PV Cells Technologies
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PV Cells Technologies. Characterization criterion : Thickness: Conventional – thick cells (200 - 500 μ m) Thin film (1 – 10 μ m). Tend to be less costly than conventional (think) cells but they also tend to be less reliable and efficient. Crystalline configuration: Single crystal

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PV Cells Technologies

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PV Cells Technologies

  • Characterization criterion:

    • Thickness:

      • Conventional – thick cells (200 - 500 μm)

      • Thin film (1 – 10 μm). Tend to be less costly than conventional (think) cells but they also tend to be less reliable and efficient.

  • Crystalline configuration:

    • Single crystal

    • Multicrystalline: cell formed by 1mm to 10cm single crystal areas.

    • Polycrystalline: cell formed by 1μm to 1mm single crystal areas.

    • Microcrystalline: cell formed by areas of less than 1μm across.

    • Amorphous: No single crystal areas.

  • p and n region materials:

    • Same material: homojunction (Si)

    • Different material: heterojunction (CdS and CuInSe2)


  • PV Cells Technologies

    Uni-Solar solar shingle

    BP SX170B Polycrystalline

    BP SX170B Monocrystalline

    Uni-Solar Laminate PVL-136 Amorphous

    Mitsubishi PV-TD 190MF5 Multicrystalline

    PV Modules at ENS


    PV Cells Technologies

    • Thick film fabrication techniques:

      • Czochraski’s (CZ): for single-crystal silicon. Costly.

      • Float zone process (FZ): also for single-crystal silicon. Costly

      • Ribbon silicon

      • Cast silicon: for multicrystalline cells. Less costly.

    • Thin film

      • Can be used embedded in semitransparent windows.

      • Techniques:

        • Amorphous Silicon: can achieve higher efficiencies (in the order of 42% thanks to the multijunction (different multiple layers) in which each layer absorb photons with different energy.

        • Gallium Arsenide (GaAs): relatively high theoretical efficiency (29 %) which is not significantly affected by temperature. Less sensitive to radiation. Gallium makes this solution relatively expensive.

        • Gallium Indium Phosphide (GaInP): similar to GaAs.

        • Cadmium Telluride (CdTe): Issue: Cd is a health hazard (it is very toxic).

        • Copper Indium Diselenide (CIS or CuInSe2): relatively good efficiency)

        • Silicon Nitrade (N4Si3)


    The p-n junction diode

    n-type substrate

    Bias voltage

    p-type substrate

    Id

    • Vd is the diode voltage

    • I0 is the reverse saturation current caused by thermally generated carriers

    • At 25 C:

    Ideal diode

    Real diode

    I0


    PV Cells physics

    The current source shifts the reversed diode curve upwards

    ISC

    VOC

    Same curve

    The bias source (voltage source) is replaced by a current source powered by the photons

    p-n junction is equivalent to a diode

    ISC

    Reverse v-i curve for the diode


    PV Cell steady state characteristic

    • From Kirchoff’s current law:

    • The open circuit voltage is

    Maximum power point

    Power

    Pmax 0.7 • Voc • Isc

    Current


    PV Cell steady state characteristic

    • Dependence on temperature and insolation:


    PV Cell steady state characteristic

    • More on the dependence on temperature and insolation:


    More complex steady-state models

    • For a more realistic representation we can consider the following (equivalent to a diode’s model):

      • 1) Effect current leakage

      • 2) Effect of internal ohmic resistance

    ISC

    Rp

    +

    +

    RS

    Vd

    V

    ISC

    where

    Vd = V+IRS

    This is a transcendental equation

    -

    -


    PV more complex steady-state model

    • Both effects can be combined to obtain the more realistic (and complex) steady state model:

    +

    +

    RS

    ISC

    Rp

    Vd

    V

    -

    -

    where

    Vd = V+IRS

    This is a transcendental equation


    Dynamic effects

    Capacitive effect

    • As with any diode, there is an associated capacitance. However, this capacitance is relatively small, so the effects on the output can often be neglected. Therefore, PV modules can follow a rapidly changing load very well.

    • One undesirable effect of the capacitance is that it makes PV cells more susceptible to indirect atmospheric discharges.


    Modules combination

    • PV cells are combined to form modules (panels). Modules may be combined to form arrays.

    More modules (or cells) in series

    More modules (or cells) in parallel

    • When modules are connected in parallel, the array voltage is that of the module with the lowest voltage.

    • When several modules are connected in series to achieve a higher array voltage, the array’s current equals that of the module delivering the lowest current.


    Shading

    -

    • A shadowed module degrades the performance of the entire array

    (Rp+Rs)(n-1)Imodule

    +

    +

    One module with 50% shadow

    One module with 100% shadow

    (n-1)Vmodule

    Two modules with 100% shadow

    -


    Bypass diode for shadowing mitigation

    • Bypass diodes can mitigate the effects of shadows but they don’t solve the issue completely.

    • A better solution will be presented when discussing power electronics interfaces.

    No shade

    Shaded without bypass diode

    Shaded with bypass diode


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