how to connect two 12v PVs wired in series and one 24 PV to get 24V

Hi

I am currently running a DIY solar power system with two nos of 12V 100W (Im 6.1A and VoC 22.4) panels in series, 30A PWM charge controller (Max PV Voltage <48V), 4 nos of 40AH 12v batteries in series-parallel (24V) and 800VA 230V Sine wave Digital Inverter.

I am planning to increase the PV size. But I am not able to locate similar panels.

Recently I saw 200W 24V (Im 6.2A) and 250W 24V (Im 7.1A) panels in a store nearby. (VoC 43V)

Is it possible to use any of these panels in parallel with my existing PV array?
Thanks

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: how to connect two 12v PVs wired in series and one 24 PV to get 24V

    Welcome to the forum.

    You may be able to connect a single "24 Volt" panel in parallel with two "12 Volt" panels in series. Unfortunately the specs you gave are missing the key information: Vmp, which is what matters when connecting panels in parallel.

    However, we can approximate that number from what you did give. For example A panel with a Wattage rating of 100 and an Imp of 6.1 should have a Vmp of about 16. The Voc on that panel being 22.4 indicates the "Imp" rating is really Isc, as Voc is approximately 4 Volts higher than Vmp on a "12 Volt" panel. So hopefully the actual Vmp for those panels is 17.5 +/- (16 would be a tad low).

    The "24 Volt" panels would then be (200/6.2) 32 Vmp and (250/7.1) 35 Vmp respectively. On these we would expect the Vmp to be about 8 Volts lower than Voc: 43 - 8 = 35. (I expect you left out the Voc for the 200 Watt panel which would be about 40.)

    So the two "12 Volt" panels in series would comprise a string with a Vmp of (17.5+17.5) 35, which is an exact match to the 250 Watt panel's expected Vmp. Together the three panels would provide about 13 Amps @ 35 Volts through a PWM type charge controller.
  • portman
    portman Registered Users Posts: 4
    Re: how to connect two 12v PVs wired in series and one 24 PV to get 24V

    Thanks Cariboocoot for the detailed tech info. It is amazing to know that I can connect 250W panel.
    Let me get more info on the 250w panel and get back to you.
    Thanks again.

    Welcome to the forum.

    You may be able to connect a single "24 Volt" panel in parallel with two "12 Volt" panels in series. Unfortunately the specs you gave are missing the key information: Vmp, which is what matters when connecting panels in parallel.

    However, we can approximate that number from what you did give. For example A panel with a Wattage rating of 100 and an Imp of 6.1 should have a Vmp of about 16. The Voc on that panel being 22.4 indicates the "Imp" rating is really Isc, as Voc is approximately 4 Volts higher than Vmp on a "12 Volt" panel. So hopefully the actual Vmp for those panels is 17.5 +/- (16 would be a tad low).

    The "24 Volt" panels would then be (200/6.2) 32 Vmp and (250/7.1) 35 Vmp respectively. On these we would expect the Vmp to be about 8 Volts lower than Voc: 43 - 8 = 35. (I expect you left out the Voc for the 200 Watt panel which would be about 40.)

    So the two "12 Volt" panels in series would comprise a string with a Vmp of (17.5+17.5) 35, which is an exact match to the 250 Watt panel's expected Vmp. Together the three panels would provide about 13 Amps @ 35 Volts through a PWM type charge controller.
  • portman
    portman Registered Users Posts: 4
    Re: how to connect two 12v PVs wired in series and one 24 PV to get 24V

    Hi

    The 200w & 250 W panels were out of stock all these days and new stock came last week.
    Here are the details of the panels

    200w (54 Cells) 24 V
    Vmp 26.9
    Imp 7.44
    Voc 32.8
    Isc 8.24


    250W 24 V
    Vmp 30.72
    Imp 8.14
    Voc 37.80
    Isc 8.63


    Existing setup:

    two 12V panels in series.

    Panel 1

    100W 12 V
    Isc 6.492A
    Voc 22.464V
    Pm 109.128W
    Im 6.120A
    Vm 17.832V


    Panel 2
    100W 12V
    Isc 6.530A
    Voc 22.511V
    Pm 108.061W
    Im 6.097A
    Vm 17.723 V

    Please give your advice.


    Welcome to the forum.

    You may be able to connect a single "24 Volt" panel in parallel with two "12 Volt" panels in series. Unfortunately the specs you gave are missing the key information: Vmp, which is what matters when connecting panels in parallel.

