Mixing PV panels

[Marvin]

I have 16 Longi 320W panels and wish to add some more but they are now discontinued I have found the  LONGi Solar Hi-MO 6 435WP Black Frame PV Module LR5-54HTH-435M (435Watt)

 

Technical details:

 

320Watts panels				435Watt panels
	STC	NOCT			STC	NOCT
Power	320	237.1		Power	435	325
Voc\V	40.9	38.2		Voc\V	39.33	36.93
Vmp\V	33.9	31.3		Vmp\V	33.04	30.15

 

Is it ok to mix them?

If not, what specification would be better if I cannot obtain exactly the same.

If so what are the pros and cons.

 

Any guidance would be appreciated.

 

M 

[Mattg4321]

I wouldn’t mix those on the same MPPT, but somebody more experienced than me with PV might come along and say that’s wrong. I’d imagine that the output of the newer panels would be compromised, and dragged down to the output of the old panels. You may even be nearing the max MPPT voltage rating anyway. 
 

On a separate MPPT or inverter, of course, crack on - subject to DNO approval if grid tied. 

Edited September 16 by Mattg4321

[Dave Jones]

one of the many many advantages of using microinverters.

[JohnMo]

First check the amps generated are ok on the new panel for the max allowable amps for the inverter.

 

A write up here, worth a read.

https://explorist.life/using-mismatched-solar-panel-sizes/

[Dillsue]

Easiest way to add the bigger panels is to wire them to a separate MPPT input on your existing inverter. Next easiest, but may not be ideal, is to get someone to calculate the output for the various panel combinations to see if they can be wired to your existing inverter, if no spare MPPT input. Next is to replace your inverter with one that has a spare MPPT input. Lastly add optimisers/micro inverters which allows you to mix and match panels freely

[SteamyTea]

I am not sure about this (as not an electrical engineer) but I think that PV modules are fixed voltage, it is the current that varies with solar intensity.

If that is the case, then as long as the module voltages match, there should not be a problem.

If that is not right, then maybe individual module optimisers can overcome the mismatch.

[Nick Thomas]

Both voltage and current vary, although I only knew this from watching them go, not from electrical principles. https://electricalacademia.com/renewable-energy/photovoltaic-pv-cell-working-characteristics/ goes into gory, gory detail, but i'm going to need to read it several times before I understand it ^^.

 

Your inverter will have a maximum it can accept per MPPT anyway, and you light find 16x320W panels is close to, or at, the limit. Check its docs.

[Dillsue]

56 minutes ago, Nick Thomas said:

 https://electricalacademia.com/renewable-energy/photovoltaic-pv-cell-working-characteristics/ goes into gory, gory detail, but i'm going to need to read it several times before I understand it ^^.

Last few pages from fig 6 onwards show the varying voltage current. The voltage looks very low as the curves are for a single cell, not a full panel!

[Marvin]

Thanks for all the replies.

 

I am a little ahead of the game with some items:

 

The DNO permission is based on the maximum the inverter will allow to supply back to the grid which is 6000W

The Inverter can cope with 7980 Watts of panels in the UK.

The Inverter has 2 strings (MTTP) which can run up to 4000W max each.

 

The existing set up has 16 x 320Watt panels = 5120W which is 64% of potential

String 1	7 panels	2,240	Watts	56%
String 2	9 panels	2,880	Watts	72%

 

The Inverter is capable of taking the amperage.

The solar DC cable is 6mm and about two separate 27m loops

The AC cable from the inverter is 22m of 6mm clipped direct with some conduit/trunking - easily capable of taking 6kW.

 

Hi @JohnMo

Thanks for the link. I have now read it and it is clear to me.  I will now calculate what the results would be.

 

[SteamyTea]

10 hours ago, Nick Thomas said:

Both voltage and current vary

Did I get it the wrong way around, they are fixed current devices (I think LEDs are the same).

 

[Mattg4321]

They are wired in series, so the current will be constant. The voltage rises when you add panels. 
 

You need to check what the max input voltage/MPPT range of each MPPT  is, as well as the current

Edited September 17 by Mattg4321

[SimC]

On 15/09/2024 at 22:15, Marvin said:

320Watts panels     435Watt panels     STC NOCT     STC NOCT Power 320 237.1   Power 435 325 Voc\V 40.9 38.2   Voc\V 39.33 36.93 Vmp\V 33.9 31.3   Vmp\V 33.04

30.15

Another significant parameter is the current under STC and NOCT conditions.

As the new and old panels will be wired in series for each string, the same current value will pass through all the panels in a string.

The best match will be if the new and old panels would naturally produce the same current at the various insolation levels say 200W/m2 to 1000W/m2.

Some manufacturers show an insolation/current/voltage graph on the data sheet, but not Longi.

Have you got the full data sheet for the existing panels?

 

Current of the new panels, Imp = 13.17 A (STC) and 10.78 A (NOCT).

 

The string voltage is then the summation of all the panels in that string which must not exceed the capabilities of the inverter, even under open circuit conditions at low temperature (Voc - (Voctempco * (25 - - 20 degC)). For the new panels this equates to:-

 

39.33 - ((-0.23% * 39.33) * (45)) = 39.33 + 4.07 = 43.40 V per panel.

 

 

Edited September 17 by SimC

[Dillsue]

13 hours ago, SteamyTea said:

Did I get it the wrong way around, they are fixed current devices (I think LEDs are the same).

 

The current varies with sunlight levels hence the difference in rated current for STC and NOCT

[Marvin]

On 17/09/2024 at 19:07, SimC said:

Have you got the full data sheet for the existing panels?

the 320 Watt panels are also Longi. 

 

Longi 320W.pdf