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Photovoltaics

Photovoltaics. Noor Shazliana Aizee bt Abidin. HISTORY OF PV. DISCOVERY OF PHOTOVOLTAIC EFFECT – 1839 – EDMOND BECQUEREL – WET CELL BATTERY SELENIUM – FIRST SOLID PV MATERIAL – 1877 FIRST SELENIUM SOLAR CELL – 1883 CHARLES FRITT LESS THAN 1% EFFICIENT SILICON PV CELLS IN 1953 – BELL LABS.

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Photovoltaics

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  1. Photovoltaics Noor Shazliana Aizee bt Abidin

  2. HISTORY OF PV • DISCOVERY OF PHOTOVOLTAIC EFFECT – 1839 – EDMOND BECQUEREL – WET CELL BATTERY • SELENIUM – FIRST SOLID PV MATERIAL – 1877 • FIRST SELENIUM SOLAR CELL – 1883 CHARLES FRITT • LESS THAN 1% EFFICIENT • SILICON PV CELLS IN 1953 – BELL LABS

  3. SOLAR CELL HISTORY • EARLY SILICON EFFICIENCIES – 6% • 1958 – SILICON SOLAR CELLS USED TO POWER VANGUARD I SATELLITE TRANSMITTER • SINGLE PV CELL PRODUCES ABOUT 1.5 WATTS • CELLS CONNECTED TOGETHER TO FORM ARRAYS • CURRENTLY, LABORATORY CELL HAVE EFFICIENCIES OF 24%

  4. What is “Photovoltaics”? Word origin: – “Photo” Light – “Voltaics” Electricity – Photovoltaics electricity from light Usual abbreviation: “PV” Common usage refers to: – Cells and modules that produce electricity from light – Systems that include PV cells and modules – A clean technology that captures a free source energy and changes this into a versatile and valuable commodity

  5. BASIC SOLAR CELL • SILICON SUBSTRATE IS 180 – 240 MICROMETERS THICK (~200 x 10-6 = 0.2 MILLIMETERS) • ALUMINUM FOIL = 0.2 – 0.006 mm • DEMO CELL WAS SINGLE CRYSTAL MATERIAL • MATERIAL IS SAWED INTO WAFERS, POLISHED AND PROCESSED

  6. MEASURING CELL CHARACTERISTICS

  7. STANDARD TEST CONDITIONS FOR PV CELLS AND MODULES • TEMPERATURE OF CELL = 25 OC • SOLAR RADATION ON CELL SHOULD HAVE A TOTAL POWER DENSITY OF 1000 WATTS PER SQ. METER • SPECTRAL DISTRIBUTION OF “AIR MASS 1.5” (AM 1.5)

  8. AM 0 • JUST OUTSIDE THE EARTH’S ATMOSPHERE • SOLAR RADIATION POWER DENSITY = 1365 W m-2 • SPECTRAL POWER DISTRIBUTION OF RADIATION BEFORE IT ENTERS THE ATMOSPHERE IS CALL AIR MASS 0 • AIR MASS DESCRIBES THE WAY THE SPECTRAL POWER DISTRIBUTION OF SUN’S RADIATION IS AFFECTED BY THE ATMOSPHERE

  9. AM 1 AND AM 1.5 • WHEN SUN IS DIRECTLY OVERHEAD (ZENITH) WE HAVE AM 1 • HIGHEST VALUE • WHEN SUN IS DOWN 48 DEGREES FROM THE ZENITH WE HAVE AIR MASS 1.5 • AIR MASS = 1/COSq

  10. WHERE SHOULD YOU OPERATE THE CELL? • MAXIMUM POWER POINT – MAX OUTPUT POWER INTO A LOAD • MAXIMUM POWER IS THE POINT WHERE I x V IS A MAXIMUM – i.e. RESISTANCE IS VARIED TO MAXIMIZE THE VOLTAGE AND CURRENT PRODUCT (WATTS)

  11. Silicon PV Cells • PV cells are made of a Semiconductor Material • The most common Semiconductor used in PV is silicon • Semiconductor atoms form covalent bonds to make a crystalline structure • In covalent bonds, valence electrons are shared by atoms

  12. Silicon PV Cells • Electrons in the valence band are restricted to movement around the atom • If the electrons are excited by an energy source, they can break the bond and reach the conduction band • Electrons in the conduction band are free to move anywhere in the material • That’s why they’re called ‘semiconductors’

