PV Technology1
-
Upload
amrsah61083 -
Category
Documents
-
view
25 -
download
0
description
Transcript of PV Technology1
-
6/4/2013
1
Introduction to Photovoltaic (PV) Technology
1
Eng. Firas Alawneh
Outline
History
Semiconductors
From Sand to Solar Cells
Semiconductors & Photovoltaic phenomenon
Silicon PV Cell Operation
Properties of the PV Cells
Standard Test Conditions (STC) of PV Cells &
performance parameters
Types of PV Cells
2
-
6/4/2013
2
History
3
1839 Photovoltaic effect discovered by Becquerel.
1870s Hertz developed solid selenium PV (2%).
1905 Photoelectric effect explained by A. Einstein.
1930s Light meters for photography commonly employed cells of copper oxide or selenium.
1954 Bell Laboratories developed the first crystalline silicon cell (4%).
1958 PV cells on the space satellite U.S. Vanguard (better than expected).
Semiconductors
Solar cells are fabricated using semiconductors.
Semiconductors are made from crystal and can act as conductors or insulators in different circumstances, according to the amount of energy that is given to the material.
Silicon is the most common semiconductor crystal.
4
Silicon
-
6/4/2013
3
semiconductor silicon
(hyper pure)
reduction
Solar cell processing slicing
purification(several steps)
Cast ingot
wafers
From Sand to Silicon Solar Cells
SiO2Quartz Sand MetallurgicalSilicon
Solar cell
1500-2000 C 300 C
1100C for ~200 300 hours
Photovoltaic Technology
6
Photovoltaic (PV) is the technology of converting light directly
to electrical energy (photo = light, voltaic = electricity).
Commonly known as solar cells.
The simplest systems power the small calculators we use every
day. More complicated systems will provide a large portion of
the electricity in the near future.
PV represents one of the most promising means of maintaining
our energy intensive standard of living while not contributing to
global warming and pollution.
-
6/4/2013
4
Photon Energy
7
Silicon Chemical Properties
8
Melting Point:
1410 C
Boiling Point:
2355 C
-
6/4/2013
5
9
Energy Bands for materials
Conduction
Band
Valence
Band
Metal
Conduction
Band
Valence
Band
Semiconductor
Eg
Conduction
Band
Valence
Band
Insulator
Eg
The semiconductors in general lies between metal and insulator properties, it needs a
small energy related to insulator to be in conduction band.
E
Eg(eV)
Element
1.14Silicon0.67Germanium0.1Tin0Copper
All at 20C
Photon
E = h.c/
e -
The response of the silicon due to the
incident Photons
e-e- e+e+
e- e-e+ e+
e-e- e+e+
e- e-e+ e+
Conclusion: we have to reengineer the material, so that we can separate the electrons (e) from the holes(e) to prevent the recombination inside the material.
10
-
6/4/2013
6
11
e+e-
e+e-
e+e-
e+e-
Photosensitivity?
12
2. Doping of Silicon : positive (p)
and negative (n) layers
-
6/4/2013
7
What is Doping?
13
Answer: Adding foreign atoms to the silicon crystal to produce
negative or positive free charge carries (electrons or holes).
Why Doping?
Answer: As mentioned before, electrons freed and energized by
photons will wander for a short time and then recombine with a
wandering hole. The energy originally transferred to the electron
from the photon is simply lost as heat. The key to producing
usable output current is to sweep the freed electrons out of the
material before they recombine with holes.
Doping the silicon
14
Pure silicon wafer is doped with a small amount of
another atoms at temperature (1000-1200)C, which
creates a valence bond between it and the silicon.
The most common impurity atoms are the Boron (B5)
and the Phosphorus (P15).
The Boron has three electrons in its outer level (less
than the silicon by one electron).
The Phosphorus has a five electrons in its outer level
(more than the silicon by one electron).
The Boron is doped by one atom for every 10,000,000
silicon atoms to form the P-type silicon.
The Phosphorus is doped by one atom for 1000 silicon
atoms, to form the N-type silicon.
-
6/4/2013
8
The P-type silicon
15
The silicon atom creates four
covalent bonds with other
neighboring atoms in the pure silicon
crystal.
When the crystal is doped with
Boron atoms, the silicon will make
three covalent bonds with it with the
forth bond missing, which represents
a hole (e+), so this type of
semiconductor is called P-type.
This hole is waiting for a free
electron to fill its location to create
the forth bond, so the impurity
atoms then is referred to it as
acceptor atoms.
Hole
The N-type silicon
16
Silicon is doped with Phosphorus
which has five electrons in its
outer orbit. So one electron (e-)
will be free. This type of
semiconductor is called N-type.
Phosphorous atoms (P) can donate
this electron to another bond that
needs it, so it is referred to as
donor atoms.
Electron
-
6/4/2013
9
Doping in 3D view
17
P-type N-type
18
Doping in 2D view
N-type semiconductorP-type semiconductor
-
6/4/2013
10
19
3. Photovoltaic Effect: p-n
junction operation and its parts
20
e-e- e+e+
e- e-e+ e+
Voltage Difference
Depletion region Built-in electric field
e- e+
E
e-e+e-e+
The p-n junction
Conclusion: The goal of doping is to create the depletion region to
create the electric field that separates the electrons from the holes to
produce the potential difference.
