PV conversion [3,82 MiB]

PV conversion [3,82 MiB]
Photovoltaic conversion
Electromagnetic radiation
2
Solar Spectral Power Density
3
Solar Spectral Power Density
4
Solar Photon Flux Density
5
Solar Irradiance
6
PV Cells, modules, arrays
•A photovoltaic cell is a
semiconductor device that
converts solar radiation into
direct current electricity.
•Because the source of
radiation is usually the sun,
they are often referred to as
solar cells.
•The basic building blocks for
PV systems include cells,
modules, and arrays.
7
Semiconductor materials
•Most PV cells use variations of
silicon altered by doping to
make them suitable
semiconductors.
•Semiconductor materials with
special electrical properties
can be made by adding small
amounts of other impurity
elements to silicon crystals.
•The process is called doping
8
Semiconductor materials
• Pure crystalline silicon has four valence (outer) electrons that each
bond with the outer electrons of other silicon atoms to form a
crystalline structure.
• When a small amount of boron, which has three valence electrons,
is added to silicon crystals, the boron atoms take the places of a few
silicon atoms. The crystalline structure where boron bonds to silicon
then has an electron void at the location where a fourth electron is
absent. This void is also called a hole, since it can be filled by other
electrons. This absence of a negative charge is considered a positive
charge carrier. A p-type semiconductor is a semiconductor that has
electron voids.
• When a small amount of phosphorus, with five valence electrons is
used, surplus of electrons is obtained. A n-type semiconductor is a
semiconductor that has surplus of electrons.
9
Photoelectric effect
• The basic physical process by which a PV cell converts
light into electricity is known as the photovoltaic effect.
• The photovoltaic effect is the movement of electrons
within a material when it absorbs photons with energy
above a certain level.
• Photons contain various amounts of energy depending
on their wavelength, with higher energies associated
with shorter wavelengths (higher frequency).
• Photons of light transfer their energy to electrons in
the material surface. The extra electrons with enough
energy to escape from their atoms are conducted as
an electric current. Because of the electric result, the
photovoltaic effect is also sometimes called the
photoelectric effect.
10
Photovoltaic effect
•The photovoltaic effect produces free electrons that must
travel through conductors in order to recombine with
electron voids, or “holes.”
11
PV Materials
•Various PV materials and technologies produce different efficiencies.
•Crystalline silicon (c-Si) cells currently offer the best ratio of
performance to cost compared to competing materials and utilize
many of the same raw materials and processes as the semiconductor
industry.
•Significant research is directed toward developing new PV cell
material technologies, as well as improving efficiency and reducing
costs of existing technologies.
12
PV Technology
13
14
Physics of Solar Cells
• Semiconductor material can be p-type (hole
carriers) or n-type (electron carriers)
P+
n-type
•
•
•
•
B-
p-type
N-type has impurities with an extra electron (phosphorus)
P-type has impurities with one fewer electron (boron)
Put them together: p-n junction
A solar cell is a very large p-n junction (or diode)
Basic Physics of Solar Cells
P+
n-type
e
B-
h
p-type
• The holes from the p-type side diffuse to the n-type
side.
• The electrons diffuse to the p-type side.
• This leaves behind charged ions (missing electrons
or holes).
Built-In Electric Field
• The charged atoms (ions) create an electric
field.
• This electric field makes it easy for current to
flow in one direction, but hard to flow in the
opposite direction.
