Solar%20Electric%20Power%20generation

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Solar Electric Power generation

• Two types
• Thermal -use suns ability to heat (usually
  water) to create electricity
• Photovoltaic devices- a device which directly
  converts the Suns energy to electricity

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Solar Thermal

• Obvious idea would be to use sunlight to boil
  water and provide steam to drive a turbine
• But what happens when you place a container of
  water in the sun-it typically does not boil!
• Need to concentrate or focus the suns energy to
  achieve this goal
• How do we focus sunlight?

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Basic properties of light

• To answer this question, lets look at some basic
  properties of light in the wave description of
  light.
• Refraction-light is bent at the interface between
  two media.
• Snells law relates the angle
• of incidence and the index
• of refraction of medium 1 to
• the angle of refraction and
• index of refraction of medium
• 2.
• n1sin(angle of incidence)n2sin(angle of
  refraction)
• n1sin?1n2sin?2

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Focusing light

• If the interface is flat, the light is not
  focused.
• Example-pencil in a glass of water
• If it is curved in the correct fashion, i.e. the
  surface of a convex lens, the light can be
  brought to a focus

convex
concave
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Fresnel Lens

• For the most part, lens are very heavy, suffer
  from reflection at the surfaces, and are
  expensive to construct to the sizes needed to
  achieve the desired heating.
• There is one type of lens, a Fresnel lens that
  can be inexpensively constructed from plastic

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Fresnel Lens

• Seen in lighthouses-used to form a concentrated
  beam of light.

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Fresnel Lens at work

• Fresnel lens melting brick

• International Automated Systems Fresnel system

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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.

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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.

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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

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How they work

• 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 cogeneration-waste steam
  could be used for space heating

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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

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Trough Pictures
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Direct Conversion of sunlight to
energyPhoto-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.

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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

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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.

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Photoelectric effect explained

• But now, a behavior of light was observed that
  fit Plancks 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.

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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.

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Silicon

• 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.

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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.

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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

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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.

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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!

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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.

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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.

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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.

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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

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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.

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Third generation alternatives

• non-semiconductor technologies (including polymer
  cells and biomimetics)
• quantum dot technologies
• 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
• hot-carrier cells
• Reduce energy losses from the absorption of
  photons in the lattice
• 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.
• 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.

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Efficiency and cost factors

• Average cost per peak watt is 1.00-3.00. Coal
  fired plant is 1.00/watt.
• Efficiency is not great.
• 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

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Novel approaches

• UA astronomer Roger Angel
• Uses cheap mirrors to focus sunlight on 3rd
  generation solar cells (triple junction cells)
  which handle concentrated light
• 1.00 per watt achievable-competitive with coal
  plants
• Potential 1 solar farm 100 miles on a side could
  provide electricity to the whole nation
• Does not have to be all in one place