-
6/4/2013
11
Depletion Region
21
22
e-e- e-
e+
e+
e+
Solar Cell Operation
-
6/4/2013
12
23
Solar Cell Operation
Solar Cell Parts
24
(n+) & (p+) diffusions (heavily
doped silicon) used to
connect the layers with the
metal to decrease the series
resistance.
The top metal gridN layer
P layer
Top view of
the cellBottom view
of the cell
Bottom
metal
-
6/4/2013
13
Silicon Solar Cell Packaging
25
26CZ Crystallization Method
Monoc-Si
Si liquid
seed
Mono-crystalline vs. Poly-crystalline SiliconThere are two types of crystalline silicon depending on its purity and crystalsorientation obtained during the crystal growth process: Poly-crystalline: Non-uniform crystals orientation Mono-crystalline: Uniform crystals orientation (purer and more expensive andefficient)The mono-crystalline silicon ingots are prepared by the exacting Czochralski(CZ) crystal growth process (crystal pulling). While the poly-crystalline siliconingots are prepared by a simpler casting (or, more generally, directionalsolidification).
Simple Crystallization
Insulation
Electric Heaters
Poly c-Si
Si liquid
-
6/4/2013
14
27
How to distinguish between polycrystalline and monocrystalline silicon
solar cells by visual inspection?
Poly-crystalline Mono-crystalline
4. Equivalent circuit of the solar cell
and characteristic curve
28
-
6/4/2013
15
Equivalent Circuit for Solar Cell
29Real Solar cell
Standard Solar cell
Equivalent Circuit for Solar Cell
Where:
Iss : Reverse saturation current (depends on: Material, Geometry, & temperature)
q : Electron charge (1.6*10-19 C)
n : Diode quality factor
(1 for ideal diodes and >1 up to 2 for real diodes)
k : Boltzmann constant (1.38*10-23 J/K)
T: Absolute cell temperature in Kelvin degrees
For real solar cells with finite values for RS and Rsh:
30
-
6/4/2013
16
Characteristics and Power for Solar Cell
0
0.5
1
1.5
2
2.5
3
3.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Cell Voltage (V)
Cell
Curr
en
t (A
)Iph
Id
Power
IV-Curve
Isc
Voc
31
I = IPH - ID
Operating Point & Maximum Power Point
0
0.5
1
1.5
2
2.5
3
3.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Cell Voltage (V)
Cell
Curr
en
t (A)
MPP
Vmp
ImpRL
Operating Point
32
-
6/4/2013
17
5. Standard test conditions (STC)
and main performance parameters
and factors
33
Standard Test Conditions (STC)
Global Solar Irradiance (G): 1000 W/m2
Cell Temperature (T): 25 C
Air Mass (AM): 1.5
34
-
6/4/2013
18
PV Performance Parameters
Open-circuit voltage (Voc)
Short-circuit current (Isc -(Iph))
Maximum power voltage (Vmp)
Maximum power current (Imp)
Maximum power (Pmp)
Maximum Power Efficiency (max)
Fill factor ( FF )
35
Solar Cell Fill Factor
0
0.5
1
1.5
2
2.5
3
3.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Cell Voltage (V)
Cell
Curr
en
t (A)
(Vmp*Imp) Square
(Voc*Isc) Square
36
-
6/4/2013
19
Solar Cell Efficiency
The electrical output depends on the operating point of the solar cell and the incident radiant power depends on the solar radiation (perpendicular to the surface of the solar cell) and cell surface area.
The maximum efficiency of the solar cell is calculated at MPP, which is:
37
38
G Global Solar Irradiance
Area
Efficiency of Solar Cell at MPP
Input Power = G [W/m2] x Area [m2]
Output Power = Vmp [V] x Imp [A]
+
-
V
I
Resistor
Solar Cell
The efficiency of the solar cell is the ratio of electrical power output to the incident radiant power :
-
6/4/2013
20
PV Efficiency Losses
39
Optical losses: Not all of the light is absorbed because of finite reflectivity.
Use antireflective coating.
Use multilayer coating with different indices of refraction.
Further reduction is caused by light blocked by the metal grid which is
needed for electrical contacts.
Recombination losses: Many charge carriers recombine before they candiffuse to the device terminals.
Series and Shunt resistance: The bulk resistance of the semiconductorcontributes some series resistance. The shunt resistance can be caused bycrystal lattice defects in the depletion region and/or leakage currentsaround the edges of the cell.
Temperature Effect on Solar Cells
40
The parameter most affected by an increase in temperature is the open-circuit voltage (Voc). Accordingly, the power of the solar cell at theMaximum Power Point (MPP) decreases by increasing the cellstemperature.
-
6/4/2013
21
41
Temperature Effect on Solar Cells
6. Solar cells types
42
-
6/4/2013
22
43
44www.nrel.gov/pv/thin_film/docs/kaz_best_research_cells.ppt
-
6/4/2013
23
Thanks
45