and like it. It's not a clone and the build quality is fine, definitely not cheap. Yesterday I got as much as 1.7 kWh from my 3 partly shadowed 200kWp panels wired in parallel, so I would say you will have a good setup. If AC output is on, minimum consumption is about 30-50 W. Therefore you will need to switch the output off when being away at least in winter to not deplete the batteries. It will charge the batteries as long as there is some light on the panels even when switched off.
 
Yes the generator would be theoretically the most cost effictive, but I like the luxury of having cold beer when I arrive there and -officially- I dont have a generator there ;)


About the waterpump, yes I was thinking about DC 24 or 48V but I have around 50-60m cable to the well. Pump near the shed not really possible because of freezing and long suction times. Pump must go into the well and stay there all year long.Would be solveable with really thick cables, or a second small buthigh discharge battery near the well. Have a 1500W/220V well pump lying around which will go first I think(these things dont work a long time) but I plan on digging in the thickest cables I can get for a reasonable price.

We also have two fridges there, one 12/24V very effective compressor fridge, and a standard 220V low energy one.
The effective one uses about 300Wh/day which is really good, the AC one 370Wh according to manual(+inverter losses which makes it around 500 - 600Wh I guess)
On the other hand the AC one has almost double the volume of the compressor one.


I think if the solar is big enough to cover the inverter standby usage(on rainy days), its easier to run a few things on 220V, Lights and smaller things excluded.

stevelectric said:
I'm using a MPP Solar PIP3024MSE and like it. It's not a clone and the build quality is fine, definitely not cheap. Yesterday I got as much as 1.7 kWh from my 3 partly shadowed 200kWp panels wired in parallel, so I would say you will have a good setup. If AC output is on, minimum consumption is about 30-50 W. Therefore you will need to switch the output off when being away at least in winter to not deplete the batteries. It will charge the batteries as long as there is some light on the panels even when switched off.
Click to expand...

Yeah, that standby usage is a LOT! 1.2kWh/day in worst case.
In reality more like 600Wh I guess because with daylight you almost always get more than 50W with 1.2kWp panels.
In winter AC will be turned off probably, because theres no way of getting that usage in with solar, but thats ok for me.
I wish there would be an option to turn off AC automatically for example from 01:00 to 04:00 at night.
 
Looks like you have several bases covered already, great!

One thing you could do about the DC well pump is to either have a soft start installed, or you could use some super caps right at the pump to help with the surge. That would allow you to go down in 1 or 2 sizes on the cable to compensate for voltage drops.
The other thing could be to convert the DC to pulsed DC and transform it up to a higher voltage for that transmission, then drop it back down on the other end. Basically you'd boost the voltage up, then buck it back down. Altho, I don't know what the requirements would be of such a heavy amp usage device.
I'm sure there are other ideas as well.
 
runsnbunsn said:
Yes the generator would be theoretically the most cost effictive, but I like the luxury of having cold beer when I arrive there and -officially- I dont have a generator there ;)
Click to expand...

I'm just try to encourage you to think outside of the box a little,as they say on trendy management courses.

A 12v travel fridge would allow you to have a cold beer half way to your destination, and when you get there, and as many as you want on the way home. Asplit charge system in your automobile, along with a secondary battery would negate the need for your existing DC fridge, and provides the means of charging additional batteries if required.

There's always different ways to get to where you want to be, it's not always wise to base new plans on having to make use of existing appliances and equipment.
 
"The new panels need to be all parallel, because I have no other choice." - good option... all in parallel.

If your going 24V pack then just buy a cheap PWM controller and spend the difference on an extra panel as it will be more cost effective (and reliable if it is a good unit) in the longer term. PWM controllers have less components and very few capacitors to fail, so over the years they will last. Given the pack voltage relative to the mpvoltage of the panels and the losses between 32V to 28V (14%), an extra panel would make up the difference...

Cost wise, compare the price of a PWM to MPPT at the low voltage levels with single panels in parallel.... also consider that you can bypass the controller altogether if it fails (hook the panels directly to thre battery) just don't over charge the battery ! The PWM will also drop around 1V for a cheap unit between input and output due to the transistors/diodes.

If your remaining AC equipment is not high wattage, look at some of the 1200VA/600VA 24V based UPS units as these have lower stanby loads, my old APC 1200VA is around 0.8A at 24V (19W). They are cheap and something only valuable as scrap to a thief... I try to avoid moving things of value around by buying less valuable older stuff that does the job... :)

With the water pump go 48V and run it via a boost converter from 24V. The 1500W units listed for 14-18 will output 20A and maximum 30A input, so limited to around 750W maximum at 24V input. For a smaller well pump this could work ok, reduce your cabling requirement and provide an upgrade route to a 48V battery. Running a 1500W well pump from an inverter will potentially need a much larger than 1500W inverter due to the startup current.
 
