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

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

    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

    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)


    Pv cells technologies

    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 technologies

    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 cells technologies

    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 cells technologies

    PV Cell steady state characteristic

    • Dependence on temperature and insolation:


    Pv cells technologies

    PV Cell steady state characteristic

    • More on the dependence on temperature and insolation:


    Pv cells technologies

    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 cells technologies

    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


    Pv cells technologies

    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.


    Pv cells technologies

    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.


    Pv cells technologies

    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

    -


    Pv cells technologies

    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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