Advice for an MPPT ignoramus

lazza
lazza Solar Expert Posts: 336 ✭✭✭
in Off Grid Solar & Battery Systems #1
HI All

we're trying to size a large installation of 12.5kW. Now based on previous conversations, it seems like MPPT is a much better bet than PWM for this size of system.

However, never having installed or worked with MPPT off-grid before, what would the forum recommend in terms of numbers and types of MPPT controllers for this system??

Panel characteristics: Voc 36V, Vmp 30V, Soc 9.5A, Imp 8.17A

I was looking at 2 x Xantrex XW 60-150. With strings of 4 panels. But I'm not sure if they have a maximum wattage or only a maximum amperage.

:confused:
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: Advice for an MPPT ignoramus
    lazza wrote: »
    I was looking at 2 x Xantrex XW 60-150. With strings of 4 panels. But I'm not sure if they have a maximum wattage or only a maximum amperage.

    The amperage rating of most (all?) charge controllers is the output amperage to the batteries. If the controller is rated at 60 amps and is charging a 12 volt battery (at 14.4 volts) , it could handle 60 amps X 14.4 volts = 864 watts coming in from the array. Of course there are cable losses and controller inefficiency losses, so the array could be putting out more than 864 watts.

    Obviously in a 48 volt system that same 60 amps rating means a much larger array can be supported.
    Panel characteristics: Voc 36V, Vmp 30V, Soc 9.5A, Imp 8.17A

    I was looking at 2 x Xantrex XW 60-150. With strings of 4 panels.

    Configuring your panels four to a string may lead to exceeding the Vmax of the controller (and damaging it) when you have cool weather. Three to a string would be optimal for those panels (I am assuming a 48 volt system).

    Also, most controllers work more efficiently with less down-conversion of voltage. If you configure your panels as strings of 4 panels you have a Vmp of 120 volts which must be down-converted to 48 volts. If you put three panels in a string, your Vmp will be 90 volts, and that is easier for your controller to down-convert than 120 volts.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #3
    Re: Advice for an MPPT ignoramus
    lazza wrote: »
    I was looking at 2 x Xantrex XW 60-150. With strings of 4 panels. But I'm not sure if they have a maximum wattage or only a maximum amperage.

    You size by Voc (which also can increase at lower temperature). 4 strings would give you 36x4 = 144V, which is above 140V maximum for XW-MPPT-60. So, you need to do 3-panel strings.

    At 48V, the controller is rated at 3500W. 3500/245 = 14.2. So, you cannot do no more than 14 panels. 4 strings will give you 12 panels. You could go with 5 strings/controller, but this will decrease production - at peak, controller will derate to 3500W. Since you live in hot climate, it is probably Ok. So, it's either 12 or 15 panels/controller.

    12.5kW gives you 12500/245 = 51 panels, so you can do 3 controller with 12 panels and one with 15 panels.
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  • YehoshuaAgapao
    YehoshuaAgapao Solar Expert Posts: 280 ✭✭
    #4
    Re: Advice for an MPPT ignoramus
    NorthGuy wrote: »
    You size by Voc (which also can increase at lower temperature). 4 strings would give you 36x4 = 144V, which is above 140V maximum for XW-MPPT-60. So, you need to do 3-panel strings.

    At 48V, the controller is rated at 3500W. 3500/245 = 14.2. So, you cannot do no more than 14 panels. 4 strings will give you 12 panels. You could go with 5 strings/controller, but this will decrease production - at peak, controller will derate to 3500W. Since you live in hot climate, it is probably Ok. So, it's either 12 or 15 panels/controller.

    12.5kW gives you 12500/245 = 51 panels, so you can do 3 controller with 12 panels and one with 15 panels.

    You need to keep Voc below 120V with Xantrex controllers. Xantrex controllers start derating at 120V Voc, generally the lowest derating Voc of among the Xantrex, Outback, Midnite, and MorningStar 150V MPPT controllers.

    Voc 36V, Vmp 30V look like typical 60-cell panels (225-260 typically 235,240,245W). These fit best 3 to a string on 150V MPPT controllers. Voltage is high enough in hot summers and low enough in cold winters.

    My expansion will be 51 panels. Controller #1 will be the south roof - 4 strings, Controller #2 will be the pole mount - 4 strings, #3 the E/W roof flat-mounted panels and flush-mounted panels squeezed into free space - 5 strings, #4 the patio panels - 4 strings. I currently have 9 roof panels (6 behind AC #2, remaining 3 #1; shading turned out to be fantasy, mergeable), 12 patio panels (#4), and 6 flat panels (#3).
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Advice for an MPPT ignoramus

    Derate what by Voc?

    Voc is not an operating parameter; it is a starting parameter. The 150 Volt +/- specification is for Voc. It is not a precise number. For example MX60's will actually shut down before 150 Voc (varies with the particular software version). Remember that cold temps will increase Voc and can cause too high a Voltage for some controllers. MidNite Classics have a hyper Voc function which allows it to go as high as Voltage rating + nominal system Voltage without damage (but does not operate). That is why they are the best choice for cold climates.

    Use the string sizing tool specific to the charge controller you intend to use.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #6
    Re: Advice for an MPPT ignoramus
    Cariboocoot wrote: »
    For example MX60's will actually shut down before 150 Voc (varies with the particular software version).

