MPPT v PWM, Panel cost

Options
zoneblue
zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
Heres a kind of 'naive enquirer' type question. You see here stated often that "using cheaper grid tie panels" is one of the primary justifications for the use of MPPT controllers. Take cabling and cabling distance out of the equation, whats wrong with 60 cell panels for charging 24v banks? Their 30-32V Vmp seems quite well suited to PWM charging, even temp compensated, vmp is still better than 30V.
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar


Comments

  • WillBkool
    WillBkool Solar Expert Posts: 35 ✭✭
    Options
    Re: MPPT v PWM, Panel cost

    If you had zero voltage drop, you could probably use them. But you will have voltage drops on the wiring, charge controller, etc., and then they may not have enough voltage, especially for an equalize that is sometimes 30 or more volts. But in a perfect world, they would work.;)
    1220 Watts, 4 Evergreen 120 watt, 1 Eoplly 190 watt; 1 Sungold 200 watt; 2 175 Watt; M-Star 15A MPPT; C40 PWM; 6 105 AH AGM Configured to 315@24V
    Cotek 1500 watt/24v
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Options
    Re: MPPT v PWM, Panel cost
    zoneblue wrote: »
    Their 30-32V Vmp seems quite well suited to PWM charging, even temp compensated, vmp is still better than 30V.

    When the panel gets hot the Vmp may be too low. Some voltage is also lost in the wiring and controller. If the batteries are also hot, that will help because then they need lower voltage. Of course, if the batteries are hot they will die sooner.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Options
    Re: MPPT v PWM, Panel cost

    As long as you use 2 or more panels and MPPT, no problem, but alone you would not get a full charge....hot panels, cold battery, wire and equipment losses...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost
    zoneblue wrote: »
    Heres a kind of 'naive enquirer' type question. You see here stated often that "using cheaper grid tie panels" is one of the primary justifications for the use of MPPT controllers. Take cabling and cabling distance out of the equation, whats wrong with 60 cell panels for charging 24v banks? Their 30-32V Vmp seems quite well suited to PWM charging, even temp compensated, vmp is still better than 30V.

    Feel free to throw away your money any way you want.
    People who assume a 30 Vmp panel will charge a 24 Volt battery bank are #4 on my list of frequently corrected problems.
  • Desert Rat
    Desert Rat Solar Expert Posts: 138 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost

    I'm fixing a system right now where the installer told the client "Yeah, these are 24V panels.". No way is a panel with a Vmp of 29.8 going to bring a battery bank to an absorption voltage of 29.6. Guess what kind of condition the batteries are in! Once again:
    Most batteries don't die of natural causes; they are murdered!
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Options
    Re: MPPT v PWM, Panel cost

    In our case i would have needed 9 times the amount of pv cable, or about $800 extra, so it paid for the mppt controller without question.
    People who assume a 30 Vmp panel will charge a 24 Volt battery bank are #4 on my list of frequently corrected problems.

    So what about 72 cell panels? which are no more expensive than 60 cells. Exactly twice the 36 cells that comprise 12v pwm type panels. Not that im going to try this any time soon, but the "allows GT panels" argument per se still seems pretty tenuous.

    The argument that was put to me was this: the gain from mppt is outweighed by the cost of the controller and cheapness of panels outweighs that gain. Has this actually been modelled recently, using todays PV costs?

    What Mppt does create to my mind is considerable flexibility of panel spec, wiring and BOS. But its no longer clear to me whether in systems with short distances, an optimally designed and spec'ed system using PWM v mppt is actually more cost effective or not. Simple equation: mmpt=10% more effcient, 2kWp, 200W gain= one more panel at $1/W = $200.

    Devils advocate, like.

    BTW what is it with american pv manufacturers, is all they can do for iv curves artist impressions? Not one of the panels listed on NAWs has a full set of IV curves in their specsheets.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost

    There is no issue really. If the panel Vmp is 30 and the system charges at 28.8 that's a mere 1.2 Volt margin which can disappear in a hurry. It won't work.

    A 72 cell panel will likely have a Vmp of 35, which will work with a PWM controller on a 24 Volt system. No issue.

    Economics comes into play when you have an array need over 400 Watts and the difference in per Watt cost between GT style panels and standard ones is 2X: the $400 saved on the panels pays for the more expensive MPPT controller. The "greater efficiency" of MPPT is irrelevant and often exaggerated. Under normal conditions they only add about 10% to the power and that's not very significant in most applications. Day to day operational variations will be larger.

    IV curves of panels are largely irrelevant with MPPT because it's going to pick a V*I point of its own which is most efficient for the output needs at the time. Don't get hung up on small details.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Options
    Re: MPPT v PWM, Panel cost

    My thought is I am buying flexibility with an MPPT controller (that is not available from PWM) as I can use anywhere from 2 (min) to the max voltage the CC can handle. For a variety of reasons I settled on a 48v (4) config but... may go for 60V (5) depending on confirmation of final shade patterns of whack a few more trees.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Options
    Re: MPPT v PWM, Panel cost
    The "greater efficiency" of MPPT is irrelevant and often exaggerated. Under normal conditions they only add about 10% to the power and that's not very significant in most applications. Day to day operational variations will be larger.