n-type
P+
B-
P+
B-
P+
B-
E-field
p-type
Generating Charges From The Sun
• Light breaks silicon bonds and creates “free” electrons and holes
“missing electrons”
• Holes are positive charges
• Built-in field separates electrons and holes
n-type
P+
B-
eP+
B-
P+
B-
E-field
h
p-type
Generating Charges From The Sun
• Connect diode to a circuit
• Photocurrent goes through resistor
• Causes a voltage drop
P+
eP+
n-type
P+
B-
BB-
h
p-type
E-field
V=IR
IPC
Generating Charges From The Sun
• Forward biases the diode
• Causes a current in opposite direction
-V
n-type
P+
B-
P+
B-
Pe+
B-
+V
h
p-type
IFB
V=IR
IPC
Generating Charges From The Sun
•
•
•
•
If R is very large, V is very large
If V is very large, IFB = IPC
I=0
Open Circuit condition
-V
n-type
P+
B-
P+
B-
e
P+
+V
h
B-
p-type
IFB=IPC
V=IR
IPC
Generating Charges From The Sun
•
•
•
•
If R is very small, V is very small
If V = 0, IFB = 0
I= IPC
Short Circuit condition
-V
n-type
P+
B-
P+
B-
eP+
B-
+V
h
p-type
IFB=0
V=IR=0
IPC
Cell Structure
23
Light incident on the cell
creates electron-hole pairs
24
Cell Structure
25
Cell Structure
26
Solar cell operating principles
1. Absorption of photons ⇒ generation of electron-hole pairs
2. Separation of carriers in the internal electric field created by
p-n junction and collection at the electrodes ⇒
potential difference and current in the external circuit
3. Potential difference at the electrodes of a p-n junction ⇒
injection and recombination of carriers ⇒ losses
The resulting current in the external circuit: I
• photocurrent IL
• dark (diode) current ID
= IL - ID (V)
External parameters of a solar cell
I-V measurement
Standard test conditions:
• AM1.5 spectrum
• irradiance 1000 W/m2
• temperature 25°C
External parameters:
• Short circuit current Isc [A]
• Open circuit voltage Voc [V]
• Fill factor ff
• Maximum (peak) power Pmax [Wp]
• Efficiency η
Open circuit
voltage
Voc [V]
Peak Power
Pmax [Wp]
Short circuit current Isc [A]
Pmax = Vmp I mp = ff Voc Isc
η = Pmax PI = ff Voc Isc PI
Solar cell performance
Single junction solar cell:
Typical commercial c-Si solar cell
sunlight
solar cell
waste
heat
- 56% colour mismatch
- 9% reflection & transmission
- 13% fundamental recombination
electricity
- 7% excess recombination,
resistance, etc
15%
Solar cell performance
Optical losses:
Non-absorption
Thermalization
Reflection
Transmission
Area loss
Collection losses:
Recombination
- bulk
- surface
Φ 0 (λ )
(1 − R )
Af
At
QE el
ηg QE opt
Solar cell performance
Optical losses: Non-absorption
Non-absorption Eph<EG
EC
λg
Eph
Non-absorption λ g < λ ph
EV
λg
hc
0
(
)
dλ
Φ
λ
∫
λ
0
∞
∞
PI = ∫ Φ 0 (λ )
0
Φ 0 (λ )
hc
dλ
λ
Photon flux density: number of photons per
unit area per unit time and unit wavelength
hc
0
(
)
dλ
Φ
λ
∫
λ
0
EG
Solar cell performance
Optical losses: Thermalization
Thermalization
λg
λ g > λ ph
Thermalization Eph>EG
EC
EG
Eph
EV
λg
Eg
0
Φ
∫ (λ ) dλ
0
λg
∞
PI = ∫ Φ 0 (λ )
0
Φ 0 (λ )
hc
dλ
λ
Photon flux density: number of photons per
unit area per unit time and unit wavelength
hc
0
(
)
Φ
λ
dλ
∫
λ
0
Solar cell performance limits
EC
EC
Eph
EG
EV
EG
Eph
EV
Non-absorption Eph<EG
Thermalization Eph>EG
λg
λg
hc
(
)
Φ
λ
dλ
∫
λ
0
0
∞
hc
(
)
Φ
