SKILLS FORGREEN

JOBS

Renewable Energy Technologies

Introduction to Renewable Energy and Energy EfficiencyNQF Level 2

BookStudent

4

New curriculum for 2015 implementation

EditorSkills for Green Jobs (S4GJ) Programme

Deutsche Gesellschaft für InternationaleZusammenarbeit (GIZ) GmbHRegistered offices: Bonn and Eschborn

GIZ Office PretoriaP.O. Box 13732, Hatfield 0028Hatfield Gardens, Block C, 2nd Floor,333 Grosvenor StreetPretoria, South AfricaTel.: +27 (0) 12 423 5900E-mail: S4GJ@giz.dewww.giz.de

Responsible: Edda GrunwaldAuthors: S4GJ Team

Illustrations: S4GJ TeamGraphic Design: WARENFORMMascot and Comic Design: Björn RothaugePhotos Title, p.17, 19, 73: Ralf Bäcker, version-foto

Pretoria, May 2015

Textbook provided free of charge by the Skills for Green Jobs Programme.

For classroom use only! Not for resale or redistribution without further permission!!

3

CONTENTS

List of Figures and Tables 4 Glossary 6 Preface 11 Foreword DHET 12 Using This Textbook 13

4. Basic Principles of Photovoltaic Systems 17

4.1 Photovoltaic System Components and Operational Principles 19 4.1.1 PV System Components 20 4.1.2 Semiconductor Materials and the Photovoltaic Effect 40 4.1.3 PV Module Datasheets and Output Parameters 51 4.1.4 Factors Affecting the Performance of PV Modules 61

4.2 Photovoltaic Experiments 73

4

LIST OF FIGURES AND TABLES

FiguresTopic 4Figure 1: Installations and installed PV capacity in various countries 18

Theme 4.1.1Figure 1: Illustration of some essential components of a grid-connected PV system 21Figure 2: Illustration of a PV cell, module and array 21Figure 3: Illustration of 36 single PV cells connected in series 22Figure 4: Illustration of the encapsulation of PV cells between various layers 23Figure 5: Illustration of different inverter configurations 24Figure 6: A 500 W grid-interactive inverter (inside and outside view) 25Figure 7: A 4 kW grid-interactive inverter (inside and outside view) 25Figure 8: Diagram indicating the maximum power point of a 75 watt PV module 26Figure 9: Schematic illustration of an inverter with dual-MPPT functionality 27Figure 10: Two different types of charge controllers without MPPT functions 27Figure 11: Different 12 V sealed gel batteries 29Figure 12: DC and AC rated overcurrent protection devices (OCPD) 30Figure 13: Mounting system components 32Figure 14: Fixing stainless steel hooks or roof anchors 32Figure 15: A hook and rail system for a tiled roof 33Figure 16: PV modules being attached to the rails 33Figure 17: Cost reductions of residential PV rooftop systems in the USA 34 Figure 18: A self-sealing mounting base 35Figure 19: A mounting system 35

Theme 4.1.2Figure 1: The two main types of materials used for the construction of PV panels 41Figure 2: Average effects of technology specific temperature coefficients of power

on PV module output performance 43Figure 3: From sand to PV modules 44Figure 4: Integrated circuit (IC) chips 45Figure 5: The silicon (Si) atom 46Figure 6: A boron (B) atom takes the place of a Si atom in the crystal lattice 47Figure 7: A phosphorus (P) atom takes the place of a Si atom in the crystal lattice 47Figure 8: ‘Doped’ or impure (compounded) silicon 48Figure 9: The depleted P-N junction 48Figure 10: Electric current in a PV cell 49

Theme 4.1.3Figure 1: Relevant parameters indicated in a sample datasheet for a PV module 53Figure 2: Diagram indicating the maximum power point (MPP) of

a typical 75 Watt PV module 55

5

Figure 3: Diagram with an I-V and P-V curve plotted together 56Figure 4: I-V curves of a PV module 57Figure 5: I-V curves of a PV module 57Figure 5: A section from the datasheet contains the temperature characteristics

of the module 58

Theme 4.1.4Figure 1: Illustration of 36 single PV cells connected in series to form a module 63Figure 2: Two identical modules connected in series 64Figure 3: Two identical modules connected in series 64Figure 4: Four identical modules connected in series 64Figure 5: Two identical modules connected in parallel 65Figure 6: Two identical modules connected in parallel 65Figure 7: Four identical modules connected in parallel 66Figure 8: Eight identical modules connected in series-parallel 66Figure 9: A shadow cast on a rooftop PV system 67Figure 10: Uniform and partial shading 67Figure 11: P-I curves indicating two illumination levels 68Figure 12: The diode 69Figure 13: Different types of diodes in a PV array 70Figure 14: The function of bypass diodes in a string 71

Unit 4.2Figure 1: The training set Solartrainer Junior 74

TablesTheme 4.1.1Table 1: Comparison between PWM and MPPT charging 28

Theme 4.1.2Table 1: Global market share of wafer based crystalline silicon cells and

thin film technologies 41Table 2: Conversion efficiencies (industrial mass production) of wafer

based crystalline silicon cells and thin film technologies 42Table 3: PV technologies and their different temperature coefficients 42

6

GLOSSARY

Alternating current (AC)

Charge can vary with time in several ways, resulting in several types of current. An electric charge flowing back and forth at a set frequency will, for example, result in a time-varying current called an alternating current. AC is a current that varies sinusoidally over time, for example 100 times per second at a frequency of 50 hertz. AC is provided by most power stations and is transmitted through the national grid to residential and commercial power users.

Amorphous

Non-crystalline semiconductor material, such as copper indium diselenide, cadmium telluride, gallium arsenide, or amorphous silicon. The layer used to make photovoltaic cells usually has a thickness of only a few microns or less. Also called thin film.

Ampere (A)Ampere is the SI unit for the electric current (I) and can be defined in terms of charge (Q) and time (t), i.e. 1 A = 1 coulomb

1 second

Amperage, voltage, wattage

These words are entirely unnecessary in engineering as we have technical terms for these quantities: electrical current, electrical potential and power. Thus, we will avoid these words in this textbook wherever we can. In the context of PV technologies this is however not always practical, as technical terms such as open-circuit voltage (Voc) or voltage at maximum power (Vmp) are very common and are even used in module datasheets and guidelines.

ArrayAny number of photovoltaic modules connected together to provide a single electrical output.

Azimuth angle

The azimuth angle is the compass direction the sunlight is coming from. Simply stated, it describes the direction of the Sun from East to West in degrees (°). At solar noon, the Sun is always direct-ly South in the northern hemisphere and directly North in the southern hemisphere. The azimuth angle varies throughout the day. At the equinoxes, the Sun rises directly East and sets directly West regardless of the latitude. Throughout the year however, the azimuth angle varies with the latitude and time of year.

Balance of System (BoS)

Represents all components and costs other than the PV modules. It includes design costs, site preparation, support structures, system installation, inverter, operation and maintenance, batter-ies, and other related costs (sometimes even land).

Bypass diode

A diode connected across one or more solar cells in a photovoltaic module to protect these solar cells from thermal destruction, in case of total or partial shading or cell string failures of individual solar cells while other cells are exposed to full light. See also reverse bias.

Charge (Q)

Electric charge is a derived quantity and can be defined in terms of electric current (I) and time (t), i.e. Q = I * t . Opposite charges (positive-negative) attract each other and similar charges (posi-tive-positive and negative-negative) repel each other. Electric charge is a fundamental property of matter and is the cause of all electrical phenomena. The SI unit of charge has been termed coulomb (C).

7

Charge controller

An electronic device which regulates the electrical potential applied to a battery system from the PV array. A controller is essential to ensuring that batteries obtain the maximum state of charge and subsequently their desired cycle life.

CircuitAn electric circuit is an interconnection of electrical elements or, more precisely, a complete path through which an electric current (I) can be conducted.

Coulomb (C)

Coulomb is the SI unit for charge (Q). The coulomb is defined as the electric charge transported through any cross-section of a conductor in one second by a constant current of one ampere, i.e. 1 C = 1 ampere * 1 second . The coulomb is a large unit for charges: in 1 C of charge, there are 6.24 × 1018 electrons (a number with eighteen zeros). In contrast, the elementary charges of a single electron or proton are incredibly small, only 160 × 10−21 C.

Current (I)

Electric current is a base quantity. Simply put, current can be defined as a flow of charge, however it is more accurate to define electric current as a rate of flow of charge, i.e.

I = ∆Q∆t . The SI unit of

current is ampere (A).

Current at maximum power (Imp)

The current at which maximum power is available from a module.

Cycle lifeNumber of discharge-charge cycles that a battery can tolerate under specified conditions before its capacity decreases, e.g. to 80 percent of its nominal capacity.

Direct current (DC)

There can be several types of current as charge can vary with time in several ways. If the current does not change with time but remains constant, we call it a direct current (DC). Direct current is provided by batteries, photovoltaic cells and other DC generators.

Diode

A diode is the simplest possible semiconductor device. In PV systems, diodes are used to restrict current from flowing back-ward through solar cells, thus protecting the PV module against the risk of thermal destruction of its solar cells.

EfficiencyEfficiency is the ratio of output to input. Easy to remember as what you get divided by what you put in.

Electrical energy

Electrical energy (E) is a specific form of energy. Simply stated, electrical energy is equal to electric power (P) multiplied by time (t), i.e. E = P * t. If we place the correct SI units into this formula, i.e. watt (W) for power and hours (h) for time, we can see that electrical energy is not expressed in joule but in units of watt hours (Wh). If an appliance consumes or if a generator provides 1 kilowatt of power over a period of one hour, then 1 kilowatt hour (1kWh) of energy exists in some form over the course of this hour. Larger amounts of electrical energy can be measured in mega-watt hours (1 MWh = 1 000 kWh).

Energy (E)

Energy is a derived quantity and is defined as the capacity to do work (W ), i.e. E = P * t. As with work (W ) the SI unit of energy (E) is the joule (J). Energy is a varying property of matter and appears in different forms, for example as thermal energy (heat), electrical energy or kinetic energy (motion). Energy is never created nor destroyed, but merely transformed (energy conservation). This principle is also known as the first law of thermodynamics.

8

Grid-connectedA PV system acting as an energy generator supplying power to the national grid.

GroundingElectrical connection of one or more conductive objects to the Earth through the use of cables and metal rods or other devices.

Hot spot

An undesirable phenomenon that can appear in PV systems whereby one or more cells within a PV module act as a resistive load, resulting in local overheating or melting of the cell. Bypass diodes aim to prevent this phenomenon.

InsolationDerived from incident solar radiation – it is a measure of the solar energy received on a specific area over time (W/m2/day). Don’t confuse insolation with insulation!

InverterAn electronic device that converts DC into AC either for stand-alone systems or for grid-connected systems.

I-V curve

The graphical presentation of current versus potential from a PV module as the load is increased from the short-circuit (no load) condition to the open-circuit (maximum voltage) condition. The shape of the curve characterises module performance at constant conditions (irradiation and temperature).

Junction boxA junction box is an enclosure fixed on the back of a PV module to connect the wiring. It is where protection devices can be located (usually bypass diodes).

Load Any device in an electrical circuit which, when the circuit is energised (turned on), draws power from that circuit.

Maximum power current (Imp)

The amount of current of a given device, usually a PV module, at its maximum power point.

Maximum power point (MPP)

The point on the current-voltage (I-V) curve of a PV module where the product of current and potential is at its maximum.

Maximum power point tracker (MPPT)

A power-conditioning unit that automatically (electronically) oper-ates the PV system at its maximum power point. An MPPT can increase the power efficiency delivered to the system by 10 to 40 percent.

Maximum power voltage (Vmp)

The potential difference value of a given device, usually a PV module, at its maximum power point.

Mono-crystalline

Semi-conductor material that is solidified in such a way that individual crystals are symmetrically arranged. Compared to the multi-crystalline random arrangement of crystals, the more symmetrical structure of mono-crystalline materials increases PV cell efficiency.

Multi-crystallineSemi-conductor material that is solidified in such a way that many small and irregular crystals form. Sometimes also referred to as polycrystalline.

Multimeter

An instrument used to measure various electrical properties, including potential difference (V ) across a component in volt (V), current (I) through part of a circuit in ampere (A), and resistance (R) of components in ohm (Ω).

9

Nominal Operating Cell Temperature (NOCT)

Nominal Operating Cell Temperature (NOCT) is defined as the temperature reached by open circuited cells in a module under the conditions as listed below: Irradiance on cell surface = 800 W/m²

Air temperature = 20° CWind velocity = 1 m/sMounting = open back sideNote the somewhat lower insolation conditions. In addition, module performance is often measured at an operating tempera-ture of 45° C instead of 20° C.

Ohm (Ω)

Ohm is the SI unit used to measure electrical resistance in a conductor. The ohm is defined as the electrical resistance be-tween two points of a conductor: when a constant potential difference of one volt is applied between those points, a current of one ampere results, i.e. 1 Ω = 1 volt

1 ampere .

Ohm’s law

Ohm’s law describes the relationship between current, potential and resistance. It states that current (I) is inversely proportional to the overall resistance (R) in the circuit and directly proportional to the electric potential difference (V ) impressed across the circuit. The term ‘inversely proportional’ describes the relationship between current and resistance, i.e. ampere values decrease at the same rate the ohm values increase. The term ‘proportional’ here describes the relationship between current and potential difference, i.e. ampere values increase at the same rate as volt values increase. Written as mathematical expressions, Ohm’s law is:I = V / RV = I * RR = V / I

Open-circuit voltage (Voc)The maximum possible potential across a photovoltaic cell or module when no current is flowing.

Parallel

Connecting two or more energy generating devices such as PV cells or modules by joining their positive leads together and their negative leads together. Such a configuration increases the amount of current.

Photon A particle of light that acts as an individual unit of energy.

Photovoltaic (PV) Photovoltaic is the technique used to convert radiation energy from the Sun (light) into electrical energy.

Photovoltaic cell

An electronic device made of semiconductor material that transforms the radiant energy of sunlight into electrical energy. The electric potential (V ) generated by each cell is about 0.6 volt (DC), thus many cells are added in series to produce greater potential.

Photovoltaic moduleSeveral photovoltaic cells that are connected in series and/or in parallel to increase the electric potential form a photovoltaic module.

Photovoltaic system

Usually a photovoltaic system consists of various components, including one or more photovoltaic modules connected to an inverter/controller. The system is designed to provide electrical energy (direct current).

10

Power (P)

Power is the rate of doing work and is measured in watt (W), i.e. 1 W = 1 joule

1 second . During this process, energy is transmitted and converted into another form of energy. Thus, power indicates the rate at which:(i) An appliance uses electrical energy, or(ii) Electrical energy is provided by a generator.

Resistance (R) An impedance to the flow of charge in a circuit measured in ohm (Ω).

Reverse bias

A condition where the current-producing capability of a PV cell is significantly less than that of other cells in its series string. This can occur when a cell is shaded, cracked, or otherwise degraded or when it is electrically poorly matched with other cells in its string.

Roof penetrationRoof penetration happens when the installation process of a PV system requires a modification to the existing roof structure, e.g. holes need to be drilled or tiles require grinding and cutting.

Semiconductor

Any material that has a limited capacity for conducting electric current. Semiconductor materials generally fall between metal and insulators in conductivity. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited for the photovoltaic conversion process.

Series

A way of connecting two or more energy generating devices such as PV cells or modules by joining their positive leads to their negative leads. Such a configuration increases the amount of potential.

Short-circuit current (Isc)The current flowing freely from a photovoltaic cell through an external circuit that has no load or resistance. It is the maximum current possible.

SiliconA chemical element (Si) and a common constituent of sand and quartz. Silicon is an excellent semiconductor and the most common material used in making photovoltaic devices.

Sine wave inverter Any type of inverter that produces utility-quality sine wave power.

Stand-alone An autonomous PV system not connected to the national grid. Such systems usually have power storage capacities (batteries).

Standard test conditions (STC)

Conditions under which a module is typically tested in a laborato-ry, i.e. an irradiance intensity of 1000 W/m2 and at a cell (module) temperature of 25° C.

Volt (V)

Volt is the SI unit used to measure potential difference in a circuit. 1 volt is defined as the potential difference between two points so that the energy used in conveying a charge of 1 coulomb from one point to the other is 1 joule, i.e. 1 V = 1 joule

1 coulomb.

Watt (W)Watt is the SI unit of electrical power (P). The watt is defined as the power resulting when 1 joule of energy (E) is dissipated in one second, i.e. 1 W = 1 joule

1 second .

Watt hour (Wh)Watt hour is a unit for energy (E) and defined as the amount of power (P) in watt that is consumed or supplied in one hour (h). W and Wh are related but different units. Don’t confuse the two!

11

PREFACEOn behalf of the German Ministry of Economic Cooperation and Development (BMZ), the Skills for Green Jobs (S4GJ) programme, together with the South African Departments of Higher Education and Training (DHET) and Science and Technology (DST), jointly developed and implemented a number of activities which aim to:

1. Support qualified TVET lecturers in their continuous professional development through train-ing in Renewable Energy and Energy Efficiency Technologies.

2. Develop and support the implementation of a new optional vocational subject on Renewable Energy Technologies for NC(V) students.3. Develop appropriate training material, such as student textbooks and lecture guides, for the new subject.

