Adding PV to existing Off Grid system..HELP!

rkymtnoffgrd · August 2011

Hello all, I need a little help. Were upgrading my fathers off-grid Xantrex system by adding more panels and need a little help on the best route to go. First the current system….
1) 1560w array consisting of (8qty) BP 195w 16v PV Modules (2 strings of 4 panels ea. configured at 64v.)
2) Then routed into a combiner box with a (2qty) 15amp breakers (one for ea 64v. string)
3) Then onto one xw mppt 60-150.
4) Then charging a 48v batt bank. Two 48v strings of 8 (6v)batts (16qty T-105 w/ 225 amp/hrs ea.)
5) PV Panel Stats: BP 195watt 16v. VMP:24.4 IMP:7.96 VOC: 30.7
I don’t want to purchase another charge controller. The combiner box has room for two more breakers. I can get up to 12 additional BP195w panels (exact same as original with matching VMP, IMP, VOC…ect) at a very attractive price and if I could utilize them all I would. How many panels can I utilize and what’s the best way? I was thinking of purchasing 10 additional panels for a total of 18 panels in three strings of 96v. (6 panels per string) The total wattage would be 3510watts. The other option was to purchase an additional 8 panels (for 16 total) and create 4 strings of 4 panels each, 3120 watts. One way seems to get me dangerously close to the max amp input rating, the other seems a little over the output max…. Suggestions please.

Cariboocoot · August 2011

Re: Adding PV to existing Off Grid system..HELP!

It looks to me like the only thing you can do here is add two more strings of four (eight panels total).
The reason being you're limited to four in a string (30.7 Voc * 4 = 122.8, 30.7 Voc * 5 = 153.5 - over the input limit for the controller).

That should give you 3120 Watts of array and double the output of the existing panels. Somewhere around 40 Amps peak, which should work for 450 Amp hours (48 Volt system). Just be sure the wiring from the combiner box to the controller is sufficient to handle doubling the current (about 32 Amps by my reckoning) and won't suffer too much Voltage drop along the way.

niel · August 2011

Re: Adding PV to existing Off Grid system..HELP!

you can expand a bit, but not that much without needing another controller.

the nec limits the output of the cc to 48a and at 48v that is 48a x 48v = 2304w which allows another 744w. to put it at the controller's max is 60a x 48v = 2880w giving an expandability of another 1320w.

for 195w pvs this is,
744w/195w=3 pvs
1320w/195w=6 pvs

now 3 would not be possible for 12v pvs to get 48v as you need 4 minimally of them to get the 48v so that violates the nec, but satisfies in not exceeding the controller's abilities. if you get 12 of them this still leaves 8 of them not connected to your system. even you admitted that even the combiner box won't handle all 12 pvs either as you have enough for 2 strings and that's 8 more pvs. to use them all will be purchasing some more stuff without a doubt.

i should mention the vmp is a tad low for charging as it is with 4 in series. any heat will drop the voltage further and possibly go under your batteries' needs. if you account for this small vmp by adding another pv to each string then each string consists of 5 pvs making the max to expand, and still meeting nec rules, to 10 pvs total. 15 pvs would exceed the max of the controller's current output ability.

Cariboocoot · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Niel;

I think you must have misread something. Or I did. Vmp on panels is 24.4. Times four in series is 97.6: plenty for a 48 Volt system. Three in series is 73.2 - that could get a bit dicey under hot panel conditions but is still above the 70 Volts you'd get with four standard 12 Volt panels in series. It certainly would not work with two in series (48.8 Volts) - unlike with standard "24 Volt" panels which have a Vmp around 35.

mike95490 · August 2011

Re: Adding PV to existing Off Grid system..HELP!

I'd really consider another independent array, with its own charge controller.
You gain redundancy, at the cost of just 1 charge controller.

rkymtnoffgrd · August 2011

Re: Adding PV to existing Off Grid system..HELP!

This system is installed in Hemet California, which is inland Southern California. Just for the sake of my immediate education, lets assume I did install 8 additional panels and configured them in 4 strings of 4 panels each. The total (ideal) wattage would be 3120. The max output on the controller appearantly is 60a. and, as mentioned, at 48v would deregulate the output at 2880 watts. but I have a couple of real questions which likely only you guys who really know these systems can answer....and your input is greatly appreciated.
1) How often is the scc output actually at 48v?? isnt almost always charging the 48v bank significatlly higher than this. (our inverter is programmed to shut off @ 49v for battery longevity)
2) If in the rare circumstances that the batt voltage is so low that the charge controller is required to charge at such a low voltage then would the output simply be deregulated to 2880 watts or would it harm the controller? (This is a very important question in my mind)
3) Isn't it difficult in inland southern california for panel temps to allow to such wattage. Maybe a very few times in winter? I dont think I've ever seen the existing 1560 watt array ever charging at over 1250 watts, I always assumed it was due to the panels heating.
4) Thanks for the NEC guidelines on scc output...Is this indeed the case? 48A or less per controller? Really? very enlighting...

mike95490 · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Most MPPT controllers will safely themselves to operate in a non-destructive region, if it is really hot, they throttle back. And if you don't have tracking arrays, you will have 2 hours of charging at less optimun angle, and at noon, your batteries may not demand full power.