    However, we can approximate that number from what you did give. For example A panel with a Wattage rating of 100 and an Imp of 6.1 should have a Vmp of about 16. The Voc on that panel being 22.4 indicates the "Imp" rating is really Isc, as Voc is approximately 4 Volts higher than Vmp on a "12 Volt" panel. So hopefully the actual Vmp for those panels is 17.5 +/- (16 would be a tad low).

    The "24 Volt" panels would then be (200/6.2) 32 Vmp and (250/7.1) 35 Vmp respectively. On these we would expect the Vmp to be about 8 Volts lower than Voc: 43 - 8 = 35. (I expect you left out the Voc for the 200 Watt panel which would be about 40.)

    So the two "12 Volt" panels in series would comprise a string with a Vmp of (17.5+17.5) 35, which is an exact match to the 250 Watt panel's expected Vmp. Together the three panels would provide about 13 Amps @ 35 Volts through a PWM type charge controller.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: how to connect two 12v PVs wired in series and one 24 PV to get 24V

    Neither of the two "24 Volt" panels you give the specs on there will actually work for charging a 24 Volt system; the Vmp of either is too low.

    Note your "12 Volt" panel Vmp ratings: 17.8 and 17.7 - these are 'normal' and two in series would provide Vmp around 35 for a 24 Volt system. That works.

    Now the "24 Volt" panels: Vmp 26.9 and Vmp 30.7 - these are actually both below the maximum charging Voltage used on a 24 Volt system. The second panel might contribute some, as long as there is zero Voltage loss in the wiring between it and the battery. But there is never Voltage zero loss.

    A 24 Volt system typically has an Absorb Voltage of 28.8 (without temp compensation) and EQ around 31. See the problem? Even with Vmp 30.7 when the panel gets hot its Voltage goes down and then the wiring also will take some Voltage too. Net result is it does not produce enough Voltage at the battery to achieve charging.

    These panels are known as "grid tie" panels because they are designed to work efficiently with GT systems (twelve in a series string producing 360 Volts @ 3000 Watts). To get them to work with a battery system requires an MPPT type controller so that you can run an array above the desired Vmp for the system Voltage and down-convert the 'extra' Voltage to additional current.

    They are not going to work with a PWM controller without a very large loss of power. For example putting a 26 Vmp panel in series with a 17 Vmp panel to get 43 Vmp and then losing the power difference between that Voltage and the 'ideal' Vmp plus having the current limited to the lower Imp rating. Or 240 Watts + 100 Watts giving you not 340 Watts but about 210 (the equivalent of 35 Vmp * 6 Imp). Pretty big power drop.
  • portman
    portman Registered Users Posts: 4
    Re: how to connect two 12v PVs wired in series and one 24 PV to get 24V

    Thanks a lot for the help and key info which many people are not aware of.
    These panels looks like low quality panels came from China.
    I will continue the search for a compatible panel and your postings will be my base reference.
    Thanks again and have a wonderful weekend.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: how to connect two 12v PVs wired in series and one 24 PV to get 24V

    Regardless of the source of the panels--It will (usually) be a choice between smaller (typically 140 watt or less) solar panels that cost more $$$/Watt and can use less expensive PWM charge controllers. Vs using cheaper >140 watt panels and more expensive MPPT charge controllers.

    In general, less than 400 watt systems can use PWM controllers pretty efficiently (does depend on the distance from the solar array to the charge controller--Long runs of "12 volt" cables need to be very heavy copper--an additional cost in itself--Even 20 feet can push a system to 6 AWG cable vs using 16 AWG cable on a higher Vmp voltage array).

    For arrays over 800 watts, it is almost always better to go with MPPT type charge controllers (and higher voltage solar panels).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: how to connect two 12v PVs wired in series and one 24 PV to get 24V

    And what we really, really need are those 30 Amp MPPT controllers from Rogue and MidNite. They would fill quite a large need in the off-grid market.

    Come on guys! You've got customers waiting! :D
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: how to connect two 12v PVs wired in series and one 24 PV to get 24V

    There are still some 24 volt nominal (VMP @35v) high wattage panels out there, keep your eyes open! You might also find some 12 v nominal panels used from people replacing them with high wattage panels.

    The trouble with the high wattage panels also extends to their size which make shipping single panels expensive. Generally speaking panels above 150-180watts need to ship via freight.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.