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Title: Solar%20Electric%20Power%20generation

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

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?

Basic properties of light
  • To answer this question, lets look at some basic
    properties of light in the wave description of
  • 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
  • n1sin?1n2sin?2

Focusing light
  • If the interface is flat, the light is not
  • 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

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

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

Fresnel Lens at work
  • Fresnel lens melting brick
  • International Automated Systems Fresnel system

  • When light is incident on a surface, it can be
  • 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

Power towers
  • Use many collectors and focus the light to a
    central point.
  • Achieves high temperatures and high power
  • Each individual collector is called a heliostat
  • Must be able to track the sun and focus light on
    the main tower

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
  • Often include auxiliary energy storage to
    continue to produce electricity in the absence of
  • More costly to construct and operate than coal
    fired plants.
  • Good candidates for cogeneration-waste 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
  • Photoelectric effect
  • When electromagnetic energy impinges upon a
    metal surface, electrons are emitted from the
  • 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

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
  • 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.

  • 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

  • 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
  • Called a P-type semiconductor.
  • Doping elements could be boron, aluminum, or

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

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

Generations of Solar cells
  • First generation
  • large-area, high quality and single junction
  • 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
  • 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
  • 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
  • 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
  • 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
  • The second and third generation technologies
    discussed are designed to increase these
    efficiency numbers and reduce manufacturing costs

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
  • 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
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