Voc Temp Coefficient Both Directions?

[hitehm]

The Temp Coefficient of Voc is given on every module spec sheet. It's almost always if not always given as a negative "-" number. So it implies a fractional LOSS of V with every degree ABOVE a cell temp STC of 25deg C. However, when calculating the MAX string V, you get 2 choices per the NEC: either Table 690.7A or the listing or labeling of the module. And as mentioned, every module label includes a Voc TC. This comes up on many test questions and practice tests and they all seem different.

So the question is, do you use the label's TC in BOTH directions above and below STC, or above only and table 690.7A below STC?


Follow up question: I've seen this confusion as well. Do you add the NOTC temp above STC to the Ambient temp to calculate the LOWEST expected cell temp?

 

[ggunn]

The Temp Coefficient of Voc is given on every module spec sheet. It's almost always if not always given as a negative "-" number. So it implies a fractional LOSS of V with every degree ABOVE a cell temp STC of 25deg C.

That's correct but it's not normally a problem except maybe on exams. Normally you are trying to make strings as long as possible, not as short as possible. To calculate minimum voltage I would apply beta to Vmp.

Years ago, with older SMA inverters this could be a problem because their operational voltage windows were so narrow. I remember putting in systems where we had to schedule inspections as early in the day as we could because with the Texas heat the string voltage could fall below the inverter startup voltage in the early afternoon, although if the system was already running it would keep running all day.

 

[hitehm]

That's correct but it's not normally a problem except maybe on exams. Normally you are trying to make strings as long as possible, not as short as possible. To calculate minimum voltage I would apply beta to Vmp.

Years ago, with older SMA inverters this could be a problem because their operational voltage windows were so narrow. I remember putting in systems where we had to schedule inspections as early in the day as we could because with the Texas heat the string voltage could fall below the inverter startup voltage in the early afternoon, although if the system was already running it would keep running all day.

Thanks for reply. I honestly expected more peers to have an answer on this since it absolutely does appear on many exams. In fact, the reason I need clarity on this is b/c I'm studying for the nabcep PVIP exam and need to know how to answer. So if I see an exam ? that asks for the MAX string length due to the coldest temp and the Voc-TC is given either as a - or + number, should I use the TC to answer the question or use table 690.7A?

 

[ggunn]

Thanks for reply. I honestly expected more peers to have an answer on this since it absolutely does appear on many exams. In fact, the reason I need clarity on this is b/c I'm studying for the nabcep PVIP exam and need to know how to answer. So if I see an exam ? that asks for the MAX string length due to the coldest temp and the Voc-TC is given either as a - or + number, should I use the TC to answer the question or use table 690.7A?

To calculate maximum string length I correct Voc for the lowest expected temperature as shown in the ASHRAE data (although on an exam it will most likely tell you what Tmin to use) by using the difference between STC (25 degrees C) and Tmin and multiplying it by the beta value on the module data sheet. They are both negative numbers, so that will yield a positive number for delta V.

Voc + delta V = Vmax. The maximum string length is the maximum DC voltage allowed by the inverter divided by Vmax after discarding any digits following the decimal point, i.e., if the maximum DC voltage divided by Vmax is 18.9, then the maximum string length is 18 modules.

I'm pretty sure that the NEC directs you to only use the table if the beta for the module is not available, and these days it is virtually always available.

 

[hitehm]

To calculate maximum string length I correct Voc for the lowest expected temperature as shown in the ASHRAE data (although on an exam it will most likely tell you what Tmin to use) by using the difference between STC (25 degrees C) and Tmin and multiplying it by the beta value on the module data sheet. They are both negative numbers, so that will yield a positive number for delta V.

Voc + delta V = Vmax. The maximum string length is the maximum DC voltage allowed by the inverter divided by Vmax after discarding any digits following the decimal point, i.e., if the maximum DC voltage divided by Vmax is 18.9, then the maximum string length is 18 modules.

I'm pretty sure that the NEC directs you to only use the table if the beta for the module is not available, and these days it is virtually always available.

That makes sense, as long as you insert Tmin as a neg # in your calc it will yield a + result but sometimes on tests they leave off the beta sign which makes it confusing. Thanks for the help.

 

[ggunn]

That makes sense, as long as you insert Tmin as a neg # in your calc it will yield a + result...

Not Tmin, but the difference between 25 degrees C and Tmin, but yes.

 

[jaggedben]

Yes it goes in both directions.

The mathematical difference between the design temp and STC is a positive or negative number. E.g. if Tmin is -5C then -5 minus 25 is -35. Multiply that by the negative coefficient and you get a positive change (increase) in voltage. If you are calculating voltage at the max temp then e.g 40C minus 25C is 15, which when multiplied by the negative coefficient results in a negative change (decrease) in voltage. So just try to remember design temp minus 25.

 

[hitehm]

Yes it goes in both directions.

The mathematical difference between the design temp and STC is a positive or negative number. E.g. if Tmin is -5C then -5 minus 25 is -35. Multiply that by the negative coefficient and you get a positive change (increase) in voltage. If you are calculating voltage at the max temp then e.g 40C minus 25C is 15, which when multiplied by the negative coefficient results in a negative change (decrease) in voltage. So just try to remember design temp minus 25.

Thanks Jagg, that's a great way of thinking about it.