    I rate each of those three sentences as: "Usually True".

    On short, but very cold winter days (when I need it the most) my MPPT often harvests MORE than the 940 watt STC ratings of my panels.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost
    vtmaps wrote: »
    I rate each of those three sentences as: "Usually True".

    On short, but very cold winter days (when I need it the most) my MPPT often harvests MORE than the 940 watt STC ratings of my panels.

    --vtMaps

    Again when comparing functions you have to use 'typical' conditions and 'all other factors being equal' not extremes such as cold temps. My panels do some outrageous Voltage at -30, but by the same token there's not many hours of daylight at those times so the daily power output is still less. There is never a case where we get hyper-conducting cold temperatures and 16 hours of daylight.

    For example: 220 Amp hour 12 Volt battery bank. Two ways of charging it: with and without MPPT controller. In both cases use three 140 Watt KD panels with Vmp of 17.7 and Imp of 7.9 wired in parallel.

    With a ProStar 30 PWM controller you will get (3 * 7.9) 23.7 Amps peak.
    With a MidNite Kid MPPT you can expect (420 Watts * 0.77 / 12 Volts) 26.9 Amps peak.

    Will you be able to make use of the extra 3 Amps? Maybe. Maybe you won't even ever see it because the peak current output is partly dependent on load demand; if the batteries never require it it doesn't happen. Even if you get it that doesn't mean the batteries will charge "better".

    Now look at the economics:
    Three KD140's @ $265 each and ProStar 30 @ $118 = $913
    The MidNite Kid is $285 so that way the system would cost $1080 - not a good deal at all. Certainly not worth 3 more Amps output power.

    But change the panels to two KD220's @ $252 each and it costs $789.
    You get 20 Watts more, 28 Amps potential peak output, and you save $124 in the major components. Not to mention the simplicity of wiring two panels as opposed to three (higher Voltage = lower current thus smaller wire for the home run, no need for breakers/fuses on each panel, less mounting, etc).

    That's a direct advantage that doesn't involve fudging the circumstances with "but if it's colder" or "but if the wire run is longer" or any other variation which renders the PWM controller impractical to begin with.

    The larger the array needs to be, the greater the advantage of MPPT. :D
  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost
    vtmaps wrote: »
    I rate each of those three sentences as: "Usually True".

    On short, but very cold winter days (when I need it the most) my MPPT often harvests MORE than the 940 watt STC ratings of my panels.

    --vtMaps

    The other morning around 10am, it was typical bright and sunny Florida, but the temp was in the low 50's and my 2460w array showed a max peak output of 2680w!
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
    Options
    Re: MPPT v PWM, Panel cost

    Don't get too wrapped up in exact numbers... It is not unusual for a charge controller to read 5% high on its output.

    The controllers don't have to be "that accurate" to work correctly. The costs and complexity of added precision shunts and accuracy is something that customers are not always willing to pay for.

    To get better than 2% accuracy in power measurements gets into lab grade equipment that costs thousands to tens of thousands of dollars--And different mfg. do not always agree either (non-linear wave forms, variations in averaging/calculations, etc. all make a difference in what is displayed).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost

    Yes if your output read 2680 Watts on this occasion the question would be "what does it read on average?"

    As Bill said the meters aren't that accurate, but they are relative. On some occasions I have seen my MX60 show >700 Watts the array is rated for, but mostly that means it is putting out more than the typical maximum of 588 Watts not that either of those numbers are 'true Watts'.
  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost
    Yes if your output read 2680 Watts on this occasion the question would be "what does it read on average?"

    I'm not near the system now, but if I recall from browsing the FM80's history, On good days, it usually shows a "peak" around the 2200-2400ish range. Now that I have all the fancy new logging of the Mate3/Flexnet, I will be able to get some better data in the future.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Options
    Re: MPPT v PWM, Panel cost

    Yes, this is the kind of model im talking about. You are comparing PWM/36 cells/12V to MPPT/60 cells/12V. If you were to compare PWM/72 cells/24V to MPPT/72cells/24V that advantage drops away. The underlying premise is that 12v systems are assumed.

    I think there should be a rider placed on that model. "Where 12V systems are required" MMPT allows you to use cheaper GT panels.
    For example: 220 Amp hour 12 Volt battery bank. Two ways of charging it: with and without MPPT controller. In both cases use three 140 Watt KD panels with Vmp of 17.7 and Imp of 7.9 wired in parallel.

    With a ProStar 30 PWM controller you will get (3 * 7.9) 23.7 Amps peak.
    With a MidNite Kid MPPT you can expect (420 Watts * 0.77 / 12 Volts) 26.9 Amps peak.

    Will you be able to make use of the extra 3 Amps? Maybe. Maybe you won't even ever see it because the peak current output is partly dependent on load demand; if the batteries never require it it doesn't happen. Even if you get it that doesn't mean the batteries will charge "better".

