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Solar Cells

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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. – PowerPoint PPT presentation

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Title: Solar Cells


1
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.

2
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.

3
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.

4
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

5
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.

6
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.

7
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8
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.
  • 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

9
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

10
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

11
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