Solar mppt help!

Discussion in 'General Electronics Chat' started by supertiger, Jun 4, 2013.

  1. supertiger

    Thread Starter New Member

    Aug 22, 2009
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    Hi,
    I have a question on solar mppt stand alone system.. Is it true that the mppt(eg,p&o) we use a boost or buck converter to achieve that. But the voltage output of the converter varies based on the irradiation? Therefore we need another converter to regulate the Vdc bus?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    No. The MPPT regulates the charging current to achieve optimum efficiency based upon the condition of the battery and the current from the solar panel. It needs no other regulator.
     
  3. supertiger

    Thread Starter New Member

    Aug 22, 2009
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    Thanks for reply.
    In literature review, the original p&o algorithm calculates the duty cycle based on vpv and ipv and input to the boost converter...It does not have any information based on the output of the converter or battery .Therfore by right the voltage output of the converter will vary if I just have a load. Correct me if I am wrong.

    Do you have any example or simulation that proves the concept?
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
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    I do not think "MPPT regulates the charging current to achieve optimum efficiency based upon the condition of the battery".

    Maybe it is true in a storage based system but my grid tied PV system contains NO storage (i.e. no batteries) yet does use MPPT. In other words MPPT is not based on a battery system rather used to optimize power from the PV array.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The MPPT needs to regulate to max output power (as Crutschow said) or an inferior system it can regulate input characteristics, which requires making assumptions about its own internal losses and load issues etc.
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    An MPPT (Maximum Power Point Tracker) controller follows the communist maxim "from each according to their ability, to each according to their need."

    You charge a battery with a controlled current while monitoring the voltage, and a solar panel is a current producing device. During the charging phase the current drawn will be adjusted to the region where the product of panel current and panel voltage (ie, power) is a maximum.

    A maximum current is maximum... so if you turn on an additional load during the charging time the MPPT will not change it's output current: it's *already* at the maximum power point (by definition).
     
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  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    That sounds fine in theory, but the goal is not to draw the maximum power from the panel, but to deliver the maximum power to the load.

    A good design monitors power to the load as the main closed loop system, and optimises max power to the load. As Crutschow said, in a simple 12v solar system this is done most effectively by regulating max current out to the 12v battery.

    In a grid tie MPPT the regulation is to deliver maiximum power back to the mains grid.

    Regulating the max output power (and not panel power) compensates all efficiency issues with the SMPS conversion inside the MPPT unit.
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I don't believe you understand the theory involved here. The "power" in a "Maximum Power Point Tracker" system is the power the panel can deliver.

    The input power.

    It's a definition, go look it up.

    As a MPPT controller is an efficient switching power system the power out is essentially the power in. So by maximizing the input power, the power the panel is supplying, the output power is also maximized.

    You seem to be confusing the goal with the method.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Umm, in my "real job" I designed a commercial high efficiency MPPT, so this is an area of my personal expertise.

    The panel has a max power point, BUT the goal of a MPPT is to deliver max power to the load. That is vitally important and any efficiency percentage points are fought hard for by designers.

    As for "confusing the goal with the method", you are simply wrong. Regulating power out compensates all issues caused by the efficiency curve of the SMPS converter (which has differing efficiency at differing voltage and current points on it's own efficiency curve).

    "So by maximizing the input power, the power the panel is supplying, the output power is also maximized."

    No it is not. Read what I said above. The maximum power out in most systems is obtained at a panel voltage above the actual MPP of the panel. This is because the conversion efficiency of the SMPS unit is higher with a higher Vin and lower duty cycle. These are the kinds of things you won't learn in your classroom Ernie, but experts know from years of experience.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    So the maximum power point is actually the maximum power point, not the maximum power point.

    Such thought is so inspiring!
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You misunderstand.

    With most 12v panels the MPP occurs at about 17v.

    The overall system MPP (output power to the battery) is achieved when the panel voltage is increased to 18v or more, AWAY from the panel MPP.

    So the panel is NOT operating at it's MPP, but the overall panel+MPPT is.

    Your statement that the "output power is maximised whenever the panel power is maximised" is wrong.

    I get a bit irritated when taking the time to spoon-feed you the basics, and your replies contain arrogant insults.
     
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