Mixing Solar Panel size in System

elmauz
elmauz Registered Users Posts: 6
Hi guys. i'm new in this forum.

I just wanna ask. We have existing solar panel system with 2x110 Watt solar cell.
then we planned to add more solar cell which is 4x120 Watt.

Is it OK if we mixing solar panel size/wattage on one System..?
Tnx.

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Mixing Solar Panel size in System

    my responses in bold.
    elmauz wrote: »
    Hi guys. i'm new in this forum.

    I just wanna ask. We have existing solar panel system with 2x110 Watt solar cell.
    then we planned to add more solar cell which is 4x120 Watt.

    ok on the 2 110w solar panel system, but you are calling each panel a cell and that not correct. you can call them solar panels, which the thermal type are in the same classification, solar electric panels, or photovoltaic modules and we use either term of pv or module here most times.

    Is it OK if we mixing solar panel size/wattage on one System..?

    for particulars now that depends on how you are presently using them. what pv system voltage are you using into what controller, and what battery bank voltage at what ah capacity? in some applications it is not good to deviate either the voltage (most parallel operations usually up to about 10% as most will say here with my believing tighter would be better) or current in series configurations and some will say there's a small degree of latitude here too, but you get the gist of the trend.

    Tnx.
  • elmauz
    elmauz Registered Users Posts: 6
    Re: Mixing Solar Panel size in System
    niel wrote: »
    Tnx.my responses in bold.

    Hi guys. i'm new in this forum.

    I just wanna ask. We have existing solar panel system with 2x110 Watt solar cell.
    then we planned to add more solar cell which is 4x120 Watt.

    ok on the 2 110w solar panel system, but you are calling each panel a cell and that not correct. you can call them solar panels, which the thermal type are in the same classification, solar electric panels, or photovoltaic modules and we use either term of pv or module here most times.

    Is it OK if we mixing solar panel size/wattage on one System..?

    for particulars now that depends on how you are presently using them. what pv system voltage are you using into what controller, and what battery bank voltage at what ah capacity? in some applications it is not good to deviate either the voltage (most parallel operations usually up to about 10% as most will say here with my believing tighter would be better) or current in series configurations and some will say there's a small degree of latitude here too, but you get the gist of the trend.

    Tnx for the response.
    We Use 24 Vdc system voltage as input for Solar Controller.
    So do with battery bank we use 24 Vdc.
    The capacity of battery bank is 216 Ah @ 24Vdc

    So, there will be a problem if we mix this 2x110 Watt solar panel with new one 4x120 Watt solar panel..?
    Tnx
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Mixing Solar Panel size in System

    You also should give us the Vmp and Imp of the panels (and/or manufacturer/model number) and what kind of solar charge controller (PWM or MPPT type) and it rated output current...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • elmauz
    elmauz Registered Users Posts: 6
    Re: Mixing Solar Panel size in System
    BB. wrote: »
    You also should give us the Vmp and Imp of the panels (and/or manufacturer/model number) and what kind of solar charge controller (PWM or MPPT type) and it rated output current...

    -Bill

    Existing Panel 2x110W Mitsubishi PV-MF110EC4.
    Electrical Rating : Vmp = 17.1 V; Imp = 6.43A

    with Solar Controller PWM Type, which is Morningstar Prostar15 (Rated output current=15A) but it will be upgraded to Prostar30 (Rated output current=30A)

    The new solar panel is 4x120W Mitsubishi PV-MF120TE4N.
    Electrical Rating : Vmp = 17.2 V; Imp = 6.99A

    How about it..? tnx for your comments..
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Mixing Solar Panel size in System
    elmauz wrote: »
    Existing Panel 2x110W Mitsubishi PV-MF110EC4.
    Electrical Rating : Vmp = 17.1 V; Imp = 6.43A

    with Solar Controller PWM Type, which is Morningstar Prostar15 (Rated output current=15A) but it will be upgraded to Prostar30 (Rated output current=30A)

    The new solar panel is 4x120W Mitsubishi PV-MF120TE4N.
    Electrical Rating : Vmp = 17.2 V; Imp = 6.99A

    The ideal would be to add the new 120W panels in series parallel (2 series, and those two in parallel)... The Vmp for the strings (34.2 vs 32.4 volts) is virtually identical.

    The Imp of the two panel types is close enough (within 10%) that you could put them in series if you needed to.

