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

Introduction to Photovoltaic (Solar) Cells

Cross-Section of a PV Cell

Solar Panel Configurations

Theory of I-V Characterization

I-V Curve of PV Cell and Associated Electrical Diagram

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 .

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

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.

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.

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.

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

Fundamentals of PV

Typical Solar PV Module: 60 cells in series

PV Fundamentals: The Solar

ModuleBut what if we shaded one cell?

0V

Due to the series connection, no current can flow through the module, so it cannot produce any power!

PV Fundamentals: The Solar Module

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…

PV Fundamentals: The Solar Module

+15*0.6 = +9V

-44*0.6 = -26.4V

Voltage Across Shaded Cell = -35.4V (Reverse Bias)

0V

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

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)!!

PV Fundamentals: The Solar Module

Solution…

PV Fundamentals: The Solar Module

Solution… BYPASS DIODES

Maximum Reverse Bias: 19*0.6 = 11.4V

(OK!)

PV Fundamentals: The Solar Module

Solution… BYPASS DIODES

Normal Operation:

Voc = 60*0.6 = 36V

Isc = 8A

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.

PV Fundamentals: The Solar Module

Solution… BYPASS DIODES

Sub-Modules

String of n Modules in Series

+

Voc = n*36V

Where ‘n’ is the number of

modules

Shading : Solutions (Cont.)

Micro-inverters

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

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