Download - Solar shading and its effects
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University of Gondar
in collaboration with
Institute for Sustainable Energy, Environment and
Economy (ISEEE)
University of Calgary
D. Yeboah
Graduate Student, ISEEE, University of Calgary
Small Scale Renewable Energy SystemsHands-on Short course
July 2012
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Cross-Section of a PV Cell
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Solar Panel Configurations
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Theory of I-V Characterization
I-V Curve of PV Cell and Associated Electrical Diagram
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Ideal PV CellIn an ideal cell, the total current I is equal to the current Iℓ generated by the photoelectric effect minus the diode current ID, according to the equation:
Expanding the equation gives:
whereI0 is the saturation current of the diodeq is the elementary charge 1.6x10-19 Coulombs k is a constant of value 1.38x10-23J/K T is the cell temperature in KelvinV is the measured cell voltage that is either produced (power quadrant) or applied (voltage bias)n is the diode ideality factor (typically between 1 and 2) RS and RSH represents the series and shunt resistances respectively
Single-Diode Model .
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The I-V curve of an illuminated PV cell has the shape as shown below 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.
Short Circuit Current (ISC)The short circuit current ISC corresponds to the short circuit condition when theimpedance 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 valuein the power quadrant. For an ideal cell, this maximum current value is the total currentproduced in the solar cell by photon excitation. ISC = IMAX = Iℓ for forward-bias powerquadrantOpen 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 inthe power quadrant. VOC= VMAX for forward-bias power quadrant
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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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Fill FactorThe Fill Factor (FF) is essentially a measure of quality of thesolar cell. It is calculated by comparing the maximum powerto the theoretical power (PT) that would be output at both theopen circuit voltage and short circuit current together.
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Efficiency (η)Efficiency is the ratio of the electrical power output Pout, compared to thesolar power input, Pin, into the PV cell. Pout can be taken to be PMAX sincethe solar cell can be operated up to its maximum power output to get themaximum efficiency.
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Temperature Measurement Consideration
When a PV cell is exposed to higher temperatures, ISCincreases slightly, while VOC decreases more significantly.
Temperature Effect on I-V Curve
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Fundamentals of PV
Typical Solar PV Module: 60 cells in series
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PV Fundamentals: The Solar
ModuleBut what if we shaded one cell?
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0V
Due to the series connection, no current can flow through the module, so it cannot produce any power!
PV Fundamentals: The Solar Module
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PV Fundamentals: The Solar Module
0V
+15*0.6 = +9V
-44*0.6 = -26.4V
Furthermore, there is a reverse bias
across the shaded cell due to the voltages
produced by the other cells…
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PV Fundamentals: The Solar Module
+15*0.6 = +9V
-44*0.6 = -26.4V
Voltage Across Shaded Cell = -35.4V (Reverse Bias)
0V
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PV Fundamentals: The Solar Module
+15*0.6 = +9V
-44*0.6 = -26.4V
Voltage Across Shaded Cell = -35.4V
(Reverse Bias)
0V
Multi-crystalline Solar Cell Reverse Bias
Breakdown Voltage: -13V
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PV Fundamentals: The Solar Module
+15*0.6 = +9V
-44*0.6 = -26.4V
Voltage Across Shaded Cell = -35.4V
(Reverse Bias)
0V
Multi-crystalline Solar Cell Reverse Bias
Breakdown Voltage: -13V
Result: Cell over heats and is
damaged (hot spot)!!
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PV Fundamentals: The Solar Module
Solution…
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PV Fundamentals: The Solar Module
Solution… BYPASS DIODES
Maximum Reverse Bias: 19*0.6 = 11.4V
(OK!)
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PV Fundamentals: The Solar Module
Solution… BYPASS DIODES
Normal Operation:
Voc = 60*0.6 = 36V
Isc = 8A
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PV Fundamentals: The Solar Module
Solution… BYPASS DIODES
Partial Shade Operation:
Voc = (40*0.6)-0.5 = 23.5V
Isc = 8A
…we can still get 2/3 of the power
out of the module, but the voltage is
reduced.
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PV Fundamentals: The Solar Module
Solution… BYPASS DIODES
Sub-Modules
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String of n Modules in Series
+
Voc = n*36V
Where ‘n’ is the number of
modules
Shading : Solutions (Cont.)
Micro-inverters
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PV Fundamentals: The Solar Array
_ +Voc = n*36V
Isc = m*8AString of n Modules in Series
String of n Modules in Series
String of n Modules in Series
m S
trings In
Para
llel
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PV Fundamentals: The Solar Array
_
+
Inverter
Varies the load
on the array
to operate at
the Maximum
Power Point
(MPP)
AC Out
String of n Modules in Series
String of n Modules in Series
String of n Modules in Series
m S
tring
s In
Pa
ralle
l
Voc = n*36V
Isc = m*8A