24 panels on a new build roof. 2 strings of 12, or 3 strings of 8?

[jimseng]

I am planning my solar install and want to check a couple of things.

I have bought 24 in roof solar panels that mount landscape. The roof shape dictates 3 rows of 8 panels. No shading, all on one 43 degree roof. This is a new build so the roof isn't finished yet. I am planning on something like a Sunsynk 8.8k inverter (the 10k version seems pretty rare at the moment in the UK.) The inverter pv input voltage is stated as 370v (125-500v DC, startup/max) and has 2 mppt inputs.

So it seems to me that my best option is 2 strings of 12 panels. With a VOC of 38.15 per panel this would give me a voltage of 458v (498 at -10 deg C) per string, which is (just) within range of the inverter. It looks like a better option than 3 strings of 8 but I just want to see what people suggest.
As for connecting up the strings, I was thinking of something like the attached image. My reasoning is that it keeps the cable lengths the same and brings the connections out at the correct end of the house. Perhaps that is unnecessary but it seems like a logical thing to do in my head.
 

[JohnMo]

1 minute ago, jimseng said:

498 at -10 deg C

Almost impossible to get in the UK winter, coldest day and PV max output so wouldn't consider that.

 

You only have 2x MPPTs so unless you buy a second inverter or different inverter, you are stuck with 2x strings. Two strings is fine from the info given. The 370v is just nominal. More panels give a quicker time to start up voltage and a longer day before inverter trips off.

[jimseng]

Quote

You only have 2x MPPTs so unless you buy a second inverter or different inverter, you are stuck with 2x strings. 

Thanks JohnMo. I believe the inverter can take two strings per MPPT but it is nice to confirm the point about extending the start and stop time, I just wasn't sure if I was getting too close to the max PV input.

Do you think my series wiring layout makes sennse?

[JohnMo]

1 hour ago, jimseng said:

believe the inverter can take two strings per MPPT

Your voltage and amps limits are the limits to each MPPT.  The invert will take two strings only, via one to each MPPT.

[SteamyTea]

Never looked into this, but have heard of it being done.

You could split the system into two strings, with each string consisting of 6 panels paralleled up to another 6.  That would halve the voltage while doubling the instantaneous current.

That inverter can take up to 44A_dc, but is designed to operate at 26A_dc input.

So it really comes down to what your panels can do, what is the maximum amps they are rated at?

Edited Friday at 15:55 by SteamyTea

[jimseng]

Quote

So it really comes down to what your panels can do, what is the maximum amps they are rated at?

The panels imp is 13.43, short circuit isc 14.2. 2 strings of 6 would be comfortable but perhaps means starting later in the day.
I think it makes sense to keep them at 2 x 12 strings. The VOC gets a little close at -10 degrees but it is still, on paper within limit. And the lowest recorded temperature here in somerset was -8.5 a few years back. There is also a Growatt 10k inverter  which gives me an extra 25v of max PV input headroom. In fact the Growatt has 3 trackers with a total of 2 strings per tracker, but it must be cheaper for a reason...

[SteamyTea]

When it is really cold, it is winter. The instantaneous power from the sun is quite low, maybe maxing out at 600 W/m². As the panels will only get about 20% efficient, that is 120 W/m².

So while the voltage will be at maximum (PV modules are fixed voltage devices), the current is low, the MPPT will do it's thing and adjust the internal resistance to get the most out of the system. So should never become a problem.

Panels are rated, and tested, at a standard temperature, irradiance and air mass. It is extremely rare in the UK to breach those limits.

If you do get lucky, the inverter will 'clip' the power initially (the fan speeds up and dumps the excess power as thermal energy, and then will eventually disconnect if it cannot reduce the output voltage and current to the agreed limits (usually 253V_ac and 16A per phase.)

Edited Saturday at 20:50 by SteamyTea