Solar PV Voc Calc

ron

Senior Member
690.7 doesn't seem to permit any voltage drop fudging of the numbers.
I have a situation where 12 panels would make the physical installation easier, but when calculating open circuit voltage, using the manufacturers oc voltage rating and temperature coefficient, puts me slightly above 600V with 12 panels. I can't find a reference that shows higher "lowest expected ambient temperature" but I know in real life, voltage drop will make these panels exhibit an installed condition below the theoretical calculated voltage. I checked ASHRAE tables and Weather.com

I don't want to battle with an inspector later on. Is there anyway to include a fudge factor, or in your experience has inspectors not challenged the 690.7 calculation.
 

broadgage

Senior Member
The on load voltage will indeed be much lower than the maximum open circuit voltage.
Unfortunatly AFAIK code requires that the maximum open circuit voltage be used, not the on load voltage.
I can only see a very little wiggle room, what is the lowest likely temperature in the area, might it be a little higher than expected ? and therefore the open circuit voltage a little lower.
And it could reasonably be argued that one should take the lowest DAYTIME temperature, not the lowest at any time of day. If a lower temperature occurs whilst it is dark, then nothing is produced, and if a low temperature occurs around dawn then the low sun angle will reduce the voltage.
As the sun rises, the modules will get warm thereby reducing the open circuit voltage.
 

tallgirl

Senior Member
You must use the lowest daily temperature, not just the lowest temperature when the sun is shining. You also need to, for the equipment vendor's warranty, take into account any evaporative or wind cooling (wind chill) on the array. The vendor isn't going to care one whit why Voc exceeded their absolute limits, only that it did and you let the smoke out of the device. So, your fault, no warranty claim, tough luck.

The reason is this -- the PV junction will produce near full voltage as soon as enough light falls on the panel to start moving electrons, and that will happen before the sun is above the horizon, thanks to Raleigh scattering. This is why you can't use a Vdrop calculation, because Iout is pretty close to zero, so Vdrop = I x R is likewise zero. Remember -- PV junctions make current in proportion to the amount of light (insolation) and voltage in proportion to temperature. In a low light, low temperature situation you have high voltage and low current.

The issue with various types of evaporative cooling is that the AIR temperature doesn't matter -- a panel with dew on it, and a breeze, will be cooled to a temperature closer to the dew point than the air temperature. Which is good for a few more volts VOC and a greater chance of letting the smoke out.
 

philly

Senior Member
690.7 doesn't seem to permit any voltage drop fudging of the numbers.
I have a situation where 12 panels would make the physical installation easier, but when calculating open circuit voltage, using the manufacturers oc voltage rating and temperature coefficient, puts me slightly above 600V with 12 panels. QUOTE]

I have a design where I am running into a similar problem with the ideal # of modules per string resulting in a combined VOC slightly higher than 600V

I am now looking at the temperature that I am using to see if the right one is being used for the calculation. I have used both the record minimum and the minumum listed in the ASHRAE index. I'm still debating at this point which temperature to use.

And it could reasonably be argued that one should take the lowest DAYTIME temperature, not the lowest at any time of day. If a lower temperature occurs whilst it is dark, then nothing is produced, and if a low temperature occurs around dawn then the low sun angle will reduce the voltage.
As the sun rises, the modules will get warm thereby reducing the open circuit voltage.
This is something I have been considering as well. I think it is obviously clear that the lowest possible temperature used (no matter what "Low" temperature is used) will come at some time during the night or at right before dawn when there is not 1000 w/m2 irradance on the panels. I am wondering if I can use some reduced w/m2 irradanc on the panels at this low temperature to arrive at a lower more realistic VOC calculation since the VOC will decrease with avaliable irradiance.

You must use the lowest daily temperature, not just the lowest temperature when the sun is shining.
Yes we must use the lowest temperature however is there anywhere that indicates that we must use STC 1000W/M2 VOC rating of the panel as opposed so some lesser W/M2 Voc to account for a lower irradiance on the panels when this low temperature occurs.

I do not see this addresed in the NEC so may this is something at the disgression of the AHJ?
 

tallgirl

Senior Member
Insolation is irrelevant (more or less ...) for Voc calculations.

Current is produced when light strikes the junction. The more light, the more current. The voltage is determined by the nature of the junction ("solar cell") and temperature. That's it.

