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Energy harvesting from solar panel

Discussion in 'General Electronics Discussion' started by wdariusw, Nov 10, 2014.

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  1. wdariusw

    wdariusw

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    Nov 10, 2014
    Hi all! I'm doing some work about solar panels, power point tracking, batteries charging etc. Let's say - solar energy harvesting system . I do not understand load maching principle with PWM and buck converter...How buck changes the load depending on PWM ?
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Any regulator does not change the load. Unless it has little hands that can change the load for you. I think you mean change the current or voltage. A buck regulator will reduce the input voltage to a preset level. The PWM controls this voltage by changing its on and off times.
    Adam
     
  3. wdariusw

    wdariusw

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    Nov 10, 2014
    Yes, you right here. PWM and buck, maybe buck-boost changes the voltage. Maybe can you give simple example with numbers? I mean... PV output for example 6V 300mA, load is 50Ohm. How should dc-dc converter convert voltage for better power ?
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    You need to show us your circuit, so we know what to reccomend.
    Adam
     
  5. wdariusw

    wdariusw

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    Nov 10, 2014
    I don't have any circuit yet. I'm collecting info and then will choose components, write program, make pcb etc.. Like in picture, in my final project i want to build as simple as can autonomous wireless sensor transmitter (black squared), and wired receiver for checking what i get. Low power consumption of course. So now i'm only wondering how to regulate power point with dc-dc converter.
     

    Attached Files:

  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I think you are interested in MPPT. This is used when charging a battery because as a system it varies the load to track the point at which the solar panel produces the greatest power.

    Do you understand the voltage vs current curves for a solar panel? (Do you have one tho look at?)

    Do you understand that MPPT will result in varying power to the load and that this is unusual for most load types?

    You also seem to be interested in DC-DC converters. Possibly the simplest of these is the buck converter. Before describing that, it is important to understand that a DC-DC converter is a constant power device, that is (ignoring losses) the output power equals the input power.

    A buck converter produces an output voltage less than the input voltage. But because the power is constant, what is the general relationship between the input and output current?

    Note that a typical MPPT device had an almost constant voltage output. So while the topology of the DC-DC converter is frequently very similar to a buck converter, it is the output current, not the output voltage that is s subject to control. That control is effected by (again typically) controlling the DC-DC converter to maximise output current by varying the load imposed on the panel.

    Your understanding of the V vs I characteristics of a solar panel should enable you to see that the tracking of the maximum power point is not a simple linear process (unlike a normal DC-DC converter maintaining a constant output voltage). Typically a microcontroller is used in the feedback loop in order to control this non-linear aspect. The software often uses a continuous "trial and error" process to track a local maxima (which happily is a global maxima a a particular value of solar irradiation)
     
    wdariusw and KrisBlueNZ like this.
  7. wdariusw

    wdariusw

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    Nov 10, 2014
    Hi steave . At first - big thanks to you for the answer ! Yes, i understand voltage vs current curves for a solar panel. I think I not everythink understand about the buck converter. You say output power is equal as input power for dc dc? So..if solar output Uoc=7V, Isc=0.33A , maximum power is about ~1,28W when the matched load is ~30Ohm. If solar intensivity changes lets say 6V 0,25A, now 30Ohm load is not matched. Power P=U*I, U=P/I , I=P/U . If dc-dc buck is lowering voltage using pwm , current is growing. And thats the way how is optimized new current ant voltage levels for the load ?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    ...

    Ok, so we've established that you don't understand the V vs I curves for solar panels.

    The first thing you need to do is to find out where Voc and Isc appear on the V vs I curve, where max power is, and what the relationship between the voltage and current is at this point compared to the specified values Voc and Isc.

    A DC-DC converter doesn't lower voltage using PWM. You need to study how a buck DC-DC converter works.

    Because this sounds like homework I'm not going to just give you the answers. You need to do some study yourself.
     
    wdariusw likes this.
  9. wdariusw

    wdariusw

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    Nov 10, 2014
    Due to poor my english language i can't explain everything as i understand. And you may think, that i do not understand, like you did about mine V vs I curves understanding. Voc is in the right of x axis, where I=0, and Isc is on the top of y axis. That is the current when circuit is shorted, and voltage is equal to zero. The numbers i wrote (7V, 0,33A are not real, only examples). Look at the attached pictures. Made some calculations and graphs in matlab with real numbers. It is right now, when I use real calculation not random numbers from my head ? :) Or i do not know something more about V I curve ? Thanks !
     

    Attached Files:

  10. wdariusw

    wdariusw

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    Nov 10, 2014
    After lot of reading I realize, that is better buy microchip with integrated mppt algorithm and dc-dc converter. Because if i want to get low power consumtion, it is better to have integrated solution, like cymbet cbc915 . If i will try to make my own dc-dc and mppt , lets say, with low power atmega 328, in final result i will have bigger energy consumption. It is better to use atmega328 in deep sleep mode, and wake up only for sensors data transmitting.
     
