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green-techie asked

Can vertical PV panels ever reach STV voltage?

I am sizing a PV array using 405W panels, with some on a 20 degree roof and some mounted vertically on a south facing wall, to maximise winter generation.

Using a SmartSolar MPPT RS 450/100 the maximum string length allowed by the Excel Sizing Tool at 9 panels using STC data, charge limited to 100A.

This shows the max input voltage at minimum temperature as 407.2V (below max of 432V).

If I use NMOT values the sizing tool allows 10 panels with max input as 420.8V.

I plan to use 9 panels to a string on the roof.

Would it be safe to use 10 panels to a string for the vertical panels? I assume they will never reach the generation peak of the 20 degree roof panels?

The online utility does not allow over 8 panels in a string, perhaps due to the charge limiting?

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Any guidance appreciated,
Jamie

MPPT Controllersmppt rssolar sizing
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2 Answers
Alexandra avatar image
Alexandra answered ·

@green-techie

The sizing on the VOC for the mppt 450s is based on 8x float voltage.

So need to know that before proceeding.

You can run the figures through wither the mopt calculator speadsheet Or online calculator.

I do not know about the vertical orientation, but hazard a guess that ground reflection can also have an effect? I am hoping someone from further north or south can chip in on that.

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green-techie avatar image green-techie commented ·

Thank you. The float voltage is 54V, using 48V Pylontech batteries.

These are the outputs from the Excel Sizing Tool (NMOT & STC)...

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Best regards,
Jamie

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Alexandra avatar image Alexandra ♦ green-techie commented ·

@green-techie

Float on Pylontec is usually 51v

See here. For. Victron recommend setup.

At 54v you will get an evil amount of high voltage warnings and unhappiness.

I think when can connected to the victron the most they request is for bulk 53.2v

The other thing you can check is irradiance records for where you live.

For example where we are we expect about 70% of installed panel wattage, because of irradiance.

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green-techie avatar image green-techie Alexandra ♦ commented ·

Thank you again. After your previous comment I looked up the Pylontech float voltage and realised my mistake. I had naively assumed that, because the sizing tool defaulted to 54V float for a 48V battery, it meant that was the standard for 48V batteries.

I really want to maximise winter generation, so I am reluctant to reduce the panel count. Especially as I'm less than 30% oversize for the array and I wanted to go higher.

I haven't, so far, found an irradiance figure for where I am in the UK (mid Wales) that I can compare to the STC figure for any reassurance.

I am concerned that my 11yo 4kWp roof array can still exceed its specified power output. I have 17 x 230W Sanyo panels with a 4kW SunnyBoy inverter, so my maximum output should be 3910Wp. I have a pulse counter on the generation meter and sometimes I can still reach 4100Wp, averaged over some minute periods. I realise this may not be the same for different hardware, but I am concerned.

If that turns out to be true for my new array then, according to the Victron docs, I might get an "Over-charge Protection" error. It goes on to say "To correct this, either increase the battery float voltage, or reduce PV voltage by removing PV panels from the string to bring the voltage back within specification." It sounds like increasing the float voltage is not an option and removing PV panels from a large roof area would not be a minor task!

All the best,
Jamie

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kevgermany avatar image kevgermany ♦♦ green-techie commented ·
You could switch panels in and out with suitable switches and wiring. But there's a risk of forgetting or not being there... For me this risk is very high, YMMV. PV Voltage should always be comfotably under the MPPT rating.
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green-techie avatar image green-techie kevgermany ♦♦ commented ·
Thanks for the suggestion. As the voltage is within the theoretical limit until -10C, I suppose this means I would only need to disconnect the extra panels in the winter. How ironic!
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johanndo answered ·

Voc can be reached by any panel, regardless of the orientation. Solar panels are current sources not voltage sources. Even with low light, the open circuit voltage is reached, it depends only on cell count and cell temperature (lower temperature means higher voltage).

With higher radiance you get a higher current, Isc is almost proportional on the radiance radiance (light exposure).

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kevgermany avatar image kevgermany ♦♦ commented ·
Yes. I was told by a very senior person at Victron that you can get extremely high voltages on a cold sunny winter's day. My own experience matches what @JohannDo has said. PV Voltage climbs, MPPT outputs to batteries and will lower the PV voltage somewhat as it optimises power output.
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green-techie avatar image green-techie kevgermany ♦♦ commented ·
Only saw your answer after replying to @JohannDo but that could be my biggest problem. I wonder if "extremely high" means greater than STC figures and temperature coefficient would suggest.
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green-techie avatar image green-techie commented ·

Thanks for the explanation, that seems an important point.

I see that for my panels the Voc in low light is a little lower (38.69 vs 41.6 for NMOT/STC).

With STC figures the sizing utility says I have 7.4V of headroom at -3oC, but only 0.8V at -10oC.

With NMOT figures I am OK down to -40oC.

I have an outdoor temperature monitor and data going back to 2008. We dipped to -10oC in 2009 and -14oC in 2010.

Do you know how much headroom is normally recommended?

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Alexandra avatar image Alexandra ♦ green-techie commented ·

@green-techie

If you are needing lower light performance use panels with higher isc ratings. You can get modules with 30voc and 13A isc for 405w of power

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