Korishan said:
One thing you could do about the DC well pump is to either have a soft start installed, or you could use some super caps right at the pump to help with the surge. That would allow you to go down in 1 or 2 sizes on the cable to compensate for voltage drops.
The other thing could be to convert the DC to pulsed DC and transform it up to a higher voltage for that transmission, then drop it back down on the other end. Basically you'd boost the voltage up, then buck it back down. Altho, I don't know what the requirements would be of such a heavy amp usage device.
I'm sure there are other ideas as well.
Click to expand...

I like the supercap idea, cheap and effective for the job.

Sean said:
A 12v travel fridge would allow you to have a cold beer half way to your destination, and when you get there, and as many as you want on the way home. Asplit charge system in your automobile, along with a secondary battery would negate the need for your existing DC fridge, and provides the means of charging additional batteries if required.
Click to expand...
Good solution, I already have a small travel fridge, but with stuff for me, wife and two kids we dont use it much as I would need a bigger one. I also should add ist just 20mins to get there, I go sometimes even by bike, so the visits are more spontaneous. My brother and mother use the place too, so the usage is a bit "unplannable".


completelycharged said:
If your going 24V pack then just buy a cheap PWM controller and spend the difference on an extra panel as it will be more cost effective (and reliable if it is a good unit) in the longer term. PWM controllers have less components and very few capacitors to fail, so over the years they will last. Given the pack voltage relative to the mpvoltage of the panels and the losses between 32V to 28V (14%), an extra panel would make up the difference...

Cost wise, compare the price of a PWM to MPPT at the low voltage levels with single panels in parallel.... also consider that you can bypass the controller altogether if it fails (hook the panels directly to thre battery) just don't over charge the battery ! The PWM will also drop around 1V for a cheap unit between input and output due to the transistors/diodes.

If your remaining AC equipment is not high wattage, look at some of the 1200VA/600VA 24V based UPS units as these have lower stanby loads, my old APC 1200VA is around 0.8A at 24V (19W). They are cheap and something only valuable as scrap to a thief... I try to avoid moving things of value around by buying less valuable older stuff that does the job... :)

With the water pump go 48V and run it via a boost converter from 24V. The 1500W units listed for 14-18 will output 20A and maximum 30A input, so limited to around 750W maximum at 24V input. For a smaller well pump this could work ok, reduce your cabling requirement and provide an upgrade route to a 48V battery. Running a 1500W well pump from an inverter will potentially need a much larger than 1500W inverter due to the startup current.
Click to expand...
The comparison PVM cost saving vs add more panels is interesting, with all the shading its even more difficult calculate because more space -> the space with the least shade isnt that big... I have to measure that, thx!

Inverter is planned 3000W(6000W surge 5 seconds), that should be plenty when the coffee machine isnt running at the same time ;-) As I already have the 1500W pump(its stainless steel, cost about 80 euro), I'll try that first and see how it goes. Handpump will always stay for backup :)


Thx for all the input guys, a LOT to think about on every "front". I will continue the hunt for panels for now and in the meantime go back to battery work :)
 
If you go the SuperCap route, you will need to limit the charge current back into the caps. Otherwise they will try to draw as many amps as possible to recharge in the shortest time possible. Limiting is needed here for the distance, cable size, and required amps.

Since the visit times are so erratic, and the place is so close, I would recommend either setting up an internet connection and trigger an RPi to turn everything on, or even try LoRa broadcasting that once you get within range, it'll trigger the RPi to turn things on. LoRa can go quite a fair bit of distance (yes, many miles, especially line of sight). I'm going to guess you have trees around the place and maybe even a slightly winding road. LoRa can still make it but it just wouldn't trigger as far. I can't guess/calculate the distance that it would with trees. The modules are fairly inexpensive.
If you want to go the quick route and virtually anywhere access, have internet service to the cabin. This would probably be best as you can remote monitor the system from anywhere.
 
Following what @Korishan has to say about IoT based triggering there are some low cost IoT based sim providers that can enable such functions for fairly inexpensive. Hologram.io is one of them but there any many others. Most works in virtually all countries and on hundreds of mobile carriers.
 
Have to look at that LoRa thing closer, thx!

Rpi was planned anyway for checking battery and solar. I have 3G/4G reception there too, so that could be easier but LoRa looks interesting...

Oh, and for the solar panels.. I managed ot get an even better deal: 234Wp 49V/40.3V Mono Full Safety glass panels for 100 a piece, took 6 of them :D
 
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