    Now look at the economics:
    Three KD140's @ $265 each and ProStar 30 @ $118 = $913
    The MidNite Kid is $285 so that way the system would cost $1080 - not a good deal at all. Certainly not worth 3 more Amps output power.

    But change the panels to two KD220's @ $252 each and it costs $789.

    You get 20 Watts more, 28 Amps potential peak output, and you save $124 in the major components. Not to mention the simplicity of wiring two panels as opposed to three (higher Voltage = lower current thus smaller wire for the home run, no need for breakers/fuses on each panel, less mounting, etc).

    That's a direct advantage that doesn't involve fudging the circumstances with "but if it's colder" or "but if the wire run is longer" or any other variation which renders the PWM controller impractical to begin with.

    The larger the array needs to be, the greater the advantage of MPPT. :D
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost
    zoneblue wrote: »
    Yes, this is the kind of model im talking about. You are comparing PWM/36 cells/12V to MPPT/60 cells/12V. If you were to compare PWM/72 cells/24V to MPPT/72cells/24V that advantage drops away. The underlying premise is that 12v systems are assumed.

    I think there should be a rider placed on that model. "Where 12V systems are required" MMPT allows you to use cheaper GT panels.

    Nope. You're not paying attention.
    I was giving the example of PWM vs. MPPT with exactly the same array to begin with in order to demonstrate the power advantage of the MPPT controller.
    Then I changed the array type to show the economic advantage on the same size system.
    If you go altering lots of parameters at the same time you aren't demonstrating anything except a talent for confusion.

    You can not charge any system with less than the minimum Vmp required for that battery Voltage.

    A 12 Volt system needs 17-18 Vmp in order to still have sufficient charging Voltage at the batteries after typical expected losses. As you go up in system Voltage these losses lessen but do not disappear. So when you try to charge a 24 Volt system which has an Absorb Voltage around 28.8 with a panel capable of only 30 Vmp you will not have sufficient Voltage to charge with. Period. You may not need full 35 Vmp that you would get with a 'true' 24 Volt panel, but the losses between hot panels and V-drop in wiring will not be reduced to 1.2 Volts.

    So the Voltage 'overhead' on a 12 Volt system is around 3 Volts. Depending on how it is distributed (hot panel vs. wiring loss) the 6 Volt 'overhead' from a 35 Vmp array on a 24 Volt system may have an 'extra' Volt or two. With a PWM controller this amounts to nothing. With an MPPT controller the extra is turned into additional charging current when available. The number of cells in the panel is irrelevant.

    To get sufficient charge Voltage for a 24 Volt system you'd need to put two 30 Vmp panels in series. With a PWM controller most of the extra power of the second panel will be lost just for the sake of gaining the necessary 3 Volt 'overhead'. With an MPPT controller the power is not lost.

    When you go up to 48 Volts you need >60 Volts for charging. Again two 30 Vmp panels in series won't do the job, despite the fact the wiring losses are significantly less than with a 12 Volt system (hot panel losses are not). So now you put three in series in order to achieve minimum Voltage needed and once again the majority of the power of that third panel would be lost if a PWM type controller is used.

    If you use four "12 Volt" panels in series on a 48 Volt system you'd have Vmp 70 for the array, which is sufficient without a doubt. But you would have (4 * 140) 560 Watts at an expense of (4 * 265) $1060. You could use three of the 220 Watt panels in series and have enough charging Voltage with (3 * 220) 660 Watts at an expense of (3 * 252) $756. Saving $304 (per string) and having 10.5 Amps current vs. 7.9 Amps (per string). In this example the panel savings alone pay for the controller.

    I really don't see why you can't comprehend this. You have to work with the equipment available, and you'd best make good use of it 'cause none of it is cheap.
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Options
    Re: MPPT v PWM, Panel cost

    Perhaps i confused the issue by mentioning 60 cell panels, but my OP was intended to be about the use of grid tie panels with PWm controllers. Just to clarify, are you saying that 72 cell panels are more expensive than 60 cells, or that 72 cell panels wont charge a 24V bank using PWM?
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Options
    Re: MPPT v PWM, Panel cost
    zoneblue wrote: »
    Perhaps i confused the issue by mentioning 60 cell panels, but my OP was intended to be about the use of grid tie panels with PWm controllers. Just to clarify, are you saying that 72 cell panels are more expensive than 60 cells, or that 72 cell panels wont charge a 24V bank using PWM?

    The number of cells in a panel usually equates to Voltage: about 0.5 Volts per cell. 60 cell panel = 30 Vmp, 72 cell panel = 36 Vmp. The 36 Vmp panel will be able to charge the 24 Volt system the 30 Vmp panel won't.

    Grid-tie panels are typically 30 Vmp: it works nicely with GTI's by putting twelve in series to get the required 360 Volts while running the Watts up to near 3kW.

    Since these GT panels are the most mass-produced the price is typically 1/2 the cost per Watt of the higher Voltage 'standard' panels (which are becoming increasingly difficult to find). For example Sharp quit making anything under 200 Watts, including the 72 cell 35 Vmp 175 Watt panels that I have.