Mixing Panels

firefly
firefly Solar Expert Posts: 66 ✭✭
in Off Grid Solar & Battery Systems #1
I have a friend who has been given two Kyocera 210 watt panels, Voc 33.2, Isc 8.58, Vpm 26.6. He wants to add them to his existing array of 6 BP 125 watt panels (in series for 24 volt system) Voc 22.1, Isc 8.02, Vmp 17.3. These are on a Blue Sky SB50 charge controller.

Is this a good or bad idea?

After that has been done, he then wants to Parallel that controller with another Blue Sky SB50, to try to maximize his system.

Please any comments or suggestions would be appreciated.
Cheers
Steven M
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Mixing Panels

    I don't think he'll be happy with he result. The BP's would be wired for 34.6 Vmp on a 24 Volt system (two in series). Weigh this against the Kyocera's Vmp of 26.6 and you get a difference of 8 Volts - 23%. Rather a big difference.

    Worse, the Kyoceras are "not-quite-24-Volt" panels; on their own they won't supply enough Voltage to recharge a 24 Volt system (28.4 Volts). Put them in series and run through an MPPT they're fine. But the BlueSky won't take over 36 Volts input, if I recall correctly, so trying to rig the whole bunch together as a 48 Volt array (dropping 2 of the BPs) is out.

    Can't even run the 2 Kyoceras through a separate PWM controller - same reason as above: not enough V for 24V.

    This won't work very well.
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  • firefly
    firefly Solar Expert Posts: 66 ✭✭
    #3
    Re: Mixing Panels

    Marc

    Thanks for the quick reply, I am sure he will not be happy. He was told there would be know problem. What did you mean by, the Kyocera's are not quite 24 volt. How can you tell?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Mixing Panels
    firefly wrote: »
    Marc

    Thanks for the quick reply, I am sure he will not be happy. He was told there would be know problem. What did you mean by, the Kyocera's are not quite 24 volt. How can you tell?

    A "24 Volt" solar panel has a Vmp around 34-35 Volts which allows it to supply maximum output at a charge Voltage that works for a "24 Volt" system: about 28.4 Volts. It will also allow an equalization Voltage of around 30. The Kyoceras at 26.6 Vmp don't actually put out enough to charge a "24 Volt" system.

    What your friend will experience is the lower Voltage panels drawing the Vmp of the whole array down while the higher Voltage panels try to bring the output up, raising the V point of the smaller panels to where their current output goes down. So whereas it will add Watts over-all it will do so at the expense of the maximum output of both sets of panels. In other words, it won't add up as "750 Watts plus 420 Watts equals 1170 Watts".

    And it's pretty difficult to figure in advance what he will get. The "1170 ideal Watts" would really perform at a lower rate, say 80%, and then that figure will be further derated due to the panel mis-match. He could end up with a "real" figure of 720 Watts; better than his original 600 "usable" Watts (estimated), but a far cry from the 936 "usable" Watts one might expect.

    Caveat: I am just guessing here. I don't know any way to accurately determine the outcome in advance. There are many factors involved.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Mixing Panels

    Closest workable wiring configuration I can think of:

    Parallel these three strings;

    Two 26.6 panels in series: 53.2 Vmp
    Three 17.3 panels in series: 51.9 Vmp
    Three 17.3 panels in series: 51.9 Vmp

    But again, I don't think the BlueSky is able to handle that high an input Voltage. He'd need a Morningstar MPPT or similar.

    EDIT: The Blue Sky SB1524iX apparently will take up to 57 Volts input. http://store.solar-electric.com/sb1524ix.html Must have been thinking of the lower Voltage units. However, it won't handle more than 15 Amps on a 24 Volts system so it won't work. You need a controller capable of 30-40 Amps.
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  • firefly
    firefly Solar Expert Posts: 66 ✭✭
    #6
    Re: Mixing Panels

    Marc

    Thanks again, very much appreciated.
    Steven M
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: Mixing Panels

    Remember Voc cold is much higher than Vmp at STC.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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