Subsequently, we are very happy that the textbooks for NC(V) level 2 Renewable Energy Technologies are now available. The new subject and textbooks are for students of the technical NC(V) programmes who want to learn more about renewable energy technology, its potentials and limitations. The textbooks introduce students to the relevant technical concepts, illustrate examples from real world applications, and offer exercises and practical work/experiments.

Yours in renewable energy…

12

FOREWORD BY THE DIRECTOR-GENERAL OF THE DEPARTMENT OF HIGHER EDUCATION AND TRAINING

The Department of Higher Education and Training is pleased to introduce the subject Renewable Energy Technologies in the National Certificate (Vocational) NC(V) Electrical Infrastructure Construction programme. This new subject is the latest addition to the vocational specialisation options offered in Technical and Vocational Education and Training (TVET) colleges and has been developed for students who want to learn more about renewable energy generation and the technologies related therewith.

Outlined in Accord 4 of South Africa’s new growth path, government commits to the procurement of renewable energy, with the aim to expand and diversify the nation’s energy generation capacity, whilst lowering greenhouse gas emissions, in order to meet the challenges posed by climate change. To fully re-alize these commitments the economy needs informed and trained people in this field, which continues to be a significant driver for future employment. The Industrial Development Corporation (IDC) and the South African Development Bank (SADB) estimated in 2011 that the total employment potential in the energy generation and energy and resource efficiency categories would be 130 000 and 68000 new jobs respectively.

Under the auspices of the German Ministry of Economic Cooperation and Development (BMZ) and supported by the Department of Higher Education and Training (DHET) and the Department of Science and Technology (DST), the Skills for Green Jobs (S4GJ) programme drove the process of developing this new subject, the training material, student textbook and lecturer guide and trained TVET College lecturers on the subject matter content on new didactical training equipment as part of their continuous professional development so that they can teach the subject in a practical and progressive manner.

Thus, in January 2015 the subject Renewable Energy Technologies was successfully implemented on NC(V) Level 2 in seven TVET colleges, namely Boland, East Cape Midlands, Ingwe, Northlink, Port Elizabeth, Umfolozi and West Coast TVET Colleges.

The development and implementation of this new subject is the result of cordial collaboration and suc-cessful cooperation between Germany and South Africa and I wish the colleges, the lecturers and mostly our students a good start with Renewable Energy Technologies in 2015 and beyond.

Mr GF QondeDirector-General: Higher Education and Training

13

USING THIS TEXTBOOKFour textbooks are available for NC(V) level 2. Each book comprises one topic of the curriculum, covering the particular learning outcomes according to the curriculum of level 2. For example, the content of Book 3 is fully consistent with Topic 3 of the curriculum Introduction to Occupational Health and Safety, and Book 2 corresponds with Topic 2 of the curriculum Introduction to Electrical Energy and Energy Efficiency (Basic concepts and principles), and so forth. In other words, each of the four textbooks has been written according to a specific curriculum topic, and subsequently covers all stated learning outcomes.

Furthermore, the structure of each topic includes various units, for example Unit 1 of Topic 3, Basic Terms used in Health and Safety, and each unit is made up of several themes. In essence, the themes form the core of the textbooks. They contain keywords, the desired outcomes, technical terms and definitions, illustrative examples, as well as questions, exercises and experiments through which the students can independently check their knowledge and understanding. Lastly, each theme ends with a bibliography section which will enable students to supplement the described subject matter.

14

COM

IC

Smart solutions – aren’t they?!

15

COM

IC

17

Basic Principles of Photovoltaic Systems Topic Overview

On a national and global scale interest in photovoltaic systems (PV systems) is steadily increasing. This is due to ecological factors (climate change) as well as economic factors. PV systems are profitable solutions for the national energy mix and this technology will take part in shaping South Africa’s future energy supply through residential (1 – 20 kW) and commercial systems (10 – 500 kW), as well as via PV power plants (> 500 kW). As of January 2015 a total of 593 MW installed capacity by PV plants was connected to the national grid and more and more rooftop PV systems for residential and commercial users are being in-stalled in South Africa. Consequently, it is important to know and understand this future technology better.

This topic aims to inform you on the relevant system components and their operational principles and it will offer you exciting experiments which can be carried out in your college. Thus, this last topic of the RET subject in level 2 will provide you with the foundational knowledge and practical skills you need when work-ing on PV installations.

We however strongly encourage you to refer to the previous top-ics, i.e. Introduction to Renewable Energy, Electrical Energy and Energy Efficiency and Safe Work Practices, whenever the need arises.

TOPI

C

18

FIGURE 1: INSTALLATIONS AND INSTALLED PV CAPACITY IN VARIOUS COUNTRIES (ADAPTED FROM IEA PHOTOVOLTAIC POWER SYSTEMS PROGRAMME [PVPS])

TOP 10 COUNTRIES IN 2014 TOP 10 COUNTRIES IN 2014

FOR ANNUAL INSTALLED CAPACITY FOR CUMULATIVE INSTALLED CAPACITY

China 10,6 GW Germany 38,2 GW

Japan 9,7 GW China 28,1 GW

USA 6,2 GW Japan 23,3 GW

UK 2,3 GW Italy 18,5 GW

Germany 1,9 GW USA 18,3 GW

France 0,9 GW France 5,7 GW

Australia 0,9 GW Spain 5,4 GW

Korea 0,9 GW UK 5,1 GW

South Africa 0,8 GW Australia 4,1 GW

India 0,6 GW Belguim 3,1 GW

Image source: GIZ/S4GJ

Topic 4 covers two units:Unit 4.1 Photovoltaic System Components and Operational PrinciplesUnit 4.2 Photovoltaic Experiments

19

UNIT 4.1

PHOTOVOLTAIC SYSTEM COMPONENTS AND OPERATIONAL PRINCIPLESIntroduction

Photovoltaic (PV) is the direct conversion of solar radiation into electrical power with the help of semiconductor technology. In a variety of aspects, PV systems are different to other renewable energy technologies and of course fossil fuel systems. One fascinating aspect of photovoltaic is that the operation of this technology is silent and that no emissions are released. Photovoltaic has a wide scope of applica-tion and centralised or decentralised use is easily feasible with power capacities from a few milliwatt to hundreds of megawatt. Subsequently, photovoltaic has developed rapidly over the past few decades.

Most areas in South Africa have an average of more than 2 500 hours of sunshine per year, as well as high average solar irradiance levels compared to Europe. This makes South Africa’s local resource one of the most abundant in the world and thus, PV systems are a very promising energy-generating option for the country and the whole region. This unit covers four different themes which will introduce you to a wide range of relevant aspects of photovoltaic system components and operational principles.

Unit Outcomes

At the end of this unit, you should be able to:(i) Describe and sketch the different components of a PV system and explain their functions.(ii) State the semiconducting materials used to produce the main types of solar cells.(iii) Describe and explain the photovoltaic effect. (iv) Compare different PV module technologies. (v) Interpret sample datasheets with reference to standards, certifications and warranties.(vi) Identify and measure key electrical output parameters using multimeters.(vii) Explain and sketch the current-potential (I-V) curve of a PV module in a diagram. (viii) List and explain critical factors that affect the performance of PV modules.

Themes in this UnitUnit 4.1 covers four themes:

Theme 4.1.1 PV System ComponentsTheme 4.1.2 Semiconductor Materials and the Photovoltaic EffectTheme 4.1.3 PV Module Datasheets and Output ParametersTheme 4.1.4 Factors Affecting the Performance of PV Modules

Uni

t 4.1

20

TH

EME

4.1.

1

THEME 4.1.1

PV SYSTEM COMPONENTS Introduction

Both off-grid systems (sole power supply or stand-alone systems) and on-grid systems generate DC through PV modules and can thus be considered as the main components of any PV system. Apart from the PV generator, various other components are necessary for power conditioning and energy storage. Additional components called Balance of System (BoS) form part of a PV system. In this first theme you will be introduced to the main components of a residential (1 – 20 kW) PV system and their functions.

Keywords

Stand-aloneGrid-connectedCellModuleArrayEncapsulationStandard test conditionsBalance of SystemInverterMaximum power point trackingCharge controllerBatteriesOver-current protection devicesIsolatorsMounting systemsRoof penetrations

Theme Outcomes

At the end of this theme, you should be able to describe and sketch the different components of a PV system and explain their functions.

Definition of Terms

Stand-Alone PV SystemsStand-alone or off-grid systems rely on PV generated power only. These systems usually comprise PV modules, batteries for energy storage and a charge regulator (see Figure 1 below) and are either used as back-up systems or to provide electrical energy for remote locations away from the national grid.

Grid-Connected PV SystemsGrid-connected PV systems are connected to the national grid through inverters (see below) and do not require batteries for back-up storage. These type of systems only operate when the energy utility (Eskom) is available. In the event of power outage the system shuts down until utility power is restored.

21

TH

EME

4.1.

1

FIGURE 1: SCHEMATIC ILLUSTRATION OF SOME ESSENTIAL COMPONENTS OF A GRID-CONNECTED PV SYSTEM

Image source: GIZ/S4GJ

PV ModulesPV modules can be considered as the main components of any PV system, as they are responsible for the direct conversion of radiant solar energy into electrical energy. The DC generated by the modules can be used either directly on-site, stored in batteries, or fed into the national grid. The conversion of radiant en-ergy into electrical energy takes place in PV cells, as they are made out of light-sensitive semiconductor materials. In the next theme we will give you more information about this conversion process (photovol-taic effect).

PV cell/module symbol used in schematic circuit diagrams

FIGURE 2: SCHEMATIC ILLUSTRATION OF A PV CELL, MODULE AND ARRAY

=

Cell

Module

Array

Image source: GIZ/S4GJA schematic illustration of the progression from a single PV cell to a module to an array. The cells are con-nected in series to form a module, while module strings can be connected in parallel to form an array.

+ - + - + - + - +

DC Disconnect Inverter AC Disconnect

kWh Meter

to Grid220 VAC

AC Electrical Panelto Household Loads

-

22

TH

EME

4.1.

1

When manufacturing PV modules, the low individual electrical potential difference of solar cells (0.5 - 0.6 volt) requires that the cells are connected in series (cell stringing). The individual cells are spaced several millimetres apart and the front contacts of each cell are soldered to the back contacts of the next cell, thereby connecting them in series. Given that an individual solar cell has a potential of be-tween 0.5‒0.6 volt, and taking into account an expected reduction in PV module potential due to certain losses (temperature etc.), many common modules with a power rating between 10‒100 watt contain 36 solar cells in series (0.6 * 36 = 21.6). This usually provides an open-circuit voltage (Voc, see Theme 4.1.3) of about 21 volt and a potential difference at maximum power (Vmp) of between 17‒18 volt. Depending on the required power output of the module, some manufacturers string 48, 60 or 72 solar cells together. For example, many larger PV modules with a rating between 250‒300 watt have 60 or 72 cells connected in series and a subsequent nominal potential between 36‒42 volt or a Vmp between 55‒65 volt. Thus, the electrical potential (volt) of PV modules is determined by the number of solar cells stringed together, whereas the module current value depends primarily on the size of its cells and their efficiency.

FIGURE 3: SCHEMATIC ILLUSTRATION OF 36 SINGLE PV CELLS CONNECTED IN SERIES TO FORM A MODULE

-+

PLEASE NOTE THAT THERE IS A LARGE VARIATION IN THE SIZE OF SOLAR CELLS USED IN PV MODULES AND THEREFORE THEIR CURRENT MAY VARY WIDELY.

Image source: GIZ/S4GJ

The manufacturing process of PV modules requires high precision and quality control in order to produce a reliable product. The most critical part of the module manufacturing process is the encapsu-lation of the cells so that these are protected from the environment. Typically, the cells are laminated (sandwiched) between a tempered safety glass layer, two transparent polymer sheets (EVA- ethyl vinyl acetate) and a plastic rear surface to keep out moisture and contaminants that could cause PV modules to fail. The panel is framed at the end of the production process with a profile of anodised aluminium to improve rigidity and sturdiness. A junction box at the back of the module is the interface between the module and the PV system so that it can be wired. Bypass diodes (see Theme 4.1.5) are also located here. Module warranties on quality PV modules from well-established manufacturers are usually over 20 years, indicating the robustness of the encapsulated PV modules. Usually, a typical warranty will guarantee that the module produces 90 per-cent of its rated output for the first 10 years and 80 percent of its rated output for up to 25 years.

23

TH

EME

4.1.

1

FIGURE 4: SCHEMATIC ILLUSTRATION OF THE ENCAPSULATION OF PV CELLS BETWEEN VARIOUS LAYERS

EVA

Tempered Glass

Back

Cover

Image source: GIZ/S4GJSimplified schematic illustration of the encapsulation of PV cells between various layers to keep moisture out and contaminants away from the module.

Standard Test Conditions (STC) The performance of PV modules and arrays is generally rated according to their maximum DC power output (Pmax) under Standard Test Conditions (STC). STC are defined as an operating temperature of 25o C and an incident solar irradiance level of 1000 W/m2. Since these conditions are not always typical of how PV modules operate in the field, actual module performance parameters are usually 85‒90 per-cent of the STC rating.

PLEASE NOTE THAT PV MODULES VERY RARELY WORK AT RATED OPERATIONS.

Crystalline Silicon and Thin Film As there are many different semiconductor materials, many different types of PV modules exist. How-ever, there are two broad categories of technology, namely, crystalline silicon and thin film. In the next theme (Theme 4.1.2) we will give you a more detailed overview of these technologies.

Balance of System (BoS) BoS represents all components and costs other than the PV modules. Thus, the BoS includes:

(i) The design costs, site preparation and other related costs (sometimes even land)(ii) Support and mounting structures(iii) All other system components such as the inverter, switches/disconnects and fuses/breakers,

cabling, connectors and combiner boxes, grounding hardware, batteries and controllers etc. (iv) System installation and certification

InverterInverters are electronic components necessary for the power conditioning of the PV system. Thus, invert-ers provide not only the conversion of the DC into AC, but also control the current flow and transforma-tion of the entire PV system. Manufacturers design different types of inverters to be used as stand-alone system inverters or as utility-interactive inverters and based on their purpose, there are different types of inverter categories, e.g. grid-interactive inverters can be either string, central and/or modular (micro) inverters.

24

TH

EME

4.1.

1

FIGURE 5: SCHEMATIC ILLUSTRATION OF DIFFERENT INVERTER CONFIGURATIONS

=~~

=~~

=~~

=~~

Inverter symbol used in schematic circuit diagrams

Image source: GIZ/S4GJLeft: single grid-interactive string. Middle: multi-string. Right: multiple inverter string.

Efficiency averages of quality inverters from well-known manufacturers lie at about 97 percent for most of their series or even more. Preference should be given to true sine wave inverters as these deliver top quality AC power. Inverters are rated according to the power they can deliver, and in the inverter data sheet you can check the inverter’s specifications for accurate continuous power rating. When selecting inverters for a PV system, the following requirements need to be considered:

(i) The maximum open circuit voltage of the PV array shall not exceed the voltage requirements of the inverter.

(ii) The minimum voltage requirement of the inverter shall be met by the PV array.(iii) The maximum power output of the modules shall be less than the inverter’s rating.(iv) The maximum current at the point of operation shall be less than the inverter’s rating.

= ~~

25

TH

EME

4.1.

1

FIGURE 6: A 500 W GRID-INTERACTIVE INVERTER (INSIDE AND OUTSIDE VIEW)

Image source: GIZ/S4GJAn example of a 500 W grid-interactive inverter (inside and outside view), commonly used for smaller solar power systems or for modular applications (multiple inverter strings). These types of inverters can be used in systems with differently aligned or partially shaded roofs in residential applications.

FIGURE 7: A 4 KW GRID-INTERACTIVE INVERTER (INSIDE AND OUTSIDE VIEW)

Image source: GIZ/S4GJAn example of a 4 kW grid-interactive inverter (inside and outside view), commonly used for residential grid-connected PV systems. These types of inverters are usually used for single or multi-string applica-tions.

26

TH

EME

4.1.

1

Maximum Power Point Tracking (MPPT)MPPT is a technique that grid-connected inverters and charge controllers use to get the maximum possible power from PV arrays at any time during its operation, thereby increasing profitability. This is useful because PV modules have internal impedances that vary throughout the course of the day and result in a non-linear output efficiency which can be analysed based on the I-V curve of the module/array (see Theme 4.1.3 and Theme 4.1.4). The variations depend not only on the level of solar irradiance impinging on the modules, but also on their cell temperature. It is thus convenient that an MPPT circuit in the inverter or controller constantly monitors the array performance levels (I and V) and operates the system at the maximum power point of the array. The maximum power point (MPP) is the product of the potential difference (Vmp) and current (Imp) at MPP.

FIGURE 8: SIMPLIFIED DIAGRAM INDICATING THE MAXIMUM POWER POINT (MPP) OF A 75 WATT PV MODULE

1.0

5.0

4.0

3.0

2.0

5 10 15 2017

I

V

Potential Difference

Cu

rren

t

Short Circuit

CurrentMaximum

Power Point

(MPP)

Open Circuit

Voltage

Maximum

Power Point

(MPP)

Vmp

Imp

Image source: GIZ/S4GJA simplified diagram indicating the maximum power point (MPP) of a typical 75 watt PV module as the product of the potential (Vmp) and current (Imp) at MPP, in our case at around 17 V and 4.1 A.

In many cases, the MPPT technology provides much greater system design flexibility, significant cost savings and higher levels of power efficiency. Inverters without MPPT circuits would result in non-opti-mal operating conditions and lower efficiency operation for the array.

