pv array questions.

Re: pv array questions.

burrow_87 wrote: »

My question is this. Is there an advantage of series wiring to 125 volts with voc 31.32 panels at 7.98 amps?

The only advantage is lower voltage drop in your wiring, The charge controller will typically work optimally at 2x the battery voltage. VOC does increase in cold weather. Rated VOC is typically for @77 degrees.

burrow_87 wrote: »

I could theoretically get 6500 watts going into the system with 4 pv in series at 10 strings this way. Or does the voltage step down negate the point of the larger array?

I guess I'm not following you. I would guess from the VOC and IMP? Are you thinking that the charge controller will see 80 amps coming in and be able to handle it?

In the last thread on batteries I posted;

"Not in any real sense, charge controllers are rated for the amperage they can pass along to the battery bank, in a 48 volt system it can handle 4000 watts since 4000/48 = 83.333 Amps and it can handle 2000 watts at 24 volts 2000/24=83.333 Amps. That said it can deliver more wattage(power) at 48 volts. "

The actual output will max out at 80 amps, You can cost effectively use an array up to about 1.2 x the rated wattage handling, since the normal array output will be around 75-80% of the rated output. On optimal days you will lose some energy...

Re: pv array questions.

burrow_87 wrote:

Is there an advantage of series wiring to 125 volts with voc 31.32 panels at 7.98 amps?
I could theoretically get 6500 watts going into the system with 4 pv in series at 10 strings this way.

What panels are those? Voc of 31.32 volts is unusual... are you sure that 31.32 is not the Vmp?

burrow_87 wrote: »

So then there is no significant advantage to over voting the system while staying under the 80 amps. I was just trying to wrap my head around the reasoning for the fm-80 taking 150 vdc

Vmp is the voltage at which the panels make their maximum power. It is lower than Voc which is the voltage that a panel makes when exposed to light with no load on it.

If your panels really do have a Voc of 31.32 volts and you wire them up four in series (string Voc = 125.28 ), you will probably damage the controller. This is because when the panels are cooler than 75° F, their Voc increases. Almost any light at all will raise a panel's voltage to Voc. The first morning light on a cold day will push the Voc high enough to damage the controller.

The 150 volt maximum input on the FM80 is all about cold Voc. When the panels heat up in the sun, their Voc and Vmp are reduced. The power that the panels produce will be at a hot Vmp... much lower voltage than cold Voc.

When you decide how many panels to put in series, you must play off cable loss against controller inefficiency. For the best controller efficiency you should try have a Vmp that is as low as possible (but at least a few volts higher than your highest battery charging voltage... about 38-40 volts would be ideal for a 24 volt battery). For the least cable loss you want the Vmp to be as high as possible (but not so high that Voc damages the controller).

Bottom line: whether 31.32 volts is the Vmp or the Voc of your panels, the optimal array configuration for those panels on a 24 volt system will be two in series. How large will your array be? How far from the array to the controller? This will determine the size cable you need. If the distance is very far and you need expensive thick cable to transmit the power, it may make sense to increase the array string voltage to save on cable costs... however, this comes at a price.... decreased efficiency and power handling capacity of the controller.

--vtMaps

Re: pv array questions.

burrow_87 wrote: »

I am old and live off my va disability so I am just trying to design the most flexible system I can while being on a fixed income.

Do you have access to grid power? Reason asking - - it's usually a lot cheaper than off grid solar power. When all things are considered, off grid is not cheap.

Re: pv array questions.

inetdog wrote: »

But to maintain your expensive battery bank in response to several cloudy days will still require at least a small generator.
If you are willing to completely cut off your energy use when the battery bank gets too low, you can do without a generator under some circumstances. but most people will not be in a position to do that.

I live without a generator, and have for 12 years, It tends to require a larger system, or willingness to endure some discomfort. My system is sized to run an air conditioner in summer, so I have plenty of reserve for several days of minimal charging in the winter. While if I have hot days of minimal charging in the summer, I will have to reduce my a/c run time or run very low State of Charge (SOC) on my battery. Fortunately our heat typically comes with sun. I had a couple days late this summer when I ran the battery below 50% SOC.