Cariboocoot · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Your concerns are valid. Shows you're informed and think about things as you should!

The "typical" derating used for panels & controller in an off-grid application is 77%, so your actual array power by the time it gets to the batteries is more likely 3120 * 0.77: 2402. That can change with temperature, elevation, humidity, air pollution ... just about anything that happens in the atmosphere. Normally we suggest running the PV Watts program to see what sort of power you can expect, but it does not take temperature (a big factor) into account.

Then there's the inevitable variation in panel output caused by the changing position of the sun through the day. By the time it gets to be full-on to the panels, much of the charging has already taken place. Before that the panels will have lower potential output, which will limit the maximum Amps at any given time. The possibility of them actually hitting full current potential of even the low Wattage rating (about 40 Amps: 2402 Watts / 59.6 charge Volts +/-) is slim, especially if they are not deeply discharged. The NEC derating (which I personally feel is foolish) is for continuous use. Well you hardly ever see a charge controller putting out any sort of Amps continuously; it tends to go up and down as needed with shifting loads taking some of the current from battery charging and MPP changing moment to moment.

Your setting of LVD on the inverter to 49 Volts means that theoretically there would never be charge Voltage below that point. If you assume really good panel efficiency (mine run 82% - hurrah for 3200 feet of elevation!) and therefor 2558 Watts / 49 Volts lowest you get 52 Amps. How long would it stay there? Not very; the first part of charging usually finds the Voltage climbing very fast. It's the Absorb stage where it hangs on: highest charging Voltage, but lower Amps.

Frankly the NEC 80% rule is, in my opinion, one of the sillier ideas they've got. It doesn't really apply to real-world operation of charge controllers. Curiously, however, some of us have found the old Outback MX60 operate best at 75% capacity. They just seem to be most efficient there. (No lab tests done; anecdotal evidence only.)

So the question to ask here is: how much are you seeing now from the 1560 Watts of panel already installed? That would give you a number for your operational efficiency (and a basis for potential maximum Amps if the array is doubled).

niel · August 2011

Re: Adding PV to existing Off Grid system..HELP!

coot,
i went by this,
"1) 1560w array consisting of (8qty) BP 195w 16v PV Modules (2 strings of 4 panels ea. configured at 64v.)"

now what i said in general i based on wattage so it is valid as far as the controller is concerned.

Cariboocoot · August 2011

Re: Adding PV to existing Off Grid system..HELP!

niel wrote: »

coot,
i went by this,
"1) 1560w array consisting of (8qty) BP 195w 16v PV Modules (2 strings of 4 panels ea. configured at 64v.)"

now what i said in general i based on wattage so it is valid as far as the controller is concerned.

Yes; that's where the confusion arises. In the same post he lists the Vmp as 24.4 and Voc as 30.7.

TheBackRoads · August 2011

Re: Adding PV to existing Off Grid system..HELP!

http://www.se-renbu.com/support/xwsizing/Default_SE.aspx ???

rkymtnoffgrd · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Gentlemen, here is the specification of the panels from BP. They're stating a nomimal voltage of 16, but the Voc of 30.7. So when doing calulations am I right to assume we go with the Higher VOC? Is VOC the figure concerned when there is nothing connected to the panel? When would this ever apply to the charge controller? Perhaps when the controler is connected but the batteries are not? Thanks for all your help.

Performance
Rated power (Pmax) 195W
Power tolerance ±9%
Nominal voltage 16V
Limited Warranty 25 years

Electrical Characteristics
Maximum power (Pmax) 195W
Voltage at Pmax (Vmp) 24.4V
Current at Pmax (Imp) 7.96A
Warranted minimum Pmax 177.5W
Short-circuit current (Isc) 8.6A
Open-circuit voltage (Voc) 30.7V
Temperature coefficient of Isc (0.065±0.015)%/ °C
Temperature coefficient of Voc -(111±10)mV/°C
Temperature coefficient of power -(0.5±0.05)%/°C
NOCT (Air 20°C; Sun 0.8kW/m2; wind 1m/s) 47±2°C
Maximum series fuse rating 15A
Maximum system voltage 600V (U.S. NEC rating)

rkymtnoffgrd · August 2011

Re: Adding PV to existing Off Grid system..HELP!

