Design my setup

W3ABC
W3ABC Registered Users Posts: 19
I've got a load of 30W @ 12V running 24 hours per day.
I get a minimum of 2.37 hours of sun during December, maximum of 5.92 during July, with a average of 4.62 throughout the year (fixed tilt).
I have four 12V 150Ah AGM batteries.

The battery bank can be divided up or added to as you see fit, as long as it can power 12V electronics for two days with no sun.
How would you design a solar setup around these requirements?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Design my setup

    Right, so apply some math:

    30 Watts * 24 hours = 720 Watt hours.
    This is all DC so there is no conversion loss or inverter consumption.
    720 Watt hours on 12 VDC (nominal) is 60 Amp hours.
    If you go for 25% DOD (gives you that 'extra' day) you should have at least 240 Amp hours.
    In reality you could fudge that by upping the DOD a bit and using standard 220 Amp hour golf cart batteries (27% DOD). But this would mean going below 50% DOD if you really need the two days full power and can not rely on any panel input. Most of the time you can rely on something from the panels.

    Recharging is another matter. If you want to be sure it recharges from solar only year-round then you have to look at the shortest day sun figures. In this case you may as well say 2 Hours. That's going to double the array size you'd normally have, meaning there will be 'extra' power when the days are 5 hours long which you won't be able to make use of.

    Clear as mud?

    Normal array sizing:
    22 Amps @ 12 VDC = 264 Watts, less derating: 343 Watt array on MPPT controller.
    Now this could either be rounded up to the nearest available panel configuration or you could get away with whatever panels could give you around 22 Amps on a PWM controller (like three 140 Watt panels in parallel). This would be close enough for most cases. The harvest would be 343 Watts * 4 hours * 0.77 (efficiency factor): 1056 Watt hours per day. Compared to consumption of 720 Watt hours you're ahead.

    Now see what happens when the day shortens to 2 hours: you get only 528 Watt hours. So we can work it backwards:
    720 Watt hours / 0.77 = 935 Watts / 2 = 468 Watt array.

    Again that's with an MPPT controller; you won't get quite as much from a PWM.

    Now there are batteries bigger than the GC2's at 220 Amp hours. I'm using East Penn PS2200 which are 232 Amp hours. If you go up in Amp hours you should increase the array accordingly.

    Totally confused now?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Design my setup

    Part of the question is "do you want to use a genset during the 'dark times' of the year)... I typically use 9 months as my suggested break even point, and figure that during winter folks will use a genset when the weather is bad (and it would take a 10x larger solar array make a useful amount of power during bad weather).

    So... Assume 30 watts of DC * 24 hours per day with an efficiency of 0.77 for panels + charge controller and 0.80 for flooded cell batteries.

    Note: Batteries have a fairly wide operating voltage range--And loads behave differently to changing input voltage... There is constant power loads (V*I=30 watts), such as AC inverters and some motors. There are constant current loads (such as filament lamps), and constant resistance loads (as voltage drop, current drops).... So Radio Gear--I will assume uses most current at 13.8 volts:
    • 30 watts * 24 hours * 1/13.8 volts * 2 days no sun * 1/0.50 max discharge = 209 Amp @ 12 volt battery bank

    That is very close to a pair of 6 volt @ 220 AH "golf cart" batteries--So, I would suggest those as a start. Cheap and rugged.

    You now have two methods to calculate solar array size--One based on load and hours of sun, and a second based on size of battery bank. First the "sun".

    Assume that you will use a generator (or utility power, and generator for backup to the backup) for 3 months of the year. And assume that break point is ~4 hours of sun per day.
    • 30 watts * 24 hours per day * 1/0.77 panel+controller derate * 1/0.80 battery eff * 1/4 hours of sun per day = 292 Watt Array for 9 month of year no-generator

    Next, size of array based on battery bank size... Assume a pair of 6 volt @ 220 AH golf cart batteries and a typical 5% - 13% rate of charge:
    • 220 AH * 14.5 volts charging * 1/0.77 controller+panel derating * 0.05 rate of charge = 207 Watt Array minimum
    • 220 AH * 14.5 volts charging * 1/0.77 controller+panel derating * 0.10 rate of charge = 414 Watt Array nominal
    • 220 AH * 14.5 volts charging * 1/0.77 controller+panel derating * 0.13 rate of charge = 539 Watt Array maximum "cost effective"

