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String re-sizing of existing array


jasonvanwyk

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Good Day

Please could I ask for some guidance / advice here.

I'm upgrading my existing system from 2 x 5kw Axpert inverters to a Quatrro 10000va.

My existing array is potentially 7840w - I don't want to have to lose any panels, and or have to add any more panels.

From my existing system, I already have an array of 28 x 280w monocrystalline panels. They are divided into 4 strings of 7 panels each. I've used the Victron mppt calculator, and I'm getting an answer of the SmartSolar MPPT 250/100 MC4.

My panel specs are as follows:

Vmp = 31.4v
Imp = 8.96A
Voc = 39.1V
Isc = 9.50A
Eff = 17.2%
Temp coefficient of Voc = -0.33%/c
Temp coefficient of Isc = 0.059%/c
My questions are:

If I keep my strings as 7 x 39.1Voc = 273.7 (I'm not taking any inefficiencies into account yet), do I potentiely risk running to much voltage into my charge controller? Should I rather re-string my array as follows 7 strings of 4 panels which would be then 4 x 39.1Voc = 156.4v?
Do I need two charge controllers to compensate for the amperage? I'm running 4 x 2.4Kwh pylontech batteries on a 48v system.
Many thanks in advance

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6 hours ago, jasonvanwyk said:

My existing array is potentially 7840w

I have learn't that there is plenty space between the potential and the realized peak.

If you never intend to add panels, there maybe a possibility of getting away with 1 MPPT, or at least a big one and a little one.

 

1. Do you have any history of the peak power your panels have ever delivered?

2. Do all your panels face the same direction and are they at the same pitch? - What direction(s) & pitch(s)?

3. I there any shade on any of the panels throughout the day, and throughout the year?

 

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1 hour ago, phil.g00 said:

I have learn't that there is plenty space between the potential and the realized peak

But, if you don't want to mess with actual values, (so that you step down an MPPT  current size).

28 panels &  4 existing cable pairs, that I am going to assume are 6mm2, because you stated you are looking at an MC4 option.

MC4's are good for 30A. So you don't want more than 3 strings in parallel on a cable pair, and you don't want to be installing more cable if you can help it.

I'd configure 16 panels as two arrays of 4S2P each, with each array brought down on its own cable pair into a 250V 85A MPPT.

 

That leaves you with 12 panels and two remaining cable pairs.

Two arrays of 3S2P each, with each brought down on its own cable pair into a 150V 70A MPPT.

( In reality the 60A version, would still be adequate).

The 150V option each cheaper than the 250V option for the same power transfer capability.

And you will be running at less than 20A on any cable pair.

 

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13 hours ago, phil.g00 said:

1. Do you have any history of the peak power your panels have ever delivered?

This is what I pulled from EMONCMS which would have been recorded from my 2 x 5kw axperts via ICC

On these occasions between Oct & Nov they peaked above my potential array size which is 280w x 28 = 7840w. This is very puzzling to me, as these numbers are between 2-6% more than the rated output?? These peaks did not last more than a minute according to the data.

2/10/2019 11:10  -9501.17w
7/10/2019 7:30 -8372.06w
7/11/2019 11:30 -8319.94w
7/11/2019 9:10 -8056.32w
7/11/2019 10:41 -8046.09w
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13 hours ago, phil.g00 said:

2. Do all your panels face the same direction and are they at the same pitch? - What direction(s) & pitch(s)?

All my panels are mounted in exactly the same orientation and all facing NNE, and all on the same pitch (i'm not sure of the angle, maybe 20-30 degree angle?)C09B4F6B-3934-4D8B-B965-6CC79F7B5394.thumb.jpeg.772b6cc9817837a76996cc52a16fc484.jpeg

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1 hour ago, phil.g00 said:

OK, what about something that looks like your typical steady-state peak (max) power just before noon in high summer?

This is what I base avg during these times (See also attached spread sheet) This is data averaged over 2 months.

The closest thing I have to summer data is OCT - NOV

11-12PM 3160w
12-13pm 3970w
13-14pm 3620w
14-15pm 3010w

 

I do However peak above this as follows:

On 2 days I had a max load of above 9000w for 19 min and 29 min (I have had contractors)

on 7  different days my load was between 8000 - 9000w, for an avg of 38 minutes each

on 12 different days my load was between 7000 - 8000w for an avg of 46 minutes each

on 22 different days my load was between 6000 - 7000w for an avg of 55 minutes each

on 33 different days my load was between 5000 - 6000w for an avg of 64 minutes each

on 49 different days my load was between 4000 - 5000w for an avg of 92 minutes each

AvgMaxLoadHourOver24HourPeriodLast2Months.xlsx

Edited by jasonvanwyk
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@jasonvanwyk,

These numbers are your loads, what you need are the maximum PV production values.

I can see that because you have values at night-time.

We are trying to size your MPPT(s) to the maximum your panels deliver.

This can't really be determined from the data you have given me.

Do you any stats on what is coming from the panels to the inverter, not from the inverter to the loads or battery?

If not, do you have a DC clip-on ammeter?

 

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@jasonvanwyk,

What I can tell you from these numbers is you only need a 5kVA Multi. ( Which delivers 4kW).