    The total current will be:
    • 6.43a + 2x6.99a = 20.41 amps
    Because the total string current is larger than the series protection fuses required for the panels--you should put a fuse in each series string (prevents a short circuit from being fed by other panels and causing a fire):
    Your battery is currently 216 AH--So the charging ratio:
    • 216 AH / 20.14 amps = 10.6:1
    • 1/10.6 = 0.094 = 9.4%
    Which is just fine (5%-13% generic rule of thumb).

    The amount of power you can get from your system is around 10 MJ / 2.8 hours f sun to 20 MJ / 5.6 hours of sun per day for Jakarta (PDF download).... Looks pretty variable (season, and, I would guess, lots of variability due to weather patterns)... Perhaps 15 MJ / 4.17 hours of sun on average (assuming 1kWH per sq.mtr. is full sun--using this for MJ to kWH conversion).

    So, if your system is providing for a DC load:
    • Use 0.77 derating for panels+controllers * 0.80 for flooded cell batteries= 0.62 over all derating
    • Use 0.77 * 0.80 * 0.85 inverter efficiency = 0.52 end to end derating
    So, assuming worst case weather, 2.8 hours of sun (worst case sun :
    • 2*110w + 4*120W = 700 watts of solar panels:
    • 700 watts * 2.8 hours of sun * 0.62 = 1,215 WH per day (DC output)
    • 700 watts * 2.8 hours of sun * 0.52 = 1,019 WH per day (AC inverter output)
    About all I can guess at this time.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • elmauz
    elmauz Registered Users Posts: 6
    Re: Mixing Solar Panel size in System

    Dear BB,

    Thanx for your information.
    I wanna ask you the reference regarding some information below :
    BB. wrote: »
    The Imp of the two panel types is close enough (within 10%) that you could put them in series if you needed to.
    Do you have reference/link for maximum difference of Imp in two panel types.
    BB. wrote: »
    • 216 AH / 20.14 amps = 10.6:1
    • 1/10.6 = 0.094 = 9.4%
    Which is just fine (5%-13% generic rule of thumb).
    Do you have reference/link for this charge ratio limitation?

    Thanx again..
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Mixing Solar Panel size in System

    Elmauz,

    For Imp/Vmp matching--you can use any silicon panel I*V curve and see how far you can move off "Vmp/Imp" peaks... I use the specifications for this panel because it has lots of clear graphs, and also has some other information graphed-such as Vmp and Imp vs temperature. Some manufacturers graph Pmax, others do not--So I use the following as a generic example. Notice how "flat" the peaks are of the Pmax curve. Pick the "left side" and move 10% of the voltage to the right--not very much change in Pmax over that range.

    data sheet for a crystalline silicon panel

    You can just subsitute your Volt and Amp ratings for your panel... The actual numbers are pretty much set by the physics of the silicon material itself--so the curves are generic.

    Solar Guppy pointed out to me a year or so ago how flat the power curves are at Vmp/Imp and showed that a little mismatch is not going cause zero power output.

    The 5%-13% charging current is sort of a synthesis of the specifications of several major battery manufacturers and deep cycle battery usage...

    5% minimum comes from (at least for me) the Rolls/Surrette charge rate for equalization. That is the "low rate" (5 amps per 100 AH capacity) they recommend for equalization current to try and recover capacity from a poorly maintained cell.

    I believe I have also seen other places where 5% was recommended as giving the minimum amount of hydrogen bubbling (electrolysis) that helps mix the acid. "Tall" flooded cell batteries tend to have the electrolyte stratify--heavy/dense acid at the bottom of the battery and light/mostly water at the top. Proper charging current helps remix the electrolyte.

    The high charge rate of 13% of Amp*Hour capacity is a recommended limit for Trojan Battery Mfg. batteries. That is also the recommend charging for many forklift batteries of C/8 (12.5% charge rate in an 8 hour shift). Flooded cell batteries can be charged faster--but typically need cooling fans and/or remote temperature sensors to protect the bank. Trojan also recommends a 10% - 13% charge current range for normal cycling.

    There are also some practical reasons for the rule of thumbs. Old Lead Acid storage/deep cycle batteries can have upwards of 1% rate of self discharge near their end of life... If you assume that you have 8 hour of sun to to recharge -- that would be a 3% minimum charge rate just to keep even with self discharge.

