5KW Grid Tie Design Review Requested Asap :)

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miscrms
miscrms Registered Users Posts: 15 ✭✭
in Solar Beginners Corner #1
Greetings,

Sorry to start off like this, but I'm hoping someone can provide some quick feedback on the general design. Have a chance to get a good deal on panels, and want to make sure I'm doing the right thing.

System Overview:
~5kW DC grid tied system for installation on a freestanding shade structure at a residence in Phoenix, AZ. Array would be south facing at 12 degree slope, compromising between getting above shade sources and keeping max height reasonable while maintaining good overall production (particularly summer).

Inverter:
Sunny Boy 5000US seems to be best option based on reviews, price, system design, and communications desires.

Panels:
Canadian Solar CS6P-230P, trying to decide between 22 (2x11) and 24 (2x12).

Analysis:
Using Sunny Design 2

11 vs. 12 panel strings vs. Inverter Spec:
Nominal Power Ratio: 105 / 96 %
Energy Usability Factor: 100 / 100 %
Peak Power (in): 5.06 / 5.52 / 5.30 kWp
Typical PV Voltage: 298 / 325 / 310 (Nom) V
Min PV Voltage: 264 / 288 / 250 (mpp) 300 (start) V
Max PV Voltage: 451 / 492 / 480 (mpp) V
Max Array Current: 15.6 / 15.6 / 21 A

All things being equal I am biased toward using 22 panels (2x11) as our utility rebate maxes out at 5kW, so this is the sweet spot for net cost. I would definitely go to 24 though if there was a performance/efficiency concern.

I am concerned that the reported Min PV voltage seems to be below the start voltage in both cases. SD2 gives it a green light, but seems to be using a 131V min voltage based on 120V grid voltage rather than 240V grid. Calculating Manually, I get a Min Voltage for 70C max cell temp of 29.6V (mpp) - (45 deg * -0.34%) = 25.07 V/panel or 275V for 11 panels, 300V for 12 panels. Of course that's before any DC losses.

What happens if the string MPP DC output voltage is below the start voltage? Will the inverter drop current to get the voltage up and then re-optimize the MPP for best efficiency as long as the output voltage stays above the minimum?

Thanks very much for any thoughts!!

Rob
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  • jagec
    jagec Solar Expert Posts: 157 ✭✭
    #2
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    Re: 5KW Grid Tie Design Review Requested Asap :)

    I have a very similar system in place:
    22 x Canadian Solar 225W panels
    Sunny boy 4000US
    17 degree tilt to the south

    The high-temperature voltage dropout issue was a concern for me, too. I made sure that the array was raised enough to ensure proper ventilation, and agonized over average and record high summer temperatures in my region. I don't expect that it will become a problem for me. However, you're going to see much higher temps in Phoenix, and I'd be tempted to go with 24 panels just so that you wouldn't have to worry about it.

    If you get 22 panels, you might look into getting the SB4000 as I did...with your setup, you're never going to max out the SB5000 (too hot in the summer, too low of an angle in the winter). In my case I'm actually predicted to get more harvest from the SB4000 than I would have from the SB5000, due to higher efficiencies at low power. So I basically paid less and get more energy. Run the numbers in Sunny Design and see what you think.

    Either way, ventilation is key for getting the most out of a low-slope array.
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  • miscrms
    miscrms Registered Users Posts: 15 ✭✭
    #3
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    Re: 5KW Grid Tie Design Review Requested Asap :)

    Thanks very much! Very nice to hear from someone with a similar configuration. Sounds like 24 is the answer then. Ventilation should be good, as panels are mounted on an open framed shade structure. Good suggestion on the SB4000, I will definitely look at that .

    Rob
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