So riddle me this! CC, battery charging vs's actual PV outputs

aksalaaksala Posts: 37Registered Users ✭✭
 So I'm still in the planning stages of building an off-grid home in Alaska. I've fairly well made a design decision with help from this forum as well as advice from a local installer. 

 I don't want to discuss my loads etc., in this thread but concentrate on my question.

 My question follows the given scenaro
  • C20 1766ah battery bank @48V keeping at or above 50% DoD
  • Daily household utilization of approx 15-18KW (350 - 400Ah)
  • Generator support (18Kva)
  • Conext XW+ 6848, 2 x MPPT 80/600 CC
  • 26 Canadian Solar CS6K 295MS
  • Fixed roof mount, 45' angle 180' azimuth. 
  • Geographical location, Willow Alaska - 61-46N, 149-56W

 My question has to do with battery charging via the CC's. Specifically, given inherent losses in panels, wire, conversion etc., how can roughly 7KW in panels manage to keep a battery bank of this size charged? It seems to me, looking at the actual output of the panels that the best I can do is roughly 3000W into each CC. I was originally thinking that I could get close to 160 charge amps from the two controllers at 60V, but the only way I see getting that is to have 4800W into the CC, but I don't see how you can get to 4800W into the CC given the max VoC or Isc etc.. limits! 

 When I use pvwatts to show hourly output, the closest I see to a max DC output from the panels is 6KW. But that's based on the entire system. 3KW per CC. So I don't see how I can ever approach 80amps at 60V unless I increase the number or wattage of the panels but then, according to the minimum temperatures and temperature coefficients, blah blah blah, I end up over the max input ratings for the CC. What I think is missing is that when I hit those max low temps, the sun is so low in elevation that with my panels at 45' I'm lucky if I get 600W/hour our of the whole array! 

So, I guess it's two questions. How could you size an array to deliver 80amps at 60V out of the 80 600 MPPT that doesn't violate max input limits? And how is this 7K array going to keep a 1766Ah going? During Nov - Feb I'll be using the generator to keep things up but I hope to be able to fall back to primarily solar the rest of the year only utilizing the generator to either EQ or cover poor weather.

Comments

  • EstragonEstragon Posts: 2,947Registered Users ✭✭✭✭✭
    The "7k" array might produce about 75% of STC rating, so say 5kw. 5kw ÷58v charging = ~ 85a. A bit light for a 1700ah bank in a full time off-grid application. No matter how you slice it with string config etc., I suspect you'll be running the generator a fair bit.

    At Alaskan latitude, you could reduce summer generator use by flattening panels from 45°. Still, a 5% charging rate capacity is pretty low.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jonrjonr Posts: 1,100Solar Expert ✭✭✭✭
    edited June 27 #3
    How about just 24 panels - 12 in series to each CC?      If you need more watts under low light conditions, you could add another string and CC (36 panels).

    Consider a smaller generator, which will be more efficient.   Even better if you can make use of the heat from it.
  • mcgivormcgivor Posts: 2,314Solar Expert ✭✭✭✭✭
    Designing a system on paper with all relevant figures, array angle direction etcetera, will at best result in a theoretical result, actual performance will only be revealed when in use. Generally in full time offgrid it's better to have excess production potential rather than being on the edge, unless regular generator use is proposed. Given your geographic location along with supplied numbers my opinion is that you would be close to the edge.  
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • aksalaaksala Posts: 37Registered Users ✭✭
    jonr said:
    How about just 24 panels - 12 in series to each CC?      If you need more watts under low light conditions, you could add another string and CC (36 panels).

     The original proposal was 24 panels, 12 on each CC. I'm trying to eek as much as I can from the available roof mounting since it's up higher and relatively inexpensive to install. Why not 13 on each CC? I'd like to maximize what I can get out of each CC. 
  • aksalaaksala Posts: 37Registered Users ✭✭
    edited June 27 #6

    Estragon said:
    The "7k" array might produce about 75% of STC rating, so say 5kw. 5kw ÷58v charging = ~ 85a. A bit light for a 1700ah bank in a full time off-grid application. No matter how you slice it with string config etc., I suspect you'll be running the generator a fair bit.

    At Alaskan latitude, you could reduce summer generator use by flattening panels from 45°. Still, a 5% charging rate capacity is pretty low.
    Right. I got that. That's why I'm asking....how can you get 80A out of a Conext 80 600? I was originally looking at 2 of these MPPT before I asked this vendor for a quote. He basically quoted the same system I had come up with on my own except changing the panel mfgr and quoting different batteries (my original concept had 1882AH). He of course also quoted the materials to put the thing together. So I thought it should be good to go but as I learn more and investigate I'm thinking it's light in the solar department. It's already rather heavy on the budget, so I don't see adding more panels at this point but if I know that I may very well NEED to add another array I can at least plan for it in the overall design. Like leaving room for another CC, stub conduit and figure out WHERE another array is likely going to go! Frankly I'd love to put another 12 panels up on a 3rd CC but it's not in my budget currently. 

