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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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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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johanndo avatar image johanndo green-techie commented ·
Extremely high is exaggregated, you can calculate with 10-15% over nominal Voc in perfect conditions like cold weather, and clear sky after some clouds, where the panels are stil cold, but exposed to high radiation.
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green-techie avatar image green-techie johanndo commented ·
Thanks, that's worth bearing in mind. I also see from @Alexandra, in reply to my other question, that the panels I have been suggested have a high Voc to Isc ratio. Other panels I have checked would easily accommodate the strings of 9 I would prefer, even with higher output.
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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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green-techie avatar image green-techie Alexandra ♦ commented ·
The Jinko Tiger Pro 405W, which you mentioned, would certainly be suitable. As would the Tiger 420W.
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