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Photovoltaics Systems Sizing

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science inthe Graduate School of The Ohio State University

By

Eyup Taymur

Graduate Program in Electrical and Computer Science

The Ohio State University

2009

Master's Examination Committee:

Professor Ali Keyhani, Advisor

Professor Charles A. Klein


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Copyright by

Eyup Taymur

2009


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Abstract

World energy consumption and the resulting CO 2 emissions are increasing

substantially and this increase puts in danger the ecological stability of our Earth.

Growing scarcity and rising prices of fossil fuels may lead to economical and

political instability in many countries in the near future. These problems can be

solved by contributing significantly the use of renewable energy resources. The

renewable energy resources are sufficient enough to meet the world energy

requirement. Most of the countries have recognized the new energy policy to

encourage the investment photovoltaic energy system which is one of the biggest

renewable energy resources, the government policies are providing credits,

reducing taxes or applying feed-in tariff which is agreement of energy purchase

for certain period of time between the government and the investors. In this thesis,

it is desired to have a comprehensive study on energy output calculation of the

solar system. The solar radiation effect is articulated and based on monthly

average global irradiation data the daily average irradiation is found on an

inclined surface; the sun rotation relative to the Earth is analyzed in order to

calculate the optimum angle in any latitude on the Earth surface to receive the

maximum annual irradiation. Based on the given data a graphical user interface is

modeled by MATLAB GUI which calculate the energy yield by using data set for

different PV module technologies to be presented to the end-users for decision

making in PV energy investment. Based on power system and power electronicknowledge different design proposals are modeled by using component

specification provided by the manufacturers.

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Dedication

This document is dedicated to my family.

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Acknowledgments

First of all I would like to thank to my parents and my wife for their support

during completion of my graduate study. I also would like to thank my thesis

exam committee: Professor Ali Keyhani and Professor Charles A. Klein for

giving me the opportunity to present this thesis and for their time, patience and

understanding. Special thanks to my advisor Professor Ali Keyhani for his greatsupport and inspiration to be able to understand Photovoltaics System, design and

application. Professor Keyhani, it has been an honor to work with you. I would

like to thank all the professors in Power area for sharing their deep knowledge and

experience with me.

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Vita

January 1982 ..................................................Born in Batman, Turkey

2007................................................................B.S. Electrical and Electronic

Engineering, Gaziantep University

Fields of Study

Major Field: Electrical and Computer Engineering

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Table of Contents

Abstract .................................................................................................................. iii

Dedication .............................................................................................................. iv

Acknowledgments................................................................................................... v

Vita ......................................................................................................................... vi

Fields of Study ....................................................................................................... vi

Table of Contents .................................................................................................. vii

List of Tables ........................................................................................................ xii

List of Figures ...................................................................................................... xiv

Chapter 1: Solar Radiation ...................................................................................... 1

1.1 Angle Definitions ..................................................................................... 3

1.2 Some Astronomical Information ................................................................... 5

1.3 Calculation of the Irradiation on an Inclined Surface ................................... 7

1.3.1 PV House Example ............................................................................... 13

Chapter2: Power Electronics ................................................................................. 15

2.1 Introduction ................................................................................................. 15

2.2.1 Step-down converter ............................................................................. 17

2.2.2 Step-up converter .................................................................................. 18

2.2.3 Buck-boost converter ............................................................................ 20

2.2.4 Inverters ................................................................................................ 21

Chapter3. Design Overview of Stand-Alone PV Systems .................................... 33

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3.1. PV generators, Batteries and Loads Coupling ........................................... 33

3.2. Calculating electricity consumption ........................................................... 35

3.3. PV generator sizing .................................................................................... 36

3.3.1. Calculation of the PV generators yield .............................................. 36

3.4 Consideration of Cable and Conversion Losses .......................................... 39

3.4.1 Cable losses .......................................................................................... 40

3.4.2 Conversion losses ................................................................................. 40

3.5 Mismatch losses .......................................................................................... 40

3.6 Determination of Summer Excess and Winter Reserve .............................. 42

3.7 A PV House Design Example ..................................................................... 43

3.7 Cable cross-section sizing ........................................................................... 44

3.8 Sizing the battery ......................................................................................... 45

3.9 Charge Controllers ...................................................................................... 45

3.9.1 Series controllers .................................................................................. 47

3.9.2 Shunt controllers ................................................................................... 48

3.9.3 MPPT charge controllers ...................................................................... 49

3.10 Stand-alone PV operation principles ......................................................... 50

3.10.1 Sizing Example of Stand-Alone System ............................................ 51

Chapter 4: Design and Installation of Large Scale PV Systems ........................... 53

4.1 Introduction ................................................................................................. 53

4.2 Grid-Tied PV System .................................................................................. 54

4.2.1 Grid-Tied Inverter ................................................................................. 55

4.2.2 Grid-Tied Inverters and PV Module Configuration ............................. 57

4.2.3 Selecting Inverter .................................................................................. 58

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4.3 Design Considerations and 2 MW System Installations ............................. 62

4.3.1 Module Selection .................................................................................. 62

4.3.2 Inverter Selection .................................................................................. 64

4.3.3 First Proposal of PV system ................................................................. 66

4.3.4 Description of AC Transmission Side and AC Line Model ................. 71

4.3.5 Sizing the DC Main Cable from the Array ........................................... 73

4.3.6 Second design proposal of 2.16MW system ........................................ 75

Chapter 5: Annual Energy Yield Calculation ....................................................... 80

