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 October 24 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

[mk1_man]

I have just installed a Sunsynk 8.8 kW inverter, it has 2 mppt's with both fed by two sets of mc4 terminals so 2 strings per mppt or 4 strings in total which is great.  I have 28 x Aiko 460 panels and decided 10 panels was the max I wanted to have per string. A string of 12 would be too close for comfort to max voltage in my view. Mine has 2 x string of 9 panels on one mppt (parallel) and second mppt has 10 panels, all working great.

 

In your case i think the three strings of 8 panels would be best option (safest)

[JohnMo]

22 minutes ago, mk1_man said:

Mine has 2 x string of 9 panels on one mppt

That isn't really correct, you have a single string which is a series-parallel array. So in this case voltage 9x a single panel and amps is 2x a single panel.

 

So question, is the amperage within limits of the inverter? Possibly not a concern at this time of year but could be in summer, could be where you may exceed the 26A MTTP limit.

[mk1_man]

Google AI response :  Two strings of panels wired in parallel are consideredtwo separate strings

 

All inverters can cope with a theoretical over current situation as all they do is throttle back, you cannot kill the inverter with too much current, its the voltage that is the killer. This is exactly the same as when you over panel the array  i.e say 13kW of panels as in my situation with the inverted only rated for 10.4 kW  i know in summer I will lose power but great for the winter when sun is scarce.  

[-rick-]

Overpanelling by up to 50% is common and helps generate more electricity in winter but imposes a cap on generation in summer. Inverters do have ratings for the extent of overpanelling they support though. I think 50% is common but 100% options are available. Given the amount of sun we get in the cooler months and the price of panels it makes sense to overpanel as much as possible if you have the space.

[JohnMo]

1 hour ago, mk1_man said:

Google AI response :  Two strings of panels wired in parallel are consideredtwo separate strings

Doesn't make the answer correct! Which it isn't.

 

There is an over panelling allowance in the total Watts you can generate, but you still need to stay within the specified amps and volts limits.

[-rick-]

Just now, JohnMo said:

There is an over panelling allowance in the total Watts you can generate, but you still need to stay within the specified amps and volts limits.

 

That was not my understanding. My understanding was that you could go over the amp rating by the allowed amount for the inverter. The inverter will limit the amps it draws from the panels to the amount in the specifications, but the panels being capable of outputting more is not an issue (within a limit).

 

The inverter can manage this through adjusting the MPPT point.

[-rick-]

Here is a Fronius doc exploring this though only by a small oversize. I've seen others discussing much larger ones but not got time to search for them.

 

https://www.fronius.com/~/downloads/Solar Energy/Whitepaper/SE_WP_Yield_Impact_Current_Oversizing_EN.pdf

 

I note the comment at the end about short circuit current limits. That one does have to be complied with but it is much higher than the MPPT string limit.

[jimseng]

So it seems I have a choice of getting close to the max PV ISC with 3 strings of 8 or close to the max PV input voltage with 2 x 12. Even at the never attained -10 degrees I am still 3 volts below the stated max input voltage with 2 strings of 12. Unless I go for a Growatt which, on paper gives me +25v of max voltage headroom. There must be a reason why the Growatt is cheaper though. 
I still can't make up my mind!