Solar Power
Photovoltaic Power Generation
Photoelectric Effect, P and N type Layers, Band Gap
Solar Cell Manufacture
Types of Solar Cells
Solar Vehicles
Photovoltaic Systems
Energy in the News
Contribution of Solar to U.S. Power
Growth rate of solar is 15% per year, when will it reach 0.15% of total
Power supply?
Change in Cost of Solar Cells
Solar power:
Already competitive
For peak power
Production
In New York area.
Why??
Present price $3.50 per peak watt-needs to go to $1.50 per
Peak = cost of generation of 12 cents per kilowatt hour, then
Economically competitive for typical power production.
Photoelectric Effect
Light hits a metal plate
In an evacuated tube,
Result is the emission of
Electrons. The electrons
Have kinetic energy.
Circuit diagram-shows that
There is an electrical potential
Generated, therefore, a
Possible electrical current.
Solar cells: use the
Photoelectric effect in
Semi-conductors.
Quantum Nature of Light
Light behaves like a wave and like a particle. Particle
Nature of light demonstrated by photoelectric effect.
If energy of light is not enough, no photoelectric effect.
(No electrons knocked out of outer shell of electrons in
An element)
Conversion of light energy into electrical energy is quantized
That is discrete.
(these amounts of energy are called photons, their energy
Is measured in electron volts (eV). )
Energy of a photon of light = h* f where h is Plancks constant
And f is frequency.
Solar Radiation vs Photon Energy
Available energy
From sunlight, has
A certain range of
Photon energies
(measured in eV)
Materials
In solar cells: exhibit
Photoelectric effect
Have certain energies
In their electron shells
These match energies
Of incoming photons
Of light.
Materials in Solar Cells: Band Gap
Band gap: energy in incoming photon needed to produce
Photoelectric effect (knock electrons out of outer shell)
Two tradeoffs: higher band gap = more electrical energy per photon
Photons with not enough energy= no photoelectric effect
What do photons with not enough energy do to the solar cell?
Inside a Solar Cell
Solar Cells: at least
3 layers:
P-type semiconductor
With electron holes
(positive charge)
N-type semiconductor
With extra electrons
(negative charge)
Active layer
In between: generates
Electric field
Blowup of 3 layer Solar Cell
Top: N-type semi-conductor
Middle: junction or active layer
Bottom: P-type semi-conductor
Incoming Photons: Possibilities
Photons enter solar cell. Some absorbed by P-layer,free
Electrons for circuit.
Some pass through, reflected off bottom of cell.
Some meet up with holes, recombine within cell.
Ultimate result: conduction of electricity.
Photoelectric Effect
Incoming photons: hit
Electrons in valence band.
If enough energy, knock them
Up to the conduction band.
Incoming photon, knocks electron
Out, makes a hole: region
Of positive charge.
Within a Solar Cell
Holes float upwards in n region
Electrons move downwards in
P region.
Recombine in the active layer.
Introducing Boron
Boron has a +3 charge,
Silicon has +4 charge.
Blue=silicon
Red=boron
Doping silicon with boron:
Makes P-type semi-conductor.
Why?
Introducing Phosphorus
Silicon +4 charge
Phosphorus +5 charge
Doping with phosphorus
Get extra electrons
N-type semi-conductor
Picture of a Solar Cell
Layers in a Solar Cell
Common Types of Solar Cells
Single crystal silicon: up to 23% efficiency
Amorphous silicon: 5 to 10% efficiency (40% of market:cheapest)
Polycrystalline silicon: > 10% efficiency
GaAs cells: 25% efficiency
Stacked cells: higher efficiencies
Highest so far: stacked GaAs:GaSb: 34% efficiency
Stacked silicon: 28% efficiency
Polycrystalline silicon: 18%
Single Crystal
Silicon
Single crystal silicon solar cells;
Refined from sand (SiO2)
Reduced at 900 deg C,
Heated to 1500 C (Czochralski process)
To produce silica for growing
Crystals.
Crystals sawed up into wafers.
Wafers polished and coated.
Assembled into solar cells.
Expensive to make because of high
T Czochralski process
Amorphous Silicon Cell
Top contact: tin oxide at base of glass
SiO2: protects the tin oxide.
P-type layer
Amorphous silicon (undoped)
N-type layer
Bottom contact: Aluminum
Multi-junction Amorphous Silicon Cell
Multiple layers increase number of photons
Whose band gaps are matched. (Each part of
Cell tailed to part of visible spectrum).
Silicon alloy with carbon; increases band gap:
Better response to blue light.
Silicon alloy with germanium: decreases band
Gap: better response to red light.
Amorphous Silicon
Absorbs solar radiation
40 times more efficiently
Than single crystal silicon
A one micron thick film
Absorbs 90% of usable solar
Energy.
Current Voltage
of Solar Cell
Maximum power pointOpen circuit:cell not connected
Short circuit:no resistance
Sometimes add resistance to
Circuit to increase cell
Efficiency.
Solar Cells Connections
Connect in series:
Increases voltage
Connect in parallel:
Increases current
Residential Photovoltaic System
Stand Alone Solar System
Solar System:Grid Connected
Solar Powered Car: Sunraycer
Solar Plane:Sun Seeker
Plane flew 4060 km across the United States
Ultralight plane piloted by designer Eric Raymond
Power from amorphous silicon cells. Cells charge
A Ni-Cd battery that runs an electric motor.
Motor turns a propeller for take off.
Plane is a glider during flight.
Mirage Solar Car
Solar Oven
Reflects 97%
Of solar energy
Solar Power Plant in the Desert
Solar Trivia
Sunniest city in the U.S.A.: Yuma, Arizona
Gets 90% of potential sunlight
Cloudiest city in the U.S.A.: Quillayute, Washington
(241 cloudy days per year)
Time it takes for sunshine falling on the U.S. to equal
Energy in all fossil fuel consumed by the U.S. in one
Year: 40 minutes
Estimated area of land needed to meet US energy needs
From photovoltaic cells: 58,360 square miles
Land area of Georgia: 58,060 square miles