Florida Climate and PWM/MPPT

TinaRay&AmySoehlerTinaRay&AmySoehler Registered Users Posts: 5
Hey guys! I've been researching solar charge controllers and I have a few questions. I understand that PWM and MPPT controllers are the two that are currently on market and the general traits of each: PWM is cheaper and works best when the incoming and outgoing voltages are the same; MPPT is more expensive and works better when the incoming voltage is higher than the outgoing.

I read that in many cases MPPT controllers are better than PWM, but I've also seen in a number of places that any benefits MPPT controllers have over PWM controllers tend to decline in warmer temperatures. I've done some "in-depth research" on the topic (I went to at least the fourth page of results when I searched online), but have not found solid technical data to back this up. If you guys have info or resources that would help me read up on this, I'd appreciate the reference.

Since I live in Florida, I was wondering whether MPPT controllers would still have any operational advantage over PWM in our warm climate.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Can you tell us more about your system/needs (loads, AH @ XX Volts or Watt*Hours per day)? What voltage/AH rated battery bank? Type of batteries (Flooded Cell, AGM)?

    Size of array (Watts), how far from Array to Charge controller/battery shed?

    What is your experience (electrical in general, solar power specifically).

    In general, for less than ~400 Watt solar arrays, a PWM system may be less expensive too install (controllers cost less, 36 cell solar panels tend to cost more and are limited to around 140 Watt maximum at this time).

    If you have a long wire run from Array to Charge controller, have >800 Watt of panels, want networking/computer logging/live in a hot climate--Then MPPT is usually a better (more cost effective) choice. Reasons include:
    • Large solar arrays running at low voltage (i.e, an 1,800 Watt array at Vmp-array 18 volts to charge a 12 volt battery will be ~100 amps with less than 1 volt or so of voltage drop allowed)--Requires a lot of heavy copper cable if you are more than a few 10's of feet from the array to the battery bank.
    • Large solar array at high voltage (i.e., 1,800 watt array at ~100 Volts Vmp-array; will be around 18 amps and can have many volts of drop without problem). You can use standard size copper cabling (typically in the 8 to 14 AWG size).
    • New controllers with lots of processor power to support data logging/even a web server for Internet connectivity. MPPT controllers tend to have more features than low cost PWM controllers (they also "use more power" too to run the switching supply and electronics for MPPT functions).
    Look at your needs and desires. Do a few different paper designs and see what works out best for you.

    Until you know your needs, it is like trying to say a smart car is better than a pick up or a semi-tractor-trailer.... They are different tools for different jobs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TinaRay&AmySoehlerTinaRay&AmySoehler Registered Users Posts: 5
    I'm looking at using a 1500 W array at 24 VDC with a 12 or 24 VAC battery pack (I'm still deciding what AH rating to get for the battery). The system needs to produce 1200W. What do you think?
  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    OK, still not much about loads... Based on a 1,500 Watt array and a 5% to 13% rate of charge:
    • 1,500 Watts * 0.77 panel+controller derating * 1/0.05 rate of charge * 1/24 volt battery bank = 963 AH @ 24 volt battery bank (weekend/seasonal usage)
    • 1,500 Watts * 0.77 panel+controller derating * 1/0.10 rate of charge * 1/24 volt battery bank = 481 AH @ 24 volt battery bank (daily/full time off grid)
    • 1,500 Watts * 0.77 panel+controller derating * 1/0.13 rate of charge * 1/24 volt battery bank = 370 AH @ 24 volt battery bank (about the smallest with 1,500 watt array)
    Next, how much power can you generate with a 1,500 watt array... Using the Solarelectrichandbook for Orlando FL, and optimum year round panel tilt: [h=3]Orlando
    Average Solar Insolation figures[/h] Measured in kWh/m2/day onto a solar panel set at a 62° angle (from vertical):
    (For best year-round performance)


    Jan
    Feb
    Mar
    Apr
    May
    Jun


    4.29

    4.76

    5.36

    5.79

    5.75

    4.99



    Jul
    Aug
    Sep
    Oct
    Nov
    Dec


    4.96

    4.87

    4.74

    4.89

    4.55

    4.16




    If you get a minimum of 4.16 hours of sun per day (long term average) for December, such a system would generate:
    • 1,500 Watts * 0.52 typical off grid system eff * 4.16 hours of sun for December = 3,245 Watt*Hours per day of AC power
    And a battery bank that is setup to store 2 days of energy with 50% maximum discharge (seems to be an optimum sizing for Lead Acid batteries):
    3,245 WH * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 541 AH @ 24 volt battery bank

    The largest AC inverter I would suggest for such a bank (with flooded cell batteries) is:
    • 541 AH * 24 volts * 0.85 AC inverter eff * C/5 discharge rate (max recommended) = 2,207 Watt AC inverter (recommended maximum)
    Or roughly a 2-2.2 kWatt AC inverter--Any larger and the battery bank will have a difficult time supplying enough current for extended run time/high starting load current (like well pump).

