renewable energy lecture help for other allah will help you in life

1. EE421 Renewable Energy Systems
Solar PV System
Mr. Ijaz Husnain

2. Photovoltaics (PV)
Photovoltaic definition- a material or device that is capable
of converting the energy contained in photons of light into an
electrical voltage and current
Rooftop PV
modules on a
village health
center in West
Bengal, India
http://www1.eere.energy.gov/solar/pv_use.html
"Sojourner"
exploring Mars,
1997
University of Illinois Solar
Decathalon House – 2nd place
overall in 2009
http://www.solardecathlon.uiuc.edu/gallery.html#
2

3. PV History
• Edmund Becquerel (1839) observed PV effect
• Adams and Richard Evans Day (1876) observed that
illuminating a junction causes photo voltaic effect
• Albert Einstein (1904)
• Czochralski (1940s) pronounced as chokh-RAHL-skee
• Vanguard I satellite (1958)
• Costs have recently dropped quite substantially
3

4. PV System Overview
Shadows
• Solar cell is a diode
• Photopower coverted to DC
• Shadows & defects convert
generating areas to loads
• DC is converted to AC by an
inverter
• Loads are unpredictable
• Storage helps match
generation to load
4

5. Some General Issues in PV
• The device
• Efficiency, cost, manufacturability
automation, testing
• Encapsulation
• Cost, weight, strength, yellowing, etc.
Yellowing” of PV modules is defined as the optical
degradation of the ethyl vinyl acetate (EVA) where
the clear encapsulant becomes visibly yellow or even brown.
• Accelerated lifetime testing
• 30 year outdoor test is difficult
• Damp heat test, light soaking test, etc.
• Inverter & system design
• Micro-inverters, blocking diodes, reliability
5

6. 6
Damp Heat Test machine
Problems that can be identified with the pv damp
heat test are:
• Corrosion
• De-lamination
• Junction box and module
connection failures

7. What are Solar Cells?
Current
Voltage
Open-circuit
voltage
Short-circuit
current
Maximum
Power Point
n-type
p-type
-
+
Load
• Solar cells are diodes
• Light (photons) generate free
carriers (electrons and holes)
which are collected by the
electric field of the diode
junction
• The output current is a fraction
of this photocurrent
• The output voltage is a
fraction of the diode built-in
voltage
7

8. Standard Equivalent Circuit Model
Photocurrent
source
Diode
Shunt
resistance
Load
Series
resistance
Where does the power go?
(minimize)
(maximize)
8

9. Photons
• Photons are characterized by their wavelength
(frequency) and their energy
• In this context energy is often expressed in
electronvolts (eV), which is defined as 1.6 10-19 J
– This is the amount of energy gained by a single electron
moving across a voltage difference of one volt
c v


hc
E hv

 
9
Planck's constant (h) is
6.626 10-34 J-s
Velocity of light (c) is
3108 m/s
1.242
, EV
m
hc
E E

 
 
Above equation for E can be
rewritten with E in eV and  in m

10. Band-Gap Energy
• Electrical conduction is caused by free electrons
(electrons in conduction band)
• At absolute zero temperature metals have free electrons
available and hence are good conductors
– Metal conductivity decreases
with increasing temperature
• Semi-conductors, such as
silicon, have no free
electrons at absolute zero
and hence are good
insulators
10

11. Band-Gap Energy, cont.
• Silicon conductivity increases with increasing
temperature; at room temperature they only have
about 1 in 1010 electrons in the conduction band
• Band gap energy is the energy an electron must
acquire to jump into the conduction band
11
Quantity Si GaAs CdTe InP
Band gap (eV) 1.12 1.42 1.5 1.35
Cut-off wavelength (μm) 1.11 0.87 0.83 0.92
Band Gap and Cut-off Wavelength Above
Which Electron Excitation Doesn’t Occur
1.242
EV
m
E




12. Silicon Solar Cell Max Efficiency
• Wavelengths above cutoff have no impact
• This gives an upper bound on the efficiency of a
silicon solar cell:
– Band gap: 1.12 eV, Wavelength: 1.11 μm
• This means that photons with wavelengths longer
than 1.11 μm cannot send an electron into the
conduction band.
• Photons with a shorter wavelength but more energy
than 1.12 eV dissipate the extra energy as heat
12

13. Silicon Solar Cell Max Efficiency
• For an Air Mass
Ratio of 1.5,
49.6% is the
maximum
possible fraction
of the sun’s energy
that can be
collected with a
silicon solar cell
• Efficiencies of
real cells are in the
~20-25% range 13

14. Solar Cell Efficiency
• Factors that add to losses
– Recombination of electrons/holes
– Internal resistance
– Photons might not get absorbed, or they may get reflected
– Heating
• Smaller band gap - easier to excite electrons, so more
photons have extra energy
– Results in higher current, lower voltage
• High band gap – opposite problem
• There must be some middle ground since P=VI (DC),
this is usually between 1.2 and 1.8 eV
14

15. Maximum Efficiency for Cells
15
• The maximum efficiency of single-junction PV
cells for different materials was derived in 1961 by
Shockley and Queisser

