Advice on solar string size.

I am in process of preparing plans for modest 2-3kW grid tie in system. Battery bank will be 48V. I will be using Xantrex 6048 as my inverter and Xantrex XW MPPT charge controller. Panels that I am considering are Evergreen 190W with Vmp of 17.4 and Imp 10.92. Voc on this particular panel is 22V.

Now I trying to figure out how many panels would make an optimal string. In order to charge 48V battery bank charge controller will have to output around 57V. Maximum input Voc that charger can handle is 150V. Does that mean that I can have six panels on the same string with 132 Voc and controller will take that and convert it down to proper charge voltage.

Or it would be better to keep number of panels to a minimum say 4 per string, in that case 4*17.4= 69.6V going in to charge controller? Way I leaning right now is try to put 5 panels per string 5*17.4=87V going in XW charger. This way I have some room if voltage drops on cloudy day.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,721 admin
    Re: Advice on solar string size.
    lesnoy wrote: »
    I am in process of preparing plans for modest 2-3kW grid tie in system. Battery bank will be 48V. I will be using Xantrex 6048 as my inverter and Xantrex XW MPPT charge controller. Panels that I am considering are Evergreen 190W with Vmp of 17.4 and Imp 10.92. Voc on this particular panel is 22V.
    Just as an aside--That is a pretty large off grid system (and nice!).

    You are correct to do your research first before you start spending money. It is easy to make expensive mistakes with solar power...

    Towards that end, we always recommend conservation as the first three steps before buying/installing solar. It is almost always less expensive to conserve a watt than to generate a watt.

    Generally, a "small to average" pure off-grid system would be around 100 kWH per month (~3.3 kWH per day).
    Now I trying to figure out how many panels would make an optimal string. In order to charge 48V battery bank charge controller will have to output around 57V. Maximum input Voc that charger can handle is 150V. Does that mean that I can have six panels on the same string with 132 Voc and controller will take that and convert it down to proper charge voltage.
    Xantrex/Schneider Electric has an online sizing tool for their solar charger/Grid Tied products. And what you are designing for is:
    • Vmp falls with increasing temperature--So Vmp-hot>Vbatt-charging. For a 48 volt system that charges the battery to near 60 volts, Vmp-stc > ~70 volts
    • Voc rises in cold temperature--So Voc<Vinput-max when cold. Typically less than 140-150 VDC (depending on brand/model). It works out, roughly, that your panel Vmp range should be around 70<Vmp-stc<~100 VDC.
    • Charge controllers tend to be a bit less efficient at high Vmp--Wiring tends to be more efficient (less $$ copper) at higher Vmp... I would tend towards Vmp>>Vbatt-charging to keep copper size/costs lower and allow for longer wiring runs from Array to Charge controller.
    See this thread to access the Xantrex/Schneider string sizing tool (currently it is difficult to give you a direct link).
    Or it would be better to keep number of panels to a minimum say 4 per string, in that case 4*17.4= 69.6V going in to charge controller? Way I leaning right now is try to put 5 panels per string 5*17.4=87V going in XW charger. This way I have some room if voltage drops on cloudy day.
    For a first approximation... The panel voltage (under operation) reaches very close to Vmp when weak sunlight hits the panel. More sunlight, for the most part, simply supports more current. Cloudy weather does not cut Vmp, it only cuts available current.

    What does cut the Vmp-array is shading... If you have a string with shading on one panel (morning/afternoon) from a chimney (for example)--Having a higher voltage Vmp-array will support one or two panels being shaded.

    If you have severe shading (trees, houses, hill, etc.)--Solar PV array power output will be severely affected. Panels need full, shade free sun, to generate full power.

    For panels in mottled shading--they will probably not produce any useful power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lesnoylesnoy Registered Users Posts: 8 ✭✭
    Re: Advice on solar string size.

    Much appreciated, great info.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Advice on solar string size.

    generally speaking it is a good compromise to go 1 pv up from the minimum number of pvs to operate at the battery voltage you wish. this considers some possible reduction due to minor shading of 1 pv, lowering the voltage drop with higher voltage, and controller efficiency which decreases with larger input/output differentials. this minimum +1 can go right out the window if shading is worse or that you can't work with that number of pvs or a number of other possible reasons. as the number of pvs in each string increase you must now work with multiples of that many pvs for each additional string and that sometimes does not work out right and this can waste some of the power capability of the controller. each situation will determine what is best and it is good to start off with what i said and modify from there if necessary.
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