Noor Shazliana Aizee bt Abidin 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

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%

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

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

MEASURING CELL CHARACTERISTICS

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)

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

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

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)

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

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’

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

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

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

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

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

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

Mono-Crystalline Silicon

Poly-Crystalline Silicon

Poly- Crystalline PV Cell Close-up

Amorphous Silicon

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

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.

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)

EFFICIENCY EXAMPLE 100 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%

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

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

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

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

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

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

Air Building-Integrated PV/Thermal