My initial instict was to utilize the Vmp figure when doing calulations, which is where I came up with the possibility of 6 panels per sting @ a max voltage of 146.4. I'm learning alot here... The sizing tool link (thanks for posting that) also shows the max "unregulated" array size of 4 strings of 4 panels, what Coot originally suggested.

Vic · August 2011

Re: Adding PV to existing Off Grid system..HELP!

rkym,

The 16 V Nominal (in my opinion) is in the context of a "12 V Nominal panel" has a Vmp of about 17.5 V. IGNORE this Nominal value, and use only:
Vmp, Imp, Voc, Isc and Max Fuse Rating data from the panel data.

Personally, I think that there is no problem in running a single CC with the proposed new panel additions.

This might not fly during some very critical inspection ... But ...
I have heard from an installer that the X SCC 60-150 is a 45 or 48 Amp CC. It may limit itself to that max value.

And as noted above, the CC will protect itself from over-current.

Good Luck, Vic

rkymtnoffgrd · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Thanks Vic, thanks coot, that explains alot, appearantly I threw everyone off when I originally posted 16v panels and 64v strings... Sorry guys. I'm going to purchase 8 additional panels and configure two more strings. Since this is an addition, and I've already passed NEC inspection four years ago, my concern is simply reliabilty and functionality. I'll heed Coots advise to insure my wireing size from the combiner box to the controller is sufficient.
PS: They told me I could not permit a third bath becasuse my septic field was too small....that was 10 years ago, and I've enjoyed my master bath ever since and never had to pump my tank.

techntrek · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Pumping a septic system isn't about removing extra liquid, its to get out the solids that separate in the septic tank.

Cariboocoot · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Don't get me started on septic systems! Most of those regs are 50 years out of date. It's all based on expected water consumption - and therefor output. In some areas they count the number of bedrooms and use that as a basis for the number of occupants which is multiplied by 100 US gallons per day. Both the tank and field need to accommodate this volume. In adding the extra bath they probably figure "two more occupants = 200 more gallons per day" or some such, rather than the same number of people using the same volume of water just in a different location. None of them seem to have heard of low-flow shower heads or low-flush toilets or any of the other conservation measures that have been introduced, and sometimes enforced by the same people who write the septic permits.

Wait ... I said not to get me started on septic systems. I'll have to ban myself now for disobeying my own cautions. Again. :roll:

rkymtnoffgrd · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Understood Tech, perhaps I should have specified tank/field, instead of field...My point was merely that "code" demanded I install a completely new "larger" tank and field based solely on number of toilets and baths in the house, not occupants. This has nothing to do with solar...I was merely alluding to the issue with the NEC output rating on SCC as concerning only my application...After Coots brief explantion, I can understand the purpose of the code, I see its intent and necessity, but after considering my application and situation, I'll sleep ok. Cheers...

niel · August 2011

Re: Adding PV to existing Off Grid system..HELP!

if you change your system it still needs an inspection for the changes made. putting a xw mppt 60 mppt cc at or above the limits specified by the nec for the cc is against nec rules. what i said still stands as i was not basing what i said on the 16v nominal, but rather on the pv wattage and the wattage the cc can have as per nec and cc wattage limits both based on current output with various battery bank voltages. you will not make 1 cc work for 20 195w pvs. i told you so in the first place and i'm telling you again. what you do will be your decision and your consequences.

rkymtnoffgrd · August 2011

Re: Adding PV to existing Off Grid system..HELP!

No not twenty, 16, 4 strings of 4..... Here is the final question Neil, can I put four strings of four on one cc? Can this be done safely, reliably, and without danger...forget code, can it be done safely without harm and be with the limits of the cc, the wireing, and the panels.

niel · August 2011

Re: Adding PV to existing Off Grid system..HELP!

sorry for all of the confusion on my part. old age settin in.;)

:p
16 pvs should work just fine.
it will have the following specs in a 4x4 arrangement,
122.8v voc
97.6v vmp
31.84a imp

don't forget it may hold back on some of the current as downconverting will up the current to probably around the cc output limit if it is in fact limited at 60a.

rkymtnoffgrd · August 2011

Re: Adding PV to existing Off Grid system..HELP!

Niel, its obvious from your previous posts you are a scholar, and a gentleman as well...thanks sincerly for your input. I weighed every word.

Adding PV to existing Off Grid system..HELP!

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