    Note--Because your load is 24x7 at 30 watts--Then, the above numbers should have 30 Watts * 1/0.77 = 40 watts of solar panel added to the array size to make up for the constant load (remember the above is based on battery charging requirements). Or:
    • 247 watt array minimum
    • 454 watt array nominal
    • 579 watt array maximum cost effective

    (note, the numbers are not really that accurate--just carrying enough digits to avoid sever round off error, and so you can reproduce my math)

    So, my recommend array size based on my guesstimates would be around 292 to 539 watts.

    And something like a 20-30 amp AC Battery Charger for grid/genset charging during bad weather (you could go as high as 50 amp @ 12 volt charger--But, as always, I like to match battery to charging to charger to power source--I.e., a huge genset and a small charger--lots of gasoline per hour. A huge charger to smaller battery bank, possible overheating of battery bank and too large of generator, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Design my setup

    one could confuse this even more by going with at least a 5% charge rate. 600ah x .05 = 30a. if through a pwm cc this would be multiplied by a nominal 18v vmp for 540w stc. of note the 600ah battery bank will go for roughly 5 days to 50% soc. with weather that can go longer than 5 days being cloudy or overcast it can be advantageous to take the pvs even higher in wattage and you can go to a 13% charge rate without too much worry. less maintenance though at 10% which is the best overall charge rate for lead acid.
  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    Re: Design my setup
    W3ABC wrote: »
    I get a minimum of 2.37 hours of sun during December, maximum of 5.92 during July, with a average of 4.62 throughout the year (fixed tilt).

    Did you get these values from a solar-insolation chart or other source for your location? If these are simple visible daylight hours, your calculations could be quite a bit off.
  • W3ABC
    W3ABC Registered Users Posts: 19
    Re: Design my setup
    PNjunction wrote: »
    Did you get these values from a solar-insolation chart or other source for your location? If these are simple visible daylight hours, your calculations could be quite a bit off.

    I got them from http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/
  • W3ABC
    W3ABC Registered Users Posts: 19
    Re: Design my setup
    Now see what happens when the day shortens to 2 hours: you get only 528 Watt hours. So we can work it backwards:
    720 Watt hours / 0.77 = 935 Watts / 2 = 468 Watt array.

    There seems to be quite a few different ways to calculate the size of the array. Why isn't this formula used exclusively? Why do some calculations use the size of the battery bank rather than the size of the load? Does it make a difference if I have a 1000Ah bank even if I draw the same 720Wh worth of power?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Design my setup

    Several reasons... More or less, 5% is the floor (for rate of battery bank charging) because you need somewhere around 2.5 to 5% current to equalize a battery bank. Some mfg actually recommend 10% as a minimum (not sure it has to be that high).

    Flooded Cell Battery Bank, especially Fork Lift type cells, can have self discharge on the order of 1-2% per day near the end of their or need near a 2-4% rate of charge (during daylight hours). Other types of batteries have lower self discharge--But it still can get signifant at lower rates of charge.

    Nominally, around 1% rate of charge is used just to "float" a lead acid battery bank.

    Also--We suggest that batteries not spend too much time at lower states of charge (75% is one number some folks suggest). So, if you have a battery at 5% rate of charge and down to 50% state of charge, it will take 5 hours just to recover from 50% to 75% state of charge (without loads)--Or a relatively sunny summer afternoon. It could take several days during winter. Lower rates of charge will streatch the charging time out and expose the battery bank to more sulfation (and earlier death).

    AGM batteries have much lower rates of self discharge, and are more efficient when charging--So, for lightly loaded banks, you probably can get down to 2.5% rate of charge and still be successful.

    In the "olden days", when batteries were "cheap" and solar panels were expensive ($10 per watt vs the ~$2 per watt or less today), it was thought that it was a good idea to install a larger battery bank if the system produced insufficient amount of power for the cabin/home... In reality, it really did not help and people ended up with big battery banks that were slowly dieing from sulfation.

    Anyway--My view of the rule of thumb regarding minimum charge rates. Basically trying to size the system to deliver expected loads over the life of the battery bank/system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • W3ABC
    W3ABC Registered Users Posts: 19
    Re: Design my setup

    Alright, that makes sense. Now, let's assume I'm running two 12V 150Ah AGM batteries in parallel.

