PV System Design and Installation

LO 5B - Solar Energy Fundamentals

PV module Current-Voltage Curve

I-V Curve

Reference 3

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.

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.

Reference 2

Module Performance Parameters

Maximum Power Point

Vmp

Imp

Module Design Parameters

Isc – Short Circuit Current, the maximum current where there is no voltage.

Voc – Open Circuit Voltage, the voltage where there no current.

Voc

Isc

Reference 2

A different view

STC – Standard Testing ConditionIrradiance = 1,000 watts/m2Module Temperature = 25 deg C (77 deg F)

PTC - PVUSA (utility consortium)Irradiance = 1,000 watts/m2Ambient Temperature = 20 deg CWind Speed = 1 meter/sec

NOCT – Nominal Operating Cell Temperature Irradiance – 800 watts/m2Ambient Temperature = 20 deg CPV Array: Open CircuitWind Speed = 1 meter/sec

Measurement Conditions for PV Cells and Modules

PV module Current-Voltage Curve

I-V Curve

Reference 3

Solar Intensity

Module output is reduced with decrease in solar intensity (irradiance)

Reference 2

Module output is reduced with increasing temperatures

Reference 2

Temperature

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

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.

In Class Work Assignment

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 series

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

PV modules with different current outputs connected in parallel

Impact of shading on PV array performance

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.

Bypass 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.

More on Bipass Diodes

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,

• PV module is in open-circuit condition in a series,

• PV modules of different output current are put in series.

.Reference Class Book by Dunlop

Breakdown Voltage