    What is bugging me is that I can't even figure out how I'd get 4800W DC input into the CC to produce 80A at 60V output. Given that the CC stops charging above 550V and has a max Isc of 28A. If anyone can tell me of an array that I can put together to feed the CC at 4800W and stay within those maximums I'd love to hear it! 

    Perhaps I'm missing the summer length of day? I mean if I pick  random day in the first week of July as example; the array can produce over 44KW for the day. I just don't see how the longer time at such low current is beneficial for the longevity of my batteries. 
  • mcgivormcgivor Posts: 2,314Solar Expert ✭✭✭✭✭
    What is bugging me is that I can't even figure out how I'd get 4800W DC input into the CC to produce 80A at 60V output. Given that the CC stops charging above 550V and has a max Isc of 28A. If anyone can tell me of an array that I can put together to feed the CC at 4800W and stay within those maximums I'd love to hear it! 

    By using a series parallel array instead of all series is how to increase total power, it may be at lower voltage so conductors would need to be sized appropriately.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • jonrjonr Posts: 1,100Solar Expert ✭✭✭✭
     > Why not 13 on each CC?  
    Might be OK, depends on just how low a temp you might get (ie, hitting the max voltage limit).

    > how I'd get 4800W DC input
    Maybe 8x2 panels on each controller for a total of 32 panels? 

    Re layout, you can't always get everything you want. 
  • mike95490mike95490 Posts: 7,964Solar Expert ✭✭✭✭
    With your possibility of cold weather spiking the panel Voc, I'd be very careful about adding too many panels in series strings

    You better be sure your PV strings output at least 120VDC, there are times, when batteries are cold, you may need +65V to charge  60V charging is NOT always enough in cold weather
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • aksalaaksala Posts: 37Registered Users ✭✭
    jonr said:
     > Why not 13 on each CC?  
    Might be OK, depends on just how low a temp you might get (ie, hitting the max voltage limit).

    > how I'd get 4800W DC input
    Maybe 8x2 panels on each controller for a total of 32 panels? 

    Re layout, you can't always get everything you want. 

     Yes, perhaps 8 in series x 2 strings per controller. I can get 32 on the roof but 8 of them would have to be on a third row split into two groups of 4 and those 4 would suffer some morning and afternoon shading. 

  • PhotowhitPhotowhit Posts: 4,742Solar Expert ✭✭✭✭
    @mcgivor has it, multiple strings in parallel. 

    Something to consider is with a fixed array that far north, the sun 'boxes the compass'. Moving from East - Northeast to west-northwest in the summer time. Depending on when you need the most energy you might find that angling your panels south east and south west, to give you a longer charging curve might be advantageous. I'm sure winter will be a nightmare with lots of generator time.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • EstragonEstragon Posts: 2,947Registered Users ✭✭✭✭✭
    Is the "up higher" roof mounting space easily accessed? Depending on climate, it may be necessary to manually clear snow off an array at 45° tilt.

    If adding panels prone to shade at some point, you may want to consider putting them on a pair of individual cheaper pwm controllers instead of a single larger mppt.

    If at higher altitude, you may need to derate controller output and not max out the 80a controllers.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • aksalaaksala Posts: 37Registered Users ✭✭
    Photowhit said:
    @mcgivor has it, multiple strings in parallel. 

    Something to consider is with a fixed array that far north, the sun 'boxes the compass'. Moving from East - Northeast to west-northwest in the summer time. Depending on when you need the most energy you might find that angling your panels south east and south west, to give you a longer charging curve might be advantageous. I'm sure winter will be a nightmare with lots of generator time.
    Yes, I've looked at that quite a bit. However in my case it seems to me that I really want to maximize collection during Sept and Feb. October through January the sun starts to get so low in the sky not to mention it's only there for a few hours that there's no way it's going to keep up, so I don't worry about it and instead have been concentrating on a good diesel generator that will charge at about 1gph. During the months you're referring to I have more than enough power with the planned array. 

    I often see mention in the forum to plan for your lowest winter sun availability, but for me that's obviously not cost effective. I'm just trying to make sure I make the most of my 2 CC's and roof space initially, so as to get as much as I can out of the panels before we just switch to charging by generator. 
  • aksalaaksala Posts: 37Registered Users ✭✭
    Estragon said:
    Is the "up higher" roof mounting space easily accessed? Depending on climate, it may be necessary to manually clear snow off an array at 45° tilt.

    If adding panels prone to shade at some point, you may want to consider putting them on a pair of individual cheaper pwm controllers instead of a single larger mppt.