    XW SCC also shuts down below 150V - at 140V.
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  • silvertop
    silvertop Solar Expert Posts: 155 ✭✭✭✭✭
    #7
    Re: Advice for an MPPT ignoramus

    I read somewhere that for say a 24volt battery bank you should step up to the next higher voltage with the panels 48volts? Anybody have a comment? I have these specs - VOC 37.6 volts / VPM 30.5 I am trying to decide between series 2 to a string, or series 3 to a string I have 6- 260 watt renesola panels JC 260M-24/Bb. I can go either way 3 x VOC 37.6 =112.8 x1.16(my nec cold voltage correction)=130.848volts this kind of makes me uncomfortable for my XW MPPT60-150 close but ok? Or back to 2 to a string, but I have also heard about down converting loss.....comments.......
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  • YehoshuaAgapao
    YehoshuaAgapao Solar Expert Posts: 280 ✭✭
    #8
    Re: Advice for an MPPT ignoramus

    Xantrex derates when voltage hits 120V. Not isure if it that was live voltage or Voc. From my observations, Xantrex raises voltage when it needs to throttle back the solar wattage due to the batteries being full (absorption/float), so I think its safer to keep Voc under 120V.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
    Re: Advice for an MPPT ignoramus

    silvertop,
    it is more a minimum function of the vmp coupled with what losses will be experienced that will maintain the highest voltage needed to charge the batteries. the rule of thumb you site was more for the 12v pvs of years past when vmps were around 17-17.5v per 12v of battery. those were based on 36 cells per 12v and with 60 cell pvs now many could get confused. unless you are in a very hot area i think 18v vmp per 12v battery should work fine. it certainly does not hurt going 1 extra when dealing with 12v pvs.

    along that line that means about 36v vmp for 24v battery banks and 72v vmp for 48v battery banks. now one has to watch adding one extra pv when dealing with the larger voltage pvs as it could approach limits on many controllers. on mppt controllers you have to watch as one voltage rating may be a shutdown of operations voltage and another would be the damage point to the cc. the damage point can be hit with the open circuit voltage and could even go higher during cold weather so one must know the lowest recorded temp for their area to avoid high voltage damages to the cc when having the strings higher in voltage by more in series.

    that kind of strayed a tad as you were asking of the lower end and ones highest temp need to be known for this. pv string calculators like midnites are good to use and if the temp and other losses do not pull the pv voltages too low to function then you can go with it. now here's the thing, say it is a bit cloudy and your batteries are in bulk on a very hot day, your voltage from the pvs could sage further than you figured for. that's why if it is borderline that adding one extra pv is good to do as that cushion almost guarantees an output that won't sag too low for the cc input needed. technically the cushion needed isn't as high as another pv, but we don't get the luxury of adding 3 or 4 volts at at time do we?

    i hope i didn't over explain this as this is as close to a bb mode i've been in awhile.:p kidding bill.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Advice for an MPPT ignoramus
    silvertop wrote: »
    I read somewhere that for say a 24volt battery bank you should step up to the next higher voltage with the panels 48volts? Anybody have a comment? I have these specs - VOC 37.6 volts / VPM 30.5 I am trying to decide between series 2 to a string, or series 3 to a string I have 6- 260 watt renesola panels JC 260M-24/Bb. I can go either way 3 x VOC 37.6 =112.8 x1.16(my nec cold voltage correction)=130.848volts this kind of makes me uncomfortable for my XW MPPT60-150 close but ok? Or back to 2 to a string, but I have also heard about down converting loss.....comments.......

    This is a rule-of-thumb to balance minimum required array Vmp for a system against maximum charge controller efficiency. You will need greater than the 30.5 Vmp of a single panel for proper charging on a 24 Volt system. Above 2X nominal Voltage (at the input) the controller efficiency starts to fall off.

    As Niel said you have to consider wiring distances too; is higher Voltage needed to overcome long resistance @ the expected current? Or can reasonably sized wire be used to handle the power?

    Short form on temperatures:
    Cold temperatures will affect the Voc which matters for the controller's maximum input.
    Hot temperatures will affect the Vmp which matters for having correct charging Voltage available.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Advice for an MPPT ignoramus
    YehoshuaAgapao wrote: »
    Xantrex derates when voltage hits 120V. Not isure if it that was live voltage or Voc. From my observations, Xantrex raises voltage when it needs to throttle back the solar wattage due to the batteries being full (absorption/float), so I think its safer to keep Voc under 120V.


    Are you referring to the efficiency drop in converting higher array Voltage to lower system Voltage?

    The raise in array Voltage you see when the batteries are full is due to lower current demands; more rapid PWM of the current results in high Voltage read on the array (and less current) as over-all power is reduced.
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  • silvertop
    silvertop Solar Expert Posts: 155 ✭✭✭✭✭
    #12
    Re: Advice for an MPPT ignoramus

    Thanks all, I think I'll just go 2 to a string, I don't have a long wire run I just wanted to simplify the fussing and wiring as much as possible. VPM of 61 volts will charge my 24 volt bank fine. I just started reading threads where other members we trying to keep VPM up around 100 volts and had to kick the idea arround.....
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