It will be important to have a good idea of your loads to enable sizing the system properly.

It will also be good to know if you have excellent southern exposure and the number of hours per day of sun. Here's one link to a solar insolation (amount of sun you can expect by season and panel angle.

Re: pv array questions.

burrow_87 wrote: »

Made my purchases today. System consists of;
5x 435 watt panels at 60 volts open at 7.25 amps
Outback FM-80 controller
vfx 3524 outback inverter
And 12 trojan l16 re 2volts.

Came out to a tad under $7000.00 just going to keep my little generac gp3300 for a while. Hopefully I didn't miss something ordering this. I got 2/0 wire for the bank. 4/0 wire for the inverter. And I will have a 60 foot run with #2. Did I miss anything?

What does "open" mean in the specs for your panels?

burrow_87 wrote: »

Did I miss anything?

You need a mate, combiner, numerous circuit breakers, and ground electrodes and ground electrode conductors. It would be smart to have an inverter bypass switch, lightning arresters and a battery monitor.

--vtMaps

Re: pv array questions.

burrow_87 wrote: »

Did I miss anything?

You will want to figure out your number of strings, since you have a 24 volt system, and 5 panels, I'll assume 5 panels in parrellel, You need a combiner box (that holds fuses or breakers) for the panels, likely wire at the array to run to the combiner box, typically you figure out how long the wire run is for each panel and buy a single wire with the correct connectors(MC4 or Tyco, I think there is one other now) and cut the wire so you have the correct connector and bare wire at the other end. Some combiners use connectors as well now so this will depend on your combiner box. Combiner boxes typically Do NOT come with breakers or fuses or strain reliefs for the wires passing through the box.

From the combiner box you will normally want to run to a Breaker box of some sort at the 'power center' either a DC disconnect or Epanel from Midnite, makes things easier. again you will need to add breakers, DC rated for the DC wiring, Since you have such a long run, a single breaker ahead of the Charge Controller(CC) would be handy, as well as a breaker from the CC to the battery. You will also want a large breaker for the inverter a 3500 watt 24v inverter will call for a 250 watt breaker I think, your spec sheet will let you know. I'm fine with just this breaker but many people like to have a breaker or fuse on the battery. If you go with a DC disconnect box, you will want to have a small AC box for a breaker at the inverter, the Epanel will have a place for an AC breaker.

Some would suggest a battery monitor, like the Trimetric, which would require a shunt in the Breaker Box. I would have preferred the Midnite classic CC with their whiz bang Jr, but I guess you made your choice.

Grounding lugs for your panels, and ground wire, grounding rod... Also surge protectors if you're in an area with a good bit of lightning.

I saw a mate suggested, don't know if you need a mate, but others will know more about that, I didn't think I was going to need one when I had planned on an outback inverter.

Maintenance things as others have described, a meter, a DC clamp meter is nice. A hydrometer to check your batteries

If batteries are stored inside your home, a battery compartment with exhaust, Mine live outside, if you have very cold temp batteries have less capacity in the cold, it returns when they warm up.

Re: pv array questions.

Photowhit wrote: »

I saw a mate suggested, don't know if you need a mate,

The OB Mate is needed in order to program the VFX inverter/charger. You could use the defaults, I suppose, but a Mate is needed to program any changes, set the charger amp limits, EQ settings, etc. . At best only half an inverter/charger without a Mate. Since there is an OB charge controller and an OB inverter a Hub would also be in order. That enables everything to be inter-connected; control/program and read data from both the FM080 and the VFX 3524 through the Mate.

Maybe a FLEXnet DC for data purposes?

Re: pv array questions.

burrow_87 wrote: »

The combiner box is already mounted to the solar tracker pole. As is the grounding rod and straps. The breakers are already in place minus the battery to inverter and all the solar is in its own shed

Solar tracker? I thought those needed to have hardware/brackets for the panels? 5 - 4 foot by 7 foot panels might be interesting to mount.