5.1 Temperature Dependency of Efficiency ..................................................... 81

5.2 Energy Production ....................................................................................... 88

5.2.1 Cumulative solar irradiance .................................................................. 88

5.2.2 Module power rating at standard test condition ................................... 88

5.2.3 Operating temperature .......................................................................... 89

5.2.4 Maximum power point voltage dependency of irradiation .................. 89

5.2.5 Soiling ................................................................................................... 89

5.2.6 Variation of Solar Spectrum ................................................................. 90

5.2.7 Peak Solar Hours Concept and Definitions .......................................... 90

5.2.8 Temperature Dependency of Array Output .......................................... 91

5.2.9 Module orientation ............................................................................... 92

5.2.10 Mismatch Losses and Blocking/Bypass Diodes ................................. 92

5.2.11 Inverter Efficiency .............................................................................. 95

5.2.12 Cable Losses and Transmission Losses .............................................. 97

Chapter 6: Solar Cell Types and Data Sheets ..................................................... 100

6.1 Technologies ............................................................................................. 100

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6.1.1 Crystalline silicon cell (market share 93%) [10] ............................. 101

6.1.2 Thin film solar cell ............................................................................. 101

6.2 Different technologies and their specifications ......................................... 103

Chapter 7: Economic Evaluation and Cost Estimation Techniques ................... 104

7.1 Introduction ............................................................................................... 104

7.2 Present-worth Factor ................................................................................. 105

7.3 The Major Cost and Performance Elements of the Utility Scale PV Power

Plant ................................................................................................................. 107

7.3.1 The Performance of PV Plant ............................................................. 107

7.3.2 Initial PV Plant Investment................................................................. 109

7.3.3 PV Operation Expenses ...................................................................... 112

Chapter8: PV Sizing Simulator ........................................................................... 114

8.1 Introduction ............................................................................................... 114

8.2 Simulator Inputs and Component Prices ................................................... 114

8.3.1 DC-AC Derates Factor ....................................................................... 118

8.4 Case Study for Mono-crystalline............................................................... 120

8.4.1 1 km Transmission Line ..................................................................... 120

8.4.2 25km Long Transmission line Losses ................................................ 123

8.5 Case Study for Poly-Crystalline ................................................................ 125

8.5.1 1km Transmission Line ...................................................................... 125

8.5.2 25km Long Transmission Line ........................................................... 127

8.6 Case Study for Thin-Film .......................................................................... 129

8.6.1 1km Transmission Line ...................................................................... 129

8.6.2 25km Long Transmission Line ........................................................... 131

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8.7 Comparison of Case Studies ..................................................................... 133

Conclusion .......................................................................................................... 136

References ........................................................................................................... 137

Appendix ............................................................................................................. 140

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List of Tables

Table 1: Typical Reflectivity .................................................................................. 9

Table 2: Irradiation on Different Inclination Angle .............................................. 12

Table 3: The Optimum Angle and Maximum Irradiation for One Year ............... 13

Table 4:Average Yearly Irradiation Data ............................................................. 13

Table 5: Input-Output Relation of DC-DC Converter with Duty Ratio Range .... 32

Table 6: Seasonal Electricity Usage of Equipment [19] ....................................... 36

Table 7: Z2 Factor Calculated for Different Locations [19] ................................. 37

Table 8: Z3 Factor for Deviation from Horizontal [19] ........................................ 38

Table 9: Temperature Correction Factor Z4 [19] ................................................. 39

Table 10: Design Parameters for Stand-Alone PV Systems ................................. 41

Table 11: Electrical Parameters and their Units ................................................... 44

Table 12: Characteristics of AS-300 Module ....................................................... 63

Table 13: Characteristics of PV-Powered Inverter ............................................... 64

Table 14: 13.2132 kV Class One phase Neutral Return Line Model .............. 72

Table 15: Satcon PV Inverters PV-135 ................................................................. 75

Table 16: PV Cell Efficiencies in Different Technologies ................................. 102

Table 17: PV Module Specifications .................................................................. 103

Table 18: Payment Schedule ............................................................................... 105

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Table 19: Module Types and Specifications ....................................................... 115

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List of Figures

Figure 1: Global irradiation values for the world (kwh/m2) [16] ........................... 2

Figure 2: World global irradiation source: www.bpsolar.com ............................... 2

Figure 3: The PV panel inclination angle on the Earths surface under sunshine .. 3

Figure 4: Values of Air-mass calculated for specific location [26] ........................ 5

Figure 5: Sun and Earth rotation and angles description ........................................ 6

Figure 6: Irradiation components [12] .................................................................... 8

Figure 7: Semiconductor devices developments [20] ........................................... 16

Figure 8: Buck Converter Circuit ......................................................................... 17

Figure 9: Boost Converter Circuit ......................................................................... 19

Figure 10: Buck-boost Converter Circuits ............................................................ 20

Figure 11: Three Phase Inverter Topology [21] ................................................... 21

Figure 12: Wave Form of PWM Sinusoidal [21].................................................. 23

Figure 13: Base Switching Vectors ....................................................................... 25

Figure 14: Determination of the Region ............................................................... 26

Figure 15: Vector Projection for Region Determination ...................................... 27

Figure 16: Vector Allocation for the Dwelling Times T a,T b and T c [23] .............. 29

Figure 17: Battery Charging and Discharging Conditions ................................... 34

