Transcript Photovoltaic cells

Session 5
Photovoltaic cells
JP TEASEL VERSION 1/02/11
Photovoltaic cells
How PV Cells Work: Photons to Electrons
•Photovoltaic cells are made of high-grade silicon, a
semi-conductor.
•When sunlight shines on a PV cell electrons break
free and create an electrical current.
•When light strikes the cell, some energy is absorbed
by the semiconductor and energy is transferred.
•The energy dislodges electrons allowing them to
move freely.
•PV cells have one or more electrical fields that force
freed electrons to flow only in one direction.
•By placing metal contacts in the top and bottom if a
PV cell electric current can be drawn off.
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Atomic structure of silicon
• Under stable conditions Si: 14 protons,14
neutron,& 14 electrons
• When a photon of solar radiation strikes the
outer shell ,an electron is released
• The incoming photon loses an amount of
energy required to eject an electron from its
shell (photovoltaic effect)
• Note 1: if the photon’s energy is more than
the electron’s binding energy, the remainder
of the energy will appear as heat in the silicon
• Therefore not all solar radiation is used to
produce free electrons
• Note 2: max. conversion efficiency of silicon
PV cells is less than 40%
2 electrons in
inner shell
2 electrons in
outer shell
8electrons in
second shell
Nucleus 14
protons &
14electrons
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The photoelectric effect
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There is N-type silicon and P-type silicon,
standing for negative and positive respectively.
They are created by using different sorts of
impurities in the silicon to produce different
electrical charges. The N-type has free
electrons in the atomic structure, and the Ptype has free spaces (or holes) for electrons.
Therefore, when they are put together, an
electrical field is produced by the free
electrons in the N-type silicon going to fill the
gaps in the P-type. Not all the gaps are filled
however, but an equilibrium is reached at the
junction. At the point when photons hit the
solar cell, if it has the right amount of energy,
it pops free an electron. Then, because of the
electrical field, an electron will flow between
the N-type and the P-type, creating voltage
and current and hence electrical power.
JP TEASEL VERSION 1/02/11
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Photovoltaic cells
JP TEASEL VERSION 1/02/11
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Photovoltaic cells
For traditional PV solar
panels two halves of
one pure silicon crystal
are doped with two
different dopants (e.g.
arsenic, gallium,
aluminium,
phosphorus). One half
of the crystal is left
electron deficient - i.e.
the atoms it contains
are short of electrons.
This is called the p-type
layer. The other half of
the crystal has an excess
of electrons - this is
called the n-type layer
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PN junction,
• the half labelled p- has a shortage of electrons so
contains acceptor atoms each with a hole which could
be 'filled' by an electron.
• the half labelled n- has excess electrons and so
contains donor atoms which have an extra electron
• electrons have moved from the n-type
(negative) side to the p-type (positive) side of
the crystal recombining with holes.
• likewise holes have moved from the p-type
side to the n-type side.
•Material close to the junction in the n-type side is positive and material close to the
junction in the p-type side is negative
• a potential between the two sides of around 0.6-0.7 volts in a silicon PN junction.
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Summary PN junction
solar cells work with a semiconductor that has
been doped to produce two different regions
separated by a p-n junction.
Across this junction, the two types of charge
carrier – electrons and holes – are able to cross
and transfer their charge to the new region.
This migration of charge results in a potential
gradient or electrical slope
When sunlight strikes a solar cell, atoms are
bombarded with particles of light called
photons, and give up electrons. When an
electron is kicked out of an atom, it leaves
behind a hole, which has an equal and
opposite (positive) charge.
JP TEASEL VERSION 1/02/11
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, as described in the sections below.
Photovoltaic cells
• The I-V curve of an illuminated PV cell has the
shape shown in Figure 3 as the voltage across
the measuring load is swept from zero to VOC,
and many performance parameters for the cell
can be determined from this data,
• At the open circuit situation R=Rmax an open
circuit voltage Voc is measured and circuit
current is zero
• As the resistance decreases the electrical
current increases and voltage decreases
• Maximum current is called the short circuit
current Isc and is measured s/cct situation R=0
and the voltage is zero
• Every solar cell has a characteristic-V curve , I
sc and Voc are quoted to help characterise a cell
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Short Circuit Current (ISC)
The short circuit current ISC corresponds to the
short circuit condition when the impedance is
low and is calculated when the voltage equals
0.
I (at V=0) = ISC
ISC occurs at the beginning of the forward-bias
sweep and is the maximum current value in
the power quadrant. For an ideal cell, this
maximum current value is the total current
produced in the solar cell by photon excitation.
