Solar cells and MPPT

Discussion in 'General Electronics Chat' started by antennaboy, Jan 25, 2014.

  1. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
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    Hello Forum,

    In the case of a solar cell, the more light is incident on the cell the more current the cell generates. The solar cell voltage instead remains constant, regardless of the illumination level.

    The solar cell electrical power output is given by the product of the current and voltage:

    P= IV

    The maximum power transfer theorem tells us that the source and load need to have impedance matching for the max power to go from the source (solar cell) to the load.

    I guess the MPPT serves to dynamically adjust the two impedances (solar cell and load) so that the matching always takes place and the max possible power goes to the load.

    Not all electronic applications require the delivery of the max power available. Some circuits/loads work well if provided with the suitable current and voltage.
    Home appliances don't require the most power from the source (power plant). They would explode otherwise.

    But in the case of solar cells we try to catch the most solar energy (radiation), convert it into electricity and transfer the most electrical power to the load or battery. If we don't satisfy the max power transfer theorem part of the available electrical energy goes wasted as heat.

    Is that correct?

    Thanks,
    antennaboy
     
  2. crutschow

    Expert

    Mar 14, 2008
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    That's basically true. With a matched load half the power is wasted as heat in the solar cell and half goes to power the load. That is the maximum power that can be delivered to the load. Seems like there should be some way to get more than 1/2 the power at the output, but there isn't.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    While possibly anecdotal there's a story of a physicist getting fired by Edison on his first day for giving the 1/2 power answer for max power to the load.

    It matters not what theoretical power one may get to a load, what matters is how efficient the transfer is, and the best case is 1 watt at the generator getting all the way to the load, or a 1:1 transfer.

    1 to .5 is right out!

    So one will always minimize the losses from generator to load, not try to match them.

    In the case of a MPPT device the load current is kept at the point where the cell voltage just begins to fall, yielding a maximum in the product of I x V, voltage at the converter times current out of it.

    In solar applications, you basically have two types of loads: batteries requiring recharging, and AC grid supply. The grid can be seen as an infinite acceptor of power, so a MPPT device will run full out. Batteries will hopefully eventually get fully charged, and at such a time the MPPT device will scale itself back and work way off the max power point.
     
    Last edited: Jan 25, 2014
  4. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
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    Thanks for your comments.

    This is how I see it: there are loads that require a specific power P to operate optimally, no more no less. However, all loads can tolerate a max power input before they explode.
    Power indicates the ability to receive energy over time.

    Some loads are picky about the voltage V that they require: cell phones require 5 V usually.The phone internal electronics will draw the current I that is needed at that moment by the phone to provide the suitable power. Of course, the source needs to be able to supply that requested current.

    In solar applications, we want to recharge batteries (which then will charge small electronics) or charge small electronics directly. If the electromagnetic radiation contains 10 W (losses aside), the best scenario according to the max power transfer theorem is that we can only send 5W to whatever load is attached to the solar panel/cell.
    Is that correct?

    A transformer is a device that converts a voltage input V_in into a different voltage output V_out. Ideally (ignore losses for now) a transformed should transfer all the input power on one side to the other side: P_in= P_out with V_in *I_in = V_out * I_out

    How would that be possible? Doesn't a transformer need to obey the max power transfer theorem too? In that case it should be able to pass only 50% of the input power.

    The best we can do is 50% power transfer efficiency.

    thanks,
    antennaboy
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The 50% power match discussion above presupposes that the output current vs output voltage of a solar panel is a straight line; the I vs V characteristic of a solar panel is anything but straight.

    Finding MPPT is all about dynamically finding the optimum point to operate the panel in light of the non-linear I vs V curve.
     
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    There is a difference between the 'extraction' of power from a current source like a solar cell and the transmission of AC power to a load. The power transformer is a EM field transmission device that uses magnetic fields to change impedances between two points in space. A 100W of power can be expressed in a infinite number of voltage and current combinations that each can be re-factored into an impedance at that point. That efficiency of 'transformation' to different electric/magnetic field ratios is not related to the max power transfer theorem.
     
  7. antennaboy

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    Jan 31, 2008
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    Ok, thanks.

    I overlooked the fact that a power source (either current source or voltage source) is somewhat different from a transformer.

    For different loads and applications there are different possible scenarios: some applications will be about delivering the most power to a load, some applications will be about delivering the highest voltage, some the highest current. More often, it is about delivering the suitable power (with the suitable voltage and current) to a load, correct?

    A solar cell has a nonlinear I-V curve. Why is the solar cell described as a current source instead of a voltage source? A current source is a source whose output current is constant regardless of the connected load and its impedance. Does a solar cell really work that way?
    I think a load will try to draw the current it needs, as long as the solar cell is able to provide that much current....

    As the illumination changes, the solar cell maintains a constant voltage (almost), very close to the open circuit voltage, while the output current is varies according to the illumination.

    thanks,
    antennaboy
     
  8. MikeML

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  9. nsaspook

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    The solar cell EMF is created when it reacts to the change in equilibrium at the PN junction due to the creation of charge carriers (electron-hole pairs) in the semiconductor due to light energy in excess of the band-gap (A direct gap is usually needed for a photo-diode junction). This EMF is voltage limited by the forward bias voltage of the diode formed at that junction but the current flow remains mainly constant down to zero volts as the load resistance is decreased.

    This link is a good primer: http://en.wikipedia.org/wiki/Electromotive_force#Solar_cell

    Semiconductor links: http://www.doitpoms.ac.uk/tlplib/semiconductors/direct.php
     
    Last edited: Jan 27, 2014
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    matched impedance? this is a dc circuit. how dould you ijmput 0 Hz into the impedance formulas? the quasi constant current of solar cells is due to internal resistance and heat. the internal resistance is dymanic, not static like a resistork, varies with current erawn and light.
     
  11. nsaspook

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    Aug 27, 2009
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    You are describing the effect, not the cause.
    From the Wiki link:
     
    Last edited: Jan 27, 2014
  12. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
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    Thanks.
    Well, in the DC regime the impedance simply becomes a real valued quantity (resistance).
    Impedance matching and the max power transfer theorem still apply to DC.

    The small solar charger I bought online surely does not have a MPPT, I guess. It must simply charge the internal lithium battery which will in turn change my phone. The phone has a USB port which needs 5V.
    The point is that the solar cell is charging the internal battery but not in an optimal way since impedance matching between the solar cell and the battery is not accomplished.

    thanks
     
  13. Bernard

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    Aug 7, 2008
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    Impure speculation: Now if you had a V controlled buck regulator,BR, solar panel, SP, OC V higher than battery, & using 75% of SP OC V as a fixed referance, use differance between ref. & SP to control BR. If SP V rises, BR output also risec, increasing charging current bringing SP V back to ref. V, almost MPPT. If battery is fully charged, MPPT must still prevail, & charging V still goes up to hold SP V @ ref.- not so good on battery.
     
  14. ErnieM

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    Apr 24, 2011
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    Too many funny acronyms and abbreviations to make any sense.
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Oh come on, the only acronym he did not introduce properly was OC, which we should all recognise as Open Circuit. And V for Voltage.

    To Bernard; You are right, a MPPT does the job of putting maximum power into the battery.

    And if the battery is fully charged the MPPT can overcharge it if it is still operating at the MPP.

    They are separate tasks really. Think of the MPPT as a fuel hose/pump that can put fuel at the max speed into the fuel tank. Then you need a SEPARATE system to detect that the fuel tank is full and deal with that.
     
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