String sizing confusion

[Drewby80]

Hi everyone

I’m currently designing an 8kW PV system and am trying to figure out how to configure my string/s

I am installing 20x Trina 400w vertex panels all fitted with tigo optimisers.

I would like to put all 20 of these panels on a single string ideally because this will mean that the minimum DC voltage for the inverter will be satisfied early in the day and get the maximum generation time out of the system.

The optimisers and panels all have 4mm tails.

I have entered all of the info into an online calculator and this is the specifications of the system:



Will this be ok on 4mm DC tails as a single series string?

Many thanks
Andrew

 

[binky]

I wouldn't do that as 800V DC is likely to blow your inverter up. Do you actually need optimisers? To me, unless you really have shading issues, they are a waste of money and merely add failure points to the roof. Reading up on those Tigo units, they just bypass the panel anyway, whereas Solaredge system effectively moved the MPPT to the panel, so even a part shaded panel will still produce power. I'm also pretty sure that Solaredge would cope with 20 panels on a single string. 4mm cable will be fine unless cable run is very long.

 

[Drewby80]

I wouldn't do that as 800V DC is likely to blow your inverter up. Do you actually need optimisers? To me, unless you really have shading issues, they are a waste of money and merely add failure points to the roof. Reading up on those Tigo units, they just bypass the panel anyway, whereas Solaredge system effectively moved the MPPT to the panel, so even a part shaded panel will still produce power. I'm also pretty sure that Solaredge would cope with 20 panels on a single string. 4mm cable will be fine unless cable run is very long.

Yes I have some shading issues the sun sweeps over the array from 9:30am. The whole array isn’t in full sun till 11:00am so I want the panels to come online as this happens.

The inverter is a SolaX X3 10kW unit:

https://www.solaxpower.com/wp-content/uploads/2017/10/X3-Mic-4-10kW-Data-Sheet-V3.pdf

It’s max voltage is 1000v
It’s operating voltage range is 160v - 900v
It’s MPPT voltage range is 470v - 800v

I don’t really want to have to split the string and use both inputs as I plan to fit another 2kw of panels that face in a different direction later and use the second tracker for that.

Also the array should hit the startup voltage in fairly low light conditions with all the panels on one string so that should really maximise my yield.

The tails run is only about 4 meters from the panels to the inverter, will this inverter handle all of these panels on just 1 string and on 4mm cables?

 

[binky]

You won't have any issues with the starting voltage. What is the nature of the shading, is it hard shading from a chimney or shading at a distance from trees?

 

[Drewby80]

Hard shading the array has a wall to the east that shades the whole roof until the sun swings across it starting at 9:30am

This photo was taken at 10am

 

[binky]

OK, I would definetty look at Solaredge, it costs more for the inverter at around £1200 inc VAT. The difference is that solaredge optimiers make each panel completely independent of the next panel, and even a shaded panel will output energy from background irradiance. Those tigo units seem to just bypass a panel to keep the string working better, but it's still a string! So solaredge will give you a higher output, plus it removes any differences between panel outputs (manufacturing tolerances) and will cope with bird poo and leaves on the panels far better. Solaredge also do a single and 3 phase 8000w inverters. Solax is OK stuff, but not as good. Solaredge will take 25 optimisers per string / 6kW, but has 3 inputs on its 10kW inverter.

https://midsummerwholesale.co.uk/pdfs/se-case-study-photon-summary.pdf https://www.deegesolar.co.uk/solaredge_vs_tigo_optimisers/

Going back to your question, personally I would not try to put the entire energy of your array through a single MPPT, although the spec seems to show it will just about cope, you will just be pushing it very hard and I suspect this will ultimately shorten its lifespan. I would use an 8kW inverter and 2 strings to balance the loads out, strings arranged as 2 sets, the panels nearest the wall as one string, the less shaded / unshaded panels as the other. If you get around to installing a small 2 kW array, then a small inverter is only around £400. It also means if you have any issues further down the line, then you can isolate the faulty string and keep the system working, and it makes fault finding easier (keep a diagram of the strings)

 

[Bob Smith]

Talking about minimum and maximum number of panels per string
Here is a guide on how to calculate the min and max number of panels for a given inverter.

https://solardesignguide.com/calculating-string-size/
You need your inverter datasheets, your panels datasheet, and find min and max mean ambient temperature, but a link is provided for this.

However, I am no expert and I cannot say if this guide is complete or missing some bits.
I did you use it though and came out with the same results than in the explanation of a quote I have received (so either pure luck or the guide is good )