PLEASE NOTE

MPPT circuits can only operate within a certain potential difference range, for example many quality inverter series with MPPT technology have a potential range between 125 V and 450 V. If the potential provided by the PV array is too low, then the system is unable to transform the current in an optimal manner due to the fact that the MPPT will not start, and if the potential difference is too high, the excess potential is lost and thus wasted. Some installations even require inverters with dual-MPPT functionality, for example arrays that are on differently aligned roofs, resulting in different solar azimuth or tilt angles. Another applica-tion is in systems which use PV modules with different electrical parameters (Voc, Vmp etc.).

27

TH

EME

4.1.

1

FIGURE 9: SCHEMATIC ILLUSTRATION OF AN INVERTER WITH DUAL-MPPT FUNCTIONALITY

Image source: GIZ/S4GJSuch a configuration is used to connect two arrays from two parts of the roof (resulting in them hav-ing different azimuth or tilt angles of each string), without compromising the power output of the PV system. Dual-MPPT functionality also allows strings of PV modules with different electrical parameters (Voc, Vmp etc.) to be connected.

Charge ControllerCharge controllers are electronic devices used in stand-alone (off-grid) systems. Their purpose is to pre-vent the energy storage devices, usually batteries, from becoming overcharged. Thus, a charge controller monitors the state of charge of the batteries and disconnects the array from the batteries when these become fully charged. Usually the controller will simply open the circuit between the PV array and the batteries. Charge controllers have maximum input voltage and current ratings specified by the manu-facturer and it is required that the PV array power capacity will not exceed the charge controller’s power limits. Exceeding the power ratings of your controller can destroy it.

FIGURE 10: TWO DIFFERENT TYPES OF CHARGE CONTROLLERS WITHOUT MPPT FUNCTIONS

=

~~

M

P

P

T

M

P

P

T

Image source: GIZ/S4GJThese controller types are commonly used for small 12 or 24 V stand-alone systems and at around ZAR 500.00 they offer reasonably priced solutions. Both controllers use PWM (pulse width modulation) as their charging mode. The first controller is an industrial-grade controller fully encapsulated (IP68) to prevent corrosion and can be used in PV-Systems exposed to extreme weather conditions. The second controller is a multi-purpose residential device with a LCD display and a user friendly interface to set various control parameters to meet application requirements.

28

TH

EME

4.1.

1

Pulse Width Modulation (PWM)PWM is one of the two principal algorithms used in charge controllers for smaller stand-alone PV systems, the other being MPPT. PWM controls the average potential (V) values by turning an electronic switch between supply and load on and off at a very fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. The result is that the potential (V) of the PV array will be pulled down to nearly that of the battery. A MPPT controller is more sophisticated but also more expensive. It will adjust its input potential (V) to harvest the maximum power from the PV array and then transform this power to supply the varying voltage requirement of the battery plus load. How-ever, PWM controllers are a good low cost solution for smaller PV systems when PV cell temperatures are moderate to high (45° C ‒ 75° C). Below is a table comparing both charging methods.

TABLE 1: COMPARISON BETWEEN PWM AND MPPT CHARGING

Charge Controller PWM MPPT

Module/array potential Potential values (volt) of PV array and battery should match.

Potential values (volt) of PV array potential (V ) can be higher than battery values.

Battery Performs well in warm climate and when the battery is almost full.

Can provide boost in cold temperatures and when the battery is low.

System size

Smaller systems (<200 W) where MPPT benefits are minimal.

200 W or more to take advantage of MPPT benefits.

Off-grid or grid-tied Off-grid with PV modules typically with Vmp 17–18V.

Larger off-grid or grid-tied PV systems.

Array sizing

PV array size based on current (I) produced when PV array is operating at battery potential.

PV array size in watt based on controller maximum charging current (I) * battery potential (V ).

Your own notes

29

TH

EME

4.1.

1

Rechargeable BatteriesBatteries can accumulate the electrical energy originating from PV systems and store it as chemical ener-gy, which can be converted back to electrical energy and be used at any desired time. A battery’s capacity for storing energy is rated in watt or amp-hours at a given potential, e.g. 12, 24 or 48 volt for residential installations. There are many different rechargeable battery types available for stand-alone or mixed PV systems and preference should be given to deep-cycle type batteries. The disadvantage of using batteries for storage is that they are sensitive to temperature and charge/discharge cycle history (age). In South Africa, battery storage for limited or total autonomy is still relatively expensive and a reasonable storage system, usually based on deep-cycle lead type batteries, for a residential PV installation would proba-bly require similar investments to the PV system itself, thus doubling the total costs. However, PV with storage is nearing its payoff point in the German residential market and most sources agree that PV-con-nected battery storage is set to take off in residential settings.

At grid level, some renewable energy developers are also starting to investigate the potential of large-scale battery storage tied to renewable energy plants. In Japan, for example, a 4 MW facility from Sumi-tomo is used to smooth out wind-generated energy peaks. Another example is Primus Power, a company which is planning to build a 25 MW storage plant in California, again for grid-scale renewable energy integration. 25 MW of electrical storage is a respectable size when considering the average capacity of a concentrated solar power (CSP) plant (approximately 50 ‒100 MW). Here, lithium-ion batteries are the technology of choice for most of these early projects, but other electrical storage concepts could also become more economical in future (redox flow batteries for example). More information on batteries for renewable energy systems and electric cars will be given in RET NQF level 4 (2017).

FIGURE 11: DIFFERENT 12 V SEALED GEL BATTERIES

Image source: GIZ/S4GJDifferent 12 V sealed gel batteries commonly used for small stand-alone (off-grid) systems.

30

TH

EME

4.1.

1

Overcurrent Protection Devices (OCPD)The safe construction and operation of a PV system is of great importance. For small-scale PV systems, South African standards are still being compiled and relevant documents within the municipal and national context are expected soon, e.g. Eskom’s proposed simplified utility connection criteria for con-nected generators, which defines the maximum size PV installations that may connect to the distribution grid without requiring additional network studies. International guidelines for installation and operation of PV systems can be found in DIN VDE 0100-712 and IEC 60364-7-712 (CD). BoS components, par-ticularly system design and the mechanical and electrical components of the system, must also comply with the respective national or even local standards and regulations. For example, as already mentioned in RET Student Book 3, NQF Level 2 (Theme 3.1.5, p. 72‒73), the installation of DC and AC disconnects requires appropriately rated overcurrent protection devices (OCPD). Please note that common OCPDs for AC are not compatible with the DC side of photovoltaic systems and vice versa.

FIGURE 12: DC AND AC RATED OVERCURRENT PROTECTION DEVICES (OCPD)

Image source: GIZ/S4GJ DC and AC rated overcurrent protection devices (OCPD) commonly used for the safe construction and operation of residential PV systems (grid-connected). Left: automatic DC rated circuit breaker (6 A). Right: automatic AC rated circuit breaker (10 A).

DC circuit breakers operate very differently to AC breakers. Short circuits in the DC side of the systems are likely to result in a short-circuit current (Isc, see Theme 4.1.3, PV Module Datasheets and Output Pa-rameters), which is not so much higher than the normal operating current (Imp) of the system’s module/array. This is due to the fact that PV cells/modules are current-limited sources and thus the short-circuit current (Isc), which can appear under faulty conditions, requires a more sensitive overcurrent protection device which triggers faster than AC breakers. OCPDs for AC circuits are not sensitive enough for this task and usually function only when a very large short-circuit current occurs in the system.

IsolatorsIt is strongly recommended that residential and commercial PV systems feature suitable load-switching capacity on both the DC and AC sides of the system. Since the PV array or its individual modules gen-erally cannot easily be disconnected under load, for example for maintenance work or in case of system faults, it is absolutely necessary that switch-off equipment is provided. DC isolators, designed with a suitable switching capacity for direct currents, enable functions such as safe disconnection under load. Often, DC isolators are already integrated into inverters. However, DC isolators are also recommended in connection boxes in order to enable selective disconnection of a PV string, allowing the rest of the sys-tem to continue producing electrical power. According to most standards, isolating equipment must also be provided on the AC side of the system to safely disconnect the AC circuit under load. Thus, isolators with suitable AC switching capacity are recommended.

31

TH

EME

4.1.

1

Mounting SystemsAlmost all types of PV systems require some mounting structures to hold the PV modules securely in place. Exceptions to this rule are integrated products which blend completely into the roof structure, resulting in the PV array being an integral part of the roof. While these integrated products can considerably improve the aesthetics of the homes and offices, they are usually far more capital intensive than conventional PV systems that require mounting structures. Mounting structures on residential roof-top installations are usually fixed parallel to the roof surface and thus determine the azimuth and tilt angle of the array. It is important to mount the rack and rails in such a way that the structure is slightly above the roof for cooling purposes (wind passing under the array!), given that cell temperature is a factor that affects the performance of PV modules (see Theme 4.1.4).

FIGURE 13: MOUNTING SYSTEM COMPONENTS

Image source: GIZ/S4GJMounting system components (rail, clamps etc.) commonly used for grid-connected rooftop PV systems.

32

TH

EME

4.1.

1

FIGURE 14: FIXING STAINLESS STEEL HOOKS OR ROOF ANCHORS INTO THE RAFTERS OF THE ROOF CONSTRUCTION

Image source: GIZ/S4GJ One of the first steps of mounting a PV array on tiled roofs is fixing stainless steel hooks or roof anchors into the rafters of the roof construction without damaging the tiles. Here this is done on a tiled training roof commonly used in TVET colleges.

FIGURE 15: A HOOK AND RAIL SYSTEM FOR A TILED ROOF

Image source: GIZ/S4GJA more detailed view of a hook and rail system for a tiled roof. Once the roof hooks have all been se-cured, the mounting rail is attached to them. Please note: The profile of the rail can vary to suit different roof hook designs.

33

TH

EME

4.1.

1

FIGURE 16: PV MODULES BEING ATTACHED TO THE RAILS

Image source: GIZ/S4GJ Once the mounting system has been secured, the individual PV modules are attached to the rails to form the array. Here this is done on a tiled training roof commonly used in TVET colleges. Mounting PV systems on racks and poles closer to the ground are more common in larger commercial applications. Some ground mounted systems may also have the ability to track the Sun across the sky, following its course over the day and the year (optimised azimuth angle). Sometimes ground-mounted PV modules are fixed on a sun-tracking system, making the system more profitable.

Your own notes

34

TH

EME

4.1.

1

Examples

Balance of System (BoS)Worldwide and across all PV market segments, including residential, commercial and PV plants, system costs have considerably decreased in the last years. This trend mainly accounts for module cost reduc-tions and less for BoS components. At present and on average, costs for BoS accounts for 50‒60 percent of a residential PV system. However, the overall cost reduction trend for PV systems will most probably continue over the next few years, including the residential system market, which is the costliest PV mar-ket segment. Subsequently, today a 1.5 kW residential PV rooftop system (grid-connected) can be pur-chased for around ZAR 42.000, i.e. ZAR 28/watt. Market research forecasts that system prices could fall to ZAR 24/watt within the next couple of years due to BoS innovation that will drive costs down, whilst services and other soft costs will remain at current levels. Please note that albeit these assumptions are based on projections that use the best available information, they are subject to considerable uncertainty.

FIGURE 17: COST REDUCTIONS OF RESIDENTIAL PV ROOFTOP SYSTEMS IN THE USA BETWEEN 2008 AND 2012

Image source: GIZ/S4GJDevelopment of costs (US $ per installed capacity/watt) of residential PV rooftop systems (PV mod-ules, BoS hardware and services) in the American market between 2008 and 2012, adapted from Chakraborty and GTM Research.

Roof Penetration Mounting structures on residential rooftops usually requires roof penetrations and these are often the most difficult part of installing a PV array. Roof penetration is when the installation process requires a modification to be made to the existing roof structure. The conventional method of drilling a hole into the roof is the obvious example of roof penetration, but grinding a tile or raising a tile from its original position is also an example.

All modifications to a roof structure must be undertaken carefully, as roofs are designed to keep wind and rain out of the building. Any installation on a roof must not compromise the roof’s function and longevity. By modifying a roof during the installation process, the installer shoulders some of the responsibility for the roof’s functions, e.g. waterproofness, and can therefore be liable for leaks. Ensuring that any penetrations in a roof are appropriately sealed, secure and as long lasting as possible will help the installer to meet these requirements.

It is strongly recommended to refer to the mounting system and roofing manufacturer’s guidelines and to ensure that all penetrations are suitably sealed and waterproof.

0

10

9

8

7

6

5

4

3

2

1

2008 201220102009 2011

US Dollar per watt

Services

BOS Hardware

Module

35

TH

EME

4.1.

1

Corrugated and Other Metal RoofsA good method of sealing penetrations around small entries such as conduits on corrugated and other metal roofs is to use small durable rubber rings or silicon flashing. Ensure that the seal is UV stabilised and freeze proof.

FIGURE 18: A SELF-SEALING MOUNTING BASE

Image source: GIZ/S4GJA more detailed view of how to screw a self-sealing mounting base onto a corrugated metal roof.

FIGURE 19: A MOUNTING SYSTEM

Image source: GIZ/S4GJA mounting system has been fixed onto a corrugated training roof commonly used in TVET col-leges.

36

TH

EME

4.1.

1

Tile RoofsThe previous advice for corrugated and other metal roofs also applies to tile roofs, but installations on tile roofs typically require grinding out numerous tiles to fix the hooks onto the beams of the trusses. Sealing these holes properly can be tedious, but it is one of the most important activities in the installation process. Replacing all broken tiles with new ones or fixing large flexible metal sheets that cover the penetrated tile is the required standard.

WHAT NOT TO DO!

Simply using silicon to seal roof penetrations is not a sufficient method for sealing any roof penetration. Silicon may be easy to apply, but over the life of a PV system this material becomes hard and is unlikely to maintain a secure waterproof bond. Roof shapes usually change over time and hardened silicon will not flex sufficiently with the roof. This is of particular concern for corrugated and other metal roofs which are subject to thermal expansion due to diurnal temperature differences. Over time, the hardened silicon will become loose, leaving gaps for water to penetrate.

Your own notes

37

TH

EME

4.1.

1

Exercises

Answer the following questions:

Question 1What kind of material are PV cells made of?

Question 2State the individual electrical potential difference of a typical solar cell.

Question 3Considering the low individual electrical potential difference of solar cells, how are PV cells connected in a PV module to produce at higher potential and thus a greater usable amount of power?

Question 4Describe the different layers of PV modules.

Question 5How is the electrical performance of a PV module typically rated?Answer 5

Question 6How reliable are quality PV modules and what is their lifetime/warranty?

Question 7Explain the term Balance of System (BoS).

38

TH

EME

4.1.

1

Question 8What are the main differences between stand-alone and grid-connected systems?

Question 9The purpose of an inverter is to:

Question 10Sine wave inverters are recommended because:

Question 11Explain why DC circuit breakers operate very differently to AC breakers.

Question 12Why is it recommended to install suitable load-switching devices on both the DC and AC side of residential and commercial PV systems?

Question 13Why is it important to mount the rack and rails in such a way that the structure is lifted slightly above the roof?

Question 14Why is it important to appropriately seal any penetrations in a roof?

39

TH

EME

4.1.

1

Further Information (all materials are on the CD)

(i) Chakraborty (Centrosolar America) and GTM Research, http://www.greentechmedia.com/articles/read/Its-Solar-Balance-of-System-Innovation-That-Will-Drive-Cost-Reduction.

(ii) Trinamount I for Tiled Roofs Installation (Video) https://www.youtube.com/watch?v=cwKG7Yrj3ug

(iii) SunLock mounting system instructional (Video) https://www.youtube.com/watch?v=SzW5X1ZOL5w

(iv) Renewable Energy Solar Panel System Components (Video) https://www.youtube.com/watch?v=zLLBJfdqPVM

(v) MPPT (Video) https://www.youtube.com/watch?v=0ItjKs7aJFM

(vi) Charge Controllers (Video) https://www.youtube.com/watch?v=qGmEz58Ixk4

(vii) Inverters (Video) https://www.youtube.com/watch?v=sZx6qDj2EOo

Your own notes

40

TH

EME

4.1.

2

THEME 4.1.2

SEMICONDUCTOR MATERIALS AND THE PHOTOVOLTAIC EFFECT

Introduction

The following theme explains in simplified terms how semiconductor materials are used in solar cells and how the process of converting sunlight into electrical current works.

Keywords

PV cell typesCrystalline siliconThin filmConversion efficiencyTemperature coefficient Poly (multi)-crystallineMono-crystalline Amorphous thin filmCopper indium gallium selenide thin filmCadmium telluride thin filmSemiconductorsDopingP–TypeN–TypeP-N JunctionPhotovoltaic effect

Theme OutcomeAt the end of this theme, you should be able to:

(i) List the semiconducting materials used to produce the main types of solar cells/panels.(ii) Describe and explain the photovoltaic effect. (iii) Compare different PV module technologies.

41

TH

EME

4.1.

2

Definition of Terms

PV Cell and Thin Film TypesThere are two broad categories of technology used for PV modules, namely, wafer based crystalline silicon cells and thin film technology. The ‘family tree’ in Figure 1 gives you an overview of the most es-tablished and commonly available technologies, and Table 1 and 2 offer a comparison regarding market share and conversion efficiency (radiation energy to electrical energy).

FIGURE 1: THE TWO MAIN TYPES OF MATERIALS USED FOR THE CONSTRUCTION OF PV PANELS

Image source: GIZ/S4GJTwo main types of materials used for the construction of PV panels can be differentiated, i.e. wafer based crystalline silicon cells and thin film technologies.