    According to your formula, 300Ah * 14.5V / 0.77 * 0.10 = 565W array. Do you think three 190W panels URL="http://www.grapesolar.com/index.php/products/modulesandkits/gs-s-190-fab3/"]link[/URL wired in series (for a total of 108.6V @ 5.25A) and a Morningstar TS-MPPT-45 controller (or Outback FM60?) will work for this setup?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Design my setup

    The issue with three panels in series is that the Voc-cold voltage can be higher than the controller's maximum allowable input voltage (around 140-150 volts for these controllers).

    Vmp-hot (solar panel voltage falls when hot--so need to know Vmp-hot-array vs battery charging voltage)
    Voc-cold (solar panel voltage falls when cold--so need to know Voc-cold-array vs maximum acceptable controller input voltage)

    Most MPPT type charge controllers have a solar array "string" calculator to help decide what will work or not. For example, here is MorningStars:

    http://www.morningstarcorp.com/en/strings/calc.php

    I believe the numbers on the right are "per panel"... 3x that for for array voltage with 3 panels in series.


    User Defined Information


    Specifications (@ STC)

    Pmax
    190.00
    Watts


    Voc
    45.40
    Volts


    Isc
    5.44
    Amps


    Vmp
    36.20
    Volts


    Imp
    5.25
    Amps


    Voc Coefficient
    -0.1590
    V/C


    NOCT
    45.00
    C




    Operation

    Minimum Voc (average high temp.)
    31.09
    Volts


    Max. Operating Voc (record low temp.)
    45.40
    Volts


    Max. Voc (record low Temp., morning)
    49.38
    Volts


    Minimum Vmp (average high temp.)
    21.89
    Volts


    Maximum Vmp (record low temp.)
    36.20
    Volts





    If I entered all of the data correctly... at 32F, the Voc is around 148.125 volts... So, if you get below freezing where you live, it would probably over voltage the MS TS 45 MPPT charge controller--Probably too close for comfort in most areas of the continental US. So you would be limited to 2 of these panels in series for the TS 45 MPPT.

    The maximum "cost effective" array a TS 45 MPPT would support would be:
    • 45 amps * 14.5 volts charging * 1/0.77 panel+controller derating = 847 watts max "cost effective array" (very rough number)
    • 4 * 190 watt array = 760 Watt array

    So, from a TS 45 MPPT perspective, you can run either a 380 watt array or a 760 watt array (or larger--but much more clipping of output current in middle of day). But >~579 Watt array is on the overkill side for a 220 AH @ 12 volt battery bank.

    The maximum rate of charge that would probably "comfortable" for a deeply cycled 220 AH @ 12 volt battery bank at 25% rate of charge (highly recommend a remote battery temperature sensor) would be an array around:
    • 220 AH * 14.5 volts charging * 1/0.77 controller+panel derating * 0.25 rate of charge = 1,036 Watt Array @25% rate of charge

    And, if we add the 30 watt 24x7 load (40 watt derated array), upwards of 1,076 watts could make sense for you (may not make $$$ sense though).

    Certainly would help you with "extra power" deeper into winter. Probably would not need to use a genset except during stormy/dark weather that last longer than 2 days at a stretch (very dark days can produce less than 5% of rated solar panel output).

    Does that all make sense?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Design my setup

    Oops--Forgot that you wanted a 300 AH @ 12 volt array... Those "rules of thumbs" charging rates would be:
    • 300 AH * 14.4 volt AGM charging * 1/0.77 panel+controller derate * 0.05 rate of charge = 281 watt array minimum
    • 300 AH * 14.4 volt AGM charging * 1/0.77 panel+controller derate * 0.10 rate of charge = 561 watt array nominal
    • 300 AH * 14.4 volt AGM charging * 1/0.77 panel+controller derate * 0.13 rate of charge = 729 watt array "cost effective max"
    • 300 AH * 14.4 volt AGM charging * 1/0.77 panel+controller derate * 0.25 rate of charge = 1,403 watt array "try not to go above max"

    And, add 40 watts (30 watt 24x7 load, derated by 0.77 to solar panel) to each of the above.

