PV Strings & Array design

Arkansasoffgrid

Have most of my solar parts picked out, but need input on panel array design. 48V system, Magnum 4448pae, with Midnite classic 200mppt CC. Panels are 250W ReneSola JC250M-24/Bb, need to put 16 panels on a 10'x30' area of roof. Roof is actually 14'x36' but I want room to walk around array. Any ideas on panel layout and best way to "group into strings" ???

Cariboocoot

Re: PV Strings & Array design

Well first of all is the fact the panels are 30Vmp. That means for a 48 Volt system you need at least three per string. You have sixteen panels so I presume you were planning on four parallel strings of four in series.

Dimensions seem to be 65" by 39" so in portrait mode you'd have 65" by 156" per string. And the four strings would be 260" 'wide'. But there's the rub; which way is 10' and which way is 30' on the roof? Ordinarily the shorter dimension is the vertical and the longer the horizontal, but not always.

Well 156" is more than 10' and so is 260" so it looks like you're down to fitting them as best you can and then keeping track on wiring to make sure electrically you have the 4x4 configuration.

Arkansasoffgrid

Re: PV Strings & Array design

What dictates # of panels per string, apperantly 90 or 120 Vmp is acceptable?

BB.

Re: PV Strings & Array design

Minimum Vmp-array for a 48 volt battery bank is ~70 VDC.

For a typical ~140 to 150 VDC maximum input MPPT charge controller, Vmp-std rating is around 90-100 VDC maximum for relatively cold climates.

2x30 V Vmp <70 volts
3x30 V Vmp >70 volts and <~100 VDC (Voc-cold less than ~140-150 VDC)
4x30 V Vmp means that Voc-cold >~140-150 VDC for cold climates.

You need to check the specifications of the MPPT charge controller you wish to use vs the coldest temperatures you expect the system to be operating. Most vendors will have a string sizing tool that will give you the Min/Max number of series panels for proper and safe operation.

-Bill

Surfpath

Re: PV Strings & Array design

BB. wrote: »

Minimum Vmp-array for a 48 volt battery bank is ~70 VDC.

You need to check the specifications of the MPPT charge controller you wish to use vs the coldest temperatures you expect the system to be operating. Most vendors will have a string sizing tool that will give you the Min/Max number of series panels for proper and safe operation.

-Bill

Hi Arkansawoffgrid,
No doubt you have checked out Midnite's string sizing tool here.

I have less panels than you do (9 for now), but a similar size roof area (10 x 30). Basically long and narrowish. I found that it was easier to mount the panels in one long run, stacked on the narrower width, since I needed space on the roof to move around them comfortably. Because of the wind/structural issues I also did not want to put them too close to the roof edge. I have just enough room to expand to 12 panels (my panels are also around 39" in width). I went with a 3 panel series arrangement (108 VDC max after one year use) because 4 would have brought me up to the Outback FM80's CC limit.

Arkansasoffgrid

Re: PV Strings & Array design

With 16 of these 250W ReneSola JC250M-24/Bb panels and a Midnite Classic 200 would it be better to have 4 strings of 4 panels, or a Classic 150 using 18= 6 strings of 3 panels? Battery bank will be 420ah. Hot Springs AR, a few days of single digits,but no severe northern weather.

vtmaps

Re: PV Strings & Array design

Arkansasoffgrid wrote: »

With 16 of these 250W ReneSola JC250M-24/Bb panels and a Midnite Classic 200 would it be better to have 4 strings of 4 panels, or a Classic 150 using 18= 6 strings of 3 panels? Battery bank will be 420ah. Hot Springs AR, a few days of single digits,but no severe northern weather.

Your panels are on the roof.... not too far from the Classic. Therefore you do not need high string voltage to overcome cable loss.

There is no doubt that you would be better off with three panels per string and a classic 150. The Classics run cooler and more efficiently when the input voltage is closer to the output voltage.

If you go four in series I predict that your Classic will be running VERY hot.
This is because:
1) the Classic is less efficient at the higher voltage
2) you are operating at its upper power limit and more power = more heat (for any given efficiency)
3) the Classic is less efficient at its highest power (for any given input voltage)

With 6 strings (three panels per string) your combined Isc is over 50 amps... you will want #4 copper cable between the combiner and the Classic. You will be pushing the limits of a Classic 150... there might be some clipping of the output as your approach the Classic's limit of 96 amps output.

btw, with a battery bank of 420 ah, why do you want/need so much panel? Do you have unusually high loads during the day? Most folks would be doing very well with 15 of those panels (5 strings of 3 panels per string).