TABLE 1: GLOBAL MARKET SHARE OF WAFER BASED CRYSTALLINE SILICON CELLS AND THIN FILM TECHNOLOGIES

Technology Market Share (%)

Mono-crystalline 54

Poly-crystalline 36

Amorphous silicon 6

CIGS 2

CdTe and other thin film 2

Adapted from International Renewable Energy Agency, IEA-ETSAP and IRENA Technology Brief E11, January 2013 (see Further Information on CD)

Crystalline Silicon

(wafer based)Thin Film

Poly-crystalline

Mono-crystalline

Amorphous-Si

(a-Si)

CIGS

(Copper Indium

Gallium Selenide)

CdTe

(Cadmium Telluride)

. . . . . .

42

TABLE 2: CONVERSION EFFICIENCIES (INDUSTRIAL MASS PRODUCTION) OF WAFER BASED CRYSTALLINE SILICON CELLS AND THIN FILM TECHNOLOGIES

Technology Module efficiency(%)

Mono-crystalline 14–20

Poly-crystalline 14–18

Amorphous silicon 8

CIGS 10–14

CdTe and other thin film 9–12

Adapted from http://en.wikipedia.org/wiki/Crystalline_silicon

Apart from different appearances and structural properties (rigid or flexible), the most obvious differ-ence amongst PV technologies is their conversion efficiency, which subsequently will result in different area/space requirements for the different technologies. For example, a thin film amorphous silicon PV array may need twice the area/space of a mono-crystalline array due to their different nominal electrical capacity under Standard Test Conditions (STC). In any case, module efficiency is a primary concern in the PV industry. Sufficient cost savings from module manufacturing can be suitable to offset reduced efficiency in the field, such as the use of larger solar cell arrays compared with more compact/higher effi-ciency designs. Thus, even though thin film technology has a lower efficiency, it can be attractive because of the low production costs. On the other hand, modules with a higher efficiency have the advantage of occupying less space and the issue of efficiency versus cost is an important decision based on whether one requires a more efficient device due to area constraints or not. Consequently, and due to their various properties, performance parameters and price per watt, each PV technology has characteristic con-straints and advantages.

Another important distinction between the different PV technology types is their temperature coefficient of power (Table 3 and Figure 2). Generally, PV module performance declines as module temperature rises (see module datasheets), but thin film technologies have a lower negative temperature coefficient compared to crystalline technologies and tend to lose less of their rated capacity as temperature rises. Thus, under hot climatic conditions, thin film technologies will generate 5‒10 percent more power per year compared to crystalline technologies.

TABLE 3: PV TECHNOLOGIES AND THEIR DIFFERENT TEMPERATURE COEFFICIENTS

Technology Temperature Coefficient(% per degrees Celsius)

Crystalline silicon -0.4 – -0.5

Amorphous silicon -0.21

CIGS -0.32 – -0.36

CdTe and other thin film -0.25

Adapted from various sources, for example Performance of Photovoltaics Under Actual Operating Conditions, G. Makrides, B. Zinsser, M. Norton and E. Georghiou, 2012 or Overview of Temperature Coefficients of Different Thin Film Photovoltaic Tech-nologies, A.Virtuani*, D. Pavanello, and G. Friesen, 2010 (see Further Information on CD).

43

TH

EME

4.1.

2

FIGURE 2: AVERAGE EFFECTS OF TECHNOLOGY SPECIFIC TEMPERATURE COEFFICIENTS OF POWER ON PV MODULE OUTPUT PERFORMANCE

Image source: GIZ/S4GJ Adapted from Overview of Temperature Coefficients of Different Thin Film Photovoltaic Technologies, A.Virtuani*, D. Pavanello, and G. Friesen, 2010 (see Further Information on CD). The average effects of technology specific temperature coefficients of power on PV module output per-formance under different climatic conditions (module temperature between 25° C ‒ 85° C) relative to performance under STC (25° C).

Crystalline SiliconMost solar cells produced today are made from silicon (Si), the second most abundant element on Earth and the primary ingredient in sand. The first distinctive process step in solar cell manufacturing is silica processing (chemical purification) which creates silicon chunks. The next step focuses on the production of ingots, i.e. blocks or bars of high-purity silicon. To do this, large amounts of silicon chunks need to be crushed to a powder. The useful electrical properties of crystalline silicon as semiconductor materials are based on extremely small amounts of impurities (see doping) added to the silicon powder. The mixture is then heated at high temperatures and ingots are formed. The ingots need to cool down before they are sliced into thin wafers which are usually only 0.2 mm (200 microns) thick, similar to the thickness of a piece of paper. These wafers are tested, inspected and processed further, including a chemical surface texturing process to reduce the reflectivity of the wafer. Additional mechanical and chemical process-es will then transform the wafer into a photovoltaic cell to which metal contacts on the front and rear surfaces are attached.

44

TH

EME

4.1.

2

FIGURE 3: FROM SAND TO PV MODULES

Image source: http://en.wikipedia.org/wiki/File:Polysilicon_compilation.jpg, http://en.wikipedia.org/wiki/SandThe most common constituent of sand is silica (silicon dioxide, or SiO2). Mechanical and chemical pro-cesses transform silica into the silicon wafers which are used for the production of solar cells, integrated circuits and other semiconductor devices.

Poly-crystalline (or Multi-crystalline) Poly-crystalline cells are effectively a slice (wafer) cut from a block of silicon (ingot), consisting of a large number of crystals which have a speckled reflective appearance (see Figure 3). Poly-crystalline cells are slightly less efficient (see Table 2) and less expensive compared to mono-crystalline cells and need to be mounted into a rigid frame.

Mono-crystalline In appearance, mono-crystalline PV cells have a more uniform, smooth texture compared to poly-crys-talline cells. Mono-crystalline cells are more efficient but also more expensive to produce when com-pared to poly-crystalline. Like poly-crystalline, mono-crystalline PV cells must be mounted in a rigid frame to protect them.

Amorphous/Thin FilmAmorphous cells are manufactured by placing a thin film of amorphous, non-crystalline silicon onto a wide choice of surfaces. They are the most well-developed thin film technology to-date, but also the least efficient (see Table 2) and least expensive silicon-based cells to produce. Due to the amorphous nature of the thin layer, the material is flexible and if manufactured on a flexible surface, the whole solar panel can be flexible. However, one inconvenient characteristic of amorphous solar cells is that their power output significantly reduces over time, about 10‒30 percent during the first six months of operation (stabilisa-tion). Thus, the quoted electrical output of amorphous cells/modules should make reference to the power output after stabilisation.

Copper Indium Gallium Selenide (CIGS)/Thin FilmCIGS is one of three mainstream thin-film PV technologies and although not as efficient as mono-crys-talline silicon cells, CIGS has the benefit of lower priced material costs. CIGS has a high light absorption coefficient and requires a much thinner film than other thin film semiconductor materials.

45

TH

EME

4.1.

2

Cadmium Telluride (CdTe)/Thin FilmCadmium telluride has potential problems with the high toxicity of cadmium (Cd) and the limited avail-ability of tellurium (Te). Te production and reserves are subject to uncertainty and thus vary consider-ably. Tellurium is primarily used as an additive to steel and almost exclusively obtained as a by-product of copper refining. On the other hand, CdTe technology can efficiently be produced and has a shorter energy payback time compared to other PV technologies. CdTe module efficiency is similar to CIGS.

SemiconductorsSemiconductors are the foundation of modern electronics and the most common semiconductor materials are silicon (Si) and germanium (Ge). Most semiconductor chips and transistors are based on silicon and you may have heard the expressions ‘Silicon Valley’, a nickname for the southern portion of Northern Califor-nia’s Bay Area (USA). The term originally referred to large numbers of silicon chip innovators and manu-facturers of the bay region, but eventually was used to refer to all high-tech businesses in that area.

FIGURE 4: INTEGRATED CIRCUIT (IC) CHIPS

Image source: http://en.wikipedia.org/wiki/File:Three_IC_circuit_chips.JPGSimilar to crystalline PV cells, most integrated circuit (IC) chips, like the three ICs in the image, are based on silicon due to its semiconductor qualities.

Semiconductors have had a massive impact on modern industrialised societies which rely on micro-processors (IC chips) and transistors. Any device that is computerised or uses radio waves depends on semiconductors. A semiconductor is a substance, usually a chemical element or a compound, with an electrical conductivity between that of a conductor, such as copper, and an insulator, such as glass (see also RET Student Book 2, NQF Level 2, Theme 2.2.1, page 52). Semiconductors can display a range of useful properties, such as passing current more easily in one direction than in the other, showing vari-able resistance, and sensitivity to light or heat. Electronic and photovoltaic devices made from semicon-ductors can be used for amplification, switching, and energy conversion.

Why use Silicon?Silicon is a common choice for semiconductors due to its inherent physical properties. You may recall from RET Student Book 2, NQF Level 2 (Theme 2.2.1, page 51) that all matter is made up of fundamental building blocks known as atoms and that each atom consists of electrons, protons and neutrons. The electrons and the protons carry a charge, and the charge of an electron is negative, while a proton carries a positive charge of the same magnitude as the electron. Electric charge, either negative or posi-tive, is the most basic quantity in an electric circuit. The silicon atom has 14 electrons, but their electron arrangement allows only the outer four of these 14 electrons to be used for bonds with other atoms. These outer four electrons, called ‘valence’ electrons, play an important role in the semiconductor properties and the photovoltaic effect.

46

TH

EME

4.1.

2

Through their valence electrons, large numbers of silicon (Si) atoms can bond together to form a crystal. In a crystal, each Si atom normally shares one of its four valence electrons in a bond with each of the four neighbouring Si atoms (see Figure 5). The basic unit of the crystal structure or lattice consists of five Si atoms, i.e. the original Si atom plus the four other atoms with which it shares its four valence electrons. Thus, in a crystal, each Si atom is bonded to four other Si atoms and each Si atom shares each of its four valence electrons with each of the four neighbouring Si atoms. In this arrangement, each Si atom has eight electrons in its outer orbit or shell and is very stable. With eight electrons in the outer orbit, silicon is an almost perfect crystal and has no free electrons and thus cannot conduct a charge.

FIGURE 5: THE SILICON (SI) ATOM

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Image source: GIZ/S4GJA silicon (Si) atom has four valence electrons and bonds together with other Si atoms in a lattice type structure, where every atom in the structure bonds with four other Si atoms.

DopingThe useful electrical properties of semiconductor materials are based on very, very small amounts of impurities (dopants) added to it. The amount of dopant added to silicon has a ratio of 1 part in 108. This is 1 gram of dopant material to 100 metric tons of silicon. However, the effect is quite amazing and will change the electrical properties of silicon significantly. There are two types of impurities:

P–type In P–type doping, boron (B) or gallium (Ga) is used as the dopant. Boron, for example, has three elec-trons in its outer shell and when a B atom takes the place of a Si atom in the lattice structure, the single impurity B atom creates a missing bond in the lattice. The missing bond or incomplete bond is also called a ‘hole’. Thus, when boron is mixed into the silicon crystal, ‘holes’ will form in the silicon lattice due to the fact that one electron per B atom is deficient or missing for perfect bonding. Consequently, and given, that a ‘hole’ can accept an electron from a neighbouring Si atom, ‘holes’ can conduct a negative charge. The moving electron subsequently creates a new ‘hole’, resulting in another incomplete bond, thus trig-gering a kind of a chain reaction where electrons are moving around in the crystal and creating ‘holes’ which can accept moving electrons. Due to this effect, P–type silicon can conduct a negative charge and is a viable conductor.

47

TH

EME

4.1.

2

FIGURE 6: A BORON (B) ATOM TAKES THE PLACE OF A SI ATOM IN THE CRYSTAL LATTICE

B

Si

Si

Si

Si

B

Si

Si

Si

Si

‘Hole’ or incomplete bond

Image source: GIZ/S4GJWhen substituting a Si atom with a boron (B) atom in the crystal lattice, the three valence electrons of the B atom can only bond with three of the four Si atom neighbours. Thus, the bond with the fourth Si atom neighbour remains incomplete and a ‘hole’ appears. The incomplete bond attracts electrons from the neighbouring Si atoms, which will move to ‘fill the hole’.

N–typeIn N–type doping, phosphorus (P) or arsenic (As) is added to the silicon (Si) in small quantities. Phos-phorous has five electrons in its outer shell and when a P atom takes the place of a Si atom in the lattice structure, an additional electron is available in the lattice structure. Thus, a minute amount of either N–type or P–type doping turns a silicon crystal from a good insulator into a viable, but not great conductor, hence the name ‘semiconductor’.

FIGURE 7: A PHOSPHORUS (P) ATOM TAKES THE PLACE OF A SI ATOM IN THE CRYSTAL LATTICE

Image source: GIZ/S4GJWhen substituting a Si atom with a phosphorus (P) atom in the crystal lattice, four of the phosphorus va-lence electrons form bonds with the neighbouring Si atoms. But the fifth additional phosphorus electron remains unbounded or only weakly bonded and can move around in the Si crystal and as a result, the phosphorus atom becomes positively charged (ionised).

P

Si

Si

Si

Si

P

Si

Si

Si

Si

Additional Electron

48

TH

EME

4.1.

2

FIGURE 8: ‘DOPED’ OR IMPURE (COMPOUNDED) SILICON

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

N-TypeAdditional

Electron (q-)

P-TypeIncomplete

bond or ‘hole’

(q+)

Image source: GIZ/S4GJ‘Doped’ or impure (compounded) silicon has a lattice with atoms which either contain more electrons to create negatively (q–) charged silicon (N–type) or a lattice with atoms which contain less electrons to create positively (q+) charged (P–type) silicon.

P-N JunctionNow you know that the N–type and P–type silicon create unique properties of semiconductors. N–type and P–type silicon are not so amazing by themselves, however, when both types are placed together, some very interesting behaviour appears at the P-N junction. We already mentioned in the above section that P‒doped semiconductors, i.e. boron doped silicon, can be conductive. The same is true for N‒doped semiconductors, i.e. phosphorous doped silicon. The P–N junction between them is, however, non‒conducting for the reason that the excess electrons in the N‒type silicon layer move into the P‒type layer and eliminate each other’s charge. As N‒type silicon layers move, they leave positively charged ‘holes’ in the N‒type layer. During this process, while the charges are neutralised, a potential difference, called built-in potential or built-in electric field, is created with a magnitude of approximately 0.5‒0.6 volts. The built-in potential creates an electric field across the P–N junction and is the significant factor in the operation of a semiconductor, including PV cells, as it drives the current across the P–N junction into the N‒type layer and through an external electrical circuit.

FIGURE 9: THE DEPLETED P-N JUNCTION

+

+

+

+

+

+

+ ++

+

+

+

+

+

++

+

+

+

+

+

+ +

++

-

-

--

-

-

- -

--

-

-

-

-

-

-

-

-

-

- - -

-

-

-

--

--

-

-

--

-

-

--

-

+

+

+

+

+

++

+

+

+ +

+

+

-

-

-

-

-

-

+

+

+

+

+

+

+ -

+

-

VoltDepletion

Layer-

+

+

Depletion

Layer- Free Electrons

Positive Ion (P)

Negative Ion (B)

+ Holes

N-type P-type

Potential difference

across the junction

-

Image source: GIZ/S4GJThe depleted P–N junction with separated (neutralised) charges has a potential difference, called built-in potential or built-in electric field, with a magnitude of approximately 0.5‒0.6 volts.

49

TH

EME

4.1.

2

The Photovoltaic EffectMany semiconductors can convert sunlight into electrical energy, a process called ‘photovoltaic effect’, and PV cells are explicitly designed and manufactured to exploit this effect. The word ‘photovoltaic’ is a combination of the Greek word for light (photo = light) and the name of the Italian physicist A. A. Volta (1745–1827) who invented the first battery (voltaic = electrical energy).The SI unit of electrical potential difference is called volt (V), in honour of Mr. Volta. Thus, photovoltaic refers to the direct conversion of sunlight into electrical energy by means of semiconductor material in PV modules.

The Photovoltaic Effect: A Simplified ExplanationWhen sunlight is absorbed in a PV cell it transfers its energy to atoms, energises them and knocks elec-trons out of their orbit. The electrons then move through the PV cell layers and create a charge flow (a DC current).

The Photovoltaic Effect: A More Detailed ExplanationWhen photons are absorbed into the atomic structure of the silicon lattice, their energy is transferred to electrons. These energised electrons can then move from their normal positions in the atoms of the semiconductor, triggering a negative charge flow (current). A special electrical property of the P–N junc-tion in the PV cell, called the built-in electric field, provides the force or potential difference necessary to drive the current across the P–N junction of the PV cell into the N‒type layer and through an external electrical circuit.

FIGURE 10: ELECTRIC CURRENT IN A PV CELL

Image source: GIZ/S4GJWhen photons are absorbed in the P‒type silicon, electrons will be dislodged and a negative charge will flow from the cathode (N-type silicon) to the anode (P-type silicon) creating an electric current in a short-circuited PV cell.