On the systems I monitor, I see Voc max around daybreak, before the charger controllers even turn on, or before they are going to any reasonable extent. The other time Voc is near a maximum is wet panels, dry air and high winds. And that goes to my comment about dew point.
 

philly

Senior Member
Current is produced when light strikes the junction. The more light, the more current. The voltage is determined by the nature of the junction ("solar cell") and temperature. That's it.
Yes but the amount of light also determines the Voc open circuit voltage. If you look at any specification for a PV module you can see that Voc decreases with solar irradiance. A particular module may have a Voc of 39V at 1000W/m2 where it only has a Voc of 36V at 1000W/m2.

This is what I am saying can possibly be considered when using the lowest temperature which will occur at night when there is less solar irradiance.
 

ron

Senior Member
The only fudge factor I can manufacturer it seems is "lowest expected ambient temperature" since there is no authoritative source that must be used.
 

K8MHZ

Senior Member
Yes but the amount of light also determines the Voc open circuit voltage. If you look at any specification for a PV module you can see that Voc decreases with solar irradiance. A particular module may have a Voc of 39V at 1000W/m2 where it only has a Voc of 36V at 1000W/m2.

This is what I am saying can possibly be considered when using the lowest temperature which will occur at night when there is less solar irradiance.
How can open circuit voltage decrease when there is more light? It will increase as the temp lowers, but increase with irradiance. If Voc decreases with irradiance, then maximum Voc would be in total darkness.

"A particular module may have a Voc of 39V at 1000W/m2 where it only has a Voc of 36V at 1000W/m2."

You lost me on this one. The panels we use are labeled 36 volts. We will be actually checking the Voc on them soon. How do you come up with two different voltages with nothing else changing?
 

tallgirl

Senior Member
Yes but the amount of light also determines the Voc open circuit voltage. If you look at any specification for a PV module you can see that Voc decreases with solar irradiance. A particular module may have a Voc of 39V at 1000W/m2 where it only has a Voc of 36V at 1000W/m2.

This is what I am saying can possibly be considered when using the lowest temperature which will occur at night when there is less solar irradiance.
You can say it all you want, but that doesn't make it so. The panel may be below ambient temperature (and probably is ...) for some length of time in the morning, and above ambient temperature well into the evening.

Stick with STC conditions and you'll avoid blowing something up. Also, a lot of string inverters and charge controllers monitor their maximum Voc. The last thing you want is a warranty claim to be denied because you decided to use some lower Voc value.
 

tallgirl

Senior Member
The only fudge factor I can manufacturer it seems is "lowest expected ambient temperature" since there is no authoritative source that must be used.
The weather service in your area. You need to find the record low temperature for the installation area and use that.
 

philly

Senior Member
How can open circuit voltage decrease when there is more light? It will increase as the temp lowers, but increase with irradiance. If Voc decreases with irradiance, then maximum Voc would be in total darkness.

"A particular module may have a Voc of 39V at 1000W/m2 where it only has a Voc of 36V at 1000W/m2."

You lost me on this one. The panels we use are labeled 36 volts. We will be actually checking the Voc on them soon. How do you come up with two different voltages with nothing else changing?
Oops I realize I typed my response incorrectly and is definitely confusing.

What I meant to say is that for a given temperature (keeping temp constant) the Voc of a module DECREASES with DECREASING solar irradiance. So a module that may have a Voc of 39V at an STC of 1000W/m2 may only have 36V at a solar irradiance of 200W/m2 (less sunlight) for the same temperature. So as the sun decreases in intensity to Voc of the module decreases as well. This can be seen on usually with curves provided by the panel manufacturer showing Voc values for different intensities of solar irradiance .
 

K8MHZ

Senior Member
Oops I realize I typed my response incorrectly and is definitely confusing.

What I meant to say is that for a given temperature (keeping temp constant) the Voc of a module DECREASES with DECREASING solar irradiance. So a module that may have a Voc of 39V at an STC of 1000W/m2 may only have 36V at a solar irradiance of 200W/m2 (less sunlight) for the same temperature. So as the sun decreases in intensity to Voc of the module decreases as well. This can be seen on usually with curves provided by the panel manufacturer showing Voc values for different intensities of solar irradiance .
Gotcha.

I would think that we would drop more than 3 volts with only 1/5 of the irradiance.

Gotcha.

I wold
 

c_picard

Senior Member
oh, and keep in mind that some inverters input voltage range is well below 600V(ex.PVPower), so if you try to push the limits the inverter won't turn on...

Great question by the way
 
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