Short circuit one panel to reduce max voltage.

[SveinHa]

I'm about to install 18 PERC solar panels, each 450W, but in the "worst case" scenario with open circuit voltage compensated for worst case low temperature, the total voltage exceeds the inverter maximum 500V PV input with about 3.5%. This MAY not be a problem but I need to have tought it thru to be on the safe side. The inverter is EASun ISolar SMW 8K.

I can bypass one panel with a relay but to do it simpler with less wiring and fewer possible fault spots, I came up with the idea to short circuit one panel when needed:

Do the solar panels survive being short circuited over time or is it better to use open circuit?

 

[timselectric]

Not sure, but I would think.
In full sunlight, a short circuit could cause excessive heat build up.

 

[SveinHa]

The specified max open circuit voltage for 9 panels in series is 450V but to compensate for low temperature increase by 15%, I end up with 517.5V. I don't know if "cold" means +10deg.C or -30 deg.C but if we are talking below zero, I guess overheating may not be an issue.

 

[timselectric]

Panels rarely meet their ratings. If factoring inefficiencies. Even with cold temperatures, it's probably not a problem. But, I try to be very cautious about over voltage to an MTTP.
I like your relay idea. If you added a diode, on the single panel. I think it would work long term without issue. (In parallel mode)

 

[timselectric]

I have the same situation. I'm running 11 panel strings, until I see how it performs this summer.
I'm set up for 12. Just waiting to see how close it is.
Math says that I will be 2 volts over. Probably not going to get that close. But, waiting to be sure. I really want those extra 500w.

 

[acdoctor]

Panel electrical specs should have temperature coefficient in volts per deg C. . That is number of deg C. From 25C. So example 15 deg C is 10 deg from 25.
So 10 x temperature coefficient per panel x 18 panels. And I would expect to see the compensated VOC unless wiring is sub standard.

 

[SveinHa]

One other thing... Is it advicable to install 500V surge protectors or is it better to go for 690V with a system maximum at 500V?

I have asked the panel supplier the same initial question, will post the answer here when it appears...

 

[timselectric]

I believe that you just need it to be higher than the max solar output voltage. Plus a little for cushion, probably wouldn't be a bad idea.

 

[rmaddy]

I'm about to install 18 PERC solar panels, each 450W, but in the "worst case" scenario with open circuit voltage compensated for worst case low temperature

Post a link to the full specs of your panels.

What is the actual worst case temperature you might ever experience?

 

[SveinHa]

Not too much info on these panels, quite a bit better for other panels from the same supplier...

Rated power: 450 watts
Power tolerance: 0 to + 5W
Maximum charging current: approx. 10.9 A (approx. 37.5A charging current for battery with MPPT regulator at 12Volt)
Short-circuit current: approx. 11.5 A
Voltage at open circuit: approx. 50 Volts **
Voltage at maximum power: approx. 41.4 Volts **
Uses PERC technology
Length: about 210 cm
Width: about 104 cm
Thickness: about 3.5 cm
Number of cells: 144 pcs 83x166mm cells
Weight: about 24 kg

In a normal winter we rarely get below -10degC but every now and then we may reach -20. The supplier says that one should add 15% safety margin on open circuit voltage to compensate for cold weather.

 

[rmaddy]

Too bad on the vague specs.

But assuming a Voc of 50.0V and a typical temperature coefficient of -0.3%/ºC, your 9S panels will reach 500V at a temperature of -12ºC.

If the Voc is even 50.2 (or higher) or if the temperature coefficient is -0.32%/C (or lower) then your panels will hit 500V at -10ºC (or warmer).

Due keep in mind that this is panel temperature, not air temperature. But this can happen first thing on a cold morning when the sun first hits the panels. The panels can jump to Voc before the sun can warm them at all.

 

[timselectric]

Also
If max VOC is 500. Then, the operating efficiency range is probably 450v at the top. An extra panel might reduce efficiency, on a sunny day.

 

[SveinHa]

An extra panel might reduce efficiency, on a sunny day.

I don't worry too much on a sunny day but would like to get as much power as possible on a cloudy day...

 

[timselectric]

That's just my OCD for efficiency talking. lol

 

[Bobert]

Not sure, but I would think.
In full sunlight, a short circuit could cause excessive heat build up.

Has anyone done experimentation on if solar panels get hotter when shorted or open? It seems to me that the energy from the sun should create a similar amount of heat regardless of wether it has been converted into electricity or not.

 

[timselectric]

Has anyone done experimentation on if solar panels get hotter when shorted or open? It seems to me that the energy from the sun should create a similar amount of heat regardless of wether it has been converted into electricity or not.

I believe that shorted, would compound the heating.
But, I have never tried doing it.
Open, wouldn't do anything.

 

[pellicle]

I'm about to install 18 PERC solar panels, each 450W, but in the "worst case" scenario with open circuit voltage compensated for worst case low temperature, the total voltage exceeds the inverter maximum 500V PV input with about 3.5%. This MAY not be a problem but I need to have tought it thru to be on the safe side. The inverter is EASun ISolar SMW 8K.

I can bypass one panel with a relay but to do it simpler with less wiring and fewer possible fault spots, I came up with the idea to short circuit one panel when needed:

Do the solar panels survive being short circuited over time or is it better to use open circuit?

firstly when I played with a mates Solar setup on his summer cottage in Eastern Finland (I lived there for a number of years) we didn't see the panels get much over voltage (and we went there in snow conditions) because by the time there was significant sunlight the panel had also warmed up a bit (them being black). However I like your idea of being cautious.
I'd alter the circuit to remove one panel by isolating it rather than shorting it out. An appropriately sepcified switch should do, an SPDT would be easy to wire in to throw so that you take 8 or 9 panels.

You may wish to take some measurements on the ground first though to check if you even need that.

PS it was about -20C or so and because we were there (in the winter) didn't heat the place up to check and then changout the PWM to an MPPT.


Finns don't normally dress like this inside ;-)

Best wishes

 

[SveinHa]

regardless of wether it has been converted into electricity or not.

Initially, I imagined that a 450W panel would produce 450W of electrical heat also when shorted but you only get the max amps and close to 0 volts so that multiplies to close to zero watts... I think...

 

[Bobert]

I believe that shorted, would compound the heating.
But, I have never tried doing it.
Open, wouldn't do anything.

I did a quick internet search on shorting solar panels and the following seemed to indicate that heat is not a significant problem with shorted panels as the maximum power point is extremely close to the same resistance as a complete short. The following quote is extrapolated from this link

Solar panel short circuit

In trying to measure the current output from a solar panel I've inadvertently short circuit the panel. Did I damaged the panel? How can I test if everything is ok?

electronics.stackexchange.com

“In trying to measure the current output from a solar panel I've inadvertently short circuit the panel.

Did I damaged the panel? How can I test if everything is ok?

No - you will not damage a solar panel by shorting it. Solar panels are designed to be continuously operated at very very close to their short circuit current.”

 

[pellicle]

0 volts so that multiplies to close to zero watts... I think...

I don't think so, but why do it if you can just side step this question with a simple Y adaptor and a switch?