The Photovoltaic Effect: An In–Depth ExplanationSunlight is composed of both particles (photons) and waves. The photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum. When photons strike a PV cell, they may either be reflected, pass right through the cell or get absorbed into the cell’s crystal lattice. Absorption of photons can create a light‒generated current. The generation of such light‒generated cur-rent happens inside the depletion zone of the P‒N junction, the contact area between the N-type and the P-type semiconductor. The P-type material has incomplete bonds or ‘holes’ in their crystal lattice holes, creating a positive charge. The N-type material has mobile negative charges, i.e. additional electrons. Near the junction, the N-type material electrons diffuse across the junction, combining with ‘holes’ in P-type material. The region of the P-type material near the junction takes on a net negative charge because of the electrons attracted. Since electrons departed the N-type region, it takes on a localised positive charge. The thin layer of the crystal lattice between these charges, the P‒N junction, has been depleted of charges and thus becomes

Light energy (Photons)

q-

+

-

Electrode

P-N junction

Current

N-type

silicon

Glass Cover with

Anti-reflection coating

P-type

silicon

q- q-

Current

q-

Electrode

50

TH

EME

4.1.

2

Exercises can be found in Unit 4.2

Please carry-out:(i) Experiment 2: The solar cell as energy converter (ii) Experiment 3: The solar cell as energy converter and/or diode

nonconductive semiconductor material (separation of charges). In effect, this is almost an insulator separating the conductive P and N doped regions. Due to the separation of charges, an electric field (and a potential difference) has been established across this region. The separation of charges at the P‒N junction, or more precisely, the built-in electric field, constitutes a potential barrier for charge flow. This barrier must be overcome by an external energy source to make the junction conduct. Thus, the problem now is that an electron (negative charge) would require some extra energy to cross the depleted P‒N junction. The formation of the P‒N junction, which causes this poten-tial charge barrier, happens during the semiconductor manufacturing process. The magnitude (potential difference) of the barrier is a function of the materials used in manufacturing, in PV cell silicon P‒N junctions approximately 0.5‒0.6 volts. Due to the depleted P–N junction, the built-in electric field is always present across the PV cell creating a force, or potential difference, and promoting only one‒directional current to flow across the junction (barrier). In PV cells, the extra energy required by an electron to cross the charge depleted P‒N junction towards the N-type layer can be provided by photons. When a photon is absorbed by one of the atoms in the P‒type silicon, it will dislodge and free an electron. Due to photon absorption, the free electron now has sufficient energy to cross the depletion zone, resulting in a negative charge flowing towards and out of the N-type silicon. Thus, if a circuit is established from the cathode (N-type silicon) to the anode (P-type silicon), i.e. if the PV cell is short-circuited, a negative charge will flow through the external circuit creating an electric current.

Further Information (all materials are on the CD)

(i) How it’s made - Solar panel (Video) https://www.youtube.com/watch?v=BKrOZ6OogmQ

(ii) From sand to silicon (Video) https://www.youtube.com/watch?v=jh2z-g7GJxE

(iii) Silicon Wafer Production (Video) https://www.youtube.com/watch?v=AMgQ1-HdElM

(iv) Semiconductor junction (Video) https://www.youtube.com/watch?v=oU_aTkPzaN8

51

TH

EME

4.1.

3

THEME 4.1.3

PV MODULE DATASHEETS AND OUTPUT PARAMETERSIntroduction

At some point, everybody involved in planning, sizing and/or installing a PV system (or any other mechanical, electrical or electronic components) needs to check the datasheets of the selected system components. This theme will introduce you to the facts of why datasheets are so important and what information you can expect to find in them. You may also recall that in RET Student Book 3, NQF Level 2 (Safety) we introduced you to Material Safety Data Sheets (MSDS) which provide you with important product information. Melting or boiling points, toxicity, health effects, storage, disposal and handling procedures of products and materials are all important for occupational health and safety. The same applies for the technical components of a PV system and a technical datasheet will provide you with product information required to work in a safe and efficient manner.

Datasheets of similar components, say PV modules, may differ from manufacturer to manufacturer and some may be more difficult to read and understand than others. Manufacturers of quality components want you, the planner or installer, to have a successful experience with their components and thus try to be as helpful as possible. As a rule, if there is no or only a poor quality datasheet provided by the manu-facturer, you can almost bet that the product is of poor quality as well! Quality products from established manufacturers will, for example, always provide you with a good installation- and user manual. As the saying goes: You usually get what you are paying for...

Keywords

DatasheetStandard Test Conditions (STC)Nominal Operating Cell Temperature (NOCT)Maximum power point (MPP)Open circuit voltage (Voc)

Voltage at maximum power (Vmp)Nominal voltage Short circuit current (Isc)Current at maximum power (Imp)Maximum or peak power (Pmax or kWp)Module efficiencyMaximum system voltageSeries fuse ratingType of output terminalI‒V curves

Theme Outcome

At the end of this theme, you should be able to: (i) Interpret sample datasheets with reference to standards, certifications and warranties.(ii) Identify and measure key electrical output parameters using multimeters.(iii) Explain and sketch the current-potential (I-V) curve of a PV module in a diagram.

52

TH

EME

4.1.

3

Definition of Terms

DatasheetsIn general, a datasheet is a document that summarises the performance and other technical characteris-tics of an electric or mechanical machine, component or software in sufficient detail, enabling a skilled worker or engineer to integrate the machine or component into a new or existing system or technical structure. Typically, a datasheet is created by the manufacturer and begins with an introduction, fol-lowed by a listing of specific characteristics and further information on the connectivity of the device. Datasheets are published by manufacturers to help people choose the right products and to install them correctly. Where do you find datasheets? Nowadays you can find almost any datasheet from quality man-ufacturers on the internet, often in PDF format for you to download.

A typical datasheet for PV system components usually contains most of the following information:

(i) Manufacturer’s name(ii) Product number and name(iii) Drawings or images showing physical details, such as minimum/maximum dimensions,

contact locations and sizes(iv) Important device properties(v) Functional description and installation instructions(vi) Absolute minimum and maximum ratings (power supply and/or consumption, input cur-

rents, temperatures for storage, operating etc.)(vii) Recommended operating conditions (as absolute minimum and maximum ratings)(viii) DC and AC specifications (various temperatures, input/output potential and currents etc.)(ix) Safety instructions, warranties and liability disclaimer

Simply put, a well designed datasheet of a quality product will tell you everything you need to know about it and we strongly recommend reading and using this information. Many design and installation errors are due to overlooking or disregarding (deliberately or not) certain information in the datasheet. Again, consider that manufacturers want you to have a successful experience with their products. The reason they are going to the trouble of producing a datasheet is that they are trying to be helpful. Un-fortunately they do not always succeed - some datasheets are not very user‒friendly and present a large amount of information in an incomprehensible manner.However, we advocate reading the datasheet before you buy the product. It is really worth taking the time to read it, particularly if you more or less know what you are looking for or what electrical and mechan-ical parameters are required. You don’t have to understand everything on a datasheet. There might be a lot of information that is not of any particular use to you, but the annotations that follow try to direct you to the datasheet parts and parameters of PV system components that you should pay particular attention to.

PV Module Sample DatasheetsA typical datasheet for PV modules usually contains the electrical output parameters under STC (Stan-dard Test Conditions) and NOCT (Nominal Operating Cell Temperature), including peak power (Pmax), potential (voltage) and current at open circuit (Voc and Ioc), maximum power point (Vmp and Imp) and the I-V curve etc. Other electrical parameters include module efficiency, temperature coefficients, mechan-ical parameters such as dimensions, weight etc. and certifications and warranties. We will illustrate this information on a sample datasheet and will explain some important electrical parameters in more detail below.

53

TH

EME

4.1.

3

FIGURE 1: RELEVANT PARAMETERS INDICATED IN A SAMPLE DATASHEET FOR A PV MODULE

Sample Data Sheet for a PV ModuleElectrical Specifications Mechanical & GeneralSTC

Peak power

Voltage at max power

Current at max power

Voltage at open circuit

Current at short circuit

1,000 W/m , 25°C, 1.5 AM2

Pmax

Vmp

Imp

Voc

Ioc

220 watts

29.8 volts

7.39 amps

36.8 volts

8 amps

NOCT

Peak power

Voltage at max power

Current at max power

Voltage at open circuit

Current at short circuit

800 W/m , 47,2°C, 1.5 AM2

Pmax

Vmp

Imp

Voc

Ioc

159 watts

27 volts

5.9 amps

34 volts

6.47 amps

99 cm

166.37cm

3.81 cm

92.25 cm

Leads:101.6 cm

166.37 x 99 cmDimensions

Area

Thickness

Weight 17.96 kg

3.81 cm

17.7 k2

60 monocrystalline siliconCells

Cell dimensions

Glazing

Backsheet Double-layer, high-perfomancepolyester

High-transparency, low-iron,tampered glass with

antireflection treatment

15.24 x 15.24 cm

Encapsulation

Frame Black anodised aluminium

Ethyl vinyl acetate

Connectors 12 AWG, PV Wire, Tyco connector

Junction box

Bypass diodes 3 diodes

Tyco Solarlak

Modules/pallet;Pallets/container

20 modules/pallet28 pallets / 40 k. container

Design load

Maximum wind speed 193.12 kph

34.02 kg / k2

Other Electrical ParametersPower tolerance Percent x3%

Efficiency Cell 15.5%

Module 13.5%

Temperaturecoefficients

Pmax -0.45 per C°

Voc

Vmp

Isc

-0.35 per C°

-0.42 per C°

+0.05 per C°Maximum system voltage

Max. series fuse rating 15 amp

600 volts

Warranty90 % rated power 10 years limited

80 % rated power 25 years limited

Workmanship 5 years

Listing

Fire safety class 193.12 kph

Ul 1703

Certifications & Ratings

1

5432

5 10 15 20Voltage

Amperage

0250

STC

Peak Power =159 W

876

109

30 35 40

Peak Power =220 W

NOCT

Isc = 8Imp = 7.39Isc = 6.47

Imp = 5.9

Vmp =27

Vmp =29.8

Voc =34

Voc =36.8

Proudly powered by the sun

Image source: GIZ/S4GJ

54

TH

EME

4.1.

3

Cell TypeThe type of technology used in the PV module, e.g. either mono-crystalline, poly-crystalline or thin film.

Numbers of Cells and ConnectionsThis parameter tells you how many PV cells are contained within the module and how they are con-nected (usually in series).

Standard Test Conditions (STC)STC represents standardised test conditions that all PV modules are tested and rated at. STC is done at a temperature of 25° C, irradiance of 1000W/m2, and an air mass value of 1.5.

Nominal Operating Cell Temperature (NOCT)NOCT is defined as the temperature reached by open circuited cells in a module under the following conditions:

Irradiance on cell surface = 800 W/m2, air temperature = 20° C, wind velocity = 1 m/s, mounting = open back side.

Note the somewhat lower insolation conditions compared to STC. In addition, module performance is often measured at an operating temperature of 45° C instead of 20° C. At 45° C, NOCT is much more realistic and probably close to the cell temperature PV modules are likely to operate at. Consequently, and given that PV modules typically operate at higher temperatures and possibly at somewhat lower insolation conditions, more realistic power outputs are stated under NOCT.

Maximum Power Point (MPP)This is the point on the current-voltage (I-V) curve of a PV module where the product of current and potential is at its maximum.

Open Circuit Voltage (Voc)This is the potential measured at the module terminals when no load is applied. In other words, the circuit is ‘open’ and not connected to any load. Voc is the maximum possible potential across a photo-voltaic cell or module.

Voltage at Maximum Power (Vmp)This is the potential difference magnitude of a given device, usually a PV module, at its maximum power point.

Nominal Voltage Nominal voltage refers to the voltage of the battery that the module is best suited to charge; this is a leftover term from the days when solar modules were only used to charge batteries. The actual voltage output of the module changes as lighting, temperature and load conditions change, so there is never one specific voltage at which the module operates. Nominal voltage allows users to, at a glance, make sure the module is compatible with a given system.

Short Circuit Current (Isc)This is the current (I) that flows through the module terminals at short circuit. Isc is the maximum amount of current that the solar panel can provide.

Current at Maximum Power (Imp)This is the amount of current of a given device, usually a PV module, at its maximum power point.

Maximum or Peak Power (Pmax or kWp)Pmax is the amount of power (P) that can be produced under standard test conditions (STC). For ex-ample, a Pmax rating of 315 W = 0.315 kW. DC output power of PV modules typically ranges from 10 to 400 watt. Pmax is calculated by multiplying open-circuit voltage by short-circuit current (Voc * Isc).

Module EfficiencyEfficiency is the most commonly used parameter to compare the performance of one PV module to another. Efficiency is defined as the ratio of energy output from the solar cell to input energy from

55

TH

EME

4.1.

3

the Sun. In other words, if the solar irradiance is 1000W/m2, and the panel is a square meter, a 13.74 percent efficiency would give a module output of 137.4 W. The efficiency of a module also determines the area of a module given the same rated output. For example, a 230 watt module with only 8 percent efficiency will require twice the area of a 230 watt module with 16 percent efficiency.

Maximum System VoltageThis is a parameter used to determine how many modules of the same type can be connected together in series. If your maximum system voltage is, for example, 600 V and you connect 10 modules with an individual Voc (each module) of 36.6 V in series, a string voltage of 366 V would result, which would still be under the limit of 600 V.

Series Fuse RatingThis parameter gives the appropriate fuse/breaker size for each string.

Type of Output TerminalThe output terminal describes the type of electrical connector installed on the back of the module.

I–V CurvesThe I‒V (current-voltage) curve of a PV module describes its energy conversion capability at the existing conditions of irradiance and temperature. Conceptually, the curve represents the combinations of cur-rent and potential difference at which the module operates, if the irradiance and cell temperature could be held constant. In other words, I‒V curves are used to indicate the performance of a module allowing determination of the maximum power point (MPP).Referring to the I-V curve in Figure 2, you will notice that the curve ranges from the short-circuit cur-rent (Isc) at zero volt to zero current at the open-circuit voltage (Voc). The maximum power point (MPP = Imp * Vmp), the point at which the module generates its maximum electrical power, is usually at the ‘knee’ of a normal I-V curve. In an operating PV system, one of the jobs of the inverter with MPPT is to constantly adjust the load, seeking out the particular point on the I-V curve at which the module or array as a whole yields the greatest DC power.

FIGURE 2: DIAGRAM INDICATING THE MAXIMUM POWER POINT (MPP) OF A TYPICAL 75 WATT PV MODULE

1.0

5.0

4.0

3.0

2.0

5 10 15 2017

I

V

Potential Difference

Cu

rren

t

Short Circuit

CurrentMaximum

Power Point

(MPP)

Open Circuit

Voltage

Maximum

Power Point

(MPP)

Vmp

Imp

Image source: GIZ/S4GJA simplified diagram indicating the maximum power point (MPP) of a typical 75 watt PV module as the product of the potential (Vmp) and current (Imp), in our case at around 17 V and 4.1 A.

56

TH

EME

4.1.

3

FIGURE 3: DIAGRAM WITH AN I-V AND P-V CURVE PLOTTED TOGETHER

Potential Difference

Cu

rren

tCurrent vs. voltage

Maximum

Power Point

(MPP)

Vmp

Pow

er

Voc

Pmax

Imp

Isc

Maximum Power

Power vs. voltage

Image source: GIZ/S4GJSometimes you find diagrams where the I-V and the P-V curve of a module are plotted together. The P-V curve is calculated from the measured I-V curve.

How is an I-V curve for a module obtained?An I-V curve represents an infinite number of current-potential operating points. These current-poten-tial operating points are plotted between the short-circuit current point (Isc) where the module pro-duces maximum current and zero potential, and the open-circuit voltage point (Voc) where the module produces maximum potential and zero current. The point at which a PV device delivers its maximum power output and operates at its highest efficiency is referred to as its maximum power point (Pmax). The potential and current values at the maximum power point are referred to as the maximum power voltage (Vmp) and the maximum power current (Imp), respectively.

Considering the above, one can say that the I-V characteristic of a module is the basic descriptor of its performance. Thus, well designed datasheets indicate the I-V characteristic of a module und various irradiance and temperature conditions, usually 5 irradiance levels ranging from 1000 W/m2 to 200 W/m2 (see Figure 4) and three temperature levels ranging from 75° C to 25° C (see Figure 4). Obviously, differ-ent irradiation and cell temperature levels will alter a module’s performance. Increased irradiance levels will increase current but will not alter potential and vice versa. Increased cell temperature will lower potential and vice versa, but has little effect on current.

57

TH

EME

4.1.

3

FIGURE 4: I-V CURVES OF A PV MODULE

0.5

2.5

2.0

1.5

1.0

5 10 15 20

Potential Difference

Cu

rren

t

Maximum Power Points

0.0

3.5

3.0

250

G = 1,000 W/m2

G = 800 W/m2

G = 600 W/m2

G = 400 W/m2

G = 200 W/m2

Range

Image source: GIZ/S4GJThe five I-V curves of a PV module clearly indicate that current will dramatically change between around 0.5 and 3 A as irradiance varies, but potential remains relatively constant at around 20 V.

FIGURE 5: I-V CURVES OF A PV MODULE

Potential Difference

Cu

rren

t

50 C°

25°C

0 C°

Image source: GIZ/S4GJThe three I-V curves of a PV module clearly indicate that potential will dramatically change as tempera-ture varies, but current remains relatively constant.

58

TH

EME

4.1.

3

Examples

Apply your knowledge!Question: Does a PV module with a rated power (Pmax) of 190 W really produce 190 W when it is on your roof? The simple answer is: No!

Let us look into the matter step by step: 1. The maximum power the module is rated at (190 W) is based on the power output mea-

sured under Standard Test Conditions (STC) and these are, unfortunately, in most cases a long way from real operating conditions.

