Increasing the pwm controller's efficiency

solarking
solarking Registered Users Posts: 67 ✭✭
in Solar Beginners Corner #1
Can a pwm charge controller's efficency  increases If a inverter is running loads coz the voltage drops as the load requires more amps and voltage drops eventually which inturn reduces the voltage coming in to the pwm.
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #2
    Not really following your question 100% but if the load current  is greater than the PV array can supply, the make up current will be drawn from the battery which will in turn drop both the battery and array voltages. There is no change in efficiency happening, just a matter of supply and demand.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • solarking
    solarking Registered Users Posts: 67 ✭✭
    #3
    Does a pwm charge controller drops PV voltage after sensing the battery voltage or the pv voltage is cut down once it reaches pwm charge controller?
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #4
    Think of the controller as  switch,  which controls the flow of current  from the PV array  to the battery, in order for there to be flow the potential , or voltage,  of the array must be higher than that of the battery, in the case of PWM it is slightly higher. During bulk charging the switch is closed allowing maximum flow the array can provide, whilst maintaining the difference in potential. Once the battery reaches the bulk voltage, the controller (switch) opens and closes, allowing pulses of current, these pulses are long on short off to begin with, as the demand for current lowers, the on time shortens, thereby effectively reducing the current, this is the absorption stage. Depending on the controller, a more complex 3 stage one will have a float stage, which will lower the voltage allowing small pulses of current to maintain the float voltage. Cheaper 2 stage controllers will maintain the absorption voltage with pulses having longer off cycles. Both will continue in their respective state until either the array voltage drops below that of the battery, or a load is introduced. If a load is introduced whilst there is sufficient capacity from the array, the controller will be pulled out of the float, or PWM back into bulk, or closing the switch, if you like, allowing current to support the load. If however the available current from the array is less than that of the load, the battery will supply the deficit.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    edited June 2017 #5
    solarking said:
    Can a pwm charge controller's efficiency  increases If a inverter is running loads coz the voltage drops as the load requires more amps and voltage drops eventually which inturn reduces the voltage coming in to the pwm.
    If your Batteries are fully charged and you have sunlight, then it is more "efficient" to run the inverter during the day.
    Example: Operate the Washer and Dryer when the sun is shining.
    You should focus on efficiently utilizing all of the potential watt-hours from the Solar Panels.

    Minimizing the losses inside the PWM should be done in the project design phase, not so much in actual operation.

    solarking said:
    Does a pwm charge controller drops PV voltage after sensing the battery voltage or the pv voltage is cut down once it reaches pwm charge controller?
    For a given Battery Voltage ...
    The output Voltage of the Solar Panel decreases and the Amperage flow increases as the On-Time of the PWM increases.
    The output Voltage of the Solar Panel increases and the Amperage flow decreases as the On-Time of the PWM decreases.

    This inverse relationship between voltage and the amps, is due to the I-V Curve of the PV Panel.

    The Watts consumed by the PWM, over some time period, is ...
    Voltage across PWM during on-time X  Amps during on-time  X  % On-Time
    - OR -
    Average Voltage across PWM  x  Average Amps
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  • solarking
    solarking Registered Users Posts: 67 ✭✭
    #6
    So the load(either dc or via inverter) connected to a PWM charge controller is directly related to the efficiency.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #7
    As more current goes through a circuit, pwm control in this case, the losses from wire resistance go up. In that sense, running loads makes the controller less efficient.

    As mvas pointed out, overall system efficiency is better if loads can be time shifted to run off solar input to avoid losses involved in battery charge/discharge.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited June 2017 #8
    Even if your batteries are charging but not yet fully charged, it's usually more efficient and better for the batteries to operate loads during the day. 

    But in the case of PWM, the maximum panel efficiency is achieved at 100% duty cycle and when the loads (battery and/or inverter) pull the panel voltage down to exactly Vmpp.  If the loads pull it lower than Vmpp, then less load would be more efficient.

    I am available for custom hardware/firmware development

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #9
    The "efficinecy" of a PWM controller does not really matter... It is the battery voltage divided by the Vmp array voltage.

    I.e., when a panel is cool and the battery is charging (100% duty cycle from the charge controller):
    • 14.75 volts battery charging / 17.5 volts Vmp-array-std = 0.84 eff
    In other conditions such as when panel is under hot sun:
    • 14.75 volts batt charge / 14.875 volts Vmp-array-hot = 0.99 eff
    Or, how about cool panel with float charging voltage and full load by AC inverter (again PWM at 100% duty cycle):
    • 13.7 volts float / 17.5 Vmp-array-std = 0.78 eff
    Now--Say you have a "24 volt panel" (Vmp~30 volts standard test conditions) charging a 12 volt battery with a pwm controller:
    • 14.75 volts charging / 30 volts Vmp-array-std = 0.49
    Of course, all of these efficiency calculations are based on the design of the system... basically panel Vmp, temperature of panel (and battery bank), and assuming wiring+controller losses are near zero (heavy/short wires, a 99% efficient PWM controller).

