Energy from the SunYearly Solar fluxes & Human Energy Consumption• The total solar energy absorbed by Earths atmosphere, oceans and land masses is approximately 3,850,000 exajoules (EJ) (1018 joules) per year. (70% of incoming sunlight) (1 Joule = energy required to heat one gram of dry, cool air by 1˚ C)• The amount of solar energy reaching the surface of the planet is so vast that in one year it is about twice as much as will ever be obtained from all of the Earths non-renewable resources of coal, oil, natural gas, and mined uranium combined.• As intermittent resources, solar and wind raise issues.
Solar Cells Background• 1839 - French physicist A. E. Becquerel first recognized the photovoltaic effect.• Photo+voltaic = convert light to electricity• 1883 - first solar cell built, by Charles Fritts, coated semiconductor selenium with an extremely thin layer of gold to form the junctions.• 1954 - Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light. Daryl Chapin, Calvin Fuller and Gerald Pearson, invented the first practical device for converting sunlight into useful electrical power. Resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6%.• 1958 - First spacecraft to use solar panels was US satellite Vanguard 1
PV Solar for ElectricityPhotovoltaics• For the 2 billion people without access to electricity, it would be cheaper to install solar panels than to extend the electrical grid.• Providing power for villages in developing countries is a fast-growing market for photovoltaics. The United Nations estimates that more than 2 million villages worldwide are without electric power for water supply, refrigeration, lighting, and other basic needs, and the cost of extending the utility grids is prohibitive, $23,000 to $46,000 per kilometer in 1988.• A one kilowatt PV system* each month: – prevents 150 lbs. of coal from being mined – prevents 300 lbs. of CO2 from entering the atmosphere – keeps 105 gallons of water from being consumed – keeps NO and SO2 from being released into the environment * in Colorado, or an equivalent system that produces 150 kWh per month
How Solar Cells Work1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.2. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity.3. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.http://teams.eas.muohio.edu/solarpower/video/solarcell2.mpeg
PV Cells HistoryFirst Generation – Single Junction Silicon Cells89.6% of 2007 Production 45.2% Single Crystal Si 42.2% Multi-crystal SI• Large-area, high quality and single junction devices.• High energy and labor inputs which limit significant progress in reducing production costs.• Single junction silicon devices are approaching theoretical limit efficiency of 33%. Achieve cost parity with fossil fuel energy generation after a payback period of 5–7 years. (3.5 yr in Europe)• Single crystal silicon - 16-19% efficiency• Multi-crystal silicon - 14-15% efficiency
PV Cells HistorySecond Generation – Thin Film CellsCdTe 4.7% & CIGS 0.5% of 2007 ProductionNew materials and processes to improve efficiency and reduce cost.As manufacturing techniques evolve, production costs will be dominated by constituent materialrequirements, whether this be a silicon substrate, or glass cover. Thin film cells use about 1% of theexpensive semiconductors compared to First Generation cells.The most successful second generation materials have been cadmium telluride (CdTe), copper indiumgallium selenide (CIGS), amorphous silicon and micromorphous silicon.Trend toward second gen., but commercialization has proven difficult. 2007 - First Solar produced 200 MW of CdTe solar cells, 5th largest producer in 2007 and the first to reach top 10 from of second generation technologies alone. 2007 - Wurth Solar commercialized its CIGS technology producing 15 MW. 2007 - Nanosolar commercialized its CIGS technology in 2007 with a production . capacity of 430 MW for 2008 in the USA and Germany. 2008 - Honda began to commercialize their CIGS base solar panel.CdTe – 8 – 11% efficiency (18% demonstrated)CIGS – 7-11% efficiency (20% demonstrated)Payback time < 1 year in Europe
Solar Cells BackgroundThird Generation – Multi-junction Cells• Third generation technologies aim to enhance poor electrical performance of second generation (thin-film technologies) while maintaining very low production costs.• Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. They can exceed the theoretical solar conversion efficiency limit for a single energy threshold material, 31% under 1 sun illumination and 40.8% under the maximal artificial concentration of sunlight (46,200 suns).• Approaches to achieving these high efficiencies including the use of multijunction photovoltaic cells, concentration of the incident spectrum, the use of thermal generation by UV light to enhance voltage or carrier collection, or the use of the infrared spectrum for night-time operation.• Typically use fresnel lens (3M) or other concentrators, but cannot use diffuse sunlight and require sun tracking hardware• Multi-junction cells – 30% efficiency (40-43% demonstrated)