2. Under STC, the solar panel is subject to an irradiance level of 1000 W/m2. That in itself is not a problem, as irradiance levels in most parts of South Africa are even higher. The prob-lem is, however, that the STC power rating is based on a panel temperature of 25° C and an air mass of 1.5. Think about that for a second and consider particularly the temperature that a module will have when it is on your roof. Remember, temperature is critical because all modules lose efficiency at high cell temperatures (potential will dramatically decrease as temperature rises, but current will remain relatively constant). Realistically, cell tempera-ture will generally be between 45° C ‒ 60° C and thus, the real maximum power output of your module will differ from the rated power value.

3. So unless you install a module in a very sunny and cold place, like Antarctica, you are never going to get the rated power indicated by the manufacturer. Thus, let us work out the real maximum power of the 190 W module by using some information from the datasheet. The section that contains the temperature characteristics, the Nominal Operating Cell Temperature (NOCT) and the temperature coefficient of Pmax will assist us.

FIGURE 6: A SECTION FROM THE DATASHEET CONTAINS THE TEMPERATURE CHARACTERISTICS OF THE MODULE

Image source: GIZ/S4GJA relevant section from the datasheet contains the temperature characteristics of the module, including the Nominal Operating Cell Temperature (NOCT) and temperature coefficients (Isc, Voc and Pmax).

As indicated in an earlier section, NOCT is a much more realistic measure of the temperature that your panels are likely to operate at. In our case, NOCT is set at 45 °C ± 2 °C. The temperature coefficient of Pmax indicates how much power is lost for every degree Celsius above 25 °C (STC). Our 190 W module will lose 0.45 percent of its maximum power for every degree above 25 °C. NOCT indicates a typical 45 °C and power loss at this temperature can be calculated as follows:

(45 °C ‒ 25 °C) * (‒ 0.45%) = 20 * ‒ 0.45% = ‒ 9%

Temperature

coefficients

Pmax -0.45 per C°

Voc

Vmp

Isc

-0.35 per C°

-0.42 per C°

+0.05 per C°

59

TH

EME

4.1.

3

Thus, the real maximum power output of the module will be 9 percent lower at a cell temperature of 45 °C than the module rated power. Our module was rated at 190 W which equals 100 percent:

100% ‒ 9% = 91%.

Consequently, the real maximum power output of the module at a cell temperature of 45 °C would be approximately:

91% * 190 W = 172.9 W

However, please consider that there will be a number of additional power losses due to wiring, fuses, switches and inverters, and total losses can often reach 25 percent. Thus, system designers often oversize the PV system to ensure reliability. How much the system needs to be oversized is determined by the relative safety margin required and the amount of information available on solar insolation for the site. A 25 percent margin is recommended, although a site with well documented solar insolation and well known system components might require only a 15 percent margin.

Exercises can be found in Unit 4.2

Please carry out the following:(i) Experiment 6: The off-load voltage and the short-circuit current at different irradiance

values(ii) Experiment 11:Voltage-current curve of a solar cell(iii) Experiment 12: Efficiency determination/MPP

Further Information (all materials are on the CD)

(i) Guide To Interpreting I-V Curve Measurements of PV Arrays, Solmetric Corporation, 2010(ii) PV Modules I (Video)

https://www.youtube.com/watch?v=8-U7_sWu63g(iii) PV Modules II (Video)

https://www.youtube.com/watch?v=v8nbwvC8aBg

60

TH

EME

4.1.

3

Your own notes

61

TH

EME

4.1.

4

THEME 4.1.4

FACTORS AFFECTING THE PERFORMANCE OF PV MODULES

Introduction

Electrical energy produced by a PV system depends on several external and internal factors. External factors, such as the amount of solar radiation (irradiation/insolation), depend on the local climatic con-ditions and can only be influenced via optimal site selection. However, usually flexibility regarding selec-tion of sites is limited in most projects, for example for residential rooftop systems. Here the setting is by default predetermined (roof arrangements etc.). Other factors such as load resistance, module efficiency, array direction and tilt, cell temperature, shading, module mismatch and inverter conversion losses can, at least to a certain degree, be considered and attended to. Any of these factors can have a major impact on the PV system’s performance (power output). In this theme we will focus on the effects of series and parallel circuits, shading and partial shading of modules, as well as module direction and tilt, as these factors can have dramatic output effects.

Keywords

Series and parallel connectionsModule circuit designArray circuit designShading Bypass diodes

Theme Outcome

At the end of this theme, you should be able to: (i) List and explain critical factors that can affect the performance of PV modules.

62

TH

EME

4.1.

4

Definition of Terms

Series and Parallel In RET Student Book 2, NQF Level 2, Theme 2.3.1 (Simple DC Circuits) we introduced you to testing and measuring of electrical quantities and illustrated the two main types of electric circuits, i.e. the series or the parallel circuit and the effects they have on electrical quantities such as electric current, potential and resistance. The understanding of these topics now comes in handy and we strongly recommend consult-ing RET Student Book 2, NQF Level 2 to recall certain facts.

Let us briefly repeat the main findings from RET Student Book 2, NQF Level 2 below:

Series ConnectionWhen PV cells or modules are connected in series, the nominal potential of the PV system is increased and cur-rent remains constant. You have a series connection if you connect positive (+) leads to negative (-) leads (PV circuits in series). Usually the sequence in which PV cells are arranged in a PV module is in a series circuit.In a series circuit, the three resistors appear as a single equivalent resistance with the value of Req. Simply put, when resistors are connected in series, the total resistance (Req) is equal to the sum of resistance of the individual resistors. Therefore, let us conclude that in a series circuit the:

Total resistance (Req) = R1 + R2 + R3

Which can read as: The sum of the three resistors R1, R2 and R3 is equal to the total resistance (Req).

Total potential (VT) = V1 + V2 + V3

Which can read as: The sum of potential drops V1, V2, and V3 is equal to the total potential in the circuit current (IT). Thus, we can determine total potential (VT) as:

VT = I * Req

Current remains: I = constant, and can be expressed as: I = V / Req

Based on the above, we can finally describe resistor loads connected in series as potential dividers, due to the fact that the total potential (VT) divides among resistors. This is called the principle of potential division in a series circuit.

Application: We can conclude from these results, that when PV cells or PV modules are connected in series, and the circuit has a constant resistance (R), the potential of each cell or module is added together, but current remains the same.

Parallel ConnectionWhen PV cells or modules are connected in parallel, the nominal current in the PV system is increased and potential remains constant. Connect positive leads (+) to positive (+) leads and negative leads (-) to negative (-) leads to wire the PV circuits in parallel.

Let us recall the equation for equivalent resistance (Req) in parallel circuits and conclude that in a parallel circuit:Total resistance (Req) is: 1

Req = 1

R1+ 1

R2+ 1

R3

Which can read as: The equivalent resistance (Req) is less than that of the smallest of the resistors.

Total current (IT) is: IT = I1 + I2 + I3

Which can read as: The sum of the current I1, I2, and I3 is equal to the total circuit current (IT). Thus, by substituting the above equation, we can determine the total circuit current (IT) as:

IT = V * 1Req

Potential (V) remains: V = constant,

63

TH

EME

4.1.

4

We declare two or more circuit elements (resistor loads) to be combined in parallel if an identical po-tential (V) acts across each circuit element. Thus, in a parallel circuit potential values remain constant, whereas current (I) will divide when passing through a resistor. Based on the above, we can describe resistor loads connected in parallel as current dividers, due to the fact that the total current (IT) divides while passing through the resistors. This is called the principle of current division in a parallel circuit.

-+

Image source: GIZ/S4GJ

The total current of such a module is the current of an individual cell multiplied by the number of cells in parallel.The total potential of such a module is the potential of an individual cell multiplied by the number of cells in series. To conclude:Module Isc = Cell Isc * number of cells in parallelModule Imp = Cell Imp * number of cells in parallelModule Voc = Voc * number of cells in seriesModule Vmp = Vmp * number of cells in series

Array Circuit DesignIndividual PV modules are connected both in series and parallel to form arrays. A series-connected set of solar cells or modules is called a ‘string’. When PV modules are connected in series, the potential

Application: We can conclude from these results, that when PV cells or modules are connected in parallel, and the circuit has a constant resistance (R), the potential of each cell or module remains the same, but the current of each cell or module is added together.

These two principles for connecting PV cells or modules are used to build modules from individual PV cells and arrays from individual modules. The PV cells in a module can be wired to any desired poten-tial and current, and modules can then be connected with each other to create PV arrays to any desired potential and current.

Module Circuit DesignWhile the voltage from the PV module is determined by the number of solar cells, the current from the module depends primarily on the size of the solar cells and also on their efficiency. If all the solar cells in a module have identical electrical characteristics and they all experience the same insolation and tem-perature, then all the cells will be operating at exactly the same current and potential. In this case, the I-V curve of the PV module has the same shape as that of the individual cells, except that the potential and current are increased.

FIGURE 1: SCHEMATIC ILLUSTRATION OF 36 SINGLE PV CELLS CONNECTED IN SERIES TO FORM A MODULE

64

TH

EME

4.1.

4

of each module is added together, but current remains the same. When PV modules are connected in parallel, the potential of each module remains the same, but the current of each module is added togeth-er. Let us illustrate this by using a typical module with a power rating of 75 watt containing 36 cells in series (0.6 * 36 = 21.6). This usually provides an open-circuit voltage (Voc) of about 21 volt and a potential difference at maximum power (Vmp) of 17.5 V and current at maximum power (Imp) of 5 A.

FIGURE 2: TWO IDENTICAL MODULES CONNECTED IN SERIES

+- +-

Image source: GIZ/S4GJTwo identical modules with Vmp =17.5 V and Imp = 5 A connected in series will result in VT = Vmp1 + Vmp2 (17.5 V + 17.5 V = 35 V) and IT = Imp (5 A).

FIGURE 3: TWO IDENTICAL MODULES CONNECTED IN SERIES

Module 1 Module 1 + Module 2

Image source: GIZ/S4GJTwo identical modules connected in series will result in VT = Vmp1 + Vmp2 and IT = Imp, i.e. the potential of each module is added together, but current remains the same.

FIGURE 4: FOUR IDENTICAL MODULES CONNECTED IN SERIES

+- +- +- +-

Image source: GIZ/S4GJFour identical modules with Vmp =17.5 V and Imp = 5 A connected in series will result in VT = Vmp1 + Vmp2 + Vmp3 + Vmp4 (17.5 V + 17.5 V + 17.5 V + 17.5 V = 70 V) and IT = Imp (5 A).

65

TH

EME

4.1.

4

FIGURE 5: TWO IDENTICAL MODULES CONNECTED IN PARALLEL

+

-

+

-

+

-

Image source: GIZ/S4GJTwo identical modules with Vmp =17.5 V and Imp = 5 A connected in parallel will result in VT = Vmp (17.5 V) and IT = Imp1 + Imp2 (5 A + 5 A = 10 A).

FIGURE 6: TWO IDENTICAL MODULES CONNECTED IN PARALLEL

Image source: GIZ/S4GJ

Module 1

Module 1

Module 2parallel

Two identical modules connected in parallel will result in VT = Vmp and IT = Imp1 + Imp2, i.e. the potential of each module remains the same, but the current of each module is added together.

66

TH

EME

4.1.

4

FIGURE 7: FOUR IDENTICAL MODULES CONNECTED IN PARALLEL

+

-

+

-

+

-Parallel Circuit (4 solar panels)

+

-

+

-

Image source: GIZ/S4GJFour identical modules with Vmp =17.5 V and Imp = 5 A connected in parallel will result in VT = Vmp (17.5 V) and IT = Imp1 + Imp2 + Imp3 + Imp4 (5 A + 5 A + 5 A + 5 A = 20 A).

FIGURE 8: EIGHT IDENTICAL MODULES CONNECTED IN SERIES-PARALLEL

Image source: GIZ/S4GJEight identical modules with Vmp =17.5 V and Imp = 5 A connected in series-parallel will result in VT = Vmp1 + Vmp2 (17.5 V + 17.5 V = 35 V) and IT = Imp1 + Imp2 + Imp3 + Imp4 (5 A + 5 A + 5 A + 5 A = 20 A).

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

67

TH

EME

4.1.

4

Shading and Bypass DiodesShading of PV modules, even partial shading or covering, can be a serious problem since shading of just one cell in the module can compromise the power output of the entire system. The reduction of the power output caused by partial module or array shading may not be proportional to the portion of the surface in shadow, but might be much higher. If shading was uniform over the whole system, for exam-ple due to a cloud, then everything would be simple, as the current of all PV modules would drop by the same proportion and power loss would be proportional to the shading.

FIGURE 9: A SHADOW CAST ON A ROOFTOP PV SYSTEM

Uniform Shading Single Substring Shading

Examples of partial cell shading that will

reduce a solar electric panel’s power by 50%

Image source: http://www.shutterstock.com/s/photovoltaic+installation/search.html?page=1&thumb_size=mosaic&in-line=251921545In this image the roof casts a shadow on the rooftop PV system. This will affect the performance of the whole system, as the shadow intercepts a large quantity of a module series.

FIGURE 10: UNIFORM AND PARTIAL SHADING

Image source: GIZ/S4GJDifferent types of shading (uniform and partial) that will potentially compromise a module’s or array’s power output by up to 50 percent.

68

TH

EME

4.1.

4

Let us look into the effect of partial shading, where different panels experience different illumination due to shading. Figure 11 is a diagram with two different P-I curves indicating the characteristics for a sam-ple module at two illumination levels, i.e. fully exposed to sunlight and 25 percent (partially) shaded. In this example, the fully illuminated module (blue line) has a maximum current (Isc) of 10 A and achieves its highest maximum power (Pmax = 300 W) at an Imp of 8 A (see Theme 4.1.3). The partial (25 percent shaded) illuminated module has a maximum current (Isc) of around 7.5 A and achieves its highest maxi-mum power (Pmax = 225 W) at an Imp of 6 A.

FIGURE 11: P-I CURVES INDICATING TWO ILLUMINATION LEVELS

Image source: GIZ/S4GJThe diagram shows two different P-I curves indicating the characteristics for a sample module at two illumination levels, i.e. fully exposed to sunlight and 25 percent (partially) shaded.

Let us assume that we have a string of such modules connected in series, which has only one shaded module with partial (25 percent shaded) illumination, resulting in Pmax = 7.5 A at Imp = 6 A. Thus, this shaded module only achieves 75 percent of the current that the other modules need for Pmax. Given that the modules are connected in series, they share the same current and consequently the whole string will operate at Pmax of 75 percent, resulting in a 25 percent power loss just because of one shaded module. When all modules in a string are connected in series, the module with the lowest current (due to shading or otherwise) will reduce the power output of the whole string.

Due to the fact that it is difficult to prevent shading, PV module manufacturers typically install diodes in their modules. What is the function of these diodes? When cells or modules are shaded (or damaged), diodes can give the current another path, skipping the shaded (or damaged) cell or module. How does this work? This can best be illustrated by examining how a diode functions.

69

TH

EME

4.1.

4

FIGURE 12: THE DIODE

+-

electrons holes

N-type(not pointing)

P-type(pointing)

cathode anode

+-

electrons holes

N P

N P- -

-- -

-- -

- - - -

+ + + +

++ +

++ +

+ +

electrons holes

N P- -

--

-- -

- - -

+

+ + +

++

++ +

+ +

-

-

-

-

-

-

-

+

+

+

+

+

+

+

(a) Forward (b) Reverse

{depletion region

+ -

Image source: GIZ/S4GJA diode is the simplest possible semiconductor device. It allows current to flow in one direction but not the other. The N-type and P-type silicon create these unique properties in a diode.

Left-hand side of Figure 12: In this arrangement, the diode conducts charge as the additional electrons (negative charge) in the N-type silicon are repelled by the negative terminal of the battery (similar charges repel each other) and pass through the junction between the N-type and P-type silicon (‘elec-trons filling the holes’) with the effect that current flows through the junction.

Right-hand side of Figure 12: N-type and P-type silicon are both viable conductors, but the circuit shown on the right-hand side of Figure 12 does not conduct any charge. The negative electrons in the N-type silicon get attracted to the positive terminal of the battery and the positive charge in the P-type silicon gets attracted to the negative terminal of the battery. This is due to the fact that opposite charges (positive-negative) attract each other (see RET Student Book 2, NQF Level 2, Theme 2.2.1, page 51). Sub-sequently, no or very little current flows across the P-N junction, since the negative and positive charges in the diode are moving away from each other and towards the battery.

To conclude, the P-N junction acts like a one-way path that allows current to only flow in one direction. The advantage of this is that diodes can be used to block or bypass the flow of current from other parts of an electrical circuit. Thus, and in practice, bypass diodes in PV modules are connected in parallel to shunt the current around it, whereas blocking diodes are connected in series with the PV module to prevent current from flowing back into them. Blocking diodes are therefore different to bypass diodes, although in most cases the diode is physically the same - they are however installed differently to serve a different purpose.

70

TH

EME

4.1.

4

Consider the arrangement in Figure 13, where the green diodes are bypass diodes, one in parallel with each solar module to provide a low resistance path (short-circuit current). The two red diodes are referred to as ‘blocking’ diodes, one in series with each series branch. Blocking diodes are different to bypass diodes, not physically, but rather in the way they are installed, because they serve a different purpose. The blocking diodes, also called a series diodes or isolation diodes, ensure that the electrical current only flows in one direction, i.e. out of the series array to the external load (controller or batteries).

