Solar panel string size?

System · October 2011

Hi. There!
I have 8 Shell Sp140 panels (VMP 33.0) currently on my system that I originally install as a back up power system. I want to convert to a grid tie. I would like to purchase 8 more panels of similar spec and connect in to inverter as 2 series strings. My question is if I stay with same or close VMP do I have to stay with same STC watts. I'd like to go with 175wt panels. ???

niel · October 2011

Re: Solar panel string size?

welcome to the forum.
once you get close with the voltage on the new string then the current does not have to match and therefore the wattages do not have to match. some may say to get within 10% on the original voltage, but i recommend a tighter tolerance of within 5% and these figures are somewhat arbitrary as the closer in you are in voltage to the original the better. so you have 8 pvs at 33v and that totals 264v. at 10% that is 237.6v to 290.4v. at 5% it's 250.8v to 277.2v. this means you don't have to match another 8 pvs as it could be 9 kc245 pvs for 268.2v or 7 suntech 190s for 277.2v just to illustrate a few. of course the inverter has to handle the power you throw at it from both strings.

solarix · October 2011

Re: Solar panel string size?

I found in a similar situation it was cheaper to add on larger modules than the original even though their output would be limited by the older ones. The newer (230 to 240w) modules are so much cheaper per watt nowadays.

niel · October 2011

Re: Solar panel string size?

solarix,
the only way he would lose anything is if he places newer pvs in series with older pvs that are mismatched on their current ratings, as i believe you think he's doing, or if the string's vmps are far mismatched. the series mismatch is not the case with the op as he will keep the original string intact and just wants to add another string to be paralleled with his older string. the only requirement will be that the new string's total vmp be semi close in voltage to the old string's total vmp and can then be any current rating so long as his inverter is capable of the extra power presented to it.

System2 · October 2011

Re: Solar panel string size?

Niel,
Thanks for your reply!
Correct I plan to have the older panels on a string all by themselves, and the new string parallel with them. I really did not want to replace the older string. Although the Shell panels are only been in use about 7 years (installed 2005) so they are not that old. Plus they are on a Wattsun Tracker which I would have make modifications to work with other panels. My 8 Shell sp140s look like they will just barely make enough power to meet the start-up voltage of grid inverter. That is why I'm looking to add more panels. Better to get answers now then to late later. The Panels I was looking at are Trina TSM175DA01 and the VMP is 36.2 X 8 = 289.6 Just under the 10%. Now I suppose the power of my Shell panels is still close to original specs. The Inverter I was thinking of to use is Fronius IG 2000 with recommended PV 1500-2500Wp or SMA SB2000 Recommended Pv Max 2500. With both strings Wp would be 2520 does the inverter need to be bigger? better? Which brings up 1 more obstacle the 2nd string I was planning on either a stationary or passive tracker mount, what happens if I put the second on stationary mount, the Tracker mounted panels will be producing power earlier and later in the day then is our Vmp going to be out of balance again.

niel · October 2011

Re: Solar panel string size?

if it is only a 20w difference between the inverter max and the pvs i don't think there will be any problem there. you could be on the safe side and subtract 1 of the proposed trina pvs to give you a voltage of 253.4v for a percentage of 96% which is within 5% of the original 264v for the other string.

it's possible to have a problem with one string tracking and the other stationary as this will set up differences that will cause the mppt to compromise between the 2 strings much more than as 2 strings fixed, but in differing directions would create differences. this means there could be a loss at times, but i really can't say if having one track while the other be stationary would be any worse than 2 fixed arrays and their losses for off times of day. maybe somebody else more familiar in that hypothetical circumstance could comment.

System2 · October 2011

Re: Solar panel string size?

It was just a thought! never occurred to me until I looked at the sticky thread at the top of the forum of the 8kw Illustration showing 1 string completely shaded.

solarvic · October 2011

Re: Solar panel string size?

I think the fronuis ig 2000 inverter will be overloaded and will lose power searching all the time with your disimilar set up. I have one that was conected t0 14 kyocera 158-g panels and that was the limit. Since I had 16 of the panels I switched them out to a 3.1 IG Plus that could handle the 16 panels I have as it is 600vdc. It doesn,t search as much as the ig 2000 did. I was thinking of getting a power-one inverter as it can have arrays pointed in separate directions. I think this might be what you need. here is the link. Since our host doesn,t sell them I hope the moderator doesn,t delete the link.
http://www.atensolar.com/m5/200-330--power-one-pvi-3-8-high-frequency-isolated-3800w-inverter.html The main reason I didn,t get it was I already have the communications display and also have a fronuis ig-3000. :Dsolarvic:D

System2 · October 2011

Re: Solar panel string size?

Solarix,
Thanks for the reply, that does sound like a possible cure, If i read the spec's right that Inverter will treat each strings Mppt independent. I guess i should research inverters a bit more.

solarvic · October 2011

Re: Solar panel string size?

Another point to think about is you won,t have to worry about matching the panels you have. :Dsolarvic:D

Solar panel string size?

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