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Author Topic: Advice please - One string, or 2?  (Read 554 times)
pjs.g
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« on: July 16, 2017, 03:04:12 PM »

Hi,

I'm looking to deploy 10 x 245w panels on a flat roof (only place I can put them for now). Its not ideally facing the right direction, but it's all I have. This time of year, by 4pm, the top part of the roof becomes shaded and the sun moves around the house. This will start off covering 5 of the 10 panels (maybe 6), and work it's way down to the remaining 5 over the next hour or so.  My initial thought was to wire them as 2 x 5, in 2 separate strings and feed them into the inverter (it can take 2 strings). The inverter is a growatt. It has a starting voltage of 100v and takes 70-500v from the PV. MPP Voltage range is 80-500v. The panels are 27v nominal each.

It may simply be too much hassle to go to in order to get an hour extra from the panels (I'm assuming that once the first 5/6 begin getting shaded, it'd really stop producing soon after), but also it strikes me that the voltage on a string of 5 might be a bit low for the inverter to do it's job in less than full sun? Some sites seem to be recommending resolving any shading issues with parallel wiring between the panels.

Sorry for my lack of knowledge here! I've been reading a lot, but not 100% clear on what might be the best option. Does anyone have any advice?

Thanks.
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Westie
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« Reply #1 on: July 16, 2017, 06:48:25 PM »

2 stings, put the panels that go into shade in string1 , the remainder in string2.  Given the min string volts for your inverter is 80v I would use 4 panels as a minimum.

If your doing this from scratch the new approach would be to use an optimiser on each panel, Tigo is the brand to look at.  The other method is to use a micro inverter per panel but as you already have
the inverter I guess that's out.

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4kwp south facing array  SMA 4000TL grid connected.  2x30tube Navitron solar thermal panels (east/west). Arada 5kw S/C WBS. 25000Ltr underground rain water tank. KTM E-Bike  Cool
M
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« Reply #2 on: July 17, 2017, 07:05:42 AM »

2 stings, put the panels that go into shade in string1 , the remainder in string2.  Given the min string volts for your inverter is 80v I would use 4 panels as a minimum.
Hiya, if there's only one MPPT, then the strings will need to be balanced with the same number of panels.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
Westie
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« Reply #3 on: July 17, 2017, 08:28:24 AM »

2 stings, put the panels that go into shade in string1 , the remainder in string2.  Given the min string volts for your inverter is 80v I would use 4 panels as a minimum.
Hiya, if there's only one MPPT, then the strings will need to be balanced with the same number of panels.

Yep, quite right Mart,  if it's an old inverter it may have 2 sting inputs but only 1 tracker, the old growatt sungolds were like that. 
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4kwp south facing array  SMA 4000TL grid connected.  2x30tube Navitron solar thermal panels (east/west). Arada 5kw S/C WBS. 25000Ltr underground rain water tank. KTM E-Bike  Cool
pjs.g
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« Reply #4 on: July 17, 2017, 11:32:47 AM »

Thanks for the replies. 2 x 5 it is...  (maybe 1x6 & 1x4, given roof is not square, but sounds like it will be OK) The inverter has 2 MPPT's. I guess it's just a case of series them down below if I ever wanted to.  1 more DC switch on the way!  Just waiting for the pallet to arrive now...
« Last Edit: July 17, 2017, 11:43:25 AM by pjs.g » Logged
M
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« Reply #5 on: July 17, 2017, 12:16:58 PM »

2 stings, put the panels that go into shade in string1 , the remainder in string2.  Given the min string volts for your inverter is 80v I would use 4 panels as a minimum.
Hiya, if there's only one MPPT, then the strings will need to be balanced with the same number of panels.

Yep, quite right Mart,  if it's an old inverter it may have 2 sting inputs but only 1 tracker, the old growatt sungolds were like that. 

But you guessed right it turns out, so a 4/6 (or 5/5 or 6/4) split might be better depending on how the shading works.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
freddyuk
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« Reply #6 on: July 26, 2017, 03:50:18 AM »

If the shade hits across the short side it will kill the panel dead or if it hits across the long side it will kill cells column by column only which are linked in series. It may give you a bit more generation......
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