    If at higher altitude, you may need to derate controller output and not max out the 80a controllers.
     Nothing on a 12x12 pitch roof is easily accessible IMHO! In this case once the snow comes there's so little sunlight it's not worth clearing panels anyway. At that angle facing south, it will self clear pretty well. But again, during those dark days of winter it's easier to just compute your fuel costs to run the generator and enjoy the fire. If there's snow on the panels in mid February then we'll have to see what we can do but I'll deal with that when and if I see it. 

     No problem with altitude, we're only about 300' above sea level. 
  • PhotowhitPhotowhit Posts: 4,742Solar Expert ✭✭✭✭
    aksala said:
    • C20 1766ah battery bank @48V keeping at or above 50% DoD
    Perhaps you are saying you want to keep your discharge less than 50%, but in general you want the batteries to reach about 85% each day to prevent sulfates from hardening.
    aksala said:
    • Daily household utilization of approx 15-18KW (350 - 400Ah)
    Looks like you are shooting for a max 20% discharge. Did you include the losses due to inverter inefficiency? I believe the Conext XW+ 6848 is very effiecent peaking around 95% and may average around 93% still if not taken into account that is a large additional loss.
    aksala said:
    • 26 Canadian Solar CS6K 295MS
    7670 watt array is smaller than we would normally like to see for a battery bank of 1766 @ 48 volts. Usually like to see the ability of charging at 10-13% of capacity of the battery bank. Your 7670 will likely produce close to name plate during cold months or 7670/58(charging volts for 48v bank)=132 amps. 132 amp/1766= 7.5%, this figure will be worse during summer months when the panels are warm/hot and producing only 75% of the panel rating.

    aksala said:

     When I use pvwatts to show hourly output,....
    I think PVwatts is setup it's default parameters for Grid tied systems. You can change the parameters under loss calculator but the set parameters only use 1% for the difference from name plate rating, while panels when hot will produce around 75% of their panel rating or a 25% loss.



    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • EstragonEstragon Posts: 2,947Registered Users ✭✭✭✭✭
    When my panels get covered and it sticks on, they will stay that way until spring unless manually cleared. Mine are a bit steeper (~65°) and it depends on the nature of early snow whether they get stuck-on covered. Cold fluffy stuff falls off, and any that doesn't melts off, even when it's cold. Wetter stuff sticks on, then freezes into a multi-inch thick mass, so the sun can't warm the panels.

    Personally, I'd rather rig up anchor points and ladder tie-offs in nice weather, but maybe you'll never need them.

    Even at my latitude (~49°), solar doesn't do much from Nov through Jan. Tree shadows etc would be mighty long at yours.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • aksalaaksala Posts: 37Registered Users ✭✭
    @Photowhit

    Thanks for the reply. Yes, shooting for about 20% DoD daily on the bank, max 50% before running the generator. We'll just have to get in the house and live for a while to see how things measure out obviously. And yes to losses. I include losses in all my calculations except for getting into the weeds on wire loss. I tend to be conservative in my estimates and used a 93% efficiency rating to calculate my measured loads to my needed wattage. Used that figure and again took loos of efficiency of the battery bank into account as well which is how I ended up with a rather large bank. I know others who use smaller banks but they also have to run their generators daily in winter. I can't speak to how well their maintaining those banks either. 

     I am well aware of the target 10 - 13% charge rate which is really why I started this thread. I was finding it difficult to figure out how I'm going to get near that number with the currently quoted system. And I'm a complete NOOB to solar power so keep that in mind.

     If I understand @mcgivor correctly and read the spec sheets and do the math I should be able to use two series strings of 8 panels and then parallel them into one controller, then double that with the second controller. IF I can find space on the roof that is. That should give me 9KW array and get me close to getting max out of the CC's. At 80A DC out each, I can at least get close to 160A and .09% charge rate.

     I'm rather curious why the vendor didn't make that recommendation in the first place, though he might have been trying to keep the system within my stated budget. The math seemed simple to me. 80A @ 60V bulk charge = 4800W which is stated as max output. To get 4800 out you need at least 4800 in. 4800 x 2 controllers is a minimum of 9600W array right? 

     I keep feeling what I'm missing here is the real world operation over time. While .05% is not ideal, what bugs me is that if I put up a 9600W array, during the summer months I'm making almost triple the KWh I use in a day! I'm not selling to the grid so there's no getting money back on that investment. Do I really NEED 9600W to keep that bank healthy or can I expect to charge at the lower rate and keep the bank at a PSOC most of the time and use the generator once or twice a month to get it to full charge or run an EQ? 

     And PVWatts is pretty good if you pull an hourly report. I wish I was building where they have new data but their new data set only goes to 60N, above that they use legacy NREL data I believe. 
  • BB.BB. Posts: 28,073Super Moderators, Administrators admin
    edited June 29 #18
    To a degree, it also depends on how you use your system... If you only use the system at night, then 5% can work OK (although, many deep cycle flooded cell batteries recommend 10% minimum rate of charge). All the daytime harvest goes to charging.

