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PV System Design and Installation
LO 5B - Solar Energy Fundamentals
PV module Current-Voltage Curve
I-V Curve
Reference 3
Module Performance Parameters
Imp – Maximum
Power Current, the
current at the point
On the I-V curve
where the PV module
produces
Maximum power. At
“knee” on I-V curve.
Maximum Power Point
Imp
Vmp – Maximum
Power Voltage, the
voltage at the point
On the I-V curve
where the PV module
produces the
Maximum power. At
the “knee on the I-V
curve.
Vmp
Reference 2
Module Design Parameters
Isc – Short
Circuit Current,
the maximum
current where
there is no
voltage.
Isc
Voc – Open
Circuit Voltage,
the voltage
where there no
current.
Voc
A different view
Reference 2
Measurement Conditions for
PV Cells and Modules
STC – Standard Testing Condition
Irradiance = 1,000 watts/m2
Module Temperature = 25 deg C (77 deg F)
PTC - PVUSA (utility consortium)
Irradiance = 1,000 watts/m2
Ambient Temperature = 20 deg C
Wind Speed = 1 meter/sec
NOCT – Nominal Operating Cell Temperature
Irradiance – 800 watts/m2
Ambient Temperature = 20 deg C
PV Array: Open Circuit
Wind Speed = 1 meter/sec
PV module Current-Voltage Curve
I-V Curve
Reference 3
Solar Intensity
Module output is reduced with decrease in solar intensity (irradiance)
Reference 2
Temperature
Module output is reduced with increasing temperatures
Reference 2
Temperature Correction Formula
I-V Curve PV modules in Series
Voltage of Array = Module Voltage x Number in Series
Reference 2
I-V Curve - PV modules in Parallel
Array Current = Module Current x Number in Parallel
Reference 2
Where is Maximum Power Point for this array
on the I-V Curve?
Reference 3
In Class Work Assignment
Breakout into groups
1. Calculate for String and
Array
Isc (Short Circuit Current)
Voc (Open Circuit Voltage)
Imp (Max Operating Current)
Vmp (Max Operating Voltage)
2. Provide information on
temperature correction factors
for voltage and current for the
PV modules being used in the
array.
PV modules with different current outputs
connected in series
PV modules with lower current output will absorb current from modules with
higher current output. Will lead to loss of power and potential
overheating/damage of lower current PV modules
Reference Class Book by Dunlop
PV modules with different current outputs
connected in parallel
PV modules with different
output currents can be
connected in parallel with
no harmful effect if string
voltages are the same.
Reference Class Book by Dunlop
Impact of shading on PV array performance
Bypass Diodes
Bypass diodes are placed
in PV modules to prevent
damage from shading.
Limits reverse bias voltage
to 0.7 volts
Diodes allow current to
pass around shaded or
open circuit PV cell.
However, negative impact
on power production as
string voltage is less.
More on Bipass Diodes
Bipass diodes are typically
installed across every 18 to 20
cells in a PV module
Bypass diodes must be able to
a) Handle maximum operating
voltage for the number of
modules/cells bypassed,
b) Must be rated in excess of
the maximum circuit current.
Breakdown Voltage
Shading will cause PV cell to act as a
resistance in circuit.
The cell develops a “reverse bias”.
Cell continues to pass current but the
voltage is negative in direction.
High currents can lead to damaging
levels of power dissipation through
heat generation.
Cell will dissipate power in the form of
heat, possibly damaging PV module.
Reverse bias conditions can also
occur if,
a) PV module is in open-circuit condition
in a series,
b) PV modules of different output current
are put in series.
.
Reference Class Book by Dunlop