Figure 18: Series Charge Controller ..................................................................... 47

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Figure 19: Shunt Charge Controller ...................................................................... 48

Figure 20: MPPT Controller ................................................................................. 49

Figure 21: Stand-Alone PV Operation [1] ............................................................ 50

Figure 22: Application of PV systems [12] .......................................................... 54

Figure 23: General Structure ................................................................................. 55

Figure 24: Maximum Power Point under Different Irradiation [27] .................... 56

Figure 25: Basic Configuration for the given Example Showing the Modules,

Strings and an Array ............................................................................................. 58

Figure 26: Central Inverter with Large Scale Array Configuration ...................... 59

Figure 27: Efficiency of the Inverter under Different Irradiation and Durations [4]

............................................................................................................................... 61

Figure 28: Configuration of the Array (112 strings with 8 modules) with the

Central Inverter ..................................................................................................... 68

Figure 29: General Structure of the Arrays with the Inverters ............................. 69

Figure 30: Network Model of 2.15MW System, Single Line Diagram................ 70

Figure 31: 13.2-132kV Class One Phase-Neutral line Model .............................. 72

Figure 32: Parameters for Sizing DC Main Cable [12] ........................................ 73

Figure 33: Sub-Array Configuration ..................................................................... 77

Figure 34: 2.16 MW PV System Line Diagram (Complete System) .................. 79

Figure 35: Shows the Error by Using the Equation (1) Calculate The PV Output in

Different Location In The World. Different Places with Their Latitude and the

Associated Error is Illustrated ............................................................................... 91

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Figure 36: By-pass and Blocking Diodes in PV Array ......................................... 93

Figure 37: Shading Effect on I-V Characteristics of PV [16] ............................... 94

Figure 38: Inverter Efficiency [16] ....................................................................... 96

Figure 39: PV Cell Types [12] ............................................................................ 100

Figure 40: PV Panel under 20 Year Exposure of Sun [18] ................................. 108

Figure 41: Relative Solar Cell Conversion Efficiencies [18] ............................. 109

Figure 42: Area related cost [18] ........................................................................ 110

Figure 43: Land Consumption versus Capacity Factor in Different Technologies

[18] ...................................................................................................................... 111

Figure 44: O&M Cost with Different Technologies [18] ................................... 113

Figure 45: Location of Batman City with Annual Irradiation per meter-square [25]

............................................................................................................................. 117

Figure 46: Irradiation Data and Sunshine Duration for Batman City [24] ......... 117

Figure 47: Shading Effect [9] ............................................................................. 120

Figure 48: Mono-crystalline for 1km Transmission Line ................................... 122

Figure 49: Mono-Crystalline for 25km Transmission Line ................................ 124

Figure 50: Poly-Crystalline for 1km Transmission Line ................................... 126

Figure 51: Poly-Crystalline for 25km Transmission Line .................................. 128

Figure 52: Thin-Film for 1km Transmission Line .............................................. 130

Figure 53: Thin-Film 25km Transmission Line.................................................. 132

Figure 54: Area Comparison of Different Technologies .................................... 133

Figure 55: Cost Comparison ............................................................................... 134

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Figure 56: Payback Time Comparison ............................................................... 135


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Chapter 1: Solar Radiation

Nomenclature:

KT: the clearness index of the sky

AM: air mass factor which determines the total distance where the sunlight has totravel to reach the Earth surface

S: the resulting coverage energy flux incident on a unit area perpendicular to the

beam outside the Earths atmosphere

The energy comes from the sun to the Earth is in the form of Radiation. The

amount of energy in the sunlight reaching the Earth surface is equivalent to

around 10,000 times of the worlds energy requirements. [12]

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Figure 1: Global irradiation values for the world (kwh/m2) [16]

Figure 2: World global irradiation source: www.bpsolar.com

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1.1 Angle Definitions

In order to make a good estimation for energy yield of photovoltaic systems, the

angle definitions and solar radiation should be understood perfectly. The totalirradiations which are provided in different official sources are the irradiation of

the certain flat surface on the world. When sun moves around the angle of the sun

reaches the Earth will change every second of the motion. In this case the

radiation will also change depending on the sun motion.

Figure 3: The PV panel inclination angle on the Earths surface under sunshine

s= solar azimuth angle

= azimuth angle of the PV generator

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= tilt angle of PV generator

s= solar elevation from the horizon

Due south is given by 0 degree, east is -90 degree and west is +90 degree. Solar

altitude is very important parameter to estimate the real irradiation on a horizontal

surface, because the intensity of the solar radiation on any surface is dependent on

the solar elevation angle. While the Earth is rotating the elevation angle of the sun

changes every month in the year and every day in the months and every hour in

the day. The air mass factor determines the total distance where the sunlight has

to travel to reach the Earth surface.

1/

gives the relation between air mass and elevation angle.

The resulting coverage energy flux incident on a unit area perpendicular to the

beam outside the Earths atmosphere is known as a solar constant,

S 1367 /[11]. As it is known the total power falling on a unit area fromradiant source is called irradiance. When the solar radiation travels through the

atmosphere to reach the Earths surface, a part of incident energy will be lost by

scattering or absorption by air molecules, clouds and particles which are called

aerosols.

