Power Generation from Renewable Energy Sources - PowerPoint PPT Presentation

Loading...

PPT – Power Generation from Renewable Energy Sources PowerPoint presentation | free to download - id: 827ee8-ZmM4Y



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Power Generation from Renewable Energy Sources

Description:

Title: 1 Author: User Last modified by: admin Created Date: 9/16/2011 11:37:56 AM Document presentation format: (4:3) Company – PowerPoint PPT presentation

Number of Views:7
Avg rating:3.0/5.0
Slides: 32
Provided by: educ5500
Learn more at: http://course.sdu.edu.cn
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Power Generation from Renewable Energy Sources


1
Power Generation from Renewable Energy Sources
  • Fall 2013
  • Instructor Xiaodong Chu
  • Emailchuxd_at_sdu.edu.cn
  • Office Tel. 81696127

2
Flashbacks of Last Lecture
  • There are two conditions for the actual PV and
    for its equivalent circuit
  • The current that flows when the terminals are
    shorted together (the short-circuit current, ISC)
  • The voltage across the terminals when the
    terminals are left open (the open-circuit
    voltage, VOC)

3
Flashbacks of Last Lecture
  • The PV equivalent circuit includes both series
    and parallel resistances as

4
Flashbacks of Last Lecture
  • The basic building block for PV applications is a
    module consisting of a number of pre-wired cells
    in series, all encased in tough,
    weather-resistant packages
  • Multiple modules can be wired in series to
    increase voltage and in parallel to increase
    current, the product of which is power referred
    to as an array

5
Flashbacks of Last Lecture
  • Example 8.3 of the textbook

6
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • The output of a PV module can be reduced
    dramatically when even a small portion of it is
    shaded
  • External diodes can help preserve the performance
    of PV modules
  • The main purpose for such diodes is to mitigate
    the impacts of shading on PV I V curves
  • Such diodes are usually added in parallel with
    modules or blocks of cells within a module

7
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • Consider an n-cell module with current I and
    output voltage V shows one cell separated from
    the others
  • In the sun, the same current I flows through each
    of the cells
  • In the shade, the current source of the shaded
    cell ISC is reduced to zero the voltage drop
    across RP as current flows through it causes the
    diode to be reverse biased, so the diode current
    is also zero

8
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • Consider the case when the bottom n - 1 cells
    still have full sun and still carry their
    original current I so they will still produce
    their original voltage Vn-1
  • The output voltage of the entire module VSH with
    one cell shaded will drop to
  • The voltage of the bottom n - 1 cells will be
  • Then

9
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • The drop in voltage ?V at any given current I ,
    caused by the shaded cell, is given by
  • Since the parallel resistance RP is much greater
    than the series resistance RS

10
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • At any given current, the I V curve for the
    module with one shaded cell drops by ?V

11
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • Example 8.6 of the textbook

12
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • The voltage drop problem in shaded cells could be
    to corrected by adding a bypass diode across each
    cell
  • When a solar cell is in the sun, there is a
    voltage rise across the cell so the bypass diode
    is cut off and no current flows through itit is
    as if the diode is not even there
  • When the solar cell is shaded, the drop that
    would occur if the cell conducted any current
    would turn on the bypass diode, diverting the
    current flow through that diode
  • Since the bypass diode, when it conducts, drops
    about 0.6 V, the bypass diode controls the
    voltage drop across the shaded cell, limiting it
    to a relatively modest 0.6 V instead of the
    rather large drop that may occur without it

13
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • In real modules, it would be impractical to add
    bypass diodes across every solar cell, but
    manufacturers often provide at least one bypass
    diode around a module to help protect arrays, and
    sometimes several such diodes around groups of
    cells within a module
  • These diodes do not have much impact on shading
    problems of a single module, but they can be very
    important when a number of modules are connected
    in series
  • Just as a single cell can drag down the current
    within a module, a few shaded cells in a single
    module can drag down the current delivered by the
    entire string in an array

14
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
15
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
16
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • When any of the cells are shaded, they cease to
    produce voltage and instead begin to act like
    that cause voltage to drop as the other modules
    continue to try to push current through the
    string
  • Without a bypass diode to divert the current, the
    shaded module loses voltage and the other modules
    try to compensate by increasing voltage, but the
    net effect is that current in the whole string
    drops
  • If bypass diodes are provided, current will go
    around the shaded module and the charging current
    bounces back to nearly the same level that it was
    before shading occurred

17
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
  • Bypass diodes help current go around a shaded or
    malfunctioning module within a string
  • This not only improves the string performance,
    but also prevents hot spots from developing in
    individual shaded cells
  • When strings of modules are wired in parallel, a
    similar problem may arise when one of the strings
    is not performing well
  • Instead of supplying current to the array, a
    malfunctioning or shaded string can withdraw
    current from the rest of the array
  • By placing blocking diodes, the reverse current
    drawn by a shaded string can be prevented

