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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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Presentation Transcript
slide1
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)
slide2
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

slide3
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)
slide4
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

slide5
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

slide6
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

slide7
PV Cell steady state characteristic
  • Dependence on temperature and insolation:
slide8
PV Cell steady state characteristic
  • More on the dependence on temperature and insolation:
slide9
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

-

-

slide10
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

slide11
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.
slide12
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.
slide13
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

-

slide14
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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