Aditional 4200w of PV

[offgridQLD]

I am upgrading current 3960w of PV with a additional 4200W of PV .We just purchased an electric car so need to cover its recharging  loads each day on our off grid system.

At the moment I have one classic 150 wired the 3960w of PV charging 48v battery. The system is doing fine in a two series configuration with the classic 150. The cable runs are shortish though 10 - 15m MAX depending where you measure from.

I have ordered 21 x 200w panels for the additional system. I wanted to purchase another classic 150 and run them in follow me mode charging the same battery.

My issue is the new 4200w of panels will be around 25 - 30m away from the new charge controller. I was initially going to use 20 panels in total for 4000w and run them in two series of 10 but I  purchased the extra panel so I could run 3 series strings of 7 panels for higher voltage.

My question is. Is there a voltage that each classic is the most efficient at. If I was to wire the panels in two stings of 10. I would have roughly 72v or if I did 3 stings of 7 I would have roughly 108v. both are within specs on the midnite calculators but is one better than the other? What i mean by that is dose the classic 150 have a voltage its most efficient at? My only reason for going 105v would be cable size on the relatively long run.

Or should I purchase a classic 250 and wire the panels in 5 series and up the voltage more. Or is it better to stick with two classics of the same line ( as in two 150's) when using follow me feature?

Specs of new  panels.

Open Circuit voltage Voc (V)    45.6
Short circuit current Isc (A)    5.89
Maximum power voltage Vpm (V)    36.9
Maximum power current Ipm (A)    5.69
System Voltage (V DC)    1000 VDC
Power temperature coefficient    -0.40%/K
Operating temperature (C)    -40 - +85C
Storage temperature (C)    -40 - +85C

Kurt

[Westbranch]

If I was to wire the panels in two stings of 10. I would have roughly 72v or if I did 3 stings of 7 I would have roughly 108v.

the closer the PV voltage is to your required bank charge voltage the more efficient the conversion, less heat generated etc

hth

add: bold words for clarity

[TomW]

the closer the PV voltage is to your bank voltage the more efficient the conversion, less heat generated etc

hth

I agree with this but not "too close".

I seem to recall you need battery volts X 1.33 to get MPPT charging with the Classic so be sure you are over that on string voltage after cable losses.

I have been wrong a lot lately so "your mileage may vary", etc.

Tom

[Westbranch]

Tom added a few clarifying words to original post.

Should also add that there should also be a few extra PV volts above the "recommended/required charge voltage" for line losses and equipment losses as well as to cover heat related PV voltage droop so that a charge can actually occur.

[mtdoc]

You need to look at the VOC of the series strings.  Unless you live in the tropics you also need to pay attention to temp coeffficients - since VOC will be higher for temps < 25 C.

Looking at your panels - 3 in series gives a string VOC of  137 which is pushing it for a Classic 150.

2 per string would be better for a 150, 3 per string for a 200, 4 for a 250.

Better efficiency for the reasons outlined by Westbranch above  and less concern about exceeding the controller input limit (and going into hyperVOC mode) in cool temperatures.

[dgd]

For a different viewpoint...
I would go with three of these panels in a series string. It's unlikely that they will be at opv very often, if at all and the 150 will be ok with 135v input.
Also the Mpv of 36.9  is actually quite variable as various factors like angle of sun NS and EW, partial cover, atmospheric conditions and panel temp will often mean the actual voltage from the panel is lower so MVP for a series of 3 PVs may be anywhere from 85v to  110v
Wire size is probably not an issue.

Just crunching the numbers....
48v * 1.33 = 64v.    2pv series at MVP = 74v. But if PV drops below 32v per PV then v too low for classic to charge battery. Not sure if 48v bank is at higher v, say 54 then does 1.33 rule still apply?
If so then v with 2 panels is too close and you could see Classic not charging battery

So  to create more charge in lower PV v ranges series 3 together.
Just IMHO

Dgd

[offgridQLD]

Thanks for the reply's.

I should explain my climate conditionsnd add some more info .

They will be installed in Subtropical QLD Australia. Winter min temps 2 deg C (average min more like 12 deg C over winter) with average day time winter temps of 18 - 25 deg C. Summer temps min 20 deg C max 38 deg C and average day temp 30C. Cold weather isn't a issue in my area.

They will be facing north/ west (So peek output a little after mid day) in my part of the world. fixed at about 15 deg (the roof is at this angle and I am lazy LOL)

My system I have now has two series and performs well and the panels are actually a little lower voltage than the new ones.

I have ordered 21 panels so I have the option to go 3 series and if not I just have one spare panel. (always a good thing)

I always consider average charging voltage of a 48v flooded led acid system to be 51 - 61v range and the bulk of the charge is done around  55V. So I can see how there is always inefficiency's in converting DC-DC so I was thinking two series would be best. Then One person mentioned my 30m cable run and how unless I used some quite thick cable I might see cable losses more than the controller losses I would get at 3 series 108v.