    More or less, I would suggest around a 3.3 kWH per day system for a full time off grid home (energy star refrigerator, LED lighting, Laptop computer, washing machine, well pump, etc.).

    So--That is sort of what I see... Your thoughts?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    The system needs to produce 1200W.

    Is this a continuous output or is it just the max load for a fridge or ? that is an intermittent load?
     
    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
  • PhotowhitPhotowhit Solar Expert Posts: 5,658 ✭✭✭✭✭
    Hey guys! I've been researching solar charge controllers and I have a few questions. I understand that PWM and MPPT controllers are the two that are currently on market PWM is cheaper and works best when the incoming and outgoing voltages are the same; MPPT is more expensive and works better when the incoming voltage is higher than the outgoing.
    Welcome to the forum, glad you are researching ahead!
    I read that in many cases MPPT controllers are better than PWM, but I've also seen in a number of places that any benefits MPPT controllers have over PWM controllers tend to decline in warmer temperatures.

    Since I live in Florida, I was wondering whether MPPT controllers would still have any operational advantage over PWM in our warm climate.

    I use to Live in Tallahassee... Not that, that relates too much....

    There is a lot of good info in past posts on this forum. Searching 'On Line' / Google searches will be largely useless unless you know what you're searching for!

    Let me present some of the reasons for MPPT in a 1500 watt system. Please note I'm a fan of simple PWM type charge controllers!

    With a 1500 watt system, a PWM charge controller might work fine if you choose a 24 volt system, but in a 12 volt nominal system It likely will NOT be cost effective. Panels charging voltage is the VMP number, Larger panels will almost without exception be 26 volts and higher VMP.

    It is key to understand what the proper charging voltage panel would be, if setting up a 24 volt system, When charging daily you will need to present to your batteries 29-30 volts, this is after loss of voltage from heat, system wiring and drops across the charge controller. So anything less than 34-35 volts is too low. Some Solar retailers call 28-31 volt panels '24 volt' panels, but they WILL NOT WORK for a 24 volt system with a PWM charge controller. Solar Panel companies have been producing more 35-37 volt panels lately as they search for ways to increase the output of a single panel. 28-30 volt (VMP) panels will have 60 cells, so Manufactures have gone to 72 (and even higher number of) cells on each panel. Smaller panels require more wiring/fusing and tend will be higher cost than the larger panels. It's hard to fine 12 volt (nominal) panels that are larger the 150 watts.

    At 1500 watts, you are in a sweet spot for PWM charge controllers. You would have 5-6/250-300watt panels so 5-6 lines into a combiner box. You wouldn't save much using a MPPT system (which work most effectively with an array about 2x the system voltage) You would need likely 3 lines into a charge controller, so you would need 2 more breakers and or fuses and a bit more wiring. 1500 watts at 35 volts has a theoretical out put of @42 amps well with in a 60 amp charge controller, with a bit of room to grow.

    MPPT charge controllers will increase your arrays production, but not by a huge factor, for most solar installations that are using solar panels in the correct charging range and used typically in the top 30% of battery capacity. They will produce more charging current in Bulk charging which is below 85% of capacity. I suspect in typical system they produce less than 10% addition charging current.

    There are other reasons to buy MPPT type charge controllers though! Some of them can turn on 'opportunity loads' when you batteries near full charge. Water pumping, water heating, air conditioning...could be these loads... I don't know of a PWM that is designed to start loads, though some are designed to have dump loads, largely pure resistance loads like water heating...

    As Bill has eluded to, when designing a system, loads are paramount... a 1500 watt system likely won't produce enough current for a home in Florida if you want a/c. 1200 watt hours would likely not be enough to run a couple large fans in a stable, or circulate water in a fish farm...

    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.
  • 2manytoyz2manytoyz Solar Expert Posts: 373 ✭✭✭
    I'm in central FL, and I have 3 PWM type MorningStar charge controllers. They live in the garage all year. It's typically 90-95 most of the day this time of year. I've had them for several years, and have suffered zero failures. I wouldn't be too concerned about temperature with these. FWIW, these charge controllers parallel quite nicely. They do each require their own dedicated solar panels, and I have them tied to a common bus bar.
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