18
Photovoltaic Materials and Electrical
CharacteristicsShading impacts on IV curves
19
Photovoltaic Materials and Electrical
CharacteristicsCrystalline Silicon Technologies
  • There are a number of ways to categorize
    photovoltaics
  • Thickness of the semiconductor
  • Extent to which atoms bond with each other
  • Heterogeneity of p and n material

20
Photovoltaic Materials and Electrical
CharacteristicsCrystalline Silicon Technologies
  • On the thickness of the semiconductor
  • Conventional crystalline silicon solar cells are
    relatively thickon the order of 200500 µm
  • An alternative approach to PV fabrication is
    based on thin films of semiconductoron the order
    of 110 µm

21
Photovoltaic Materials and Electrical
CharacteristicsCrystalline Silicon Technologies
  • On the extent to which atoms bond with each other
    in individual crystals
  • Single crystal, the currently dominant silicon
    technology
  • Multicrystalline, in which the cell is made up
    of a number of relatively large areas of single
    crystal grains, each on the order of 1 mm to 10
    cm in size
  • Polycrystalline, with many grains having
    dimensions on the order of 1 µm to 1 mm
  • Microcrystalline, with grain sizes less than 1 µm
  • Amorphous, in which there are no single-crystal
    regions

22
Photovoltaic Materials and Electrical
CharacteristicsCrystalline Silicon Technologies
  • On whether the p and n regions of the
    semiconductor are made of the same material
  • With the same material, they are called
    homojunction PVs
  • When the pn junction is formed between two
    different semiconductors, they are called
    heterojunction PVs

23
Photovoltaic Materials and Electrical
CharacteristicsCrystalline Silicon Technologies
  • Multiple junction solar cells are made up of a
    stack of pn junctions with each junction
    designed to capture a different portion of the
    solar spectrum
  • The shortest-wavelength, highest-energy photons
    are captured in the top layer while most of the
    rest pass through to the next layer
  • Subsequent layers have lower and lower band gaps,
    so they each pick off the most energetic photons
    that they see, while passing the rest down to the
    next layer
  • Very high efficiencies are possible using this
    approach
  • Lab examples of multi-junction cells have
    demonstrated performance over 42

24
Photovoltaic Materials and Electrical
CharacteristicsCrystalline Silicon Technologies
25
Photovoltaic Materials and Electrical
CharacteristicsCrystalline Silicon Technologies
  • Concentrated photovoltaic (CPV) technology uses
    optics such as lenses to concentrate a large
    amount of sunlight onto a small area of solar
    photovoltaic materials to generate electricity
  • Unlike traditional, more conventional flat panel
    systems, CPV systems are often much less
    expensive to produce, because the concentration
    allows for the production of a much smaller area
    of solar cells

26
Photovoltaic Materials and Electrical
CharacteristicsCrystalline Silicon Technologies
  • Today, the vast majority of PV modules (85 to
    90 of the global annual market) are based on
    wafer-based crystalline silicon
  • Crystalline silicon PV modules are expected to
    remain a dominant PV technology until at least
    2020, with a forecasted market share of about 50
    by that time

27
Photovoltaic Materials and Electrical
CharacteristicsThin-Film Photovoltaics
  • Conventional crystalline silicon technologies
    require considerable amounts of expensive
    material with additional complexity and costs
    needed to wire individual cells together
  • Competing technologies are based on depositing
    extremely thin films of photovoltaic materials
    onto glass or metal substrates
  • Thin-film devices use relatively little material,
    do not require the complexity of cell
    interconnections, and are particularly well
    suited to mass-production techniques

28
Photovoltaic Materials and Electrical
CharacteristicsThin-Film Photovoltaics
  • Their thinness allows photons that arent
    absorbed to pass completely through the
    photovoltaic material, which offers two special
    opportunities
  • Their semitransparency means that they can be
    deposited onto windows, making building glass a
    provider of both light and electricity
  • They also lend themselves to multiple-junction,
    tandem cells in which photons of different
    wavelengths are absorbed in different layers of
    the device

29
Photovoltaic Materials and Electrical
CharacteristicsThin-Film Photovoltaics
  • Currently, thin-film cells are not as efficient
    as crystalline silicon especially when they are
    not used in tandem devices
  • While the likelihood of significant reductions
    in module costs are modest for conventional
    crystalline silicon, many opportunities remain to
    increase efficiency and dramatically reduce costs
    using thin-film technologies

30
Photovoltaic Materials and Electrical
CharacteristicsThin-Film Photovoltaics
  • Thin films currently account for 10 to 15 of
    global PV module sales
  • They are subdivided into three main families
  • Amorphous (a-Si) and micromorph silicon
    (a-Si/µc-Si)
  • Cadmium-Telluride (CdTe)
  • Copper-Indium-Diselenide (CIS) and
    Copper-Indium-Gallium-Diselenide (CIGS)

31
Photovoltaic Materials and Electrical
CharacteristicsThin-Film Photovoltaics
About PowerShow.com