    If you have mixed usage (run pumping, tools, computer, stat TV, washer, refrigerator) during the day--Then your daytime loads will subtract from even the 5% rate of charge current... Not ideal (5% rate of charge, 20% discharge, need 4 hours per day at 5% and another 4 hours or so for absorb--Just for some very rough round numbers).

    We try to get people to understand their needs and loads... Solar power is very expensive and any conservation you can do up front (LED lighting, very efficient refrigerator, using laptop computer instead of desktop computer, a "solar friendly" water pump, etc.) to get your energy usage to a minimum is very helpful (and figuring what loads must be run every day like a refrigerator and computer for business, vs those optional loads like washing, vacuuming, etc. that can be put off for another day, or ran while the genset it turned on).

    If you have a 10% to 13% or so rate of charge, then you can run the system without having to plan every power event--Because, on average, your panels will easily recharge the battery bank on a daily basis, and you have excess energy available that you can use without thinking (hairdryers, kids electronics, laundry, watering crops, etc. anything during the day).

    For the most part, the battery bank is the thing that is easiest to "break" (or murder) if not operated within the limits of the bank (to little charging, too much load, too much charging, filling with hard/dirty water, not keeping cable connections clean/dry/corrosion free/etc.).

    Solar panels, in general, will simply work for 20-40 years (electronics will last for ~10+ years with repairs possible 5+ years).

    If you can keep your battery usage "reasonable" (the 1 day without sun to 25% loads--2 days of storage and 50% discharge)--Then the (hopefully) smaller battery bank will be cheaper, give you a higher rate of charge with the same array, and if you kill your first bank (many people do), you have less money lost (somebody forgot to turn off the water pump when going on vacation, kids came up for the week and turned on EVERYTHING 24 hours per day, or similar)...

    At this point, solar panels are about the cheapest they ever have been (and probably are not dropping that much more--Shipping and mounting is a major cost of solar array these days). Plus if you order panels by the pallet, trucking/shipping/packaging charges are much less (assuming you have some sort of ground access to your property and are not flying everything in).

    If you are the type that enjoys managing loads/system on a near daily basis--5% can work. If you are the type person that just wants things to work (and need to to work for your spouse/kids without causing family friction), then 10%-13% rate of charge (higher will work OK, but for many people, that is way more solar panels than they need for most of the year--Not always cost effective) is not a bad place to put your money.

    Doing a virtual tracking array (1/2 panels facing south east, other 1/2 facing south west, 10%+ array)--I think you would be happy with such a system.

    One question--Why on the roof for the solar array? I did it because I have a relatively small yard area where I could install such an array... In Alaska, you would seem to have a lot more land available, on average... I understand putting panels on a roof to reduce the risk of theft--But if that is an issue, adding some sort of alarm system for your major components (battery, generator sheds, solar array, etc.) could be helpful...

    But to answer one of your concerns... Yes, you will probably, on average, generate way more solar energy than you will use in the summer... But as you approach the "dark season", little sun means little solar harvest and it is difficult (or impossible) to build a "big enough" solar array--And genset+fuel is your winter power source.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • aksalaaksala Posts: 37Registered Users ✭✭
    @BB. Yes, I've already done the math for our loads using a watt meter. When you account for loss and degradation of batteries over time this is where we're at, say 1400 - 1800ah bank. Generator use for several months in winter is a given at my location.

    I'll have to discuss with the vendor again, as I just dont agree with the 7KW array for this sized bank and from what I'm seeing here, I am correct in that assessment.
  • jonrjonr Posts: 1,100Solar Expert ✭✭✭✭
    There really isn't a right answer to "how many days do you want to operate without sun or running a generator".

    > how can roughly 7KW in panels manage to keep a battery bank of this size charged?

    1KW of panels can keep this battery bank charged.  It depends on how much you use.     You  might post your usage (15-18 Kwh/day) breakdown and see if people here have ideas on how to reduce it.
  • aksalaaksala Posts: 37Registered Users ✭✭
    > @jonr said:
    > There really isn't a right answer to "how many days do you want to operate without sun or running a generator".
    >
    > > how can roughly 7KW in panels manage to keep a battery bank of this size charged?
    >
    > 1KW of panels can keep this battery bank charged.  It depends on how much you use.     You  might post your usage (15-18 Kwh/day) breakdown and see if people here have ideas on how to reduce it

    I appreciate the response, but that's not the question. If i wanted to reduce my consumption further I'd do it. It's a house, not a cabin, on 40 acres and an operating sled dog kennel. Could I run it on less? Sure. But I dont have to.
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