λ
dλ
∫
λ
0
0
Eg
0
Φ
∫ (λ ) dλ
0
λg
hc
0
(
)
Φ
λ
dλ
∫
λ
0
About 55% of solar energy is not usable by PV cells
Solar cell performance
Optical losses: Reflection and transmission
Φ 0 (λ )
(1 − R )
Reflection:
• Different refractive indices
Af
At
Transmission:
• finite thickness of a cell
• absorption coefficient
Area loss:
• metal electrode coverage
QE el
ηg QE opt
Solar cell performance
Collection losses: Recombination
Φ 0 (λ )
(1 − R )
Af
At
Recombination:
• bulk recombination
(minority carrier lifetime)
• surface recombination
(surface recombination
velocity)
λg
QE el
J max = q ∫ Φ 0 (λ ) dλ
ηg QE opt
0
Af
J sc = J max (1 − R ) QE opt η g QE el
At
Solar cell performance
∞
Efficiency:
PI = ∫ Φ0 ( λ )
0
J V ff
η = sc oc
PI
hc
dλ
λ
λg
A
J sc = (1 − R ) QE opt ηg QE el f q ∫ Φ 0 ( λ ) dλ
At 0
λg
q ∫ Φ0 ( λ ) dλ
η= ∞
0
0
Φ
∫ (λ)
0
η=
hc
dλ
λ
(1-R ) ηg QEopt QEel
λg
λg
0
0
λg
E G ∫ Φ0 ( λ ) dλ
∞
hc
∫0 Φ ( λ ) λ dλ
0
0
Φ
∫ (λ)
hc
dλ
λ
0
Φ
∫ (λ)
hc
dλ
λ
0
Af
Voc ff
At
(1-R ) ηg QEopt QEel
Af q Voc
ff
A t EG
Solar cell performance limits
λg
η=
λg
hc
0
(
)
Φ
λ
dλ
∫0
λ
E g ∫ Φ 0 (λ ) dλ
∞
λg
hc
(
)
Φ
λ
dλ
∫0
λ
0
0
0
Φ
∫ (λ )
0
hc
dλ
λ
⎛ q Voc ⎞
Af
⎟ ff
(1 − R ) η g QE opt QE el ⎜⎜
⎟
At
⎝ Eg ⎠
1. Loss by long wavelengths
2. Loss by excess energy of photons
3. Loss by metal electrode coverage
4. Loss by reflection
5. Loss by incomplete absorption due to the finite thickness
6. Loss due to recombination
7. Voltage factor
8. Fill factor
Overstraeten, Mertens: Physics, technology and Use of Photovoltaics, Adam Hilger 1986
Solar cell performance
Optical losses:
Non-absorption
Thermalization
Reflection
Transmission
Area loss
Collection losses:
Recombination
- surface
- bulk
Properties:
Optical gap
Optical gap
Refractive indices
Absorption coefficient
Metal grid design
Surface recombination velocity
Minority carriers lifetime
Diffusion coefficient
Solar cell performance
Optimal design
(
Total current: I T = I 0 e
Short circuit current (V=0):
ISC = − I L
qV kT
)
−1 − IL
Open circuit voltage (I=0):
VOC
kT ⎛ I L ⎞
=
ln⎜⎜ + 1⎟⎟
q ⎝ I0
⎠
High Isc :
Low I0:
• Minimize front surface reflection
- antireflection coatings
• Minimize transmission losses
- thick absorber
• Minimize surface recombination
- passivation layers
• Minimize bulk recombination
- large diffusion lengths
- high electronic quality material
• High doping densities
• Low surface recombination
velocities
• Large diffusion lengths
⎛ q Dn ni2 q D p ni2 ⎞
⎟
+
I0 = A⎜
⎜ L N
L p N D ⎟⎠
⎝ n A
Solar cell performance
Optimal thickness of the absorber layer:
Absorption versus collection:
- Thickness of the absorber layer
- Minority carrier diffusion length
Al
Al
SiO2
n+
Le
p-type
c-Si
p++
p++
Al
c-Si (300 µm)
Solar cell performance
Optimal thickness of the absorber layer:
Absorption versus collection:
- Thickness of the absorber layer
- Minority carrier diffusion length
Al
Al
SiO2
n+
Le
Le
p-type
c-Si
p++
p++
Al
Solar cell performance
Optimal thickness of the absorber layer:
Absorption versus collection:
- Thickness of the absorber layer
- Minority carrier diffusion length
Al
Al
SiO2
n+
Le
p++
p++
p-type
c-Si
Al
p++
p++
Al
Solar cell performance
Optimal thickness of the absorber layer:
Absorption versus collection:
- Thickness of the absorber layer
- Minority carrier diffusion length
Al
Al
SiO2
n+
p-type
c-Si
p++
p++
Al
Solar cell performance
Thin absorber layer:
Increase absorption:
- Surface texture
- Antireflection coating
Al
Al
n+
SiO
2
Avoid surface recombination:
- Surface passivation
p++
p++
Al
Solar cell performance
Equivalent circuit:
I-V characteristics
+
IL
1
2
V
Current
I
ID
-
VOC
Voltage
IT
• current source IL
• 1 diode diffusion current
• 2 diode recombination current
ISC
IL
Solar cell performance
Equivalent circuit:
I
RS
IL
1
• series resistor RS
• parallel resistor Rsh
2
Rsh
+
V
-
Solar cell performance
RP
RS
Series resistance (RS)
Shunt (parallel) resistance (RP)
• Bulk resistance of semiconductor
• Bulk resistance of metal electrodes
• Contact resistance between
semiconductor and metal
• Leakage across the p-n junction around
the edge
• Crystal defects, pinholes, impurity
precipitates
Solar cell performance
Total current:
I T = I 0 (T ) e q (V + IT RS ) kT − 1 − I L
(
)
Saturation current:
kT
−E
I0 = K T 3 e g0
Open circuit voltage:
E g 0 kT ⎛ kT 3 ⎞
⎟⎟
VOC (T ) =
−
ln⎜⎜
q
q ⎝ IL ⎠
⎤
dVOC
1 ⎡ Eg 0
=− ⎢
− VOC (T )⎥
dT
T⎣ q
⎦
o
Si dVOC dT = −2.3 mV C
External parameters
η
1.0
VOC [V]
0.9
[%]
11
10
9
0.8
ff
0.7
8
7
0.6
6
0
300
100
200
Temperature [oC]
First c-Si solar cell
First c-Si solar cell
p-type Si
• wrap-around structure
• p-n junction formed by B
dopant diffusion
• high resistive losses in the player
• efficiency 6%
n-type Si
(-)
(+)
Fabricated in 1954
(+)
c-Si solar cell
c-Si solar cell: Efficiency improvement
University of New
South Wales (Australia)
Record c-Si solar cell
c-Si solar cell: PERL structure (UNSW)
External parameters (1994):
Passivated Emitter and
Rear Locally diffused
•
•
•
•
Jsc =40.9 mA/cm2
Voc =0.709 V
ff = 0.827
η = 24.0 %
Record c-Si solar cell
Key attributes for high efficiency solar cells:
• Surface texture (inverted pyramids for light trapping)
• Selective emitter (n+-layer for contact, n-layer for active part of
surface)
• Passivation of surface (SiO2 on both sides of solar cell)
• Thin metal fingers on the front side
• Back side metalization with small contact area to the base
material
• Locally diffused regions under contact points at the back
(BSF field)
• Minority diffusion lengths well in excess of device
thickness
PV Electrical Scheme
• A PV device can be modeled
by a current source in
parallel with a diode, with
resistance in series and
shunt (parallel).
• Both series and shunt
resistances have a strong
effect on the shape of the IV curve.
27
Cell Characterization
28
I-V Characteristics
•An I-V curve is the graphic
representation of all possible voltage
and current operating points for a PV
device at a specific operating
condition.
•An I-V curve illustrates the electrical
output profile of a PV cell, module,
or array at a specific operating
condition.
•As voltage increases from zero, the
current begins at its maximum and
decreases gradually until the knee of
the curve is reached. After the knee,
small increases in voltage are
associated with larger reductions in
current, until the current reaches
zero and the device is at maximum
voltage.