    So a 760 watt array (2x series; 2xstrings in parallel for 190 watt array) would be a nice fit to the 729+40=769Watt cost effective max array.

    Obviously, this is getting in the "overkill" range for a 30 watt * 24 hour load. The minimum sun to keep up with the load would be around:
    • 30 watt * 24 hours * 1/0.77 * 1/0.90 AGM efficiency * 1/760 Watt Array = 1.37 hours of sun per day to "break even"

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • W3ABC
    W3ABC Registered Users Posts: 19
    Re: Design my setup

    Do I use Voc or Vmp for wire size calculations?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Design my setup

    Vmp and Imp for voltage drop calculations. Isc * 1.25 (or Isc*1.25*1.25 if you believe NEC) for minimum wire gauge and fuse size calculations.

    Usually, wire gauge will be larger because of voltage drop calculations vs based on Isc rating (i.e., wire has to be heavier to have low voltage drop for efficient energy transfer).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • W3ABC
    W3ABC Registered Users Posts: 19
    Re: Design my setup

    What panels would you suggest for this system if I used an Outback FM60?

    I was looking at these and I could get 5 in series according to the Outback string calculator. They have good reviews, but they're not 'name brand'. I could also get these Kyocera panels (considerably more expensive) and also wire 5 in series.

    Not exactly sure what I should look for in the panels themselves, but it seems these two options would allow me to expand a little down the road.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Design my setup

    Um, $320 for 140 Watt no-name or <$300 for 140 Watt Kyoceras? http://www.solar-electric.com/hiposopa.html

    And if you put five in series you get 700 Watts and 87.5 Vmp so why not go with fewer higher Watt panels for even less money per Watt? You'll have to be using an MPPT controller either way.

    Please look over the range of panels linked to above.
  • W3ABC
    W3ABC Registered Users Posts: 19
    Re: Design my setup
    Um, $320 for 140 Watt no-name or <$300 for 140 Watt Kyoceras? http://www.solar-electric.com/hiposopa.html

    The $320 is for two 145W panels, so 290W.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Design my setup

    Shipping included?
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Design my setup
    W3ABC wrote: »
    The $320 is for two 145W panels, so 290W.

    My mistake!
    And yes shipping is in there so you're looking at $1.10 per Watt. In order to beat that you'd have to get something like the Conergy 245 Watts for $245. Not sure what the shipping is but probably more than $24 per panel.

    The only other advantage to the higher Watt panels is the fewer number of them for the about the same array size and thus fewer connections. The downside is that they are larger, have to be truck shipped, and aren't easy to put in place.

    As for wiring five of any panel in series, the higher the array Vmp the less efficient the conversion to battery Voltage. At 80+ Volts it works okay on a 48 Volt system, but you'd be losing a few Watts more with a lower system Voltage. This is not necessarily a problem that will interfere with function.
  • W3ABC
    W3ABC Registered Users Posts: 19
    Re: Design my setup

    So there isn't any reason to buy 'brand name' panels? The build quality is generally the same throughout?
    As for wiring five of any panel in series, the higher the array Vmp the less efficient the conversion to battery Voltage. At 80+ Volts it works okay on a 48 Volt system, but you'd be losing a few Watts more with a lower system Voltage.

    I guess there is always going to be conversion inefficiencies when using an MPPT controller no matter the voltage. Is there a chart I can reference to see how much loss we are actually looking at?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Design my setup
    W3ABC wrote: »
    So there isn't any reason to buy 'brand name' panels? The build quality is generally the same throughout?

    Some on the forum have bought the DM panels. I haven't heard any complaints yet.
    I guess there is always going to be conversion inefficiencies when using an MPPT controller no matter the voltage. Is there a chart I can reference to see how much loss we are actually looking at?

    Outback has a graph somewhere. It will be different for other makes. MidNite's seem to be the most efficient, but they don't have a reference chart. Worst case you are looking at <10%, normally not more than 7%. As always, your actual experiences may vary.

    You can parallel input to controllers, you just have to do the V-drop calculations accordingly and install the necessary circuit protection.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Design my setup
    Outback has a graph somewhere.

    try this: http://www.outbackpower.com/pdf/manuals/900-0009-01-00_Rev_B.pdf --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i