    But, as soon as the controller goes to less than 100% duty cycle--Basically efficiency does not matter any more... The controller is drawing less energy from the panels because the battery bank (battery is in absorb/float charge state) + loads (and loads are less than solar array output) don't need any more power. And it would not matter if the charge controller is MPPT or PWM type once the energy from the solar array is being throttled by the charge controller.

    A properly designed PWM system can actually be more efficient than a similar MPPT charge controller system. However, the MPPT system is more versatile and can be easier to wire up (less copper wiring) for larger solar systems, and/or systems where the solar array is a long distance from the charge controller/battery shed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited June 2017 #10
    > 14.75 volts battery charging / 17.5 volts Vmp-array-std = 0.84 eff

    At voltages below Vmp, current is higher.  So more like .91 watts-efficiency in this case.

    I am available for custom hardware/firmware development

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  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    edited June 2017 #11
    solarking said:
    So the load(either dc or via inverter) connected to a PWM charge controller is directly related to the efficiency.
    Please define "PWM efficiency", as you see it.

    Are you asking ... Can a PWM operate the Solar Panels at or near Max Power Point like a MPPT Charge Controller?

    Again, that is an upfront DESIGN issue and is not something that can easily be changed during daily usage.
    How well does your design match the Solar Panels Max Power Point to the Battery Bank during Bulk Mode and/or Loads?
    The PWM does not care nor affect how well you have matched your Solar Panels MPP to your Battery Bank and/or Loads.

    Or are you asking ... How many WATTS does PWM dissipate/waste/consume during Bulk Charging Mode?

    Does this help ?
    http://www.phocos.com/wp-content/uploads/2015/12/Guide-Comparing-PWM-MPPT-Charge-Controllers.pdf
    Do see the chart on Page #3?
    The PWM will operate with the I-V Curve of the Solar Panels that you provide - the system that you designed.
    During Bulk Mode the PV Panels will go from the "Lowest Charging Voltage" point to the "Highest Charging Voltage" point.
    "Lowest Charging Voltage" = farther from Max Power Point = less Watts = less efficient
    "Highest Charging Voltage" = closer to Max Power Point = more Watts = more efficient 

    Temperature changes the IV Curve.
    Insolation changes the IV Curve.

    There is nothing that the PWM do to "fix" this - which is the whole point of MPPT.
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  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    #12
    @solarking

    I had a PWM system before and it took quite some time to understand all these.

    I would suggest you follow my old threads and you can see me asking many such questions - haha

    From my understanding, you are asking how to improve the efficiency of PWM ( it is impossible it is just a switch)
    If you were to ask about how to improve the efficiency of the whole system when using PWM, the answer is you try to use the loads more during the day and when the energy is needed the inverter will try to pull it out of the battery but before battery can supply it the PV will throw those electrons you need via the PWM. 
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited June 2017 #13
    Also,

    a) loads during absorb are better than during bulk.

    b) for loads during bulk, spreading it out over time provides some increase in panel efficiency.  Ie, 1kw for 1 hour is more efficient than 2kw for 1/2 hour.   The latter operates further to the left on the IV curve.  

    I am available for custom hardware/firmware development

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #14
    Solar panels are "use it or lose it" type power supplies. "Use it" supplying power to charging the battery bank (as fast as the batteries can take it) and as the batteries exceed 80-90% state of charge (end of absorb, approaching float), the current the batteries are not using from the solar array can now be supplied to your afternoon loads.

    If your battery is full, and you have no "optional" loads, then the energy (Power*Time=Energy) is lost for that day from the solar array.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • solarking
    solarking Registered Users Posts: 67 ✭✭
    edited June 2017 #15
    Nila said:
    @solarking

    I had a PWM system before and it took quite some time to understand all these.

    I would suggest you follow my old threads and you can see me asking many such questions - haha

    From my understanding, you are asking how to improve the efficiency of PWM ( it is impossible it is just a switch)
    If you were to ask about how to improve the efficiency of the whole system when using PWM, the answer is you try to use the loads more during the day and when the energy is needed the inverter will try to pull it out of the battery but before battery can supply it the PV will throw those electrons you need via the PWM. 
    How many years did your offgrid setup last ? After reading about the offgrid setup failure I  think I should choose  either grid tie system & use single battery inverter for backup.
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