29
Open-Circuit Voltage
•The open-circuit voltage (Voc) is
the maximum voltage on an I-V
curve and is the operating point
for a PV device under infinite
load or open-circuit condition,
and no current output.
•Since there is no current at the
open-circuit voltage, the power
output is also zero.
•The open-circuit voltage is used
to determine maximum circuit
voltages for modules and arrays.
•The open-circuit voltage of a PV
device can be measured by
exposing the device to sunlight
and measuring across the output
terminals with a voltmeter or a
multimeter set to measure DC
voltage.
30
Short-Circuit Current
•The short-circuit current is maximum
current PV cell give when exposed to
sunlight.
• If the short-circuit current is less than
the fused current rating of the meter
(typically 1 A or 10 A), the test leads
can be connected to the output
terminals. The meter short-circuits the
PV device with a very small resistance
and measures the resulting current.
•If the current is expected to be close
to or higher than the meter rating, this
in-line method should not be used.
Instead, a conductor with a switch is
used to short-circuit the output
terminals and a clamp-on ammeter is
put around the conductor to measure
the resulting current.
31
Maximum Power Point
• The operating point at which a
PV device produces its
maximum power output lies
between the open-circuit and
short-circuit condition, when
the device is electrically
loaded at some finite
resistance.
• The maximum power point
(Pmp) is the operating point on
an I-V curve where the
product of current and voltage
is at maximum.
• Maximum power is often
called peak power and the
parameter may be designated
by Wp for “peak watts.”
32
Fill factor
•Fill factor (FF) is the ratio of
maximum power to the product
of the open-circuit voltage and
short-circuit current.
•Fill factor represents the
performance quality of a PV
device and the shape of the I-V
curve.
•A higher fill factor indicates that
the voltage and current at the
maximum power point are closer
to the open-circuit voltage and
short-circuit current, respectively,
producing a more rectangularshaped I-V curve.
33
PV Efficiency
• Efficiency is the ratio of
electric power output to solar
power input.
• Solar irradiance is multiplied
by the area of the PV device to
determine watts of solar
power, which can then be
directly compared to watts of
electrical power.
• PV cell efficiencies vary
considerably among different
PV technologies, and for the
same material and technology,
efficiencies vary widely
between laboratory samples
and commercial devices.
34
Series Resistance
•
•
•
•
•
•
•
Series resistance in PV devices includes
the resistance of a cell, its electrical
contacts, module interconnections, and
system wiring.
These resistances are in addition to the
resistance of the electrical load.
Series resistance in a PV system is
unavoidable because all conductors and
connectors have some resistance.
Increasing series resistance over time
can indicate problems with electrical
connections or cell degradation.
Series resistance reduces the voltage
over the entire I-V curve.
Increasing series resistance also
decreases maximum power, fill factor,
and efficiency.
If a PV device is operated at constant
voltage (such as for battery charging),
increasing series resistance results in
decreasing operating current.
35
Shunt Resistance
• Shunt (parallel) resistance
accounts for leakage
currents within a cell,
module, or array.
• Shunt resistance has an
effect on an I-V curve
opposite to the effect of
series resistance.
• Decreasing shunt
resistance reduces fill
factor and efficiency, and
lowers maximum voltage,
current, and power, but
does not affect short-circuit
current.
• Decreasing shunt
resistance over time can
indicate short-circuits
between cell circuits and
module frames, or ground
faults within an array.
36
Solar Irradiance Response
•Voltage increases rapidly up to
about 200 W/m2, and then is
almost constant. Current and
maximum power increase
proportionally with irradiance.
•Changes in solar irradiance have
a small effect on voltage but a
significant effect on the current
output of PV devices.
•The current of a PV device
increases proportionally with
increasing solar irradiance.
•Consequently, since the voltage
remains nearly the same, the
power also increases
proportionally.
37
Temperature Response
• For most types of PV devices,
high operating temperatures
significantly reduce voltage
output.
• Current increases with
temperature, but only slightly,
so the net result is a decrease
in power and efficiency.
• Long-term high temperatures
can also lead to premature
degradation of cells and
module encapsulation.
