Series or parallel ?

addco
addco Registered Users Posts: 11 ✭✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
I’m planning a solar installation on my Motorhome. Thinking I will use, essentially this kit, with perhaps some additional  control options. (Color control). 12 volt system. 
https://www.solar-electric.com/rvkit-025-640w-12v.html

I’m leaning on wiring the two 325 watt panels in parallel for better shade resistance. The 39+ Vmp should be plenty of overhead for the 100/50 MPPT. 
Is there any benefit to running in series (other than wire sizing) that would offset the better performance if one panel is shaded?
Will the MPPT perform more or less efficiently one way or the other ? 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #2
    More or less, the Vmp-array voltage should be at least 1.3x battery voltage (15 volt charging * 1.3 = 19.5 volts Vmp-array minimum) for an MPPT type charge controller.

    And, the rough "optimum" (controller losses) is about 2x battery voltage (15 volts * 2 = 30 Volts Vmp-array). That usually works out to be the least losses for the MPPT charge controller... But you are probably comparing 96% vs 97% efficiency--Not a big deal.

    For short runs, running your two panels in parallel is probably best.

    If you had long runs for array (i.e., portable panels 50+ feet from RV/charge controller), then putting the two panels in series would save on wire costs (smaller diameter copper wire because of lower current).

    HOWEVER, you need to look at your ambient temperatures too... Voc (voltage open circuit) rises in cold weather. For places that get very cold, the typical Vmp-array max would be something like 66-70 VDC max (to prevent Voc-very-cold, way below freezing from over volting the controller). Two panels in series--You need to check. These two panels in parallel--Not a problem with Vmp-hot or Voc-cold.

    Make sure you have as little shading as possible--You don't want vent pipes, roof rack hardware, A/C unit casting shadows on the panels (or as little as possible). Even a relatively small shadow on a solar electric panel can kill output by 50% (or up to 100%) very easily.

    If you will be winter camping, being able to tilt the panels can significantly increase your harvest in winter months (vs flat mounting)--Especially the farther north you go.

    You can use a simple solar calculator (hours of sun per day... Over 3 hours of sun per day, not bad; Less than 2 hours per day, almost not worth solar.

    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Fresno
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:

    JanFebMarAprMayJun
    2.42
         		3.48
         		5.03
         		6.51
         		7.64
         		8.30
     
    JulAugSepOctNovDec
    8.00
         		7.27
         		5.89
         		4.45
         		2.96
         		2.28
     

    Vs tilted for winter harvest:

    Fresno
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 38° angle (from vertical or 52 degrees from horizontal):
    (Optimal winter settings)

    JanFebMarAprMayJun
    4.00
         		4.98
         		6.02
         		6.22
         		6.11
         		6.09
     
    JulAugSepOctNovDec
    6.10
         		6.44
         		6.58
         		6.31
         		4.93
         		4.08
     

    Tilting racks add weight, and have to be structurally sound--You don't want the panels blowing off when traveling.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Textrout
    Textrout Registered Users Posts: 5 ✭✭
    #3
    I'm probably a month too late, but If you were to run two separate pairs of wire, one for each panel, you would eliminate the shading issues by half, meaning one won't affect the other.  This increases redundancy in case you have a fault in one of your panels. You have to combine them near the Charge controller I(with the "big baby box"?  There is a lot I don't know, like how to put diodes in the circuit, but fairly sure of the above.

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