16. Review of Diodes
• Two regions: “n-type” which donate
electrons and “p-type” which accept
electrons
• p-n junction- diffusion of electrons
and holes, current will flow readily in
one direction (forward biased) but not
in the other (reverse biased), this is
the diode
http://en.wikipedia.org/wiki/File
:Pn-junction-equilibrium.png 16

17. The p-n Junction Diode
• Can apply a voltage Vd and get a current Id in one
direction, but if you try to reverse the voltage
polarity, you’ll get only a very small reverse
saturation current, I0
• Diode voltage drop
is about 0.6 V
when conducting
17

18. The p-n Junction Diode
Voltage-Current (VI) characteristics for a diode
/
0 ( -1)
d
qV kT
d
I I e

38.9
0 ( -1) (at 25 C)
d
V
d
I I e
 
k = Boltzmann’s constant
1.381x10-23 [J/K]
T = junction temperature [K]
Vd = diode voltage
Id = diode current
q = electron charge 1.602x10-19 C
I0 = reverse saturation current
18

19. Circuit Models of PV Cells
• The simplest model of PV cell is an ideal current
source in parallel with a diode
• The current provided by the ideal source ISC is
proportional to insolation received
• If insolation drops by 50%, ISC drops by 50%
ISC
Id
I
VLoad
+
-
I
VLoad
+
-
PV
cell
Vd
+
-
19

20. Circuit Models of PV Cells
• From KCL, ISC = Id + I, and the current going to the
load is the short-circuit current minus diode current
• Setting I to zero, the open circuit voltage is
/
0 ( -1)
qV kT
SC
I I I e
 
0
ln 1
SC
OC
I
kT
V
q I
 
 
 
 
I
ISC
Id
VLoad
+
-
Vd
+
-
20
The subscript SC
is for short circuit

21. PV Cell I-V Characteristic
• For any value of ISC , we can calculate the
relationship between cell terminal voltage and
current (the I-V characteristics)
• Thus, the I-V characteristic for a PV cell is the
diode I-V characteristic turned upside-down and
shifted by ISC (because I = ISC – Id)
• The curve intersects the x-axis at VOC and the y-axis
is ISC
/
0 SC
( -1)= f(I ,V)
qV kT
SC
I I I e
 
21

22. PV Cell I-V Curves
• I-V curve for an illuminated cell = “dark” curve + ISC
dark curve
22

23. PV Cell I-V Curves
• More light effectively shifts the curve up in I, but
VOC does not change much
• By varying the insolation, we obtain not a single I-
V curve, but a collection of them
23

24. Need for a More Accurate Model
• The previous circuit is not realistic for analyzing
shading effects
• Using this model, absolutely NO current can pass
when one cell is shaded (I = 0)
24

25. PV Equivalent Circuit
• It is true that shading has a big impact on solar cell
power output, but it is not as dramatic as this
suggests! Otherwise, a single shaded cell would
make the entire module’s output zero.
• A more accurate model includes a leakage
resistance RP in parallel with the current source and
the diode
• RP is large, ~RP > 100VOC/ISC
• A resistance RS in series accounts for the fact that
the output voltage V is not exactly the diode voltage
• RS is small, ~RS < 0.01VOC/ISC
25

26. PV Equivalent with Parallel Resistor
• From KCL, I = ISC - Id – IRP
• For any given voltage, the parallel leakage
resistance causes load current for the ideal model to
be decreased by V/RP
( )
SC d
P
V
I I I
R
  
Photocurrent
source
R
P
Load
(maximize)
Parallel-Only
ISC
I
V
+
-
Vd
+
-
Id
Shunt resistance drops
some current (reduces
output current)
26

27. PV Equivalent with Series Resistor
• Add impact of series resistance RS (we want RS to
be small) due to contact between cell and wires and
some from resistance of the semiconductor
• From KVL
• The output voltage V drops by IRS
d S
V V I R
  
Photocurrent
source
Load
Rs
(minimize)
Series-Only
ISC Vd
+
-
V
+
-
I
Id
Series resistance drops
some voltage (reduces
output voltage)
27

28. General PV Cell Equivalent Circuit
• The general equivalent circuit model considers both
parallel and series resistance
Photocurrent
source
R
P
Load
Rs
(minimize)
(maximize)
Equivalent Circuit
Id
I
ISC Vd
+
-
V
+
-
d S
V V I R
  
( ) d
SC d
P
V
I I I
R
  
 
/
0
( ( -1))
S
q kT V R I S
SC
P
V R I
I I I e
R
 
  
28
Load
current
Load
Voltage

29. Series and Shunt Resistance Effects
• Parallel (RP) –
current drops by
ΔI=V/RP
• Series (RS) –
voltage drops by
ΔV=IRS
29

30. Fill Factor and Cell Efficiency
Cell
Efficiency
(h) =
Isc•Voc•FF
Incident
Power
Imax•Vmax
Incident
Power
=
Fill Factor
(FF) =
VR•IR
Isc•Voc
area = VRIR
area = VOCISC
30
Fill factor is ratio at
the maximum power point

31. 31
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