1. Noor Shazliana Aizee bt Abidin
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 – 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 – 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 = 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/COS q

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

16. 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 [J] (1.1 eV) at 25oC

17. 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

18. 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

19. 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

20. Mono-Crystalline Silicon

22. Poly-Crystalline Silicon

23. Poly- Crystalline PV Cell Close-up

24. Amorphous Silicon

25. 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

26. 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.

27. 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)

28. EFFICIENCY EXAMPLE 100 cm2 CELL
PMAX = 2.6 AMPS x VOLTS = WATTS
h = Pmax / (E x AC)
= W/(1000 Wm-2 x .01 m2) = = 12.48%

29. 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

30. 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

32. 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

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

35. 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

37. 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

38. Air Building-Integrated PV/Thermal