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Power Generation from
Renewable Energy Sources
Fall 2012
Instructor: Xiaodong Chu
Email：[email protected]
Office Tel.: 81696127
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)
Flashbacks of Last Lecture
• The PV equivalent circuit includes both series and parallel
resistances as
  q(V  I  RS )    V  I  RS
I  I SC  I 0 exp 
 1  

kT
   RP
 



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
Flashbacks of Last Lecture
• Example 8.3 of the textbook: you should master it!
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V 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
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V 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
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V 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
VSH  Vn1  I ( RP  RS )
• The voltage of the bottom n − 1 cells will be
 n 1 
Vn 1  
V
n


• Then
 n 1 
VSH  
 V  I ( RP  RS )
n


Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V curves
• The drop in voltage ΔV at any given current I , caused by the
shaded cell, is given by
 1
V  V  VSH  V  1   V  I ( RP  RS )
 n
V 
V
 I ( RP  RS )
n
• Since the parallel resistance RP is much greater than the series
resistance RS
V 
V
 IRP
n
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V curves
• At any given current, the I –V curve for the module with one
shaded cell drops by ΔV
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V curves
• Example 8.6 of the textbook: you should master it!
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V 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 it—it 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
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V 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
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V curves
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V curves
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V 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
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V 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
Photovoltaic Materials and Electrical
Characteristics–Shading impacts on I–
V curves
Photovoltaic Materials and Electrical
Characteristics–Crystalline 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
Photovoltaic Materials and Electrical
Characteristics–Crystalline Silicon
Technologies
• On the thickness of the semiconductor
– Conventional crystalline silicon solar cells are relatively thick—on the
order of 200–500 μm
– An alternative approach to PV fabrication is based on thin films of
semiconductor—on the order of 1–10 μm
Photovoltaic Materials and Electrical
Characteristics–Crystalline 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
Photovoltaic Materials and Electrical
Characteristics–Crystalline 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 p–n junction is formed between two different
semiconductors, they are called heterojunction PVs
Photovoltaic Materials and Electrical
Characteristics–Crystalline Silicon
Technologies
• Multiple junction solar cells are made up of a stack of p–n
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%
Photovoltaic Materials and Electrical
Characteristics–Crystalline Silicon
Technologies
Photovoltaic Materials and Electrical
Characteristics–Crystalline 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
Photovoltaic Materials and Electrical
Characteristics–Crystalline 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
Photovoltaic Materials and Electrical
Characteristics–Thin-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
Photovoltaic Materials and Electrical
Characteristics–Thin-Film
Photovoltaics
• Their thinness allows photons that aren’t 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
Photovoltaic Materials and Electrical
Characteristics–Thin-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
Photovoltaic Materials and Electrical
Characteristics–Thin-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-GalliumDiselenide (CIGS)
Photovoltaic Materials and Electrical
Characteristics–Thin-Film
Photovoltaics