FIGURE 13: DIFFERENT TYPES OF DIODES IN A PV ARRAY

+

-

+

-

+

-

+

-

Bypass

Diodes

Blocking

Diodes

+

-

I = I + IT A B

IA

IB

Image source: GIZ/S4GJBypass diodes, one in parallel with each solar module to provide a low resistance path, are indicated in green. Blocking diodes, one in series with each series branch, are indicated in red.

71

TH

EME

4.1.

4

Shading and the Function of Bypass Diodes We already discussed earlier that it is difficult to prevent shading - this is why PV module manufac-turers install bypass diodes in their modules. We already know that:

(i) When part of a PV cell or module is shaded, the shaded cells or module will not be able to produce as much current as the unshaded cells or unshaded modules in an array.

(ii) If all cells in a module or modules in an array are connected in series, the same amount of current must flow through every cell and every module.

(iii) If the unshaded cells in the module force the shaded cells to operate at a current higher than their short circuit current, the shaded cells will cause a net potential loss in the system. In addition, the shaded cells will dissipate power as heat and can cause so-called ‘hot spots’. This situation can lead to irreversible cell damage, resulting in unintentional short circuits which, in a worst-case scenario, can trigger a fire.

(iv) One way to minimise the shading effect and potential module damage is to use bypass diodes in the junction box of the modules. Bypass diodes allow current to pass around shaded cells or modules and thereby reduce potential loss through the shaded module. When a module becomes shaded, its bypass diode becomes forward biased and begins to conduct current through itself. All the current greater than the shaded module’s short-circuit current is bypassed through the diode, thus drastically reducing the amount of heat dissipation at the shaded module area.

Image source: GIZ/S4GJThe arrangement on the left-hand side of the image indicates a normal functioning solar array with unshaded modules and a uniform Imp. The arrangement on the right-hand side of the image with the top module being (partly) shaded indicates a bypass mode where the bypass diode of the shaded module be-comes forward biased and begins to conduct current through itself (green arrows indicate current flow).

FIGURE 14: THE FUNCTION OF BYPASS DIODES IN A STRING

72

TH

EME

4.1.

4

Exercises can be found in Unit 4.2

Please carry out the following:Experiment 4: The off-load voltage of a solar cell/shading Experiment 5: The short-circuit current of a solar cell/shading Experiment 8: Series connection of solar cells/shadingExperiment 9: Series connection of solar cells/shading with bypass diodeExperiment 10: Parallel connection of solar cells/shadingExperiment 14: Charging a GoldCap capacitor / accumulator with a solar cell

Further Information (all materials are on the CD)

(i) PN Juction And Diodes (Video) https://www.youtube.com/watch?v=W6QUEq0nUH8

(ii) The PN Junction. How Diodes Work (Video) https://www.youtube.com/watch?v=JBtEckh3L9Q

(iii) Critical Factors that Affecting Efficiency of Solar Cells, Furkan Dinçer, Mehmet Emin Meral, 2010

Your own notes

73

UN

IT 4

.2

UNIT 4.2

PHOTOVOLTAIC EXPERIMENTS

Introduction

For training to be truly effective, it is necessary to practically apply your knowledge in hands-on experi-ments or real world installations. Given that the latter is often difficult to realise in some TVET colleges, we recommend certain resources and equipment for practical photovoltaic experiments. Commercially available equipment for use in practical activities ranges from low-cost components such as PV cells, motors, leads, turbine blades, etc., to kits and laboratory apparatuses and vocational training versions of ‘real’ PV training systems. We suggest using the latter as low-cost components, as ‘hobby’ kits are rather more appropriate for use in primary and secondary schools. There are a number of manufacturers who offer well-designed training sets suitable for all experiments set under this unit. The training set Solartrainer Junior, for example, includes modular experiments de-signed to demonstrate all the aspects of PV systems covered in this textbook, albeit on a miniature scale.

Uni

t 4.2

74

UN

IT 4

.2

FIGURE 1: THE TRAINING SET SOLARTRAINER JUNIOR

Image source: GIZ/S4GJ

Your own notes

75

UN

IT 4

.2

EXPERIMENT 1

Measurement of the irradiance of different sources of light

InformationThe different sources of light mainly differ in the irradiance of the colour (wavelength) of the light. The wavelength of visible light is in the range of 400 nm (blue) to 800 nm (red).For example, sunlight is far whiter due to the high share of blue light when compared to the light of an incandescent bulb, which is yellowish due to the high share of red.

EXPERIMENT 1

SET-UP

WIRING DIAGRAM

Adjust irradiance to

different levels!

Irradiation sensor

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

V

76

UN

IT 4

.2

ASSIGNMENT

Determine the irradiance of different sources of light, such as those listed in the table below. The output jacks of the sensor are connected to a multimeter as voltmeter, the range selector is switched to position DC 2000 mV as shown in the wiring diagram. The sensor face must be held in the direction of the source of light in such a way that the maximum measured value results. The sensor face and the solar cell of the sensor must not be shaded during the measurement. Display will be directly in W/m2. The sensor reacts at an irradiance of more than approx. 15 W/m2.

Source of light Irradiance (W/m2) at distance

Irradiance (W/m2) at distance

Irradiance (W/m2) at distance

Spotlight (level 10)

Torch

Room lighting

Sun

Sun (overcast sky)

Now take the measurements at different distances from the artificial sources of light.What are the differences of different sources of light regarding their performance?Which observations can be made?

77

UN

IT 4

.2

EXPERIMENT 2

The solar cell as energy converter

InformationA solar cell converts light energy into electrical energy.

EXPERIMENT 2

SET-UP

WIRING DIAGRAM

Solar cell as energy converter

Light energy

Adjust irradiance to

different levels!

Cell inclination 90°!

Spotlight

(Halogen)

Motor (Load)

M

M

Electrical energy

78

UN

IT 4

.2

ASSIGNMENT

Set up the experiment according to the diagram shown. The lamp arm is in the South position, the brightness controller is on level 10.

1: What happens when the connecting cables on the solar cell are commutated?

2: Set different irradiance values on the brightness controller and observe the electric motor while doing so.

3: Specify the energy conversions within the solar cell and the electric motor.

79

UN

IT 4

.2

EXPERIMENT 3

The solar cell as energy converter and/or diode

InformationA non-shaded solar cell converts radiation energy to electrical energy. If the solar cell is shaded completely, it loses its active role and will behave like a normal diode with P-N transition. A diode is an electronic semiconductor component, the conductivity of which depends on the current di-rection. Therefore, the electrical current can only flow through the diode in one direction.

EXPERIMENT 3

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Accumulator

Motor (Load)

M

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Accumulator Motor (Load)

M

80

UN

IT 4

.2

WIRING DIAGRAMS FOR EXPERIMENT 3

Diagram A Diagram B

Diagram C Diagram D

MA MA

MA MA

module shadedmodule shaded

81

UN

IT 4

.2

ASSIGNMENT

Set up the experiment according to the diagrams shown. The range selector switch of the multime-ter as ammeter must be set to position DC 2000 mA (1 A = 1000 mA).

Diagram AThe solar cell is initially operated without a shading plate (lamp in South position, brightness con-troller level). Which observation can be made?

Diagram B Switch the connections to the solar cell afterwards. Which observation can be made?

Diagram C and D Do the two experiments again, but with the shading plate and without irradiation. Which observa-tions can be made now?

82

UN

IT 4

.2

EXPERIMENT 4

The off-load voltage of a solar cell/shading

InformationCrystalline silicon solar cells consist of two layers of semiconductors with positive and negative charge. If light energy reaches the cell, some of the photons will be absorbed by the semiconductor. In this way, electrons in the negative layer are released and flow from the semiconductor to the positive layer via an external circuit (see also diagram for Experiment 3). At no-load condition, potential can be measured on the outer contacts, the off-load potential VL.

To which extent does the off-load potential depend on the irradiated solar cell surface area?

Irradiated surface area of the solar cell

0 ½ ¾ 1/1

Off-load potential (mV)

EXPERIMENT 4

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

83

UN

IT 4

.2

WIRING DIAGRAMS FOR EXPERIMENT 4

Diagram A Diagram B

V

module shaded

V

84

UN

IT 4

.2

ASSIGNMENT

Set up the experiment according to the diagram shown. The lamp is in the South position, the brightness controller is on the highest level 10. The range selector switch of the multimeter as volt-meter must be set to position DC 2000 mV (1V = 1000 mV).

Completely cover the solar cell with the 1/1 shading plate (set controller to 0 temporarily for this shading), measure the off-load potential, and enter the value into the table.Continue with controller setting 10, with ½ shading, with ¼ shading, and without shading, and measure the potential in each case. Record the measured values in the graph and connect the mea-suring points by means of lines.

Off

-loa

d po

tent

ial (

mV)

Off-load potential of a solar cell/shading

600

500

400

300

200

100

0 0 1/2 3/4 1/1

Relative surface area of the solar cell

Which finding can be obtained from the measurement?

85

UN

IT 4

.2

EXPERIMENT 5

The short-circuit current of a solar cell/shading

InformationCrystalline silicon solar cells consist of two layers of semiconductors with positive and negative charge. If light energy reaches the cell, some of the photons will be absorbed by the semiconductor. In this way, electrons in the negative layer are released and flow from the semiconductor to the positive layer via an external circuit (see also diagram for Experiment 3). At no-load condition, a potential can be measured on the outer contacts, the off-load potential (which is approx. 0.5 V). If the outer contacts are connected directly to a conductor, the maximum possible current will flow, the short-circuit current IK.

To which extent does the short-circuit current depend on the irradiated solar cell surface area?

Irradiated surface area of the solar cell

0 ½ ¾ 1/1

Short-circuit current (mA)

EXPERIMENT 5

SET-UP

Cell inclination 90°!

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Spotlight

(Halogen)

86

UN

IT 4

.2

WIRING DIAGRAMS FOR EXPERIMENT 5

Diagram A Diagram B

A

module shaded

A

87

UN

IT 4

.2

ASSIGNMENT

Set up the experiment according to the diagrams shown. The lamp is in the South position, the brightness controller is on the highest level. The range selector switch of the multimeter as ammeter must be set to position DC 2000 mA.

Completely cover the solar cell with the 1/1 shading plate, measure the off-load potential and enter the value into the table. Continue with ½ shading, with ¼ shading and without shading, and mea-sure the current in each case. Enter the measured values into the graph and connect the measuring points by means of lines.

Shor

t-ci

rcui

t cur

rent

(mA)

Short-circuit current of a solar cell/shading

300

250

200

150

100

50

0 0 1/2 3/4 1/1

Relative surface area of the solar cell

Which finding can be obtained from the measurement?

88

UN

IT 4

.2

EXPERIMENT 6

The off-load potential and the short-circuit current at different irradi-ance valuesInformationWhen using solar cells as energy converters, the level of irradiation is of importance. However, this depends on the time of day, the season and the weather conditions.

To which extent do off-load potential and short-circuit current depend on the irradiance?

Irradiance(W/m2) 0 20 30 40 60 80 100 120 160 200

Off-load potential(mV)

Short-circuitcurrent (mA)

EXPERIMENT 6

SET-UP

Cell inclination 90°!

Adjust irradiance to

different levels!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)Irradiation sensor

89

UN

IT 4

.2

WIRING DIAGRAMS FOR EXPERIMENT 6

Diagram A

Diagram B

V V

V A

Cell inclination 90°!

Adjust irradiance to

different levels!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)Irradiation sensor

90

UN

IT 4

.2

ASSIGNMENT

Set up the experiment according to the diagrams shown. Parallel connection of two solar cells is selected due to the improved resolution at higher currents, but shows the same result as for an individual cell. Initially, a multimeter is connected as voltmeter to the solar cells, the range selector is switched to position DC 2000 mV. The lamp is in the South position.

In order to determine the irradiance, connect the jacks of the sensor to a multimeter as voltmeter. The range selector is switched to position DC 2000 mV. Hold the back of the sensor directly to the centre of the surface of the connected solar cells. The sensor face and the solar cell of the sensor must not be shaded during measurement. Display will be directly in W/m2. The solar cell of the sen-sor serves as the power supply of an internal storage capacitor, which is why it makes sense to run the experiments from high to low irradiance.

Diagram A Set different irradiance values on the brightness controller from 10 – 0 and enter the related poten-tial values into the table above.

Diagram BAfterwards, connect the multimeter as ammeter according to the figure, range selector switched to position DC 2000 mA. Set the same irradiance values again and enter the related current value into the table above.

Please enter the values from the table into the graph below and connect the measuring points by means of lines.

Which statement can be made?

Off

-loa

d po

tent

ial (

mV)

Off-load potential short circuit current / irradianceSh

ort-

circ

uit c

urre

nt (m

A)600 600

500 500

400 400

300 300

200 200

100 100

0 20 40 60 80 100 120 140 160 180 200 0

Irradiance (W/m2)

Potential

Current

91

UN

IT 4

.2

EXPERIMENT 7

The short-circuit current of a solar cell at different angles of irradiation

InformationThe angle of incidence of the sunlight in relation to the Earth changes depending on the time of day and on the season. Therefore, in the morning the rays of sunshine fall onto a stationary solar cell at a different angle than in the afternoon.

What is the relation between the angle of incidence of the light to the solar cell and the short-circuit current intensity?

EXPERIMENT 7

SET-UP

WIRING DIAGRAM FOR EXPERIMENT 7

Adjust cell inclination from 0 to 90 !� �

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

A

92

UN

IT 4

.2

ASSIGNMENT

Set up the experiment according to the diagram shown. Connect the multimeter as ammeter. The range selector switch must be set to position DC 2000 mA. The lamp is in the South position, the brightness controller is set on the highest level.

Initially set the solar cell housing to position 90°, measure the short-circuit current, and enter the values into the table. Now, rotate the solar cell housing in increments of 15° to position 0° and enter each of these values into the table below.

Angledimension (°) 90 75 60 45 30 15 0

Short-circuit current (mA)

Enter the values from the table into the graph and connect the measuring points by means of lines.

Shor

t-ci

rcui

t cur

rent

(mA)

Short-circuit current/angle of irradiation

300

250

200

150

100

50

0 15 30 45 60 75 90

Angle dimension (°)

Which coherences between the angle of incidence of the light onto the solar cell and theshort-circuit current intensity can be derived from the aforementioned?

93

UN

IT 4

.2

EXPERIMENT 8

Series connection of solar cells/shading

InformationFor many electrical consumers, the required potential is higher than the potential delivered by an individual solar cell with approx. 0.5V. Therefore, several solar cells are connected in series. What is the behaviour of a series connection of solar cells regarding the off-load potential and the short-cir-cuit current? What effect does a shadow have on a solar cell?

EXPERIMENT 8.1

SET-UP

WIRING DIAGRAM FOR EXPERIMENT 8.1

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

V

94

UN

IT 4

.2

EXPERIMENT 8.2

SET-UP

WIRING DIAGRAM FOR EXPERIMENT 8.2

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

A

95

UN

IT 4

.2

EXPERIMENT 8.3

SET-UP

WIRING DIAGRAMS FOR EXPERIMENT 8.3

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

V V

V V

Diagram 8.3A Diagram 8.3B

Diagram 8.3C Diagram 8.3D

96

UN

IT 4

.2

EXPERIMENT 8.4

SET-UP

WIRING DIAGRAMS FOR EXPERIMENT 8.4

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

A A

A A

Diagram 8.4A Diagram 8.4B

Diagram 8.4C Diagram 8.4D

97

UN

IT 4

.2

ASSIGNMENT

Set up the experiment according to Diagrams 8.1, 8.2, 8.3 and 8.4.The lamp arm is in the South position, the brightness controller is on the highest level.

Experiment 8.1Connect a multimeter as voltmeter according to Set-up 8.1. The range selector switch must be set to position DC 20 V. Measure the off-load potential for solar cells 1-4 and enter the values into Table 1.

Experiment 8.2Connect a second multimeter as ammeter according to Set-up 8.2. The range selector switch must be set to position DC 2000 mV. Measure the short-circuit current for solar cells 1-4 and enter the values into Table 1.

TABLE 1

Solar cell 1 Solar cell 2 Solar cell 3 Solar cell 4

Off-load poten-tial (V)

Short-circuit current (mA)

Experiment 8.3 and 8.4Connect the solar cells (1 and 2), (1, 2, and 3), as well as all four solar cells in series according to Set-up 8.3 and 8.4 and measure both the off-load potential and the short-circuit current of the array with the same multimeter settings. Enter the respective values for potential and current into Table 2.

TABLE 2

Solar cell 1Series connec-tion solar cell

1+2

Series connec-tion solar cell

1+2+3

Series connec-tion solar cell

1+2+3+4

Off-load poten-tial (V)

Short-circuit current (mA)

Experiment 8.5Use the set-up of Experiment 8.3 and 8.4 and gradually shade the solar cell with the lowest short-circuit current when all four solar cells are connected in series and enter the current and potential values into Table 3.

TABLE 3

No shading ¼ shading ½ shading Complete shading

Off-load poten-tial (V)

Short-circuit current (mA)

98

UN

IT 4

.2

Which findings are obtained from the analysis of the individual tables?

99

UN

IT 4

.2

EXPERIMENT 9

Series connection of solar cells/shading with bypass diode

InformationFor many electrical loads, a higher potential than a single solar cell (about 0.5 V) can supply is required. For this purpose, several solar cells are connected in series. What are the effects of series connection of solar cells with respect to the open circuit potential, the short-circuit current and the effect of a shadow, with and without a bypass diode?