--vtMaps

Arkansasoffgrid

Re: PV Strings & Array design

Right on both accounts, plan to run a mini split/inverter AC heat pump during daytime. And the battery bank is a single string, plan to upgrade to parallel battery strings in a few years. In the future I hope to be able to charge 800 AH battery string.

zoneblue

Re: PV Strings & Array design

vtmaps wrote: »

Your panels are on the roof.... not too far from the Classic. Therefore you do not need high string voltage to overcome cable loss.

There is no doubt that you would be better off with three panels per string and a classic 150. The Classics run cooler and more efficiently when the input voltage is closer to the output voltage.

If you go four in series I predict that your Classic will be running VERY hot.
This is because:
1) the Classic is less efficient at the higher voltage
2) you are operating at its upper power limit and more power = more heat (for any given efficiency)
3) the Classic is less efficient at its highest power (for any given input voltage)

Im with vt. However re 3) in general i would say that the controllers are more efficient at higher work loads. Although i notice the latest FM80 curve is fairly flat in the top half, but interestingly less diff from 70-100V.

Was it you who laid hands on the older manuals? I swear one day im going to get another shunt for my PV current and do some classic curves of our own...

Attachment not found.

vtmaps

Re: PV Strings & Array design

zoneblue wrote: »

Was it you who laid hands on the older manuals? I swear one day im going to get another shunt for my PV current and do some classic curves of our own...

I have posted the FM80 efficiency curves on this forum, I think. The ones I posted look a bit different from the one you just posted.
Attachment not found.

--vtMaps

zoneblue

Re: PV Strings & Array design

vtmaps wrote: »

I have posted the FM80 efficiency curves on this forum, I think. The ones I posted look a bit different from the one you just posted.
--vtMaps

Interesting. I wonder if the curves were revised from production data v theoretical?

With the setup here we ought to be able to compare 72 and 104V at 24v. I doubt our system would run at 36V, the cable is too long.

Arkansasoffgrid

Re: PV Strings & Array design

Just curious, would the aforementioned equipment be sufficient to charge a 840ah battery bank. If I decided to parallel 420ah battery strings.

BB.

Re: PV Strings & Array design

Just to be clear--What voltage battery bank?

-Bill

Arkansasoffgrid

Re: PV Strings & Array design

48v, 840ah, 16-250w panels, and classic 150 or 200.

BB.

Re: PV Strings & Array design

Using various rules of thumbs (5% to 13% rate of charge)...

• 840 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 3,218 Watt array minimum
• 840 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 6,436 Watt array nominal
• 840 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 8,367 Watt array "cost effective" maximum

Maximum "cost effective array" for A Classic 150 controller on a 48 volt battery bank:

• 83 amps (max for Classic 150 @48 volts) * 59 volts charging * 1/0.77 panel+controller derate = 6,360 Watt array "maximum"

Maximum number of panels (you then have to arrange the panels series+parallel for a "workable array"):

• 6,360 Watt array / 250 Watt panels = 25.44 = 25 panels maximum

If the system is lightly used, you can probably get away with closer to a 5% array (weekends/seasonal). However, if the system is heavily used/you want to "worry less" about battery state of charge vs loads, 10%+ array is usually a better fit. However, that will force you to use more charge controllers (2 in parallel for the Classic 150) to manage the larger array.

What are your loads--No use in oversizing the battery bank if you are not going to use the capacity (2 days storage/50% maximum discharge--or 25% nominal discharge per day).

• 840 AH * 48 volt battery bank * 0.85 inverter eff * 1/2 days storage * 0.50 maximum discharge = 8,568 WH = 8.6 kWH per day

Using PV Watts for Little Rock Ark, fixed array, tilted to latitude:

Month    Solar Radiation (kWh/m 2/day)
1      3.78     
2      4.38     
3      5.19     
4      5.67     
5      5.89     
6      5.81     
7      5.93     
8      6.08     
9      5.41     
10      5.16     
11      3.69     
12      3.56     
Year      5.05      

Toss the bottom three months (use genset for backup power during bad weather) and 8.6 kWH per day energy usage (February at 4.38 hours of sun for "break even" month):

• 8,568 WH per day * 1/0.52 system end to end eff * 1/4.38 hours sun per day = 3,762 Watt array "break even for Feb"

Your array:

• 16*250 Watt panels = 4,000 Watt array

That is the rough back of the envelope math... How does this fit your needs?

-Bill