Temperature Impact of Open Circuit Voltage in PV Module

Like a standard semiconductor the open circuit voltage is dependent on temperature, but the change in voltage is very small (in the mV per degree C range) so even a lot of panels connected in parallel and run at a high temperature will not result in a large voltage change.

A link

I would just go with the maximum open circuit voltage as indicated in the solar cell or panel data sheets and add an additional safety margin if there was any concern.

Open circuit voltage per cell drops by about 0.3% per degree C of temperature increase. A series string with an open circuit voltage of 400 V at 30 C will show a bit under 440 V at 0 C.

As temp. Rises so does resistance. The lower the temp. of the conductor the more current can flow.

I asked my electrical colleagues a similar question yesterday. Apart from the issue of slight increases in the temperature, I was more worried about the following: I understand that most types of PV Panels have a materials limit wrt exposure to excessive heat and such extremes. They may even be subject to damage under extremes of heat. I read about this kind of thing occurring with PV installations in Dubai. Does anyone know more about this. Also, one must keep a look out for quality and life-span wrt PV Panels. At a recent Energy Exhibition in SA, I came across the rough capital cost of $1/ Watt of PV Panels. I have not yet done a survey to relate cost to quality. Is this a good average figure for good PV Panels? And how does it vary as size increases?

Thank you vey much for the link. I read it carefully, and while I am not a physics whiz, I was able to gather some interesting info. Let me ask the question another way, because I was still unable to grasp the answer. Is open circuit voltage a theoretical maximum that can be created by the intrinsic carrier concentration reacting with the band gap energy on a P/N junction? When you have 0 current (as VOC dictates) how does an increase in V become relevant?

The more I study this issue, I see the need to adjust VMP for temp, but I dont see the logic behind adjusting VOC.

VOC is not a theoretical concern, it is a real concern that will occur and as noted by others is a predictable coefficient for the various absorber materials (Si, ASi, CIGS etc). As you surely know, installation location is used to define what the avg min temperature of the area is and therefor the max VOC you will need to designed to. Again, this changes based on location and you can use this to your advantage say if you install in Florida vs Alaska.

UL and IEC's only goal in life is to ensure safety of components at predefined conditions which for PV are -40C- 120C. What this means is that in the US, UL uses 600VDC for all the components. There are impulse tests that are much higher, eg, 2500V but this is not sustained and has a different purpose.

Murphy's is alive and well on this point because the days of highest solar insulation are the dead of a cold clear winter day and so that is when VOC would be highest. VMP also increases and so inverter ratings need to be taken into account. The good news is that the inverter runs more efficient on cold days.

Cheers,

Thanks Tommy,

I do understamd and design to this requirement. My question is, why is NEC concerned with voltage in the absence of current (which is a requirement of VOC)

Step back for a second and think about how the agencies and component manufactures rely on each other to comply to the various certifications and codes. The primary concern of the entire chain is life and fire safety. Therefor all of the devices (wire insulation, creepage, clearance, pollution factors, exposure factors) are designed, tested and certified to the requirements and in this case it is 600VDC. If this is exceeded, and without regard to all of the multiplying safety factors, the systems will arc through the insulation and could kill somebody and finally burn the place down and kill more people. Well not really, but the standard in the US was set to 600VDC and in Europe it is 1000VDC.

The short answer to your question of, "Why is NEC concerned about VOC?" Because the insulation of the components will break down and arc through at >600VDC, independent of the flow of current. In fact the highest voltage of the system is when the disconnect is off and VOC floats up to maximum.

If you have a savy inspector you could theoretically use all IEC certified components (they have mostly harmonized to IEC) up stream and still be safe up to 1000V. However, this is considered very high voltage in the use such that the DC disconnect would likely need to be UL rated for this which is really expensive (not just a simple Square D box).