Transcript Lesson 24: Photocell Electrical Characteristic and Circuit Model

Lesson 24: Photocell Electrical
Characteristic and Circuit Model
ET 332a
Dc Motors, Generators and Energy Conversion Devices
1
Lesson 24 332a.pptx
Learning Objectives
After this presentation you will be able to:
Identify and interpret a photocell electrical
characteristic
 Find the maximum power output from a photocell
 Calculate a photocell’s efficiency
 Determine circuit model parameters for a photocell
given its characteristic curve
 Perform a calculation using the circuit model of a
photocell.

2
Lesson 24 332a.pptx
Photocell Characteristic Curve
Pm = Maximum cell power
IC
Max Power Pt.
(Vm, Im)
ISC
Fill Factor = FF
PM  IM V M
FF 
Pm
ISC V OC
0
0
Cell Efficiency
3
VOC
C 
V
PC
PI
0.7 <FF<0.85 Typical FF range
PI = incident solar power
Lesson 24 332a.pptx
Solar Cell Characteristics Example (1)
-12 A
Example: A photocell has a saturation current of 2.5 x 10
2.
and a short circuit current of 35 mA. It has an area of 1.5 cm
2 . Assume that the cell
The incident solar power is 1000 W/m
operates at room temperature. Find V
oc , P m , Fill Factor and
conversion efficiency.
I s  2.5  10
V T  0.026
 12
A
V
IL 

V OC  V T  ln  1 

Is


4
I sc  0.035
A
I L  I sc
V OC  0.607
Lesson 24 332a.pptx
V
Solar Cell Characteristics Example (2)
Find P
m
graphically
Create function for plotting
Define plot range
Calculate power as
function of V
5


I c ( V )  I L  I s   e
V
VT
V  0.0  0.02  0.62
P ( V )  I c ( V )  V
Lesson 24 332a.pptx
W


 1
V
Solar Cell Characteristics Example (3)
Cell Characteristic
Cell Current (A), Cell Power (W)
0.04
I sc
V m  0.5360
0.03

I m  I c V m
0.02
V

P m  0.0177
I m  0.0328
A
Find the Fill Factor (FF)
0.01
FF 
0
0
0.1
0.2
0.3
0.4
Cell Voltage (V)
Cell Current
Cell Power
6
0.5
0.6
Pm
V OC  I sc
0.7
V
OC
Lesson 24 332a.pptx
FF  0.8
W
Solar Cell Characteristics Example (4)
Find the cell efficiency at maximum power output
Find the incident power, P
I  1000 
W
m
A m  A 
A  1.5  cm
2
1 m
P I  A m  I

7
C

PI
A m  1.5  10
2
P I  0.2 W

2
2
10000  cm
Pm
I
C
 0.1
4
m
2
P m  0.0177  W
Cell efficiency is 10%
Lesson 24 332a.pptx
Circuit Model of Solar Cell
+
D
IL
+
Rs
VOC Rsh
RL
Vc
-
Cell Characteristic
0.04
Rs slope of characteristic near Voc
Rsh slope of characteristic near Isc
1 /R
sh
0.02
1/Rs
Values determined by cell construction
Cell Current (A)
0.03
0.01
0
0
0.1
0.2
0.3
0.4
Cell Voltage (V)
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Lesson 24 332a.pptx
0.5
0.6
0.7
Solar Cell Circuit Model Parameters
Finding Model Parameters, R
Cell Characteristic
sh
s
1.)
V 1s  0.5781
V
I 1s  0.02395
0.03
2.)
V 2s  0.6058
V
I 2s  3.9  10
0.02
G s 
4
2
1/Rs
3
Cell Current (A)
and R
Pick points on characteristic plot and compute slope.
For R s use points 1 and 2.
1/R
0.04
sh
R s 
0.01
For R
0
I 2s  I 1s
V 2s  V 1s
1
G s  0.8505
Gs
sh
S
4
A
s
R s  1.176

I 1sh  0.0343
A
I 2sh  0.0311
A
use points 3 and 4
1
0
0.1
0.2
0.3
0.4
Cell Voltage (V)
0.5
0.6
0.7
3.)
V 1sh  0.512
4.)
V 2sh  0.550
G sh 
R sh 
9
Conductance is 1/R
A
I 2sh  I 1sh
V 2sh  V 1sh
1
G sh
V
V
Conductance is 1/R
G sh  0.0842
Lesson 24 332a.pptx
S
sh
R sh  11.875

Solar Cell Circuit Model Example (1)
IRL
+
ID
VOC Rsh
RL
Ish
D
-
Vc
-
I L  100  mA
R sh  11.875  
+
R eq 
D
VOC Req
 R s  R L   R sh
 R s  R L   R sh
Parallel circuit so.....
-
I D  I L  I eq
10
R s  1.176  
R eq  8.599 
I D  17.4 mA
Lesson 24 332a.pptx
R L  30  
0
I eq 
Ieq
100 mA
IL
VOC  0.71  volt
I L  I D  I eq
By KCL
ID
IL
+
Rs
Example: Find the power delivered to a 30 ohm resistive load
by the solar cell with a light current of 100 mA and model
parameters of R s =1.176  and R sh of 11.875 . Determine
the cell load voltage for this load resistance.
VOC
R eq
I eq  82.6 mA
Solar Cell Model Example (2)
IRL
Ish
+
-
Ish
Rsh
VOC
+
Rs
Use Current Divider rule to find I
RL
Vc
-
RL
R sh
I RL  I eq 
R sh  R L  R s

P L  I RL
Find V
C
from KVL
2
R L

I RL  22.77 mA
P L  15.6 mW
V C  V OC  I RL  R s
V C  0.683 V
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Lesson 24 332a.pptx
Solar Cell Efficiency

AM1.5 Solar Intensity (Incident power density) 1000 W/m2 or
100 W/cm2
 Losses





Photon Energy -47% of photons have eV<1.1, 30% goes to heat
Voltage factor – ratio of energy given to energy required to produce
electron 0.65
Recombination – electron/holes that recombine 10%
Reflection – reduced to 4%
Overall Efficiency c = (0.47)(0.65)(.10)(.96)=.26

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26% Maximum efficiency using current technologies
Lesson 24 332a.pptx
End Lesson 24
ET 332a
Dc Motors, Generators and Energy Conversion Devices
13
Lesson 24 332a.pptx