Vmp Min Calcs ?

Vic · August 2010

OK, am getting ready to order more panels for my off-grid system.

The added panels will have their own MPPT CC, such as an OB FM-80, or hopefully a Midnite Classic (when available).

One of the candidate panels is the Kyocera 210 LPU which has a 26.6 VDC Vmp, and a 23.6 NOCT Vmp. Kyo 210 data:
http://lib.store.yahoo.net/lib/wind-sun/KD210GX-LPU.pdf

With three panels in series per string, this yields 79.8 Vmp into the CC (less drops), and an NOCT Vmp = 70.8 V.

What is the customary formula used to calc the minimum available Vmp for the CC at some given Max ambient temp ?

Derating the NOCT Vmp by, lets say 5 additional percent, leaves about 67.25 Volts available to do an EQ at, lets say 62.5 Volts. Is this considered sufficient margin for the CC plus voltage drops on a 100 degree F, no wind day ? Seems a bit close to me, but probably OK (?).

The other candidate panel is the SolarWorld 235 Mono, with 30.3 and 27.5 NOCT Vmps -- to me, clearly, this panel has enough Vmp in the above scenario.

Thoughts ? Thanks Vic

niel · August 2010

Re: Vmp Min Calcs ?

i wouldn't call it a formula, but they do say on their pdf spec sheet how much to derate the pv by for every degree c above 25 degrees c. they state -01.2 x 10 ^-1 per degree c. that looks to be -.12v per degree c upward. you can look at the voltage drop calculator to see temp conversions.
http://www.kyocerasolar.com/pdf/specsheets/KyoceraSolar_KD210GX_Web.pdf
solarworld 230 i think is making things a bit fuzzier to be sure what they spec.
http://www.powerproductionmanagement.com/TRIAL/Documentation/SW-230.pdf
hope this helps.

Vic · August 2010

Re: Vmp Min Calcs ?

Hi Niel, Thanks,

Well I have looked at the derating values vs temp in C, but the STC is at 25C CELL temp. And it is very difficult to know what is the cell temp when it is in direct sun, at 1Kw/M2, and NO wind. Some indication can be seen by the NOCT footnote, stating with 20 C ambient temp, the cell temp is 47.9 degrees, but of course this is at the lower iirradiance 800/ Msquared, AND with wind of 1.5 M/sec (IIRC).

I do not wanna make too big of a thing of this, but this must a fairly standard exercise in system design. AND, EQing places fairly large demands on the system. I am not really looking for the absoulte worst-case situation of a very cold battery bank on a very hot day with NO wind, just a reasonable approximation. Guess that the safer thing to do is just to go with the SW 235.

As another data point, a 'standard' 24 volt panel, such as my current Shell 175s have a Vmp of 35.4 volts. Two in series (70.8 V) should be adequate to charge a 48 volt bank by extension. And, 70.8 V happens to be the NOCT output of the Kyo 210s. These Shell 175 panels predate NOCT ratings, and think the PTC or whatever the predecessor to NOCT was called lacked the data on cell temp under Pacific conditions.

Thanks for the reply, have just never felt that i needed to do this calc, and with the Kyo 210s seems prudent ... but this worry must not be an common one.

Thanks Vic

mike95490 · August 2010

Re: Vmp Min Calcs ?

Just go to any GT inverter sizer, and chose your panel, and adjust the number in series. They have temps set up for you to enter, and if you size for 6 or 9, you can divide the result to match your actual array

Vic · August 2010

Re: Vmp Min Calcs ?

OH, Thanks Mike ..

Grid, what IS that ? !? Just kidding. Thanks will try that. Vic

BB. · August 2010

Re: Vmp Min Calcs ?

The Xantrex XW 60 amp MPPT charge controller calculator still works for now.

The Xantrex solar panel "Hot" calculations assume a 63F (35C) temperature rise over ambient.

Solar Guppy has said that his panels run around 150F when hot(?).

The forumula I have used is:

-0.157 V/C * 1/1.8 C per F = -0.0872 V/F
Vmp = Vmp-STC + (temp coeff * (ambient+assumed temp rise-STC temp)
Example:
34.9 Vmp + (-0.0.0872 V/F * (100F+63F-77F)) = 27.4 volts Vmp Hot
26.8 Volts * 2 panels in series = 54.8 Vmp Hot for 2 series panels
43.2 Voc + (-0.0.0872 V/F * (14F -77F)) = 48.7 volts Voc Cold\

Double check my work (this is the Internet after all :roll:).

-Bill

Vic · August 2010

Re: Vmp Min Calcs ?

Hi Bill,

Thanks for the data-rich post. Did a quick once over couple of hours a.

Seems to me that the real key question is, 'what is the temp rise of the cells on the exact panel in question. And in looking at the stated temp of the cells under NOCT conditions, there is some variability in this temp ... guess that that stems from the ability of that exact panel design to dissipate the heat. This variability is not too great.

Did run the numbers with the data you provided, and the Kyo 210 would not hack it at full output with 100 F ambient, altho, the output voltage does rise with reduced current demands. But, as it stands now, looks like the SW 230 will do the job, and I do like mono cells. And SW has a 25 year gurantee vs the Kyo 210 is 20 years ... tradeoffs, tradeoffs !

Thanks again, Vic

BB. · August 2010

Re: Vmp Min Calcs ?

Remember the Equalization (of flooded cell batteries) is 15.0+ volts... Only done once every one-three months (typically). So--if there is a 100F day during one period, just wait a few days or a week until there is cooler temperature... Otherwise, the (14.4-14.6) volts gives you another 1/2 volt of headroom.

If you go through all the "worst case assumptions" (cold batteries, hot panels with poor rear ventilation, hot climate, worst case controller+wiring drop on a 12 volt system with "12 volt" panels)--it looks like a PWM controller would never work correctly such a setup--But there are probably millions of them out there and, for the most part, people are using them successfully.

-Bill

Vic · August 2010

Re: Vmp Min Calcs ?

Hi Bill,

Yes, of course, you are correct, altho I have done EQs more often than quarterly. In Summer months EQ is usually 62.0 or 62.5, depending on battery temps. Am an 'opportunity EQer', mostly. Just would seem to be a bit embarassing to go to the trouble to make improvements, and have chosen panels which are ALMOST good enuf.

The one thing about Summer EQing, is that, now that I've discovered A/C, it is more difficult finding days where there is enough time to get the EQ done from Solar -- the panels do not track, and are fixed (at this time) at the Winter angle.

Am consideering tracking the new panels, but will see how things settle.

Yes, you are correct about absolute worst-case worst-case. In looking back, my current 24 volt panels do have too little Vmp as well, altho, we run three of these in series strings, so there is pleanty of Vmp reserve. Am trying to reduce the voltage that the CC sees as Vpv, but still with a good margin.

Thanks again, BB. Vic

Vmp Min Calcs ?

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