Direct (beam) radiation is the radiation which is not reflected or scattered and

reaches directly to the surface is called diffuse radiation. The scattered radiation

reaching to the ground surface is called diffuse radiation. The radiation which is

the reflection of the radiation reaches to the ground surface and being received by

the objects is called albedo. The sum these radiations (diffuse, direct, albedo) is

called global radiation. The air mass specifies the clearness of the atmosphere.

The integration of the irradiance over a period of time gives us the irradiation.

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Figure 4: Values of Air-mass calculated for specific location [26]

1.2 Some Astronomical Information

The Earth revolves in an elliptical orbit around the sun; the plane of this orbit is

elliptical. The Earth completes one cycle travelling around the sun in one year.

The angle between the equatorial plane and celestial plane is 23.45.The celestial sphere around the Earth shows the relative position of the sun and

Earth. The angle between line joining the centers of the sun and the Earth and the

equatorial plane is called declination angle . This angle will be zero at 20/21

march (vernal) and at 22/23 September (autumnal) equinoxes. On these days sunwill rise exactly in the east and will set in the west. At summer solstice, 21/22

June, will be 23.45 and at winter solstice will be -23.45 .5


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S

W

N

EHorizon

Equator Plane

Suns Daily path

Solar Noon

Zenith

Nadir

Figure 5: Sun and Earth rotation and angles description

The Earth rotates about the polar axis in the rate of one revolution per day. The

instantaneous position of the sun on the world can be described by , sun

angle. Solar angle is the angle between the meridian passing through the sun and

the meridian of the area. At solar noon hour angle is zero, and increases toward

the east.

in the figure shows the elevation angle and is the latitude for a givengeographical location on the Earths surface. During the daily motion, the solar

declination is assumed to be constant and equal to its middays value. The

following geographical relationship can be used to formulize the sun position and

relate them to a ithe irr d ation on an inclined surface [11].

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From these two equations it is found that the solar hour angle can be found by the

formula [11].

cos-tan.tan )(Where is sunrise hour angle and is sunset hour angle for the day of the

year.

1.3 Calculation of the Irradiation on an Inclined Surface

The solar radiation data are generally given in the form of global radiation on a

horizontal surface. Global daily irradiation is denoted by G. If the PV panels are

positioned with an angle on a horizontal surface the total global irradiation

received by the PV will change.

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Figure 6: Irradiation components [12]

The data G for the site is used to determine the diffuse and beam contributions to

the global irradiation by using Bo as a reference and KT is clearness index. The

attenuation of the solar radiation through the atmosphere at a given area on the

Earth can be described by KT=G/Bo in that month. When Bo is calculated the

variation of extraterrestrial irradiance on account of the eccentricity of the Earths

orbit is considered. The diffuse irradiation can be obtained by using the diffuse

fraction index D/G of the global irradiation. D/G is the universal function of clearness index KT. B=G-D gives the separate beam and diffuse radiation.

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Finally, angular dependent of each component will give the diffuse and beam

irradiation on an inclined surface. And also by using the reflectivity of

surrounding area the total albedo is calculated. The total is found by adding

albedo, beam and diffuse irradiation.

The inputs for calculations are daily global irradiation G for a day, the middle of

each month can be chosen to give the average monthly irradiation and the solar

constant S, which is 1367w/ , the sites geographical latitude and the solar

declination angle for the day of the year.

Table 1: Typical Reflectivity

GROUND COVER REFLECTIVITY

Dry bare ground 0.2

Dry grassland 0.3

Desert sand 0.4

Snow 0.5-0.8

Bo is the irradiation received over one day by a unit horizontal surface area

outside t s o p [11].of he Earth atm s here is calculated using the expression below

24/ . . 1 033.. .where d n is the number of day starting from January 1 d n=1 and December 31

dn=365.

Since we have the daily irradiation data G from KT=G/Bo the clearness index can

be found. For the disuse irradiation D, d from the formula [11]it can be foun

/ 1 1.13.

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The beam irradiation also can be found simply by subtracting D from G, B=G-D.

After B is found for the horizontal surface, the beam irradiation B( ) on a south-facing panel inclined at an angle to the horizontal surface is given by [28].

.cos . .sin .sin . .

1. . 1 cos 212. . . 1

Where D( ) is the diffuse and R( ) is the albedo radiation. Finally the globalirradiation can be found b h ponents.y summing up all the t ree com

Since ws (hour angle) is used for one day inclination angle is required and the

inclination angle can b f o 3].e ound by the foll wing formula [1

23.45.sin 360.

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The overall formulas to put in the program for calculation of irradiation on an

inclined surface will be as following:

1. 23.45.sin 360. 284 Sol l io for the given dar dec inat n angle is found ay n.2. cos .and costan and where is the solar declination angle and is the geographical latitudeof the location on the Earth.

Then we find = min{ ws, ws }. Since the Earth rotates one revolution per

day, this corresponds to 360 longitude makes 24 hours. This means the day lengthis 2.ws/15.

3. 24/ . . 1 033.. . Where n is the day of the year.

4. /KT is the clearness index, and G is the solar irradiation dataof e r th a ea.

5 . 1.13 .. 16. .7. .. . . . . .