    Also, because a battery bank that sets below ~75% state of charge for long periods of time (days-weeks-months), that is when the sulfates tend to harden and cause loss of capacity / battery early life failure. So--you want to recharge a battery back above the 75% state of charge fairly quickly--So as to ensure a long life. A charger with less than 5% rate of charge is going to take a long time to do this.

    The maximum charge rate is also a goo practical number... A typical lead acid battery will recharge at maximum current up to ~80-90% state of charge--past that point, the battery rate of charge will become voltage limited (14.4 volts or so). A charger which can dump huge amounts of current into bank will still be time limited for the last 10-20% final charging of the bank (~1-6 hours). So a very large charger's capacity is sort of wasted during the absorb cycle of the battery.

    And since we are a solar board here (mostly)--we are cheap and don't want to see people spend huge amounts of money for a large solar array + charge controllers when much of that capacity will be wasted during day to day usage.

    Now--like all rules of thumb, there are times when they don't apply...

    AGM type batteries is one place... AGM's tend to have very low self discharge (maybe 1-2% per month instead of per day). Also, they have much lower internal resistance/better physical chemistry which allows them to discharge/recharge at much higher rates than flooded cells. However, because AGM's are sealed batteries, they are very sensitive to over charging (can cause cells to vent hydrogen/electrolyte--which is not replaceable).

    Some rules of thumbs are just handy guidelines to do a quick sizing on the bank--and take into account other rules of thumb...

    For example, sizing a battery bank for 3 days of no sun and 50% maximum discharge. This keeps the battery above 75% state of charge most of the time--to reduce sulfate hardening. Also, on the third day, you can fire up the genset if you need to recharge because there is still no-sun. Also accounts for (roughly) a 5-13% charging rate with combined with other rules. Small battery banks with large/heavy loads/charging are not as efficient--a fair amount of energy gets turned into heat and hydrogen gas when high currents are involved.

    The 50% maximum discharge is another suggestion... The battery bank will certainly work if discharged deeper (as long as it is quickly recharged above 75%)--But the battery will not last as many cycles... However, from some back of the envelope calculations, a battery bank that is 2x larger will last a little more than 2x longer. So, the costs are almost a wash (replace 8 batteries every 8 years vs replacing 4 batteries every 4 years).

    Time to Question the 3 day Rule ?

    There are a lot of reasons to follow the various rules of thumb--but they can certainly be "bent" to the needs of the specific installation. But--usually, there should be a good reason to do it. Also the rules are very handy for us to give somebody a quick sizing of their system with generic equipment (conservative assumptions). Usually, the results are close enough to make for a very successful installation.

    Batteries will last a long time if properly maintained (including properly recharged). However if abused, they can die quickly. And the farther one goes from the "rules", the easier it is to abuse the batteries.

    -Bill

    PS: Should add the Battery FAQ's... Always a good read for basic understanding of the issues:

    Deep Cycle Battery FAQ
    www.batteryfaq.org
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Mixing Solar Panel size in System

    i think we can say it should work for you as the specs for the newer seriesed/paralleled pvs will work out fine for your application. the new ones will be of 2 in series and paralleled to one another and then paralleled to the present string. you will have a good charge rate percentage given the ah rating of about 216ah too. it is advisable to fuse each string before being combined.
  • elmauz
    elmauz Registered Users Posts: 6
    Re: Mixing Solar Panel size in System

    On which location i must put the fuse ?
    on each 2 series of solar panel or on each paralleled point o f solar panel ?

    How about the fuse size and type ? 15 Ampere is enough ?
    Thanx
    niel wrote: »
    i think we can say it should work for you as the specs for the newer seriesed/paralleled pvs will work out fine for your application. the new ones will be of 2 in series and paralleled to one another and then paralleled to the present string. you will have a good charge rate percentage given the ah rating of about 216ah too. it is advisable to fuse each string before being combined.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Mixing Solar Panel size in System

    let me clarify that 2 inputs (aka pv strings) to a controller do not need to be fused by requirement. many will get a disconnect with the fuses or circuit breakers in it. these can be up there in price and it is possible to make one up if you have that ability and know-how. i do recommend you look over our sponsor's store to see what they have and the pricing of such items. it may even be possible to go to an electrical supply store to obtain some of these things too if there is one near you that has knowledgeable employees.
    one of crewzer's recent posts, #4,
    http://forum.solar-electric.com/showthread.php?p=50253#post50253
    did clarify the current requirements, but one must observe the voltages reached too as a 32v rated car type fuse cannot go with pvs in series that exceed that voltage.