Transcript Organic Chemistry Introduction

GECH 119
Solar Power
Dr. Ralph C. Gatrone
Department of Chemistry and Physics
Virginia State University
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Solar Power
Chapter Objectives
• Energy from the Sun
• Photosynthesis
• Spectroscopy
• Energy exchange
• Photovoltaics
• The future
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Energy from the Sun
• Sun
– Primary source of energy for the planet
• Coal, Oil, Wood
– Burning produces light and heat
– Energy released due to change in chemical bonds
– Energy put there by sunlight
• Wind
• Sun provides 10X total amount of energy available from
nuclear, coal, and oil reserves each year
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Solar Power
• Available everywhere
• Free
• A few billion years of reserves
• Minimally polluting
• Best alternative energy source
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Energy from the Sun
• The Sun
– Core temperature
• Millions of degrees C
– Surface temperature
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• 6000 degrees C
H fuses into He
Nuclear fusion
Kinetic energy
Potential energy
Electromagnetic
• Heat and light
• Radiates in all directions
• Earth receives very little of sun’s energy
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Electromagnetic Energy
• Travels at speed of light (c)
• Constant in space
• Energy of light is related to frequency and
wavelength
• Energy of light is quantized (small packets
of energy)
• 1 quantum of light energy = 1 photon
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Light
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Electromagnetic Spectrum
• Gamma Rays
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High energy
High frequency
Short wavelengths
Harmful to living organisms
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Low energy
Low frequency
Long wavelengths
No ill effects
• Radio Waves
• Visible
– Region of spectrum detected by human eyes
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Energy Received
• 1022 J/day reaches upper atmosphere
• Surface receives 75% to 0% of this energy
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Weather conditions
Reflected by clouds
Absorbed by molecules in atmosphere
Scattered by water vapor
• X-rays absorbed by atmosphere gases
• UV is absorbed by ozone
• Infrared absorbed by water vapor and CO2
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Energy Striking the Surface
• Absorbed
– Converted into heat
– Photosynthesis
• Green plants
• Phytoplankton
• Algae
• Reflected
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Photosynthesis
• Starting materials
– Carbon dioxide + water
• Products
– Glucose + oxygen
• Some organisms produce Sulfur in the
process – green/purple bacteria
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Green Plants
• Photosynthesis
– Light stage – needs visible light
– Chloroplasts
– Chlorophyll-a
• Absorbs light
• Blue and red regions of spectrum
• See green light reflected (green leaves)
– Chlorophyll-b, carotenoids
• absorb other wavelengths
– Dark stage – light may be present or not
• Converts carbon dioxide into carbohydrates
• Carbohydrates are energy source for animals
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Conceptual Framework
• Photosynthesis
– Sunlight into useful energy sources
– Via chemicals
– Controlled use
• Can we capture sunlight in a similar
process?
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Spectroscopy
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Heated atoms release light
Color dependent upon identity of atom
Electron gets excited to higher energy state
Releases light as it relaxes
Emission spectroscopy
• Absorption spectroscopy
– Shine light on atom
– Look at light that is absorbed
– Electron absorbs light as it gets excited
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Aqueous Chemistry of Copper
• Solutions of copper ions in water
• Blue in color
• Blue is transmitted
• Red and orange light are absorbed
• Intensity of blue color measures amount
of copper ions present
• Use color intensity to determine [Cu(II)]
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Transition Metals
• Intensely colored solutions
• Color changes as you add different
materials
• Allows chemist to examine trace amounts
of metal ions that might be present
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Energy Exchange
• Plants can store sun’s energy in chemical
systems
– Photosynthesis with carbohydrate generation
• Release energy
– Many forms
– Release as electron flow
• Current
• Voltage
• If great enough - work
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Electric Car
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1994 – by Daimler-Benz
Uses H2 as fuel that reacts with oxygen
Produces water
H2 generated from methanol
• H2
– Desirable
– Cheap to make
– Doesn’t hold much energy
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Photoelectrolysis
• Sunlight
– Converts H2O into H2
– Effortless among plants
• Human efforts
– Requires high temperature
– Using energy from chemical reactions
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Transfer of Electrons
• H2O
H2 + O2
• Oxygen is more electronegative than hydrogen
• Oxidation number of oxygen is -2 in water
• Oxidation number of oxygen is 0 in oxygen
• Splitting water requires exchange of electrons
– Oxygen – oxidized (loss of electrons)
– Hydrogen – reduced (gain of electrons)
• Must be forced to happen
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Electrolysis of Water
• Requires energy source
– Sunlight
– Battery driven
– Chlorophyll a
• Technology is not sufficiently developed
• Political and economic factors
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Sources of electricity
• Consider
– Cu added to Zn+2 – no reaction
– Zn added to Cu+2 – reaction occurs
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Cu + Zn+2 forms
Cu is reduced
Zn is oxidized
Electrons move
Redox reaction
Energy of products is lower than SM
EMF or voltage
Power = current X voltage
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Use a salt bridge for electrical neutrality
Zn end is called anode
Cu end is called cathode
Electricity used to light an LED
– Example of a battery if we separate half-reactions (see page 495)
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Batteries
• Familiar with batteries
– Flashlight
• Alkaline battery
– Graphite – inert cathode
– NaOH or KOH surrounds graphite
 salt bridge
 electrolyte
– Zn anode
– MnO2 cathode
• Generates 1.5V
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Battery
• Car Battery
– Pb – anode
– PbO2 –cathode
– H2SO4 – electrolyte
• Rechargeable nearly continuously
• Used to start the engine not run it
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The Electric Car
• Batteries
– Must be Rechargeable
– Must Last a long time
– Must Last between charges
– Must provide sufficient energy for car to
handle well
• Ni/Cd batteries
– Limited range
– All require fuel of some sort
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Photovoltaics
• Insulators
– Unreactive materials
– Stable electron structures
– Example Diamonds
• Semiconductors
– Silicon
– Valence electrons are higher in energy
– Closer to conducting band than Carbon
– Conducts electricity under certain conditions
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Doping
• Replace some Si atoms with As
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Makes the Si crystals less stable
More electrons are present
Energy gap is smaller
Less energy is needed to conduct electricity
• Replace Si with Ga atoms
– Electron deficient regions
– Positively charged hole is produced
• Dope with both As and Ga
– Has electron rich and deficient regions
– Sunlight striking this material generates electric current
– Photvoltaic cell
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Photovaltaic (PV) Cells
• 1954 – silicon-based
• 6% of sunlight produced electricity
• Low efficiency
– Some light is reflected
– Some light wavelength’s don’t excite the e-
• GaAs cells – 26% efficiency
• GaAsSi cells – 32% efficiency
• > efficiency = > cost
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Uses of PV Devices
• Lawnmowers
• Security lighting
• Calculators
• Power tools
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The Future
• US supported solar programs
– 1974 – 1985
– Tax breaks
• Costs of solar energy
– Decreased ($5 to $0.2 per Kw hour)
• $800 billion spent on fossil fuels
• $1 billion spent on solar power
• The Debate Continues
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