  11. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    wdariusw, It sounds like you are attempting to build a solar site evaluation device, perhaps a "logger'....these are readily available already built ( http://www.solarpathfinder.com/PF?id=JX9yF4Qr as a quick example) many with very sophisticated capabilities......If you are seriously considering a large PV/RE installation then it would be fiscally irresponsible not to asses the site for at least a year....given the length of time involved in a real site assessment, it is best to use tools that you have 100% confidence in, typically not first generation DIY devices....If your intended purpose is NOT about site assessment then perhaps disclosing your actual intended purpose would help get better suggestions.

    Fish
     
    Arouse1973 likes this.
  12. wdariusw

    wdariusw

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    Nov 10, 2014
    Now i'm think that I understand solar cell curves and output characteristics. If load is too high, output is near Voc, if very low, then current is near Isc. My earlier post about IV curves was tottaly wrong.. :) . Buck converter, as i understand now controls output current, and output characteristics are V_out=V_in / Duty, I_out=I_in*Duty .Know I'm reading about MPPT now, and i'm thinkink : do MPP tracking needed only for solar panels, because they have varying impedance depending on light, or MPPT is needed for all energy harvesting methods such as thermo, wind, piezo etc. ?
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Sorry for not replying to your earlier posts. I think I ignored an alert and I don't get a second one :-(

    That would be correct if you say "load resistance" instead of just "load". A high, or heavy load is one which draws a lot of current and is thus a low resistance.

    That's an approximation of the relationship, but the actual relationship is power out = power in, so Vout * Iout = Vin * Iin.

    An MPPT charger (actually a special type of regulator) has an almost fixed output voltage by virtue of it being connected to a battery. It varies the duty cycle until the output current is at a maximum. By doing so it extracts the maximum power from the panel.

    If you wewre to look at he V vs I curve for a solar panel and graph it as power Vs current you would see an interesting relationship. I would do this if you haven't done it already.

    MPPT can be used for many energy harvesting methods. As well as solar, it is also fairly common with small wind power systems.
     
    KrisBlueNZ and wdariusw like this.
  14. wdariusw

    wdariusw

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    Nov 10, 2014
    Nice ! Example for understanding : So i connected MCU as a solar panel load. MCU wake up from sleep and measure sensors values, the current consumption will increase, and load to solar panel increase, but load resistance decrease assuming R=U/I?

    So, if there are DC-DC buck between solar cell and mcu, what it should do ? It should increase current equal to load consumed current at that moment? But then voltage drops?

    So it can be used for every energy harvester ? Because pjezo ar TEG's are constant output impedance, MPP will be at 50% from Voc and Isc ?
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I suspect so, but I'm not experienced with them.

    A DC-DC converter can keep the load voltage current constant until such time as the load demands more power than is available from the panel. Then it will drop (actually it will likely collapse).

    MPPT is almost universally used to charge a battery because the amount of power provided is independent of the load -- the load is generally assumed to have a constant voltage and a very low impedance. I recommend you do some more reading until you have an understanding of what it is and how it's not what you use to power a "normal" load.
     
  16. wdariusw

    wdariusw

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    Nov 10, 2014
    There is storm in my head ! Step by step something is coming (i hope it will be understanding...:D ).. Tried to do some calculations. Please look at attached file and comment or this time i'm thinking correctly. :)
     

    Attached Files:

  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I think you still misunderstand. Are you of the belief that you will always be able to get 4.4W from your solar panel?

    Practically speaking, you will NEVER get 4.4W from it.

    You need to get away from talking in terms of mark space ratio. It's meaningless id most circumstances. The exact ratio is variable and will vary with load voltage and current, and input voltage.

    Have you done any reading on MPPT? (Have you even done any reading on buck and/or boost DC-DC converters?) Can you point me to what you've read and think you understand?
     
  18. wdariusw

    wdariusw

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    Nov 10, 2014
    No, i think that 4.4W will be in perfect conditions, when irradiance is very high, and load is matched ... In my example, increased load is show as work point changes to the left (current increases, load resistance decreases)..

    After you answer now i'm thinking...I always thought that MPPT matches load RESISTANCE. But is not ?

    I collect information about energy harvesting, but i write everything in Lithuanian language...

    One of better source about MPPT i found here http://www.solar-electric.com/mppt-solar-charge-controllers.html
     
    Last edited: Dec 1, 2014
  19. wdariusw

    wdariusw

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    Nov 10, 2014
    In that site about MPPT are example with battery. So if solar panel gives 5V and 0.2amps and i connected 3.6V battery straight, then solar panel voltage drops to the battery level and power is P=3.6*0.2=0.72W. 5-3.6=1.4V is wasted. So if between solar panel and battery is connected buck converter with 72% duty cycle, than output will be 3.6V and 0,277 Amps, and we have power equal to 0,927W. Very difficult to understand when my english skills are not very good, and there almost no litherature in my language..
     
  20. Dan.W

    Dan.W

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    Jun 3, 2015
    Hi,
    I found another app: http://easysolar.co/ which allows azimuth calcs, shadowing simulations, etc.. I mean, I'm in a similar situation, all the calculations scare me a bit, and having an app on my phone would defenitely make things easier and they offer a trial period or sth like that... but I'm not sure if it isn't just better to ask for professional help. Have any of you used that sort of apps? Are they worth it?
    Regards,
    Dan
     
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