EXPERIMENT 9

SET-UP

Cell inclination 90°!

½ shade

Spotlight

(Halogen)

Potentiometer

Solar cell

I

U

Capacitor

GoldCap

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

100

UN

IT 4

.2

WIRING DIAGRAMS FOR EXPERIMENT 9

Diagram A Diagram B

ASSIGNMENT

Set up the experiment according to the diagrams shown above, with shading size ½ and without a bypass diode. The lamp arm is in the South position, the brightness controller is on the highest level.

Diagram ATurn the knob from the consumer (load 2) to the right (the largest resistance). Set the rotary knob to the first current value given in Table 1 and enter the missing potential value into the table. Now set the rotary knob to the next given value to obtain the missing value in the table. Continue until all values (potential and current) are entered into the table. Using the measured values, draw a curve into the graph provided.

TABLE 1: SET-UP WITHOUT BYPASS DIODE

Potential(V) 1,50 1,00 0,90 0,70 0,65 0,60 0,30

Current(mA) 22 55 80 100 120

Diagram BUse the ½ shading and the bypass diode. Turn the knob from the consumer (load 2) to the right (the largest resistance). Set the rotary knob to the first current value given in Table 2. Set and enter the missing potential value. Now enter the next given value to obtain the missing value in the table. Continue until all values (potential and current) are entered. Use the measured values to plot a curve into the graph.

TABLE 2: SET-UP WITH BYPASS DIODE

Potential(V) 1,50 1,00 0,90 0,70 0,65 0,60 0,30

Current(mA) 20 50 80 100 120

A A

V V

101

UN

IT 4

.2

I-V curve

Curr

ent (

mA)

260

240

220

200

180

160

140

120

100

80

60

40

20

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Potential (V)

With bypass diode

Without bypass diode

Which findings are obtained from the analysis of the two I-V curves (with and with-out bypass diodes)?

102

UN

IT 4

.2

EXPERIMENT 10

Parallel connection of solar cells/shading

InformationFor many electrical consumers the required current is higher than the current delivered by an individual solar cell. Several solar cells are thus connected in parallel to achieve a higher current. What is the behaviour of a parallel connection of solar cells regarding the off-load potential and the short-circuit current? What is the effect of a shadow on a solar cell?

EXPERIMENT 10.1 AND 10.2

Use the set-up of Experiment 8.1 and 8.2 and repeat these two experiments or accept the results obtained from Experiments 8.1 and 8.2 and copy them into Table 1 below.

TABLE 1 (SET-UP SIMILAR TO EXPERIMENTS 8.1 AND 8.2)

Solar cell 1 Solar cell 2 Solar cell 3 Solar cell 4

Off-load potential (V)

Short-circuit current (mA)

EXPERIMENT 10.3

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

103

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 10.3

EXPERIMENT 10.4

SET-UP

V

module shaded

module shaded

module shaded

module shaded

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

104

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 10.4

ASSIGNMENT

Set up the experiment according to the diagram shown above (Experiment 10.3 and 10.4, parallel connections). The lamp arm is in the South position, the brightness controller is on the highest level.

Experiment 10.3 and 10.4Connect the solar cells (1 and 2), (1, 2, and 3), as well as all four solar cells in parallel according to set-up 10.3 and 10.4 and measure both the off-load potential and the short-circuit current of the array with the same multimeter settings. Enter the respective values for potential and current into Table 2.

TABLE 2

Solar cell 1Parallel

connection solar cell 1+2

Parallel connection

solar cell 1+2+3

Parallel connection solar cell 1+2+3+4

Off-load potential (V)

Short-circuit current (mA)

A

module shaded

module shaded

module shaded

module shaded

105

UN

IT 4

.2

Experiment 10.5 Use the set-up of Experiment 10.3 and 10.4. Use the shading plates and gradually shade the solar cell with the highest short-circuit current when all four solar cells are connected in parallel and enter the current and potential values into Table 3.

TABLE 3

No shading ¼ shading ½ shading Complete shading

Off-load potential (V)

Short-circuit current (mA)

Which findings can be obtained from the analysis of the individual tables?

106

UN

IT 4

.2

EXPERIMENT 11

I-V curve of a solar cell

InformationIf a consumer (load resistance) is connected to a solar cell, voltage and current will adopt certain values. How do voltage and current change for different consumers?

EXPERIMENT 11

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Potentiometer

Solar cell

I

U

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

107

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 11

ASSIGNMENT

Set up the experiment according to the diagram shown above. Due to the better resolution, the measurement is taken in series connection. However, the course of the curve in the graph basi-cally shows the same course as for the measurement of an individual cell. Connect a multimeter as voltmeter to load 2 according to the diagram. The range selector switch must be set to position DC 20 V. Connect the other multimeter as ammeter according to the diagram. The range selector switch must be set to position DC 2000 mA. The lamp area is in the South position, the solar cells in position 90°.

Two measurement series will be documented:1. Brightness controller is set to the highest level. Rotate the knob of the consumer (load 2) to its

leftmost position (lowest resistance).Use the knob to set the first current value in Table 1 and enter the missing potential value. Now, set the next specified value and enter the missing value into the table. Continue until all values have been entered.

TABLE 1 HIGH IRRADIANCE

Potential(V) 1.60 1.00 0.50 0.20

Current(mA) 20 40 60 80 90 100 105 110

2. Brightness controller is set to level 8. Rotate the knob of the consumer (load 2) to its leftmost position (lowest resistance). Use the knob to set the first current value in Table 2 and enter the missing potential value. Now, set the next specified value and enter the missing value into the table. Continue until all values have been entered.

V

V

108

UN

IT 4

.2

TABLE 2 LOWER IRRADIANCE

Potential(V) 1.50 1.00 0.50 0.20

Current(mA) 20 40 60 80 90 100 105 110

A: Now, enter the table values of Tables 1 and 2 into the graph and connect the related measuring points.

Potential-Current curve

Curr

ent (

mA)

240

220

200

180

160

140

120

100

80

60

40

20

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Potential (mV)

High irradiance

Low irradiance

Which findings are obtained from the analysis of the graph?

109

UN

IT 4

.2

EXPERIMENT 12

Efficiency determination/MPP

InformationThe current/potential value pairs mentioned in Experiment 11 can be used to calculate the electri-cal output P = V * l (please observe: 1 V * 1 A = 1 W and 1 mV * 1 mA = 0.001 mW).

How large must the load resistance be for a maximum power drain from the solar cell?

EXPERIMENT 12

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Potentiometer

Solar cell

I

U

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Irradiation sensor

110

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 12

ASSIGNMENT

Repeat Experiment 11, the measurement of series 1 first. Now calculate the electrical output from the current/potential value pairs and enter the results into the table below. Afterwards, enter the current/potential value pairs and the output/potential value pairs into the graph and connect the measuring points. Highlight the point of maximum output!

Point of maximum output (maximum power point – MPP)

Potential(V) 1.60 1.00 0.50 0.20

Current(mA) 20 50 80 110 130 140 150 170

Calculatedoutput (mW)

V

V

V

111

UN

IT 4

.2

Curr

ent (

mA)

Efficiency determination/MPP

Out

put (

mW

)

260

240 300

220

200 250

180

160 200

140

120 150

100

80 100

60

40 50

20

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 2.0 2.2 2.4 0

Potential (V)

I-V curve

Output

Efficiency determination of a solar cell

Efficiency is defined as follows:

Efficiency = Total output powerTotal input power

The total output power is the maximum calculated output of the solar cell (MPP). The total input power is obtained by multiplying the irradiance value with the overall surface area of the four solar cells. In order to determine the irradiance, connect the jacks of the sensor to a multimeter as voltmeter, as shown above. Set the range selector switch to position DC 2000 mV. Hold the sensor directly with the back side to the centre of the surface of the solar cells. The sensor face and the solar cell of the sensor must not be shaded during the measurement. Display will show W/m2. The dimensions of a solar cell are 5 x 10 cm.

Maximum calculated output of the solar cell in the MPP:

Measured irradiance:

Overall surface area of the 4 solar cells:

Impinging irradiation output upon the overall solar cell surface area:

Efficiency = = x 100% = %

112

UN

IT 4

.2

EXPERIMENT 13

Emulation of a diurnal variation

InformationThe angle at which the sunlight reaches a stationary solar cell on the Earth’s surface changes from sunrise to sunset. Depending on the location (latitude) of the solar cell, the angle additionally depends on the season. Therefore, the orientation according to the cardinal direction on the one hand, and the horizontal work angle on the other hand, are decisive for the maximum possible energy yield of a stationary solar cell. Since the Sun’s orbit visible from the Earth changes on a daily basis for a specific location, it is important to find the orientation of the solar cell which results in the maximum yield over the entire year.

EXPERIMENT 13

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

E

0°

ENE

22.5°

NE

45°

NNE

67.5°

N

90°

NNW

112°

EW

135°EWE

157.5°

W

180°

113

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 13

ASSIGNMENT

Set up the experiment according to the diagram shown above. The two solar cells in the middle are connected in parallel. Connect the multimeter as ammeter, the range selector switch must be set to DC 2000 mA, the brightness controller must be set to the highest level. Place the lamp arm into the East position and enter the short-circuit current value into the table below.Gradually bring the lamp arm to the West position and document the values of the short-circuit current in each case. Afterwards, enter the related current values above the cardinal directions into the graph.

East ESE South-east SSE South SSW South-

west WSW West

Short- circuit current

(mA)

A

114

UN

IT 4

.2

Emulation of a diurnal variation

Curr

ent (

mA)

600

500

400

300

200

100

0

E ESE SE SSE S SSW SW WSW W

Cardinal direction

1. Which findings are obtained from the analysis of the graph? (see also Experiment 7)

2. At which location is the Sun’s visible orbit always the same from sunrise to sunset, regardless of the season?

3. Which horizontal work angle of the solar cell must be selected for this location for maximum energy yield?

4. What must be taken into consideration for your location?

115

UN

IT 4

.2

EXPERIMENT 14

Charging a GoldCap capacitor / accumulator with a solar cell

InformationA solar cell will only provide electrical energy when it is irradiated. If a consumer is to be operated in darkness as well, a part of the electrical energy generated by the irradiation must be accumulat-ed. Traditionally, an accumulator or, for consumers with very low energy consumption, a GoldCap capacitor is used for this purpose.

EXPERIMENT 14.1

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap

116

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 14.1

ASSIGNMENT

Experiment 14.1Connect the solar cells in series and connect them to the two upper jacks of the GoldCap using a multimeter as ammeter. The range selector must be switched to the DC 2000 mA position. Connect the other multimeter as voltmeter to the upper contacts; range selector must be switched to posi-tion DC 20 V. Bring the brightness controller to level 10 and the lamp arm into the South position. Ensure that the GoldCap is discharged (with incandescent lamp). Charge the GoldCap until only a very low current (approx. 4-5 mA) is flowing.

What is the potential now present on the GoldCap?

A

V

117

UN

IT 4

.2

EXPERIMENT 14.2

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap

Full shade

118

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 14.2

ASSIGNMENT

Experiment 14.2 Switch off the halogen spotlight and shade the solar cells completely with a folder (night situation). Observe the ammeter. What happens?

A

V

module shaded module shadedmodule shadedmodule shaded

119

UN

IT 4

.2

EXPERIMENT 14.3

SET-UP

WIRING DIAGRAM FOR EXPERIMENT 14.3

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap

LED (Load)

A

V

120

UN

IT 4

.2

ASSIGNMENT

Experiment 14.3 Completely discharge the GoldCap (potential 0 V) by additionally connecting the incandescent lamp as a consumer (load 1) to the upper connections of the GoldCap, and then disconnect the incandescent lamp.

EXPERIMENT 14.4

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap

121

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 14.4

ASSIGNMENT

Experiment 14.4Now insert the cables of the solar cells into the two lower jacks of the GoldCap (with diode). The voltmeter remains connected to the upper jacks. Charge the GoldCap until only a very low current (approx. 4-5 mA) is flowing.

A

V

122

UN

IT 4

.2

EXPERIMENT 14.5

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap

Full shade

123

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 14.5

ASSIGNMENT

Experiment 14.5Repeat Experiment 14.2. Observe the ammeter. What happens? What is the potential now present on the GoldCap? What is the assignment of the diode within the circuit?

A

V

module shaded module shadedmodule shadedmodule shaded

124

UN

IT 4

.2

EXPERIMENT 14.6

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap

125

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 14.6

ASSIGNMENT

Experiment 14.6Discharge the GoldCap completely (see Experiment 14.3). Charge the GoldCap, document potential and current at the time intervals in the table. Then transfer the values into the graph and connect the related measuring points.

Time (sec) 10 20 30 60 90 120 150 180 210 240 300 360

Potential(V)

Current(mA)

A

V

126

UN

IT 4

.2 Pote

ntia

l (V)

Charging a GoldCap

Curr

ent (

mA)

2.2 220

2.0 200

1.8 180

1.6 160

1.4 140

1.2 120

1.0 100

0.8 80

0.6 60

0.4 40

0.2 20

0 60 120 180 240 300 360 0

Time (sec)

Which findings can be derived from the graph?

127

UN

IT 4

.2

EXPERIMENT 15

Charging a GoldCap capacitor / accumulator

InformationHow does a GoldCap capacitor behave when it is loaded with a consumer?

EXPERIMENT 15.1

SET-UP

WIRING DIAGRAM FOR EXPERIMENT 15.1

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap Motor (Load)

M

AM

V

128

UN

IT 4

.2

EXPERIMENT 15.2

SET-UP

WIRING DIAGRAM FOR EXPERIMENT 15.2

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCapLED (Load)

A

V

129

UN

IT 4

.2

ASSIGNMENT

Initially, charge the GoldCap as described in Experiment 13 (using the diode).

Experiment 15.1Set up the experiment according to Diagram 15.1 (with motor). Connect the electric motor as load via a multimeter as ammeter to the upper jacks of the GoldCap; the range selector must be switched to position DC 2000 mA. Do not establish the positive connection on the electric motor yet.Connect the other multimeter as voltmeter to the GoldCap as shown; the range selector must be switched to position DC 2000 mV.Discharge the GoldCap using the electric motor (insert positive cable). Observe potential and current during the discharging procedure and enter the values into Table 1 at the specified time intervals.

Experiment 15.2Again charge the GoldCap as described in Experiment 13 (via the diode). Repeat the previous instructions of Experiment 15.1, but use the incandescent lamp as load according to Diagram 15.2. Enter the values into Table 2, transfer the values into the graph and connect the related measuring points. Which findings can be derived from the graph?

Table 1: Electric motor as load

Time (min) Potential (mV) Current (mA)

0

1

2

3

4

5

6

7

8

9

10

Table 2: Incandescent lamp as load

Time (min) Potential (mV) Current (mA)

0

1

2

3

4

5

6

7

8

9

10

130

UN

IT 4

.2

Pote

ntia

l (m

V)

Discharging a GoldCap with electric motor

Curr

ent (

mA)

2200 22

2000 20

1800 18

1600 16

1400 14

1200 12

1000 10

800 8

600 6

400 4

200 2

0 1 2 3 4 5 6 7 8 9 10 0

Time (min)

Pote

ntia

l (m

V)

Discharging a GoldCap with incandescent lamp

Curr

ent (

mA)

2200

2000 100

1800

1600 80

1400

1200 60

1000

800 40

600

400 20

200

0 1 2 3 4 5 6 7 8 9 10 0

Time (min)

Potential

Current

131

UN

IT 4

.2

Which application is the tested storage unit suitable for?

132

UN

IT 4

.2

EXPERIMENT 16

Design of the island network

InformationIf a solar cell is connected to an energy storage unit and a consumer, it is an island network of the simplest form. Depending on the irradiance, the charging condition of the storage unit and the operation of the consumers, different current flows and current intensities result within the system.

EXPERIMENT 16.1

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap

Motor (Load)

M

133

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 16.1

ASSIGNMENT

Experiment 16.1According to the wiring diagram, the solar cells are connected in series and are connected within the circuit to the GoldCap via a multimeter as ammeter using the two lower jacks. The range selec-tor must be switched to position DC 2000 mA. Bring the brightness controller to the highest level and the lamp arm into the South position. Charge the GoldCap until current is no longer flowing.Connect the electric motor via the second multimeter as ammeter to the upper jacks of the Gold-Cap within the circuit; switch the range selector to position DC 2000 mA. The halogen spotlight is switched off. Let the electric motor run for 3 minutes.

What can be observed?

A

M

134

UN

IT 4

.2

EXPERIMENT 16.2

SET-UP

Cell inclination 90°!

Spotlight

(Halogen)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Multimeter

OFFDC

V

DC

A

COM

Volt (U)

Ampere (I)

Capacitor

GoldCap

Motor (Load)

M

LED (Load)

135

UN

IT 4

.2

WIRING DIAGRAM FOR EXPERIMENT 16.2

ASSIGNMENT

Experiment 16.2Connect the incandescent lamp in parallel to the electric motor as additional load until the Gold-Cap is completely discharged. Observe the ammeters. What happens?

Now, re-activate the halogen spotlight; set the brightness controller to position 10. Observe the voltmeters. What happens?

Document the current flow directions with arrows in the two wiring diagrams (16.1 and 16.2).

A

M

A

136

NO

TES

Your own notes

137

NO

TES

Your own notes

138

NO

TES

Your own notes

Skills for Green Jobs (S4GJ)

Department of Higher Education and Training (DHET)