Matching different solar PV panels

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Matching different solar PV panels

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OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Wed Aug 21, 13 11:14 am Post subject: Matching different solar PV panels

I've got myself a solar pv installation (as seen in other threads); I have a charger/controller feeding 2 100ahr batteries. Also a 1500w inverter. I started off with a single 250w panel (to get familiar with it all) and replaced it with 6 x 170w panels (arranged as 2 strings of three).

The 6 170 panels have been installed for 4 months now and all seems to be well.

Except I have the original 250w panel sat in the shed doing nothing. I was wondering if I got another 250w panel I could have a 3rd string. Each string being approx 500w.

The 250w panel has Vsc 60.5 (Vmp 50.6). The 170w 44.0 and 35.0
Each string of 3 170w is actually running in daylight at 118-120v.
Is it possible to get another 250w and pair it up in series with the original to give me a third string?
Is there some device available that could the used to make the new string match the other 2 (in terms of voltage)?

Your suggestions/ideas greatfully accepted.

RichardW

Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales

Posted: Wed Aug 21, 13 12:35 pm Post subject:

The basics are:

panels wired in series must have very similar maximum amps

panels wired in parallel must have the same maximum power point voltage

So wired in multipul strings you need each string to have the same maximum power point voltage & all panels within that string to have the same maximum amps.

In your case the total string voltage is a long way out.

101.2v for the 250 watt panels & 118v for the 170 watt panels.

You will end up with all the strings being taken out of their MPP & reduce the total output.

You would be better off with a second controller. Also how big is your controller?

I make 2 x 3 x 170 watts + 2 x 250 watts = 920 watts.

Thats a peak of 76 amps at full charge into a flat battery.

That is a lots of amps into a 200ah bank.

OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Wed Aug 21, 13 2:15 pm Post subject:

RichardW wrote:

I make 2 x 3 x 170 watts + 2 x 250 watts = 920 watts.

My convoluted explanation may have misled. I have 2 x 3 x 170w. The extra 2 x 250 would make the 3rd string.

It might be better then to sell the original and buy another 3 x 170watts.

The controller can handle 60 amps by the way.

RichardW

Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales

Posted: Wed Aug 21, 13 6:07 pm Post subject:

So if I have this right you will then have 3 strings of 3 panels in each & each panel is 170 watts?

So 9 panels x 170 watts = 1530 watts.

Thats a stonking 127 amps at 12v.

Far to much for your controller & your bank size.

Whats the financial implications of selling the 250 watt panel & buying 3 x 170 watts compared to buying a second 250 watt panel & a suitable controller?

Edit to add

You already seem to be putting 85 amps into the controller. Not a huge issue but lots of wasted energy.

2nd edit to add

You do know that its the output amps at the battery voltage that specifies the controller size. Not the input amps or watts.

OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Wed Aug 21, 13 7:21 pm Post subject:

RichardW wrote:

Riichard

The battery bank is 24v (feeding 2 x 12v in series).

Quote:

Whats the financial implications of selling the 250 watt panel & buying 3 x 170 watts compared to buying a second 250 watt panel & a suitable controller?

Now I've looked the 170 w panels I got were £485 for 6 but the seller doesn't have them anymore.

So back to the drawing board with my spare 250w panel sat all alone and unused..

RichardW

Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales

Posted: Wed Aug 21, 13 7:57 pm Post subject:

OtleyLad wrote:

The battery bank is 24v (feeding 2 x 12v in series).

Doh.

In that case all's good if you can find some more similar panels.

Personally I would get a cheap controller to use the 250w panel.

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