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PV Cells Technologies

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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  1. 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)

  2. 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

  3. 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)

  4. 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

  5. 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

  6. 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

  7. PV Cell steady state characteristic • Dependence on temperature and insolation:

  8. PV Cell steady state characteristic • More on the dependence on temperature and insolation:

  9. 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 - -

  10. 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

  11. 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.

  12. 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.

  13. 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 -

  14. 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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