1. . 1 cos

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The average irradiation of Batman city, Turkey for each month is given below

G=[1900,2690,4070,5050,6240,7040,6840,6040,5270,3730,2410,1800]

Table 2: Irradiation on Different Inclination Angle

Inclinati

on

Angle

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Ave.

year

wh/m

0 1900 2690 4070 5050 6240 7040 6840 6040 5270 3730 2410 1800 4233

6 2054 2863 4258 5164 6261 6973 6809 6137 5510 3992 2616 1965 4550

12 2194 3016 4414 5237 6231 6851 6721 6180 5699 4221 2803 2117 4640

18 2318 3146 4533 5266 6150 6671 6574 6166 5835 4414 2968 2252 4691

24 2425 3253 4616 5252 6016 6433 6368 6095 5915 4570 3110 2371 4702

30 2513 3334 4661 5194 5830 6137 6104 5967 5940 4686 3226 2472 4672

36 2582 3390 4667 5093 5595 5787 5784 5784 5910 4760 3316 2553 4602

42 2631 3419 4636 4951 5312 5387 5411 5546 5823 4794 3379 2613 4492

54 2666 3396 4459 4546 4618 4451 4526 4922 5487 4734 3420 2671 4158

60 2653 3344 4316 4288 4214 3928 4024 4542 5241 4641 3398 2667 3938

72 2563 3164 3929 3678 3320 2807 2935 3671 4608 4337 3270 2595 3406

84 2393 2887 3421 2965 2356 1653 1795 2689 3810 3885 3035 2440 2778

90 2281 2716 3129 2582 1870 1099 1237 2175 3361 3609 2880 2333 2439

In the given table the inclination angle of the PV panel is betha is changing from 0

degree to 90 degree. It is shown that the maximum irradiation as a total of twelve

months is at 24 degree of inclination angle which is 4702 Wh/

.This amount is

almost 3343 Wh/ greater than the irradiation on a horizontal surface. If the

angle of inclination can be changed every month of the year the yield will be

almost 4930.3 Wh/ which is very substantial increase of irradiation on PVs.

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Table 3: The Optimum Angle and Maximum Irradiation for One Year

Jan Feb Mar Apr May June July Aug Sept Oct Nov DecTotal

Ave.

2667 3424 4669 5267 6261 7014 6840 6181 5941 4795 3421 26724930.3

Wh/m2

54 44 33 18 6 3 0 12 30 45 51 57(opt.)

As it is seen in the Table 3 the total irradiation falling on the surface at the

specified angles will be more than when they are at an only one fixed angle. In

later steps, the total revenue will be discussed to analyze if different technology is

applied to change the angle every month is possible and also feasible in terms of economics of PV systems. An example of a PV house is given below to show the

effect of inclination angle.

1.3.1 PV House Example

In this example is desired to show the energy yield increment by changing theangle of inclination.

Data for the location:

Irradiation Data is given for the months as a wh/m 2.

Table 4: Average Yearly Irradiation Data

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1900 2690 4070 5050 6240 7040 6840 6040 5270 3730 2410 1800

The solar house power capacity is 15kW.

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The geographical latitude, L=38 north.

Assume the PV modules used has an efficiency of 17%.

Power rating= 300w

Module area=2.43 m 2

Assume the performance ratio of the module type is 0.77 which includes wiring

losses, power mismatch, soiling loss and inverter loss.

Result:

Total modules are for 15kw system is (15kw/300w)*2.43=121.5m 2.

The calculation of irradiation for this example is done for different inclination

angles and the results are given in Table 2.

As it is seen in Table 3 the optimum angle is found at 24 degree of panel

inclination. This gives 4702 wh/m 2 average annual irradiation for the location.

The average annual irradiation for 0 was 4232 wh/m 2. The difference will be

extra which is 4702-4232=470wh/m 2.

The annual extra revenue from 15kW will be;

470x121.5x365=20,843.325kwh energy.

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Chapter2: Power Electronics

2.1 Introduction

The world energy consumption is increasing while fossil fuel is limited. The main

resource of the world energy in the near future is going to be renewable energysources such as wind, solar and geothermal energy. Although these new energy

technologies are promising, their energy production is still more expensive than

conventional energy productions. In this case power electronic technologies are

needed to reduce the losses as much as possible, and to deliver and distribute the

power very efficiently. Power electronics had changed rapidly during the last

thirty years and the number of application is increasing due to developments of

the main components of power electronics, which all semiconductor devices and

the microprocessor technology.

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Figure 7: Semiconductor devices developments [20]

The power electronics application in photovoltaic system is using the most

efficient technique to extract the maximum power from the PV cell. This is

achieved by using MPPT control unit in the converter or inverter side. The MPPT

techniques will be summarized in the following section.

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2.2 Power Electronic Circuits

2.2.1 Step-down converter

Step-down converter which is the basic converter is known as buck converter. As

it is understood from its name the main function of this converter is to convert the

input DC voltage level to another and lower voltage level. The main components

in buck converter are semiconductor switch S, diode D, inductor filter L and

capacitor filter C as shown in figure8.

Figure 8: Buck Converter Circuit

The state of the converter where the inductor current is never zero called

continuous conduction mode (CCM). According to Faradays law the voltage of

inductor in any period of time is zero then;

( 1 17


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where D is duty cycle, D=ton/T and 1-D=toff/T.

In conduction current mode the current IL is always greater than zero. When the

average value of the inductor current is low or the switching frequency is very

low the converter may enter the discontinuous conduction mode. In the buck converter the boundary between continuous conduction and discontinuous

conduction mode can be found in the following formula [21].

1 2 The minimum value for the capacitance to satisfy the continuous conduction

mode will be found by [21]

18 These two equations are very important in the design consideration of converters.

2.2.2 Step-up converter

Step-up converter is called boost converter which consists of inductor L, capacitor

C, controllable semiconductor S, diode D and load resistance R.