Advantages & DisadvantagesLarger, Si-based photovoltaic cells Typically made of a crystalline Si wafers sawed from Si ingotsDominant technology in the market More than 86% of the commercial production of solar cellshigh-efficiency Maximum theoretical efficiency of 33.7%Advantages Broad spectral absorption range (Eg=1.12eV)Disadvantages High costs: Expensive manufacturing technologies Extracting Si from sand and purifying it before growing the crystals Growing and sawing of ingots is a highly energy intensive process Much of the energy of higher energy photons, at the blue and violet end of the spectrum, is wasted as heat Not more energy-cost effective than fossil fuel sources
Thin Film Photovoltaics• Amorphous Silicon• Gallium Arenide and Indium Phosphide• Cadmium Telluride• Copper Indium Diselenide (CIS) or
Thin Film Photovoltaics• CIGS- copper-indium-gallium- selenide• Thin film growth and deposition on glass/polymer/flexible foil substrate• High efficiency-19.6% (I. Repins et al. 2008)• CdTe- Cadmium Telluride• Efficiency-16.7% (Wu X et al. Oct. 2001)• High cost due to Tellurium availability• a-Si- Amorphous Silicon• 10.1% Efficiency (S. Benagli, et al. Sept. 2009)
Thin Film Technology Silicon deposited in a continuous on a base material such as glass, metal or polymers Thin-film crystalline solar cell consists of layers about 10μm thick compared with 200-300μm layers for crystalline silicon cells PROS Low cost substrate and fabrication process CONS Not very stable
How Organic Solar Cells Work1. Photon absorption, exactions are created2. Exactions diffusion to an interface3. Charge separation due to electric fields at the interface. High Work Function Electrode4. Separated charges travel to the electrodes. Donor Material E Acceptor Material Low Work Function Electrode
Developed to reduce the costs of the first generation cells Deposition of thin layers of materials on inexpensive substrates: Mounted on glass or ceramic substrates Reduce high temperature processing Production costs will then be dominated by material requirements Compared to crystalline Si based cells they are made from layers of semiconductor materials only a few micrometers thick Reduces mass of material required for cell design
Advantages and DisadvantagesAdvantages Lower manufacturing costs Much less material require Lower cost/watt can be achieved Lighter weight (reduced mass) Flexibility: allows fitting panels on curved surface, light or flexible materials like textiles Even can be rolled upDisadvantages Inherent defects due to lower quality processing methods reduces efficiencies compared to the first generation cells
Cost Trends - Photovoltaics 100 COE cents/kWh 80 60 40 201980 1990 2000 2010 2020Current cost is 16-25 cents Levelized cents/kWh in constant $20001per kWh Source: NREL Energy Analysis Office Updated: June 2002
Residential Cost2 Kilowatt system: $16-20,000 (installed)- Could meet all needs of a very energy efficienthome. - $8-10 per Watt5 Kilowatt system: $30-40,000 (installed)-Completely meets energy needs of mostconventional homes. -$6-8 per Watt(Estimates from U.S. Department of Energy)
Energy Payback Time• EPBT is the time necessary for a photovoltaic panel to generate the energy equivalent to that used to produce it. A ratio of total energy used to manufacture a PV module to average daily energy of a PV system.• At present the Energy payback time for PV systems is in the range 8 to 11 years
References1. K. BOWER. et al., Polymers, Phosphors, and Voltaics for Radioisotope Microbatteries, CRC press, New York, (2002).2. QYNERGY Corporation, Press Announcement –Qynergy Announces Breakthrough in Power Cell Performance, September 5, 20053 H.FLICKER, J. LOFERSKI, T. ELLERMAN, IEEE Transactions on Electronic Devices, 11, 1-2-8 (1964)4 P. RAPPAPORT, J. LOFERSKI, E. LINDER, RCA Rev., 17, pp. 100-134 (1956)5. H. GUO, A. LAL, IEEE Transducers, 1B3.1, pp. 36-39, (2003)6 CREE RESEARCH INC., Product Specifications, (1998-2000)8 T. KOSTESKI, Tritiated Amorphous Silicon Films and Devices, PhD diss., University of Toronto, (2001)
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