Mixing Solar Panels

kbax
kbax Registered Users Posts: 2
Hi,

looking for a little guidance on mixing solar panels. Current system is 2 strings of 5 kyocera 135W panels routed thru a breaker box then to an outback MX60 charge controller (all components are outback). Would like to add panels however the kyocera's are rather expensive per/watt. Its been 3 years since intial setup (myself). Haven't done any electrical since (lots of mechanical).

Will I need a separate charge controller to handle the different panels? Any other issues? Did some searching but I need to make the purchase soon. Any links are appreciated.

Thanks,

KB

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Mixing Solar Panels

    More or less, you want Vmp~17.5 to 18.5 volts for PWM controllers charging a 12 volt battery bank. For 24 volt battery bank, 2xVmp-12 or ~35-37 volts, etc...

    If you have MPPT type charge controllers, you still want, as a minimum, the Vmp-array voltages listed above. Voc-array-cold may be anywhere from 45 to 150+ volts depending on brand/model of charge controller.

    If you are mixing/matching panels, you want to match Vmp-panels to within 10% or better (the farther away from each other they are, the less efficient the overall power generation will be).

    For adding panels in series, you want to match Imp-panel to within 10% of rated current or better.

    If the old and new panels are too far apart, many times you end up installing a new charge controller to manage the "new array"--Just connect the new charge controller in parallel (directly to the battery bus) with the existing charge controller(s). (Obviously, there is a bit more detail about wiring, fusing/breakers, proper battery wiring and such--but the basic paralleling of charge controllers still applies).

    So, what is your existing array and Vmp/Imp ratings (and series/parallel connections). And what type/size of charge controller do you have?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Mixing Solar Panels

    Kyocera KD 135 specs: Vmp 17.7, Imp 7.63
    Amazing how I have those numbers memorized. :roll:

    As Bill said, so long as the new panels have similar specs for the type of connection and the controller's capacity isn't exceeded it will work.

    Right now your array is Vmp 88.5 and Imp 15.26 with a total Wattage of 1350. If your system Voltage is 24, then your MX60 is already close to maximum capacity (about 1440 Watts). You may have to get a new controller anyway. If it's 48, you've got room to double it. If it's 12 you're way over the limit.

    Can you give us the details of your battery bank? (Voltage and Amp hour capacity.)
  • kbax
    kbax Registered Users Posts: 2
    Re: Mixing Solar Panels

    You are correct on the Vmp & Imp & wattage. So I need to match my new string of panels to + or - 10% of the 88.5 Volts right? My battery system is 48 Volts (2 sets - 4 X 12V batteries). The MX-60 manual states that 3200w is max recommended. Is there a tolerance to this value (what happens if I hit 3300W)?

    Also if anyone knows....why are the kyocera 135/140 watt panels price/watt so much higher?

    Thanks for the help,

    KB
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Mixing Solar Panels

    For good quality MPPT type charge controller, they simply limit their output current to the rated value (60 amp in this case). You can have a lot more power available on the PV input side of the controller, and as long as you don't violate the maximum input voltage, the controller will operate quite nicely.

    Roughly, I like to use a 1/0.77 derating (~0.81 derating for solar panel STC rating, and ~0.95 for charge controller efficiency)... That gives you a good "typical" maximum value of Array to Battery bank conversion (on very cool/clear days, the solar panels can output higher amounts of energy, but the MPPT charge controller will current limit). So, as an example:
    • 60 amps * 59 volt battery charging * 1/0.77 panel+controller derating = 4,597 watt maximum "cost effective" array

    I am not sure where Outback is getting the 3,200 watt maximum... That would be 60 amps at 53 volts... Or, in the NEC they require a *0.8 derating (or even a 0.8*0.8 derating) for solar power systems. This is an argument I have had before about if a device is rated for 60 amp output, you do not derate it by *0.8, instead, the wiring and breaker have to be over-sized by 1/0.8 per standard NEC practices (you can only run continuous current through breakers/wiring to 80% of rated capacity).

    It is sort of like saying a 100 watt light bulb can only be operated at 80 watts continuous--No, it was designed and Listed by UL/NRTL's to run at 100 watts.

    If you have Isc>60 amp rated input for the Charge Controller, you would need to up rate the wire and/or add a breaker/fuse to protect against a controller failure (just like any load). Sometimes MPPT charge controllers are attached to a battery bank to down convert to a lower voltage battery bank (48 volt battery charging a 12 volt battery for alternate voltage equipment). Works just fine.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Mixing Solar Panels
    kbax wrote: »
    Also if anyone knows....why are the kyocera 135/140 watt panels price/watt so much higher?

    Welcome to the forum. There are two reasons that the Kyocera and most other 12 volt nominal panels cost more.

    One reason is low production quantity. Nowadays most panels are for grid tie applications and are higher voltage. No problem using them off grid if you have an MPPT controller. Unfortunately, Evergreen who made high wattage 12 volt panels is out of business.

    The other reason is that it costs more to make small panels, i.e. its cheaper to manufacture one 200 watt panel than two 100 watt panels. The Kyocera panels are (relatively) low wattage.

    Small panels such as the Kyocera often are the low price choice when you factor in shipping. They can be shipped by UPS, whereas larger panels must be shipped freight.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Mixing Solar Panels

    Here's a bit of trivia about the venerable MX60 charge controller: it can actually handle 70 Amps.
    Here's another; they work best at 45 Amps (don't ask why, they just do).

    3200 Watts / 48 Volts = 66.6 Amps - about 10% over the "maximum". In reality you could put about 3700 Watts on one for 48 Volts; it will successfully 'clip' any surplus power. But there's no point in doing it if you're not using the full 70 Amp potential.