Basics of Solar Energy & the Basics of Solar Cell : Cost of Solar Energy : a concern The main barrier to widespread adoption of solar energy is cost.Solar Energy costs as much as $0.30 per KWH.This is very high compared with : Wind Energy : $0.05 per KWH Natural Gas Energy : $0.03 per KWH.But, PV Capacity is growing by 50% every year and With every doubling of capacity,PV costs come down by 20 percent. The Future is Bright : (i) Gradual improvements to the basic single-crystal silicon solar cells have already reduced the cost of photovoltaic electricity by about a factor of 20 in the past 30 years, and the continuing development of cheaper crystalline materials should see this trend continue. (ii) According to a report produced by American scientists George Crabtree and Nathan Lewis for the US Department of Energy in 2005,solar cells will become competitive enough – generating electricity at $0.02 per KWH – to be implemented on a massive scale in about 20-25 years’ time. Global Solar Irradiance : IEA Projections show that all of the world’s primary energy needs could met if 4% of the World’s Dry Desert areas were covered by PV Systems. The Sun An inexhaustible clean source of energy : (i) In a single hour the Sun delivers the same amount of energy as consumed by all of humanity in a year ~ 5 * 10^20 J (ii) The energy delivered by the sun in 36 hours is equal to the energy content of all of the earth’s known oil reserves. Solar Insolation Map of India : If one percent of the land is used to harness solar energy for electricity generation at an overall efficiency of 10% : 492 GWH/year electricity can be generated.Current installed capacity of electricity in India is 165 GW. Metals,Semiconductors and Insulators : (i) A band diagram is a diagram showing the electron energy of the valence band and conduction band edges vs some spatial dimension. (ii) The Fermi Level denotes the level at which the occupation probability is 0.5. (iii) Metals have unfilled valence band,or greatly overlapping valence & conduction bands. (iv) Semiconductors have a small band gap between them.The Fermi level is located in the middle of the band gap for an uncapped semiconductor. (v) Insulators have a large band gap, so that electrons cannot escape the valence band to enter the conduction band. Semiconductor Basics : Metals,Semiconductors and Insulators. Silicon and neighboring elements. n-type doping. p- type doping. pn junctions. Intrinsic Electrical Field. Photovoltaic Effect. Basics of Doping : If a small amount of impurity is mixed into the Silicon Crystal it can significantly change the electrical resistivity of the silicon semiconductor.Silicon can either be doped with an n-type impurity(P/As) or a p-type impurity(B/Ga). N-type Doping : In this type of doping P or As is added to the silicon in small quantities.Phosphorous and arsenic each have five outer electrons,so they’re out of place when they get into the silicon lattice.The fifth electron has nothing to bond to,so it’s free to move around.It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon.N-type silicon is a good conductor.Electrons have negative charge,hence the name is N-type. P-type Doping : In P-type doping,B or Ga is the dopant.Boron and Gallium each have only three outer electrons.When mixed into the silicon lattice,they form “holes” in the lattice where a silicon electron has nothing to bond to.The absence of an electron creates the effect of a positive charge,hence the name is P-type.Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space.P-type silicon is a good conductor. The Photovoltaic Effect : Solar Cell operation is based on the photovoltaic effect : The generation of a voltage difference at the junction of two different materials in response to visible or other radiation. (i) Absorption of Light – Generation of charge carriers (ii) Separation of charge carriers. (iii) Collection of the carriers at the electrodes. Fundamentals of Solar Cell Operation : Part of a wafer of silicon is doped to create an excess of holes(i.e. a p-type semiconductor) while another part of the wafer is doped to contain an excess of electrons(an n-type semiconductor).At the junction between these two regions,electrons and holes combine to create a potential barrier,which keeps the remaining electrons and holes apart.However, when a photon with sufficient energy strikes the cell,it promotes electrons from the valence band to the conduction band, creating electron - hole pairs.Pairs formed on or near the p-n junction are forced by the electric field to separate so that the holes pass to the p-type region and the electrons to the n-type region,thereby producing a current. Solar Cell Parameters : Efficiency,Open Circuit Voltage,Short Circuit Current Density,Fill Factor,Shunt Resistance,Series Resistance. Solar Cell Performance : (i) Efficiency is the key parameter. (ii) In 1961,Shockley and Queisser calculated that the simplest solar cell has a maximum efficiency of 31%. (iii) This is a cell that consists of a single p-n junction,generates just one electron-hole pair for each incoming photon,is exposed to unconcentrated sunlight,and wastes as heat any incoming photon energies in excess of the semiconductor band gap. Loss Mechanisms in a Si solar cell : Loss processes in a standard solar cell : Thermalization Junction & Contact Voltage Losses Recombination Loss. Technology Generations : Crystalline Silicon(covered in a separate module) Thin film multi-crystalline a-Si,CdTe,CIGS,CIS Third Generation Nanotechnology,DSSC Wafer based Crystalline Silicon Solar Cells Monocrystalline : Lab Record : 25% Industrial Record : 23% Multicrystalline : Lab Record : 20.3% Industrial Record : 19.3% Thin Film Solar Cells : CIGS Lab Efficiency Record : 20% Industrial Efficiency Record : 13% Thin Film Solar Cells : a –Si Lab Efficiency Record : 12.5% Thin Film Solar Cells : CdTe Lab Efficiency Record : 16.7% Industrial Efficiency Record : 12% Third generation Solar Cells : Nanocrystals/Quantum Dots Dye Sensitized Quantum Well Cells Organic/Plastic Solar Cells Conclusions : Solar energy will become one of the most important and cost – efficient energy sources in the future. A solar Cell is basically a semiconductor pn junction. Thermodynamics limits the solar cell efficiencies. Three technologies exist : Crystalline Silicon,Thin Film and Third Generation. Crystalline Silicon accounts for 85-90% and Thin Film 10-15%. Nanotechnology and other third generation technologies may offer a long-term solution for future solar energy technology. Thanks Subhodip Das
Posted on: Wed, 19 Mar 2014 14:50:02 +0000
Trending Topics
Recently Viewed Topics
© 2015