Download Reflection

Reflection
• When light is incident
on a surface, it can be
reflected
• An interesting result is
that the angle of
incidence (incoming
angle) equals the angle
of reflection (outgoing
angle.
Reflection from a curved surface
• When the surface doing
the reflecting is curved,
the light can be brought
to a focus.
• The curved surface can
be parabolic or
spherical.
• Spherical surfaces are
cheaper and easier to
construct.
Power towers
• Use many collectors and
focus the light to a central
point.
• Achieves high
temperatures and high
power density.
• Each individual collector
is called a heliostat
• Must be able to track the
sun and focus light on the
main tower
How they work
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Light is collected at the central tower,
which is about 300 feet tall. There are
on the order of 2000 heliostats.
Used to heat water and generate
steam
Steam drives a turbine which
generates electricity
Often include auxiliary energy
storage to continue to produce
electricity in the absence of sunlight
More costly to construct and operate
than coal fired plants.
Good candidates for cogenerationwaste steam could be used for space
heating
Solar troughs
• A parabolic shaped
trough collects the light
and focuses it onto a
receiver.
• The receiver has a fluid
running through it which
carries the heat to a
central location where it
drives a steam turbine
• May have more than a
hundred separate troughs
at such a facility
Trough Pictures
Direct Conversion of sunlight to energy:
Photo-voltaics
• Photoelectric effect:
• When electromagnetic energy impinges upon a
metal surface, electrons are emitted from the
surface.
• Hertz is often credited with
first noticing it (because he
published his findings) in 1887
but it was seen by Becquerel
In 1839 and Smith in 1873.
Photoelectric effect
• The effect was a puzzle
• The theory of light as a wave did not explain the
photoelectric effect
• Great example of the scientific method in action.
– Up until this point, all the observations of light were
consistent with the hypothesis that light was a wave.
– Now there were new observations could not be
explained by this hypothesis
– The challenge became how to refine the existing
theory of light as a wave to account for the
photoelectric effect
Photoelectric effect explained
• Einstein in 1905 explained the photoelectric
effect by assuming light was made of discrete
packets of energy, called photons.
• Not a new idea, he was building upon an idea
proposed by Planck, that light came in discrete
packets. (in fact, Newton proposed a particle like
explanation of light centuries earlier). The
problem for Planck was his discrete packets were
in conflict with the wave like behavior of light.
Photoelectric effect explained
• But now, a behavior of light was observed that fit
Planck’s energy packet idea.
• So electromagnetic radiation appears to behave
as if it is both a wave and a particle.
• In fact, you can think of light as discrete wave
packets-packets of waves which, depending upon
the measurement you make, sometimes exhibit
particle behavior and sometimes exhibit wave
behavior.
• Einstein won the Nobel prize for his explanation
of the photoelectric effect.
Semi conductors
• Devices which have conductive properties in
between a conductor and an insulator.
• Normally, the outer (valence) electrons are
tightly bound to the nucleus and cannot
move.
• If one or all of them could be freed up, then
the material can conduct electricity
• Silicon is an example of a semi-conductor.
Silicon
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Element 14 in the periodic table
Very common element (sand, glass composed of it)
8th most common element in the universe
Its 4 outer valence electrons are normal tightly bound
in the crystal structure.
• However, when exposed to light, the outer electrons
can break free via the photoelectric effect and conduct
electricity.
• For silicon, the maximum wavelength to produce the
photoelectric effect is 1.12 microns. 77% of sunlight is
at wavelengths lower than this.
But its not quite this simple
• You also need to produce a voltage within the silicon to
drive the current.
• So the silicon must be combined with another material.
This process is called doping.
• 2 types of doping: P and N
– If you replace one of the silicon atoms in the crystal lattice
with a material that has 5 valence electrons, only 4 are
need to bond to the lattice structure, so one remains free.
The doped semi conductor has an excess of electrons and
is called an N type semiconductor.
– Doping elements can be arsenic, antimony or phosphorus.
P-types
• If you dope with an element with only 3 valence
electrons, there is a vacancy, or hole left where the 4th
electron should be.
• If the hole becomes occupied by an electron from a
neighbor atom, the hole moves through the
semiconductor. This acts like a current with positive
charge flowing through the semi conductor, so it
appears to have a net positive charge
• Called a P-type semiconductor.
• Doping elements could be boron, aluminum, or indium
Creating the solar cell
• To create the solar cell, bring a p-type silicon into
contact with an n-type silicon.
• The interface is called a p-n junction.
• Electrons will diffuse from the n material to the p
material to fill the holes in the p material. This
leaves a hole in the n material.