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Figure 9: Boost Converter Circuit

As depicted in the Figure 9, when the switch is on the inductor current is

increasing and when the switch is off the inductor current will flow through the

diode to be deployed in RC circuit.

Using again Faraday r e ,s law for the inducto in boost conv rter

1

From which the converter transfer function Vo/Vin=1/(1-D) is found.

The boundary condition for CCM in boost converter is found by

12 The minimum value for the capacitor can be found by

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2.2.3 Buck-boost converter

Non-isolated buck-boost converter which consists of input voltage Vs, inductor L,

capacitor C, load R and controllable switch S is shown in the Figure 10.

Figure 10: Buck-boost Converter Circuits

The inductor current condition yields the equation below.

1 0 From this equation it is found that the transfer function equation for the inputoutput relation will be Vo/Vin=-D/(1-D)

The boundary condition for the inductor for continuous conduction mode is

12 Since the circuit structure is the same as boost converter the minimum capacitance

for buck-boost converter to stay in CCM can be found from the same formula.

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2.2.4 Inverters

The function of inverters is to convert the DC to different AC levels. Single phase

voltage source inverters are used for low-voltage application whereas three-phaseis used for medium to high voltage application. In voltage source inverter

application, the current, phase, frequency and voltage need to be always

controllable. By this control topology it is desired to give as pure as possible

sinusoidal output in desired phase, voltage and frequency.

Figure 11: Three Phase Inverter Topology [21]

The standard topology of three phase inverter is shown in Figure 11, three

switching legs are demonstrated where S1 and S4, S3 and S6 and S5 and S2 cant

be closed at the same time, because this will result in short-circuiting the DC

voltage supply.

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2.2.4.1 Sinusoidal PWM technique

In this inverter technique the triangular waveform is compared with three

sinusoidal references voltages. The peak value of the triangular waveform is V t and the peak value of reference sinusoidal is V r with desired frequency of f o.

When V a,r is greater than V t, S1 is closed and S4 is open, when V b,r is greater than

Vt S3 is closed and S6 is open, when V c,r is greater than V t, S5 is closed and S2 is

open. The waveform of sinusoidal PWM technique is shown in Figure 12 below.

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Figure 12: Wave Form of PWM Sinusoidal [21]

ma is the modulation index ich is Vr/Vt [22]. If V r is greater than V t andwh

0 1 The output of the inverter is then given by;

2 sin 2

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sin 2 2

2sin 2

If Vr is less than Vt in this case the modulation is called over modulation and

/ The output voltage in this ca lse wil be

,1 4/2.2.4.2 Space vector PWM

The output voltages achievable by the three level inverter can be represented in

the space as a set of vectors as shown in Figure 13. These vectors correspond to

switching combinations for the inverter switches. Figure 13 shows these base

vectors V 1 through V 6 and the two zero vectors which correspond to switching

positions resulting in zero output voltage.

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Figure 13: Base Switching Vectors

One cycle of the output voltage can be represented by six sectors (60 o each). The

desired output voltage is represented by a rotating reference voltage or vector V

which is approximately calculated by using three adjacent vectors .For the

purpose of specifying the base vectors used to represent the reference one, four

region in each of the six sectors are defined,

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Figure 14: Determination of the Region

Determination of the region is based upon the length and the angle of V ref and is

done by calculating its projections m 1 and m 2 as shown in Figure 15.

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Figure 15: Vector Projection for Region Determination [23]

where 1 2 2. Based upon the magnitudes of m 1 and m 2 components the region is determined as

follows:

If m 1, m2 and (m 1+m2) < 0.5, then V is in Region 1,

If m 1 > 0.5, then V is in Region 2,

If m 1 and m 2< 0.5 and (m 1+m2) > 0.5, then V is in Region 3,

If m 2 > 0.5, then V is in Region 4.

The switching or dwelling times , for the three base vectors, that are used to implement the reference vector are calculated asfollows:

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where, T s is the total switching time.

These switching times for the four regains are found as follows,

Region 1:

2. . .

.1 2

2. . . 2. 1

Region 2:

2 2

1 2Region 3:

2 1 2 2

Region 4:

2 2 128


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where m is the modulation index.

Therefore T a, T b and T c can be determined and the switching sequence

corresponding to the appropriate base vector will be applied to the inverter

switches for the specified amount of time.

The vectors which are applied to the inverter switches for T a,T b and T c periods are

shown in Figure 16 , and they are designated by a, b and c respectively.

Figure 16: Vector Allocation for the Dwelling Times T a,T b and T c [23]

The peak value of the fundamental component of the phase output voltage is

related to the dc link voltage through the modulation index m ,

3 3002 7

0 1 The maximum output voltage obtainable is when m=1;

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_700 3

401.1

The switching frequency is chosen to be 1.2 kHz.

2.2.4.2.1 A System Sizing Example

In these examples it is desired to show DC-Dc operation for varying voltage to a

fixed voltage level of the boost converter output of 120 V, 240V and 600V.

Assume a 10kW, 30kW and 100kW systems.

The switching frequency is 25-kHz.

Assume that the input voltage for the converter is varying from 0 to 48

volts.

Figure 17: The Configuration of DC to AC Conversion

For the Boost Converter:

Case 1:

P=10kW

Vin= 0-48Vdc

Vout=120Vdc

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For the boost converter e t by;th input-outpu ratio is calculated

1 0

11

Dmax=(120-0)/120=1 and D min=(120-48)/120 6=0.