  13. SEMICONDUCTORS • METALS – HIGH CONDUCTIVITY OF ELECTRICITY AND USUALLY HEAT • INSULATORS – LOW CONDUCTIVITY OF ELECTRICITY AND HEAT • SEMICONDUCTORS – CONDUTIVITY CAN BE CONTROLLED BY DOPING • CONDUCTIVITY BETWEEN CONDUCTOR AND INSULATOR • SILICON COMMONLY USED FOR SOLAR CELLS

  14. SILICON SEMICONDUCTOR • CUBIC CRYSTAL STRUCTURE • PURE SILICON HAS 4 VALENCE ELECTRONS • DOPED AS p-TYPE OR n-TYPE • n-TYPE – DOPED WITH PHOSPHORUS • PHOSPHORUS ATOMS HAVE ONE EXCESS ELECTRON -5 VALENCE e- • p-TYPE – DOPED WITH BORON – ONE LESS ELECTRON – HOLE

  15. Silicon PV Cells • The amount of energy needed to ‘free’ an electron from the valence band to the conduction band is called ‘Band Gap Energy’ • The Band Gap energy is dependent on temperature and the material • Silicon Band Gap is 1.8 x 10-19 [J] (1.1 eV) at 25oC

  16. Semiconductor Doping • PV cells are ‘doped’ with impurities to create a p-n junction - a diode • Negative (n-type) silicon is doped with phosphorus since it has one more electron in the valence bond • Positive (p-type) silicon is doped with boron since it has one less electron in the valence band

  17. PV Cell Principle of Operation • The energy in light can free electrons to create an electron- hole pair • Electrons collect in the n-type, Holes collect in p-type • The internal voltage of the junction causes the free electrons to travel through an external circuit, transferring the energy to the load

  18. Types of Silicon Cells • Three common types of silicon cells are • Mono- Crystalline • Poly- Crystalline • Amorphous • Mono- Crystalline is more efficient than Poly- Crystalline but more • Expensive to manufacture • Amorphous silicon is less efficient but much cheaper • Thin- film amorphous silicon can be made flexible

  19. Mono-Crystalline Silicon

  20. Poly-Crystalline Silicon

  21. Poly- Crystalline PV Cell Close-up

  22. Amorphous Silicon

  23. Effect of Temperature • PV cells, like any semiconductor electrical device, are sensitive to temperature • Solar cells loose 0.5% efficiency for every 1oC temperature increase • It is desirable to keep the cells at low temperatures

  24. EFFECTS OF TEMPERATURE • OPEN CIRCUIT OUTPUT VOLTAGE VARIES DIRECTLY WITH TEMPERATURE • VOC = (kT/q)exp{Isc/Io + 1} • T= TEMPERATURE IN DEGREE KELVIN • Note: Increasing temperature causes the diode leakage current to decrease Io and this causes the decrease in VOC with T.

  25. ENERGY CONVERSION EFFICIENCY • h = Pmax / (E x AC) • WHERE Pmax= MAXIMUM POWER • E = INPUT LIGHT IRRADIANCE (W/m2) UNDER STC • AC = SURFACE AREA OF SOLAR CELL (m2)

  26. EFFICIENCY EXAMPLE100 cm2 CELL • PMAX = 2.6 AMPS x 0.48 VOLTS = 1.248 WATTS • h = Pmax / (E x AC) = 1.248 W/(1000 Wm-2 x .01 m2) = 0.1248 = 12.48%

  27. From Cell to Array • Typical cell • 12.5 cm by 12.5 cm • 4 Amps x 0.5 Volts = 2 W • Module • multiple cells in series to increase operating voltage • glass cover to protect cells • various frame and backing materials to facilitate mounting

  28. Array • multiple modules in series to increase operating voltage: “string” • multiple strings in parallel to increase the current and therefore power up to several MW • physically attached to a mountings structure or building to face the sun as much as possible

  29. From Array to System Can connect load directly to cell but • cell current fluctuates with radiation and • cell voltage fluctuates with temperature • most loads cannot operate like this

  30. Therefore • add a power conditioner to control the voltage and current Challenge • keep the cell operating voltage and current at the maximum power point

  31. Stand-Alone Systems • Energy storage to power loads when the sun isn’t shining • Inverter to convert DC to AC for most loads • Energy management logic to make sure • batteries are not overcharged or discharged • only critical loads are powered when power is scarce • Backup power source, just in case

  32. Grid-Connected Systems • If grid power is available • take power from the grid when the sun isn’t shining • put power into the grid when the sun is shining • The grid acts like an infinite energy storage and backup supply (when it is working) • Grid-connect inverter makes the link between PV array and utility grid

  33. Air Building-Integrated PV/Thermal

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