Pv Module Readings

[Barx]

Hey guys.

A little question. Say a module has stats of:

    Voc - 37.4

    Isc - 8.98

Now, obviously these figures are STC values. How do you work out the expected voltage for different temps? Is it to do with the temperature coefficient found on the data sheet?

The reason I ask is this. I was asked today what V would be expect from a string of 16 panels, Now today was about 8 degrees C. The coefficient (Voc) was -0.30 % / degree C

Thus: 25 - 8 = 17 degrees difference from STC

          17 * -0.30 = -5.1 = total coefficient

           16 x 37.4 = 598.4 Voc for string

            598.4 + 5.1% = 628.9v expected

So, why on earth do the physical reading come out at < 550V

What am I missing? (probably something obvious) It could well be down to low irradiance readings, I was told that Volts changes with panel temp. Obviously there must be a point that the generated power is so low to support the voltage????

I'm probably going to be made to look a fool now, but I guess it will get it all out in the air.

Thanks

 

[binky]

VCc also varies with irradiance - you need to look at data sheet to get some ideas for that. Other than that, if the array has been 'stalled' for a while the panels will warm up so panel temp could be higher than air temp.

 

[kme]

V (oc) is , if I recall, stated under stc of 1000 W/M sq - so if your measured irradiance is less than that, you have to take that into account. 

Panel volts change quite dramatically - hence why, if we were installing in late Autumn or Winter; and it was a poor light day, or getting late; there wouldn`t be enough string voltage to kick the inverter in - which was why we had 2 x 400W HMI fittings - we could flood the array, and fool it into generation ( we used to have to take a photo of the gen. meter, showing at least 0.1 KW/h, before they`d pay for the install as "completed". 

A total ballache if the job was >100 miles away, you didn`t want to return the next day for commisioning........

 

[wunmi007]

We know that V is drastically affected by the increase in Panel Temp, my target is to make sure there is enough juicy in those babies to kick the inverter. One  thing I'm still trying to rap my head around is the difference in what energy actually charges the bank, found my self arguing between the total Watt of the PV array or the Nominal Current Imp, cause we know W = Vmp * Imp, so which value is more important cause current is what we know that charges the battery.

Let us consider the following off grid setup:

4 PV panel of (250W, 30.8Vmp, 8.1Imp), 4 PV in Parallel = (30.8Vmp 32.48Imp) = 1000.38W and in 4 in Series = (132.2Vmp 8.12Imp) = 1073.46W

  2 in Series (61.6Vmp * 8.12Imp) = 500.19W, connect in Paralell = (61.6Vmp * 16.24Imp) = 1000.38 

                                                                                                                  Vs

4 PV panels of (250W, 16Vmp, 15.63Imp), 4 in Para =(16Vmp, 62.52Imp) and 4 in Series = (64Vmp 15.63Imp), 2 in Series (32Vmp, 15.63Imp) = 500.16W connect in Para ( 32Vmp, 31.26Imp) = 1000.32W, 2 in Para (16Vmp, 31.36Imp) = 500.16 , in Series (32Vmp, 31.36Imp) = 1003.52W

connected to a bank of 4 12V 200AH either in a 12,24,48v P or S connection there are only a few PV configuration above connected through a 40A or 60A 3 statge mppt charge controller that will work for the bank. My question is what is the best PV configuration achievable for the 4 batt. bank for maximum charge in a 8-10hrs of good sun, if increase in panel/external temp is not an issue.