Then the duty ratio will be in the range of 0.6 1 Case 2:

P=30kW

Vin=0-48Vdc

Vout=240Vdc

By using the same equations above for the duty ratio, D;

Dmax=(240-0)/240=1 and D min=(240-48)/240=0.8

Then D will be in the range of 0.8 1 Case 3:

P=100kW

Vin=0-48Vdc

Vout=600Vdc

Dmax=(600-0)/600=1 and D min=(600-48)/600=0.92

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Then D will be in the range of 0.92 1 For The AC-DC Inverter:

Assume the output voltage from Case 3 is applied to the inverter for the 120 Vac

and 240 Vac output.

Case 1:

f=60Hz

Vin=120 Vdc

Vout=120Vac

For the inverter;

2 sin 2 sin 2 2

2 sin 2 For one phase 2, Vout=120* 2=169.7 VacMa=169.7/(600/2)=0.565

Table 5: Input-Output Relation of DC-DC Converter with Duty R R atio ange

0.6P=10kW Vin=0-48Vdc Vout=120Vdc f=25-kHz

0.8 P=30kW Vin=0-48Vdc Vout=240Vdc f=25-kHz 0.92 P=100kW Vin=0-48Vdc Vout=600Vdc f=25-kHz

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Chapter3. Design Overview of Stand-Alone PV Systems

When considering planning stand-alone PV system the most important factor

should be energy balance of the system which means balancing the energy

consumption with the supply. Since the solar energy is changing and fluctuating

daily the solar irradiation for the given place should be calculated realistically in

order to meet the load requirement. A PV generator leads very long payback timefor the system, because it is not used for its whole life span. For this reason using

an auxiliary power generation unit will be more effective in the energy

investment. A combination with a wind turbine can be a good solution

3.1. PV generators, Batteries and Loads Coupling

A small PV generator with reverse diode and battery is connected as shown in the

figure below. The reverse diode is a protector for reverse current when there is no

PV generation otherwise the current will flow from the battery to the array which

will dissipate as a heat. Therefore, the reverse diode is to protect the battery from

these losses and avoid the PV from thermal losses.

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Figure 18: Battery Charging and Discharging Conditions

In Figure 18, with switch change over different load can be connected to the PVsystem. In case B the current for the load is delivered completely by PVs. The

load A needs greater current, since the PV can only give part of the required

current to the load. Therefore, the battery will discharge in this case current flows

from the battery to the load. In the load C case, the current generated from the

PVs is more than what is required for the load. Thus, the generators will charge

the battery by the surplus of current from the PVs. Power electronic controllers

are used in stand-alone PV systems for higher performance. The charge controller

is used to protect the battery from over charging and also low discharging. MPPT

is used in the inverter to optimize the utilization of the PV output by extracting

the maximum power from the modules.

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3.2. Calculating electricity consumption

During the year the fluctuations in the irradiation requires stand-alone PV system.

The consumption has to be calculated in different irradiation durations such as in

different seasons or in different months or at least between the extreme values of

summer and winter when estimating the consumption.

The calculation of the solar energy is based on the weakest month of the year

based on the location and global irradiation data. The radiation calculation for

different geographical latitude and inclination angles will be calculated in the

following section. The first step in stand-alone system design should start by

calculating the energy consumptions of all the appliances and other equipment

and their power range should be listed in Table 6.

For instance the list for individual power consumers will be as follow.

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Table 6: Seasonal Electricity Usage of Equipment [19]

Consumer Power (watt) Operating hours, daily Energyused daily

Three lamps in

living room

3*12=36 w

Summer

1

Winter

3

Summer

36wh

Winter

108wh

One lamp in a

room

12 0.5 0.5 6wh 6wh

Two lamps 2*7=14 1 1 14wh 14wh

Refrigerator 50 6(effective) 0 300wh 0

TV 50 2 2 100wh 100wh

Water pump 60 1 0.33 60wh 20wh

Total 186w 11.5 6.83 720wh 248wh

3.3. PV generator sizing

After the demand for electricity is determined, the PV generator should be

determined accordingly. The sizing procedure will base on the nominal power of

the modules so that the required power rating can be meet by differentcombinations of the modules in series or parallel.

3.3.1. Calculation of the PV generators yield

Monthly average totals for daily horizontal irradiation is listed below, in Table 7

Z2 kwh/m2/day for different locations in the world.

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Table 7: Z2 Factor Calculated for Different Locations [19]