ISC = IMAX = Iℓ for forward-bias power quadrant
Open Circuit Voltage (VOC)
The open circuit voltage (VOC) occurs when
there is no current passing through the cell.
V (at I=0) = VOC
VOC is also the maximum voltage difference
across the cell for a forward-bias sweep in the
power quadrant.
VOC= VMAX for forward-bias power quadrant
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Courtesy National Instruments
Photovoltaic cells
Maximum Power (PMAX), Current at PMAX (IMP),
Voltage at PMAX (VMP)
The power produced by the cell in Watts can
be easily calculated along the I-V sweep by the
equation P=IV. At the ISC and VOC points, the
power will be zero and the maximum value for
power will occur between the two. The
voltage and current at this maximum power
point are denoted as VMP and IMP respectively.
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The Fill Factor (FF) is essentially a measure of quality of the solar
cell. It is calculated by comparing the maximum power to the
theoretical power (PT) that would be output at both the open circuit
voltage and short circuit current together. FF can also be interpreted
graphically as the ratio of the rectangular areas
A larger fill factor is desirable, and corresponds to an I-V sweep that is
more square-like. Typical fill factors range from 0.5 to 0.82. Fill factor
is also often represented as a percentage.
Courtesy National Instruments
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Efficiency (η)
Efficiency is the ratio of the electrical power output Pout, compared to the solar
power input, Pin, into the PV cell. Pout can be taken to be PMAX since the solar cell
can be operated up to its maximum power output to get the maximum
efficiency.
Pin is taken as the product of the irradiance of the incident light, measured in
W/m2 or in suns (1000 W/m2), with the surface area of the solar cell
[m2]. The maximum efficiency (ηMAX) found from a light test is not only an
indication of the performance of the device under test, but, like all of the I-V
parameters, can also be affected by ambient conditions such as temperature
and the intensity and spectrum of the incident light. For this reason, it is
recommended to test and compare PV cells using similar lighting and
temperature conditions
Courtesy National Instruments
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During operation, the efficiency of solar cells is reduced by the dissipation
of power across internal resistances. These parasitic resistances can be
modeled as a parallel shunt resistance (RSH) and series resistance (RS),
For an ideal cell, RSH would be infinite and would not provide an alternate
path for current to flow, while RS would be zero, resulting in no further
voltage drop before the load.
Decreasing RSH and increasing Rs will decrease the fill factor (FF) and PMAX as
shown in Figure 6. If RSH is decreased too much, VOC will drop, while
increasing RS excessively can cause ISC to drop instead.
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If incident light is prevented from exciting the
solar cell, the I-V curve shown in Figure 8 can
be obtained. This I-V curve is simply a
reflection of the “No Light” curve from Figure 1
about the V-axis. The slope of the linear
region of the curve in the third quadrant
(reverse-bias) is a continuation of the linear
region in the first quadrant, which is the same
linear region used to calculate RSH in Figure 7
Courtesy National Instruments
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It follows that RSH can be derived from the I-V plot obtained with or without
providing light excitation, even when power is sourced to the cell. It is
important to note, however, that for real cells, these resistances are often a
function of the light level, and can differ in value between the light and dark
tests.
Courtesy National Instruments
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Temperature Measurement Considerations
The crystals used to make PV cells, like all
semiconductors, are sensitive to temperature.
Figure 9 depicts the effect of temperature on
an I-V curve. When a PV cell is exposed to
higher temperatures, ISC increases slightly,
while VOC decreases more significantly.
For a specified set of ambient conditions,
higher temperatures result in a decrease in the
maximum power output PMAX
Courtesy National
Instruments
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I-V Curves for Modules
For a module or array of PV cells, the shape of the I-V curve does not
change. However, it is scaled based on the number of cells connected in
series and in parallel. When n is the number of cells connected in series
and m is the number of cells connected in parallel and ISC and VOC are values
for individual cells, the I-V curve shown in Figure 10 is produced.
