Panels in parallel same Vmp different Voc

stephendv

Hi all,

I'm planning to add some Sharp micro-amorph panels to my current array of Yingli poli's. At STC the Vmp between the two differs by 2.6% all OK, but the Voc differs by 6%. The purpose of adding the panels is purely to improve cloudy weather performance in autumn and winter. I don't care so much if I lose power because of high summer temps, but I do care about losing power at low irradiance and winter temps. I'd say average ambient cloudy winter temp is about 7 degrees C.

The 12 Yingli poly panels are in 4 strings of 3 with total characteristics for each string:
Vmp: 88.5V
Voc: 112.5V
Voc temperature change: -0.33 %/C
Vmp temperature change: -0.45 %/C

Because the sharp panels have higher voltages, I plan to put them in parallel with the Yinglis in 3 strings of 2, so they would have total characteristics:
Vmp: 90.8V
Voc: 119.6V
Voc temperature change: -0.3 %/C
Vmp temperature change: Not published.

So if I adjust the Voc of both to 10 degrees panel temp (?), then the Yinglis would have 107 Voc and the Sharps 114 Voc about 7% difference. I don't know what the Vmp values would be because Sharp only publishes Voc temperature change and Pmax temperature change -0.24 %/C.

Both will be connected to a midnite CC, so since there's 2.8kW of Yingli and only 760W of Sharp I expect the Vmp of the Yingli's will carry more weight when the midnite does it's MPPT sweep. Is the 6% difference in Voc significant, or should I only look at the Vmp which is much closer?

EDIT: Datasheets:
http://www.yinglisolar.com/frontend/downloads/datasheets/YGE_235_Series_US.pdf
http://www.sharp.eu/cps/rde/xtri/documents/documents/Marketing/Datasheet/Product_Flyer_NA-FxxxGK_10years_0112_eu.pdf?siteid=5&sitetype=1

vtmaps

Re: Panels in parallel same Vmp different Voc

The only concern I have is that the two types of panels may have different max power points. That could confuse your controller.
I think that overall you will make more power, but perhaps not as much as having a dedicated controller for each panel type.
--vtMaps

Cariboocoot

Re: Panels in parallel same Vmp different Voc

The Voc difference won't be of any concern functionally. The moment the charge controller activates panel Voltage will be pulled down. MPPT controllers pretty much 'ignore' panel specs when in operation and choose what they 'think' is the best V & I for charging at any given time. So the Voltages will be the same on the two panels with the controller running, and the current will be different according to what each can produce at that Voltage. Use the higher Voc for determining V-max in.

inetdog

Re: Panels in parallel same Vmp different Voc

Cariboocoot wrote: »

The Voc difference won't be of any concern functionally. The moment the charge controller activates panel Voltage will be pulled down. MPPT controllers pretty much 'ignore' panel specs when in operation and choose what they 'think' is the best V & I for charging at any given time. So the Voltages will be the same on the two panels with the controller running, and the current will be different according to what each can produce at that Voltage. Use the higher Voc for determining V-max in.

Right and well said!

There is only one major gotcha to avoid, and that is trying to parallel two strings or arrays when the the Vmp of one (Vmp1) is higher than the Voc of the other (Voc2). Some MPPT controllers will stop scanning when they see the power decrease as they go below Vmp1 and therefore will not ever see the output of array 2. But the real MP point will be closer to Vmp2, with array 1 pulled down to match. (If array 1 is much larger than array 2, on the other hand, of the two local peaks in power, the higher one, with only array 1 producing, may well be the better one.)

If you follow the thumb rule of at most a 5% voltage difference, then you are not likely to get this kind of relationship between Vmp1 and Voc2.

stephendv

Re: Panels in parallel same Vmp different Voc

Thanks all, makes sense

BB.

Re: Panels in parallel same Vmp different Voc

Solar Guppy from years ago said that his testing showed Vmp/Imp within 10% was "good enough" as the IV/Power curves are relatively flat at the peaks.

-Bill

niel

Re: Panels in parallel same Vmp different Voc

fwiw, i'd use them.