Analysing 24 hour data on 48V system with G9 Qcells ALL BLACK 80VDC PV VS 120VDC PV?

TerryTtibbs

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Mar 15, 2022
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mppt100-20-48.png


Just running a test rig with some small sla's to track the data with new panel setup before finalising install.

This is a 16-18 Degree C day with no sun and just heavy cloud northern hemisphere (London). My 2 x 345W Qcell panels on short circuit mid day facing south lying flat on the ground are giving me 80VDC @ 4.4amps averaging 3.2amps (rated at 10amps -11amps at full output STC) max and 82V output.

The current (A) is only showing charging cycles from float to bulk throughout the day as only a very small invertor is connected just to give a 15W -25W load overnight for testing purposes.

The above graph is showing 24 hour data for 2 x 345W Qcell panels series charging small 7Ah 48V pack at 1.0A max charge set in the MPPT just for testing purposes.

I am very impressed with the output from these panels in low light. Much higher than I expected. I am also very impressed with the efficiency of the Victron MPPT.

I would like opinions from people with real world experience which route to take next!

array=4 physical panels for the purpose of this analysis.

I will have 3 arrays in total, 1 facing South, 1 West and 1 East with various shading across all arrays that cannot be avoided due to surrounding buildings.

I would really like to keep the arrays in 2 strings each of 2 series panels and use a dedicated MPPT for each string. I like the idea of having a low voltage @80VDC PV solar output with dedicated tracking per string with no shading.

However I would like opinions from people with vast experience on whether I should change my array to 3 panels series of 1 string.

The current locations of the arrays are much more suited to 4 panels portrait wide as I have a limitation of 4.2 meters in mountable width for each array (4 panels wide). HOWEVER there is shading early morning and late evening across all of the arrays at about 50% vertical with the panels arranged in portrait from left to right on each array. This is why my preference has been to track each string using individual MMPT per pair of series panels.

If I go to 3 panels it will leave 1 panel on the end of each array on its own facing in different directions, south, east and west, which to me seems pointless.

I understand I am cutting it fine for a 48V system needing 51.2V + 5V for MPPT activation) for bulk phase charging. My calcs show PV voltage 134mv drop per degree above 25 Degrees C which is only 3V including efficiency 98% maximum drop from the figures shown in that chart above even in hot days. This still leaves me headroom.

Will there be any point in going to a 3 panel string with shading across 30% of the string at various times of the day? Will it make any difference to when my string will start charging in a morning having 3 panels instead of 2 in the string?

My climate rarely exceeds 28-33 degrees C in summer and down to -12 degrees C in winter.

Before I install 1 MPPT 100/20 (6 in total) for each pair of series panels please let me have your thoughts, as otherwise I will need 4 x MPPT 150/35 with a potential shading issue on all series strings.

Going to 4 panels in series 160VDC with MPPT 250/60 is not an option as I don't want to exceed 125VDC PV per string.

From what I have been reading if cost is not a problem then assigning multiple MPPTs in smaller strings is more efficient for the way the Victron MMPT software is coded.

Anyone else with a 2-3 panel series setup please post your 24 hour data so I can see the V and A figures.

Regards
 
Last edited:

sunshine_eggo

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Not sure I qualify for "vast experience," but I'll chime in.

The main reason to use higher series voltages is to reduce voltage drop due to wiring losses across larger distances.

MPPTs actually operate a little less efficiently the greater the difference between series Vmp and battery voltage. However, this is often offset by less losses due to reduced voltage drop.

You are not cutting it close on a 48V system. +5V applies to Voc, not Vmp.

If I were in your shoes, due to the multiple shading issues, I would looks to have an MPPT for each array.

Doubt it helps, but here's my 3S array (47Voc/40Vmp) under typical use in sunny AZ. Battery temp included for giggles:

1650120619289.png
 

TerryTtibbs

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Mar 15, 2022
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56
Thanks for the reply,

So today I set up a little experiment. I used 1 40VOC panel with 100/20 48V MPPT and set the charging voltage to 12V at max 20A.
I then waited until 18:15 when the sun went below the effective range of the panel. My charging current was 12V at 1.4A. I then added a second PV and made a series string of 80VOC. The current on the PV side stayed the same but the MPPT went to 12V at 2.7A. Next I will add then panels in parallel and see what happens under the same light conditions. Will post my results later.
 

TerryTtibbs

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Mar 15, 2022
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series-test.pngExactly the same in series and parallel. Both cut off at 19:38. So it seems as long as you allow for temperature increase and have decent headroom in the string voltage over Vbat, it really makes no difference. As you say the only advantage is if you are running long runs from your array then it makes sense as cable can be a lot thinner. Then you lose efficiency. From my findings I am going to run 6 strings of 2 series panels using 6 x Mppt 100/20 48V for this year and then see how they perform when it gets hot. In hindsight if I had the access to PV with 44VOC instead of the 120 cell 40.94VOC panels I have now I would have ordered those just to give me that extra headroom of maybe 5V, but currently stock is at zero here because of energy prices soaring. I also liked the 345W 120 cell panels as they are small in footprint and fit perfectly where I need them.
 
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