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irradiance
Irradiance is measured in the units
of watts per square meter (W/m2)
and milli watts per square
centimeter (m W/cm2). “Watts” is a
measure of the power of the light
or how bright the light is. In a very
clear weather at midday (12 p.m.),
the irradiance reaching a surface
that faces the sun is about 1000
W/m2 (or 100 m W/cm2). This
irradiance of 1000 W/m2 is called
full sun, one sun or AM1 intensity
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Figure above shows I-V curves for
100, 500 and 1000 W/m2 . The I-V
curve at 1000 W/m2 is for a
module that faces the sun
directly. When the sun is exactly
overhead in tropical countries, the
module should be horizontal for
maximum current. When the sun
is low in the sky at 30° above the
horizon, the module should be
tilted towards the sun at an angle
of 60° from horizontal for
maximum current
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For the 1000 W/m2 I-V curve,
figure above shows the effect
of cell temperature. As the
temperature rises above 0 °C,
Voc falls while Isc gets slightly
higher. However, the graph
shows that the current at 16 V
decreases because of the
decrease of Voc. Therefore, to
get the maximum current
output, modules should be
mounted so that air can
circulate around them freely to
keep the cell cool.
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There are three basic types of solar cell
Crystalline solar cells are wired in series to produce solar
panels. As each cell produces a voltage of between 0.5 and 0.6
Volts, 36 cells are needed to produce an open-circuit voltage of
about 20 Volts. This is sufficient to charge a 12 Volt battery under
most conditions
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CellsIma
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Photovoltaic cells
Monocrystalline – made from a single large
crystal, cut from ingots. Most efficient, but
also the most expensive. Somewhat better in
low light conditions (but not as good as some
advertising hype would have you believe).
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Polycrystalline – basically cast blocks of silicon
which may contain many small crystals. This is
probably the most common type right
now. Slightly less efficient than single crystal,
but once set into a frame with 36 or so other
cells, the actual difference in watts per square
foot is not much.
Although the theoretical efficiency of
monocrystalline cells is slightly higher than
that of polycrystalline cells, there is little
practical difference in
performance. Crystalline cells generally have a
longer lifetime than the amorphous variety.
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Amorphous - technology is most often seen in
small solar panels, such as those in calculators
or garden lamps, although amorphous panels
are increasingly used in larger
applications. They are made by depositing a
thin film of silicon onto a sheet of another
material such as steel. The panel is formed as
one piece and the individual cells are not as
visible as in other types.
The efficiency of amorphous solar panels is not
as high as those made from individual solar
cells, although this has improved over recent
years to the point where they can be seen as a
practical alternative to panels made with
crystalline cells. Their great advantage lies in
their relatively low cost per Watt of power
generated. This can be offset, however, by
their lower power density; more panels are
needed for the same power output and
therefore more space is taken up.
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Vaporware – this is a 4th type – one that pops
up in the news once in a while proclaiming to
be the next major breakthrough that will make
plastic spray on solar cells that will cost around
5 cents a watt, or some similar claim. None
have reached production yet as of this writing.
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Cause of performance loss
Explanatiom
Typical
loss
Grid coverage
The surface of the cell has to be covered with metallic grid to collect electrons
produced by photovoltaic effect
4.0%
Reflection loss
Some of the incoming solar radiation is reflected from the front surface of the cell.
2.0%
Spurious absorption
Some of the electron rejected from their electron shell will be absorbed by impurity
atoms in the crystal
1.0%
Photon energy less than
required absorption
energy(hv>Eg)
Some of the incoming solar radiation does not have sufficient energy to eject an
electron from its solar shell
19%
Photon energy greater than
required absorption
energy(hv>Eg)
Some of the incoming solar radiation has more than enough energy to eject an
electron from its solar shell The extra energy is dissipated as heat in the crystal
28%
Quantum efficiency
Of the photons with the correct energy to eject an electron from its electron shell only
approx.90% will actually strike an electron and eject it
4.5%
Absorption not near junction
Some photon are absorbed by the crystal far from the junction. The photons create
electrons hole pairs which do nothing but immediately recombine, leaving only a little
heat as their legacy
19%
Electrical resistance
(fill factor)
The solar cell and its circuit have a small but significant electrical resistance
4.75
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Take home questions
1) Why can’t all solar radiation be used to produce free electrons when is impinges on
a solar cell?
2) What is meant by;
a) p-type silicon
b) N-type silicon
3) What is a pn junction? What happens when a n-type and p type semiconductor are
fused together
4) Describe what happens at the depletion region
5)What causes the free electrons to move when there are produced by the
photoelectric effect near the pn junction in a solar cell?
6)Briefly explain the follwing term
a) Voc
b) Isc
c) Vmp
d) Imp
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Take home questions continued………
7) Why is the output of PV cells recorded and rated at 25°C
8) Which characteristic of a solar cell is reasonably constant under varying irradiance
9) What factors affect the efficiency of solar cells?
10) List the types of solar PV cell technology currently available.
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