Kurt

[vtmaps]

As has been mentioned, having three panels in series might push you into hyperVoc.  Having two in series will be easier on the classic.  Either way you will need some pretty thick cable to have an acceptable line loss. 
Here are some numbers:
If you go three in series through 30 meters of #4 cable you will have a voltage drop of 2 volts and a power loss of 80 watts at maximum power (seldom achieved).  The power loss in the cable goes as the square of the power produced, so at half power you would be losing 20 watts in the cable.

If you go two in series through 30 meters of #4 cable you will have a voltage drop of 2.9 volts and a power loss of 164 watts at maximum power (seldom achieved).  The power loss in the cable goes as the square of the power produced, so at half power you would be losing 41 watts in the cable.

If you go two in series through 30 meters of #2 cable you will have a voltage drop of 1.8 volts and a power loss of 103 watts at maximum power (seldom achieved).  The power loss in the cable goes as the square of the power produced, so at half power you would be losing 26 watts in the cable.

If you can afford it, I recommend two in series and #2 cable.

--vtMaps

[mtdoc]

Good info vtmaps.

The other option would be to run smaller wire from each string to a combiner box closer to the batteries and a short home run of larger wire. Might be cheaper.

For example 30 meters of 10 AWG for each string to a combiner box next to the CC. 

This may or may not be less expensive or more difficult to do depending on wire pricing, conduit considerations, etc.  For my system I ended up doing it this way with both my arrays.

[RossW]

The other option would be to run smaller wire from each string to a combiner box closer to the batteries and a short home run of larger wire. Might be cheaper.

For example 30 meters of 10 AWG for each string to a combiner box next to the CC. 

Not sure it's relevant, but thats what I did here - but mostly for reasons of convenience.
I have six arrays. 4 of them are tracking, 2 are simply seasonally-adjusted.
Each of the 6 arrays are wired using 4 sq mm cable. (about 11 AWG), but the arrays range between 20 and35 metres from my battery/controller/inverter area. Each array comes in to a combiner panel inside where each cable has a double-pole DC rated breaker, a schottky diode (to prevent backfeeding to any array) and a 10A DC meter. A tiny PLC measures the bus voltage and individual array currents (I use the meters themselves as the shunts). It's very nice to be able to see each arrays output to know if one isn't tracking, needs cleaning or has some other problem! (And the cheaper and easier to work with wire is nice)

[offgridQLD]

All good info I will go with two series 20 panels total.

The panels that I have now 24x 165w are wired in 2 series 2 parallel  into a combiner box that's mounted to the railing just under the panels. So in total there is 6 or the 2s 2p strings. From each of the 6 combiner boxed is a slightly thicker wire that runs down to a breaker box near the charge controller 6 beakers in total then one larger wire for 1m to the classic.

When I say my new panels will be 30m away its not quite true. The layout of the panels will be in two long rows of 10 so roughly 10m long. The closest panels on the left will be 25m and the most distant roughly 35m. I did think several thinner wires could be better.

I could go 5 strings (800w each - 4000w total ) with 4 panels in 2s - 2p for each string. Perhaps that is best like the picture below. And some kind of combiner box for each string mounted on the railing and then run 5 thin wires (only need to handle (10A - 800w each) run them back to the power room into a box with 5 breakers then larger short cable to the new classic.

lots of options I  guess.





Kurt

[RossW]

All good info I will go with two series 20 panels total.

Just realized - I left off perhaps the most critical bit.
My arrays are each 6, nominal 12V, 100W panels. Vmp around 17V, gives an array Vmp of just over 100V and a nominal Voc of 126 (although it does go significantly higher I've noticed)

I chose this arrangement because 6 panels @ 100W each are easily managed by one person and have plenty of flexibility in configuration. Also if one gets damaged, it's easier/cheaper to replace one 100W module than larger ones.

[dgd]

Just realized - I left off perhaps the most critical bit.
My arrays are each 6, nominal 12V, 100W panels. Vmp around 17V, gives an array Vmp of just over 100V and a nominal Voc of 126 (although it does go significantly higher I've noticed)

..which would be the equivalent of a string of 3 in series 36v PVs.  . Are you using a Classic 150?
So getting near hypervoc is not an issue?

I chose this arrangement because 6 panels @ 100W each are easily managed by one person and have plenty of flexibility in configuration. Also if one gets damaged, it's easier/cheaper to replace one 100W module than larger ones.

Hopefully that (damage) is never an issue, interesting though that economy of scale seems to kick in with larger wattage PVs, I see here in NZ that 295 to 320 watt PVs are around 86c/watt but 85 to 110 watt PVs are near $2 per watt.
Dgd

[Westbranch]

We are seeing similar prices this (NA) side of the pond...  I think it might be a $$ drop as GT is seeing subsidies dropping or disappearing all over the world, leading to over supply

[cpm]

well,

Here on this side of the pond (the US), folks who based their long term models on
the whims of the US government really haven't bothered to read their history,
not even the history of solar power.

if it doesn't make sense absent subsidy, then it doesn't make sense.