The Victron units work in a way that the grid absorbs the unusual peak loads, so you don't have to size your inverter for those unusual peaks. It isn't cost effective.

Edit: I am assuming you have a grid connection, I can see the poles in your photo.

 

 

Edited by phil.g00
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On 2019/12/13 at 3:38 PM, jasonvanwyk said:

11-12PM 3160w 12-13pm 3970w 13-14pm 3620w 14-15pm 3010w

These numbers are my avg base PV watts being produced during these times, averaged over oct & nov. My PV watts do however peak above these averages - I often get numbers between 4000 - 7000w when the sun is good. When I put my system in, it was designed to be more "off grid" hence the huge outlay.  think the grid tied system is going to be a much better option for me (i'm still connected to the grid) as my loads do peak over 8000w from time to time. I however have already got all the panels and batteries, so I might as well put them to the best use I can.

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2 hours ago, jasonvanwyk said:
On 2019/12/13 at 1:38 PM, jasonvanwyk said:

11-12PM 3160w 12-13pm 3970w 13-14pm 3620w 14-15pm 3010w

These numbers are my avg base PV watts being produced during these times, averaged over oct & nov. My PV watts do however peak above these averages - I often get numbers between 4000 - 7000w when the sun is good.

But, these are just the same numbers as on your load spreadsheet.

Which isn't helpful because it could mean :

a) Your PV is capable of delivering more because your battery is charged and your load is only this much.

or

b) Your PV is not even delivering this much because your battery is supplementing to match the load.

I am looking for a peak value of the PV when we know it's going all out.

Alternatively, if you have a DC clip-on ammeter, we can contrive such a scenario and measure it.

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Thanks @jasonvanwyk, for allowing  me to log in to your online monitoring history.

It would appear anecdotally that rating your total MPPT capability at 6kW, would cover about 99%ish of your PV production without any clipping.

So some maths:

All panels are at the same pitch and direction, so proportioning production: 6000/28 = 215Wp/panel.

Pylontech charging voltage = 52.5V

Assuming perfect MPPT efficiency and pricing from the Smartsolar range of Victron MPPTs:

6000W/52.5V =114A,  which is more than the biggest MPPT (100A), so more than 1 MPPT is required.

I propose a 16:12 split between MPPTs.

First 16 panels:

16 panels = 215*16 = 3440W

3440W/52.5V = 65A  therefore a 250V 70A MPPT is sufficient for 16 panels. (750 Euro)

 

For the remaining 12 panels:

12 panels:

215* 12 = 2580W,

2580W/52.5V = 49 A, therefore a 150V 60A MPPT is sufficient for 12 panels. (550 Euro)

Like I say working from actual PV production figures, and you can easily see you can step down an MPPT size.

So 750+550 = 1300 Euro and you still have plenty headroom, and you've saved a bob or two.

 

 

 

Edited by phil.g00
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9 minutes ago, jasonvanwyk said:

what happens if the panels for some reason Do manage to supply more amps than the Mppt’s are rated at? For example if an MPPT is rated at 70A and the panels push through 80A?

They clip and you don't get the benefit of that extra power.

But in your instance, based on your production figures and sizing like i have suggested, that will be seconds in a year, if ever.

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4 minutes ago, phil.g00 said:

They clip and you don't get the benefit of that extra power.

But in your instance, based on your production figures and sizing like i have suggested, that will be seconds in a year, if ever.

Ok so no harm done to the MPPT then?

also, it would make more financial sense to go with the Quattro 6500w as opposed to the 8000w then. 
 

is the reason to go with the Quattro because it’s more robust As opposed to the multiplus? Although I don’t need the extra inputs and outputs of the Quattro, as my entire load is on 1 DB. 

Edited by jasonvanwyk
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27 minutes ago, jasonvanwyk said:

is the reason to go with the Quattro because it’s more robust As opposed to the multiplus?

No, the innards are pretty similar, merely scaled for size.

Both units have two outputs, the Quattro just has an extra input so it is able to accept a generator when the grid is down.

The other major difference is that it is the multiplus that is on SA's approved list. ( Well technically CT's approved list).

The 5kVA unit is the biggest approved Multi, if you don't need the gen input, I would recommend this unit based on your typical loads and it being approved.

Both units will pass on and blend extra power from the grid if the loads require it up to the rating of their respective transfer switches.

 

Edited by phil.g00
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8 minutes ago, phil.g00 said:

No, the innards are pretty similar, merely scaled for size.

Hi Phil, some experience with both and I dont want to publicly start listing the problems my clients have experienced with the new Multi within a month after commissioning. The Quattro is definitely more robust, as were the older Multis in the Alu cases/enclosures. 

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4 minutes ago, Jaco de Jongh said:

Hi Phil, some experience with both and I dont want to publicly start listing the problems my clients have experienced with the new Multi within a month after commissioning. The Quattro is definitely more robust, as were the older Multis in the Alu cases/enclosures. 

I was thinking of the older Multi's.

So its not just a re-housing in a new form factor with the new one, OK, I wasn't aware of that.

I have a Phoenix ( based on the older Multi) and two Quattro's and they are robust alright.

Edited by phil.g00
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