section9bateau avatar image
section9bateau asked

Verifying planned design for remote cottage, expecting snow covered panels

I have an off-grid cottage that currently has two damaged panels I got a few years ago, plus various bits of gear, and I plan to expand on my power generation (still small scale this year, hopefully massively with a grid connected ESS reselling power next year.)

My goal is to have 24/7/365 power output this year, even when my family isn't there. My current setup is as follows:

2x250w (damaged) solar panels on the south facing roof of the "utility" building. While they are damaged, I have seen output levels as high as 450w.

Venus GX

MultiPlus Compact 24/2000/50-30

BlueSolar Charger MPPT 150/45


In addition, I have about 35 watts of loads I want to run 24/7/365 (small esxi server, PoE switch, a few webcams, and GSM modem). The loads are a mix of AC and 24VDC. During the summer months I would prefer to have an addition ~100 watts of load constantly, but I am fine with shedding loads in low battery situations.

The area is prone to extreme winds and snow, when I was in the area and tried to check on it at Easter, there was 130cm of snow on the ground, making it impassible. The location is not quite Arctic, but it is a hair over 63 degrees north, so the shortest day of the year will be *VERY* short. In addition, the outdoor lows over the winter sometimes get to -35c.

I am looking at getting 4 new 275w panels, and while I know it will not be ideal, I would place 2 of them on the north facing roof, and two on the south facing wall under the roof line but as elevated as possible so that hopefully they will not get covered in snow over the winter. I expect that only the panels mounted vertically on the wall will have any meaningful output in the middle of winter. I am hoping to get enough output from them that this system can remain online year round, but I do not know if that is realistic.

Question 1) My "gut feeling" is that I should hook up each of the 3 pairs of panels so that each pair is in series, and each series of 2 is in parallel with eachother. Is this correct?

Question 2) Is there any good/easy solution for automatic load shedding at low power levels (such as a Venus controlled circuit breaker)?

Question 3) If it would help, I could actually put 3 panels on a south west facing wall, where they also may produce some power during winter, but they won't have any direct sunshine at any point. Is this of any value if I would have two panels in direct sunlight? Would the MPPT be limited to the current max from the string in direct sun, or would it actually benefit from some small additional current? What problems, if any, would having this sort of missmatched string size have? (all panels are 32v, so even a string of 3 is under the voltage limit for the MPPT)

Thanks in advance!

MPPT Controllersoffgrid
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1 Answer
tilo avatar image
tilo answered ·

1) The "official" recomendation is to use a separate MPPT for each orientation. But if you have a budget limitation, you could just put the 3 strings in parallel with your existing MPPT if you use a blocking diode for each string, to avoid reverse current in strings with shadow/snow. A reverse current will lose energy, and if it gets higher than the allowed limit (usually around 15A) it can damage the panel.

When you mount the panels on the wall, try to have them horizontally oriented (in printer speech "landscape", not "portrait"). Then if there is high snow which might cover the lower part of the panels, it still can work with reduced voltage, as it will internally use the bypass diode of the shaded section. 60-cell panels have usually 3 sections with a bypass diode for each one. Each section consists of 2 rows of 10 cells.

2) Yes, you can use the Venus GX relay and/or the Multiplus auxiliary relay and/or the BMV relay for load shedding - you can implement 3 levels like this. See the max. current and voltage specification of those relays, they all can be used for DC, and some for AC (small current, enough for your mentioned loads). You can also shed loads remotely manually with the Venus GX relay over the VRM Remote Console.

3) If you mix string with 2 and 3 panels, you will not get the maximum possible total output of all panels - not recommended. But if you have just space for 3 panels, it is better than 2 panels. Use blocking diodes to avoid reverse current, then you can´t do damage, and any additional current will help. The MPPT will find the optimal working voltage of the whole set, not of each string. That means some 2-panel strings might contribute little to nothing if the sun is on the 3-panel string, as the MPPT will jump to the higher working voltage of the high current sunny 3-panel string. For a permanent installation I recommend a separate MPPT for 2 and 3 panel strings.

Consider also East and West walls, who might produce a little in winter, and receive some sun all year round and likely be without snow. Even the North wall will receive in the morning and evening in summer. As you need energy in winter, the snow free wall mounting positions are probably the best option.

If you use a small MPPT for each string/orientation, you will have better monitoring information, and know which orientations contribute throughout the year.

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section9bateau avatar image section9bateau commented ·

Thank you for the feedback!

I do want to keep it to the one MPPT for this year, I hope to get several 250/100 (or larger) models in the future and will have a much better setup when I switch to those. The panels I already have do include blocking diodes already and are setup as 3 sections internally.

As to horizontal vs. vertical, the wall is 180cm to the roof line, and 2m wide on the south facing wall, 3m wide on the south-west and the panels are ~165x100cm. The building is also elevated 30cm. If I place them horizontal, I can only place a single panel on each wall, so I suspect I would be better off with two, unless the snow cover of some parts of the cell strings will render them inoperative.

Thanks for the info about the relays, I will look into that more!

Finally, hopefully next year I will install some standing dual axis tracking systems, if any ones are recommended I would welcome the input. Ideally I would want something that can rotate at least 270 degrees to maximize output being so close to the "land of the midnight sun".

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tilo avatar image tilo section9bateau commented ·

Blocking diodes are not very common in solar panels, usully panels just come with 3 bypass diodes. Blocking diodes, in case they are used, usually have to be added externally, one per string is enough, as they have a small voltage drop (and thus heat up). Just to make sure that we are talking about the same thing.

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section9bateau avatar image section9bateau tilo commented ·

Ok, thank you. So do I install one blocking diode at the end of each "string" in series with the string, and then merge them after the blocking diode all to the MPPT input? If so, does it matter which end they are installed at?

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tilo avatar image tilo section9bateau commented ·

Correct. The end does not matter.

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