• It is desirable to install
modules and arrays in a
manner that allows them to
operate as cool as possible.
38
Module Construction
•
•
•
•
A module is a PV device consisting
of a number of individual cells
connected electrically, laminated,
encapsulated, and packaged into a
frame.
The PV cells are laminated within a
polymer (plastic) substrate to hold
them in place and to protect the
electrical connections between
cells.
The cell laminates are then
encapsulated (sealed) between a
rigid backing material and a glass
cover.
Some thin-film laminates use
flexible materials such as aluminum
or stainless steel substrate and
polymer encapsulation instead of a
glass cover.
39
Array
• An array is a complete PV
power-generating unit
consisting of a number of
individual electrically and
mechanically integrated
modules with structural
supports, trackers, or other
components.
40
PV Panel
•The term “panel” is also used
in relation to modules and
arrays.
•Sometimes panel is used as
an alternate term for a
module.
•More commonly, the term
panel refers to an assembly of
two or more modules that are
mechanically and electrically
integrated into a unit for ease
of installation in the field.
41
Series Connection
•
•
•
•
PV cells or modules are connected
in series strings to build voltage.
Individual cells are connected in
series by soldering thin metal strips
from the top surface (negative
terminal) of one cell to the back
surface (positive terminal) of the
next.
Modules are connected in series
with other modules by connecting
conductors between the negative
terminal of one module to the
positive terminal of another
module.
When individual devices are
electrically connected in series, the
positive connection of the whole
circuit is made at the device on one
end of the string and the negative
connection is made at the device on
the opposite end.
42
Series Connection
•The overall I-V characteristics of a series string are dependent on the
similarity of the current outputs of the individual PV devices.
•Only PV devices having the same current output should be connected
in series.
•When similar devices are connected in series, the voltage output of
the entire string is the sum of the voltages of the individual devices,
while the current output for the entire string remains the same as for
a single device.
•Correspondingly, the I-V curve for a string of similar PV devices is the
sum of the I-V curves of the individual devices.
43
Parallel Connection
•Strings of PV cells or modules are connected in parallel to build current.
•Parallel connections are not generally used for individual PV devices,
especially cells, but for series strings of cells and modules.
•Parallel connections involve connecting the positive terminals of each
string together and all the negative terminals together at common
terminals or busbars.
44
Parallel Connection
•The overall I-V curve of PV
devices in parallel depends on the
similarity of the current outputs
of the individual devices.
•When similar devices are
connected in parallel, the overall
circuit current is the sum of the
currents of individual devices or
strings.
•The overall voltage is the same
as the average voltage of all the
devices connected in parallel.
45
Bypass Diodes
•Bypass diodes allow current to flow around devices that develop an
open-circuit or high-resistance condition.
•A bypass diode is a diode used to pass current around, rather than
through, a group of PV cells, that are shaded or develop an opencircuit or other high resistance condition, preventing an interruption
of the continuity of the string.
•This allows the functional cells or modules in the string to continue
delivering power at a lower voltage.
46
Bypass Diodes
•A bypass diode limits reverse current
through PV devices, preventing excessive
power loss and overheating.
•Without a bypass diode, reverse voltage
may decrease until the breakdown voltage
is reached.
•Breakdown voltage is the minimum
reverse-bias voltage that results in a rapid
increase in current through an electronic
device.
•The high currents can result in potentially
damaging levels of power dissipation within
the module, and under extreme cases, the
resulting high temperatures can melt the
module laminate and pose a fire hazard.
•A bypass diode allows a reverse bias of
only 0.7 V, which limits the reverse voltage
to a level where only a small amount of
power may be dissipated.
47
Building an Array
•Modules are added in series
to form strings or panels,
which are then combined in
parallel to form arrays.
•The modules or groups of
modules are integrated to
form a complete array, using
additional series or parallel
connections.
•The result is a complete array
that integrates all the modules
into a single power-generating
unit, with one positive
terminal and one negative
terminal for connection to
other components.
48
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