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Solar Panel regulator

Discussion in 'Electronic Design' started by bxdobs, May 12, 2007.

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  1. bxdobs

    bxdobs Guest

    I was recently give a Solar Regulator that had been thrown out; its made by
    Go Power! Electric Inc.

    Doing a quick check I determined that the possible reason it had been tossed
    is it had a short across the Solar inputs ... not knowing how these
    regulators work I decided to do a bit of reverse engineering. To my dismay
    it appears that this device shorts the solar panel out when the battery is
    deemed fully charged. Because they also have a pair of blocking diodes this
    would certainly stop the charging however I wonder if this is potentially
    damaging to the solar panel.

    Attempting to find an answer to that by reading specifications on PV panels;
    most specs give a short cct current, and an open cct voltage and specify
    that the operational voltage/current is somewhere in the middle of that.
    Being that they provide a Short cct spec sort of implies they can be
    shorted.

    To make a long question short;

    I am trying to understand;
    A) Why shorting a panel is an acceptible way to shut off the charging
    B) Is there a Solar Cell Equivalent CCT (similar to a battery) that can be
    used to mathematically predict the characteristics of the cell.
    C) Why shorting out a cell doesn't fry the cell
     
  2. I think it is no problem. It just adds a tiny temperature
    rise to the silicon.
    Since the panel is completely power limited, it is cheap and
    reliable.
    A current source proportional to light intensity in parallel
    with a string of silicon diode junctions. Instead of doing
    this, mathematically, I suggest you simulate it with
    LTspice, a free circuit simulator. It has built in
    functions to display voltage, current and power dissipation.
    http://www.linear.com/designtools/software/switchercad.jsp
    Count the number of cells in series in your panel and use
    that number of diodes. Set the current source to the panel
    short circuit current rating.
    They are so inefficient that the electric energy is only a
    small part of the total heating caused by the absorbed
    light. If you want to eliminate that small additional
    heating, you could add a power resistor in series with the
    short that drops just enough voltage to keep the blocking
    diode from conducting. That will move that extra power to
    the resistor. A better way might be to have that resistor
    be the heating element in a water heater, so that you would
    make some use of the energy.
     
  3. Its a shunt type regulator.

    Solar modules are current sources. Get the data sheet of a module to model
    it.

    Cheers
     
  4. boB

    boB Guest

    It is also acceptable to simply open the PV to stop charging.

    boB
     
  5. mpm

    mpm Guest

    Many solar regulator designs simply dump the excess current to a load
    when the power is not needed for charging. Others will open the
    circuit.

    Is the short present without the solar panel connected?
    -mpm
     
  6. Chris Jones

    Chris Jones Guest


    High-efficiency (20% +) concentrator cells (intended for use with a concave
    mirror and a big heatsink) will run measurably cooler when they are under
    load. This is of course required by the conservation of energy, but can be
    a bit surprising when you see it in practice. If you measure the
    temperature carefully, then this should be observable with more common
    solar cells too. The manufacturers would almost certainly make sure that
    each individual module can be run open circuit or short circuit or anywhere
    in between, but see below for arrays made of several panels.

    Regarding the possibility of damage, I would only caution that if there are
    a lot of cells (and a lot of panels) in series then make sure that you have
    the right arrangement of power diodes connected backwards across the cells
    (or modules) according to any requirements of the panel manufacturer. The
    reason for this is that if you have say 50 volts worth of cells in series,
    and you short circuit them, and one of the cells is slightly in the shade
    (or slightly less efficient), then the less efficient cell will be subject
    to a reverse voltage produced by all of the other cells in the series
    string. So in this example, one cell could get up to 50 Volts across it
    the opposite polarity from normal (the p-type semiconductor would be
    negative and the n-type semiconductor would be positive). If the cell
    can't handle that much reverse voltage (and usually they can't), then it
    will start to leak current like a so-called Zener diode, and then it will
    get more hot and more leaky, and current crowding will take place because
    the hottest parts of that cell will hog the current, and in the worst case
    the cell could be damaged. This can be prevented by placing a diode across
    each cell with the p and n regions of the diode connected the opposite way
    around to the p and n regions of the solar cell. (For less protection but
    for lower cost, a diode can be placed in reverse across each group of cells
    or each module.) The diode should be rated for the max current of the
    solar cells. In normal operation the diode won't conduct, but if a bird
    s(h)its on one cell then the diode will prevent damage and will also help
    to maintain the output power of the array. As John says, you can simulate
    all of this in some SPICE type thing.

    Chris
     
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