• So the n-material ends up with an excess positive
charge and the p material ends up with an excess
negative charge.
• This creates an electric field across the junction.
Current in the solar cell
• Any free electrons in the junction will move
towards the n –type material and any holes
will move toward the p -type material .
• Now sunlight will cause the photoelectric
effect to occur in the junction. Thus free
electrons and holes are created in the junction
and will move as described above.
• Current flows!
Solar Cells
• Typically 2 inches in diameter and 1/16 of an inch
thick
• Produces 0.5 volts, so they are grouped together
to produce higher voltages. These groups can
then be connected to produce even more output.
• In 1883 the first solar cell was built by Charles
Fritts. He coated the semiconductor selenium
with an extremely thin layer of gold to form the
junctions. The device was only around 1%
efficient.
Generations of Solar cells
• First generation
– large-area, high quality and single junction devices.
– involve high energy and labor inputs which prevent
any significant progress in reducing production costs.
– They are approaching the theoretical limiting
efficiency of 33%
– achieve cost parity with fossil fuel energy generation
after a payback period of 5-7 years.
– Cost is not likely to get lower than $1/W.
Generations of Solar cells
• Second generation-Thin Film Cells
– made by depositing one or more thin layers (thin film)
of photovoltaic material on a substrate.
– thickness range of such a layer varies from a few
nanometers to tens of micrometers.
– Involve different methods of deposition:
• Chemical Vapor deposition the wafer (substrate) is exposed
to one or more volatile precursors, which react and/or
decompose on the substrate surface to produce the desired
deposit. Frequently, volatile by-products are also produced,
which are removed by gas flow through the reaction
chamber.
Thin Film deposition techniques
• Electroplating
– electrical current is used to reduce cations (positively
charged ions,) of a desired material from a solution
and coat a conductive object with a thin layer of the
material.
• Ultrasonic nozzle
– spray nozzle that utilizes a high (20 kHz to 50 kHz)
frequency vibration to produce a narrow drop size
distribution and low velocity spray over the wafer
• These cells are low cost, but also low efficiency
The Third Generation
• Also called advanced thin-film photovoltaic
cell
• range of novel alternatives to "first
generation” and "second generation” cells.
• more advanced version of the thin-film cell.
Third generation alternatives
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non-semiconductor technologies (including polymer cells and biomimetics)
quantum dot technologies
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also known as nanocrystals, are a special class semiconductors. which are crystals composed
of specific periodic table groups. Size is small, ranging from 2-10 nanometers (10-50 atoms) in
diameter.
tandem/multi-junction cells
– multijunction device is a stack of individual single-junction cells
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hot-carrier cells
– Reduce energy losses from the absorption of photons in the lattice
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upconversion and downconversion technologies
– Put a substance in front of the cell that converts low energy photons to higher energy ones or
higher energy photons to lower energy ones that the solar cells can convert to electricity.
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solar thermal technologies, such as thermophotonics(TPX)
– A TPX system consists of a light-emitting diode (LED) (though other types of emitters are
conceivable), a photovoltaic (PV) cell, an optical coupling between the two, and an electronic
control circuit. The LED is heated to a temperature higher than the PV temperature by an
external heat source. If power is applied to the LED, , an increased number of electron-hole
pairs (EHPs) are created.These EHPs can then recombine radiatively so that the LED emits light
at a rate higher than the thermal radiation rate ("superthermal" emission). This light is then
delivered to the cooler PV cell over the optical coupling and converted to electricity.
Efficiency and cost factors
• In 2002 average cost per peak watt was 2.90-4.00. Coal
fired plant is 1.00/watt.
• Efficiency is not great.
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Recall, 77% of the incident sunlight can be used by the cell.
43% goes into heating the crystal.
Remaining efficiency is temperature dependent
Average efficiency of a silicon solar cell is 14-17%
• The second and third generation technologies
discussed are designed to increase these efficiency
numbers and reduce manufacturing costs
Solar Cooling
• Consider a refrigeration system with no moving parts.
– Heat the coolant (say ammonia gas dissolved in water) and force it via a
generator into an evaporator chamber where it expands into a gas and cools.
Move it to a condenser and cool it back to a liquid and repeat the process.
• These systems actually have existed for a number of years, refrigerators in
the 1950s were sold with this technology (gas powered and there was/is a
danger of CO emissions).
• Energy to heat the coolant and drive it through the system comes from
burning fuel or a solar cell to provide electricity to do the heating.
– Need what is called a concentrating collector (lens or other system to
concentrate more light on the solar cell).
• Ideally, you could do this with a flat plate collector system, though you do
not obtain as much cooling.
• Devices are not widely used, due to the intermittency of sunlight