How much voltage can I use to an MX60 in Louisiana

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System
System Posts: 2,511 admin
in Advanced Solar Electric Technical Forum #1
I have 3 Evergreen spruce solar (2) 180's and (1)190 panels but plan on a 4th in the coming weeks and then later 4 more 190's.

I will be using an MX60 CC.

Here in Louisiana we rarely fall below 30-32 degrees and the our record is 24 or 26 for a day or two in a year.

I have checked the panels Voc and it is almost 40V coming out of an A/C room into the sun.

Then drops a bit as they heat up, can I use 4 to a series or would I be better to use 3 in a series and hold off till I can swing another 3?

Basically what Voc is the array going to produce here at low temps and is it safe for the MX60?

and is the CC limited (at 60 , will 70 shorten it's life ) to the current output of the array or the current input to the bank or which ever hit 60/70 amps? I am running a 24V bank.
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  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    #2
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    Re: How much voltage can I use to an MX60 in Louisiana

    Considering the weather where you are, a 72 Volt nominal array should work just fine.

    boB
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #3
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    Re: How much voltage can I use to an MX60 in Louisiana

    The MX60's operational input limit is ~141 V,and its factory-specified absolute limit is 150 V. Allowing for the NEC's 113% correction factor for temperatures in the 14 F to 31 F range, your array's Voc should not exceed 124 V. Considering that rated Vmp is roughly 80% of rated Voc, your array's STC rated Vmp should not exceed 99V.

    The Spruce 180's STC Vmp is 25.9 V, and the 190's Vmp is 26.7 V. Wiring four 190's in series will put the string STC Vmp at 106.8 V and the STC Voc at 131.2 V. Adding in the 113% factor pushes the design Voc to 148.3 V, and adding in the manufacturer's +2.5% tolerance just aggravates the situation.

    Wiring four modules in series is therefore a little risky. If the early morning Voc exceeds 141 V on a cold day, the MX60 will enter a self-protection mode until the modules warm up and the Voc drops back below 141 V. However, the MX60 should not be damaged, and no significant energy will be lost.

    A slighter safer solution might be to wire each of the 180's with three of the 190's. This will balance the two strings, and the STC Vmp and Voc numbers will drop a bit to to 106 V and 131 V. Using a 1,500 W STC array, the charge current to your "24 V" battery bank will rarely ever exceed 50 A except for the occasional and brief edge-of-cloud event. The MX60's built-in protections will limit charge current to a couple of Amps above the charger setting.

    Considering your relatively warm location, your best solution might be to consider nine modules wired as a 3 X 3 array for 1,690 W STC.

    HTH,
    Jim / crewzer
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  • System2
    System2 Posts: 6,290 admin
    #4
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    Re: How much voltage can I use to an MX60 in Louisiana

    Any more input?

    I checked and only 2 days in the last 10 years have we been under 32 during the day and I am betting it was clear that night to dump heat and then clouds moved in that morning to keep the cold so I doubt any Juice would have been incoming anyway.

    Normally when we get over night lows in the 30 range the day time high is over 40

    2 strings of 4 would be easier for me to set up and mount.

    http://www.wunderground.com/NORMS/DisplayNORMS.asp?AirportCode=KBTR&StateCode=LA&SafeCityName=Baton_Rouge&Units=none&IATA=BTR&lastyear=on&normals=on&records=on

    http://www.cityrating.com/cityweather.asp?city=Baton+Rouge
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  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    #5
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    Re: How much voltage can I use to an MX60 in Louisiana
    SKYWLKR wrote: »
    ...
    I have checked the panels Voc and it is almost 40V coming out of an A/C room into the sun.
    ...

    Sidestepping the detailed technical analysis, it seems that an air-conditioned room would be no colder than 65 degrees, and I would expect the air outside to regularly get colder than that in the morning.

    4 panels would therefore be 160 volts, which I think is too much. Even if the panels would drop to a reasonable level once loaded by the charger/inverter, the latter has to start in the first place. You could be leaving a lot of energy on the table if the equipment won't start due to overvoltage, and has to wait until the noon heat to work.

    I did a similar experiment with my grid-tied inverter, which was normally connected to two strings. I reconfigured so that it was all panels except for one in a single string. Couldn't use all the panels, because that would have exceeded the absolute maximum voltage. Result was that the input voltage slightly exceeded the operating voltage, and the inverter refused to turn on, even though, if it did turn on, the voltage would have dropped to well within limits.
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