Locations Jan Feb Mar Apr Ma

y

Jun July Aug Sep Oct Nov Dec

Birmingham,

UK

0.65 1.18 2.00 3.47 4.35 4.53 4.42 3.87 2.67 1.48 0.83 0.45

Brisbane,

Australia

6.35 5.71 4.81 3.70 2.90 2.43 2.90 3.61 4.93 5.45 6.33 6.32

Chicago, US 1.84 2.64 3.52 4.57 5.71 6.33 6.13 5.42 4.23 3.03 1.83 1.45

Dublin, Ireland 0.65 1.18 2.26 3.60 4.65 4.77 4.77 3.68 2.77 1.58 0.77 0.45

Glasgow, UK 0.45 1.04 1.94 3.40 4.48 4.70 4.35 3.48 2.33 1.26 0.60 0.32

Houston, US 2.65 3.43 4.23 5.03 5.61 6.03 5.94 5.61 4.87 4.19 3.07 2.48

Johannesburg,

south Africa

6.94 6.61 5.90 4.80 4.35 3.97 4.26 5.10 6.13 6.45 6.57 7.03

London, UK 0.65 1.21 2.26 3.43 4.45 4.87 4.58 4.00 2.93 1.68 0.87 0.48

Los Angeles,

US

2.84 3.64 4.77 6.07 6.45 6.67 7.29 6.71 5.37 4.16 3.13 2.61

Melbourne,

Australia

7.13 6.54 4.94 3.20 2.13 1.93 2.00 2.71 3.87 5.26 6.10 6.68

New York, US 1.87 7.71 3.74 4.73 5.68 6.00 5.84 5.39 4.33 3.19 1.87 1.48

Philadelphia,

US

1.94 2.75 3.81 4.80 5.55 6.10 5.94 5.42 5.37 3.23 2.13 1.68

Phoenix, US 3.29 4.36 5.61 7.23 8.00 8.17 7.39 6.87 5.97 4.84 3.57 2.97

Sydney,

Australia

6.03 5.54 4.23 3.07 2.61 2.33 2.55 3.55 4.63 5.87 6.50 6.13

Toronto,

Canada

1.58 2.54 3.55 4.63 5.77 6.30 6.29 5.45 4.03 2.68 1.37 1.16

Vancouver,

Canada

0.84 1.75 3.00 4.27 6.03 6.50 6.52 5.42 3.80 2.06 1.03 0.65

The values above in Table 7 are given for horizontal surface, if the location is

tilted then it requires to be multiplied by a factor z3, therefore z2 is given for

horizontal surface and z3 is a factor for tilt correction. For instance, the sun

supplies 4kw per meter square of horizontal area. In august the time between

sunrise and sunset is almost 14 hours. The standard irradiation 1000w/m2 can be

applied for 4 hours instead of 14 hours of fluctuating irradiation for one day. If the

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module is 50w laid flat on the ground and 1000w/m2 is applied for 4 hours then

the daily yield will be 4*50=200wh.

By interpreting the Table 7 as hours per day with a standard illumination of

1000w/m2, it is only multiplied by the nominal power of the modules or solar generators. If the total nominal power of the modules is 0.5kw for 4 hours of

standard illumination the total yield is 0.5kw*4h/day=2kwh/day in august. The tilt

correction is an important factor in yield calculation to adjust the non-south-

facing panel laid. Tilt angle correction factor (Z3) for different directions is given

below in Table 8 Z3 is the correction factor.

Table 8: Z3 Factor for Deviation from Horizontal [19]

Module

orientation

tilt angle

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

South, 30 1.44 1.40 1.17 1.08 1.00 0.96 0.97 1.03 1.17 1.30 1.47 1.42

South, 45 1.57 1.5 1.19 1.05 0.94 0.90 0.91 1.00 1.18 1.37 1.61 1.55

South, 60 1.63 1.54 1.15 0.98 0.85 0.81 0.83 0.92 1.14 1.38 1.68 1.61

South-

west/south-

east 30

1.37 1.33 1.15 1.07 1.00 0.97 0.98 1.03 1.15 1.25 1.40 1.36

South-

east/south-

west 45

1.48 1.42 1.16 1.05 0.95 0.91 0.92 1.00 1.16 1.31 1.51 1.46

West/east,

30

1.01 1.01 0.99 0.98 0.97 0.96 0.96 0.97 0.99 1.00 1.01 1.00

West/east,

45

0.99 1.00 0.96 0.95 0.93 0.92 0.92 0.94 0.96 0.98 1.00 0.98

West/east,

60

0.95 0.96 0.91 0.89 0.88 0.86 0.86 0.88 0.92 0.94 0.96 0.94

Eventually, the cell temperature deviation from standard conditions should be

taken into account. Since the standard test temperature for a PV module is 25

C, the correction factor is calculated for the average temperature over 25

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centigrade, which reduces the power generated by the PVs. The average cell

temperature deviations for the months are listed below for London [19].

Table 9: Temperature Correction Factor Z4 [19]

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec1.00 1.00 0.98 0.96 0.93 0.90 0.88 0.88 0.90 0.94 0.97 0.99

Summary,

The ideal energy yield of the generator is calculated by the product of the nominal

power of the PV generator in the following formula.

Where,

Ideal energy yield of PV generator, Eideal, in kwh/day

Nominal power of the PV generator, Ppv in kWp

Factor allow for the location and month, Z2, h/day

Factor for tilt correction, Z3

Factor for deviation of cell temperature, Z4

3.4 Consideration of Cable and Conversion Losses

The electricity yield calculation has been done before, now the net produced

electricity for the costumer has to be calculated by taking into account the cable

losses and other effects on the output.

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3.4.1 Cable losses

When sizing the cable the restriction for the losses should be 3 percent. This is the

goal for loss reduction in low voltage system applications. In stand-alone systemthe module yield is reduced twice by 3 percent. One is in the way from the

generator to the battery via charge controller and second is in the way from the

battery to the load via charge controller. Therefore the over-all losses due to

cabling can be assumed as 6 percent of the total yield.

3.4.2 Conversion losses

The conversion of electrical energy into chemical energy will take place in the

battery and from the chemical energy to back again electrical energy is a process

which has a certain loss. As this involves the temperature, age, construction

details and depth of charge and discharge this loss will reduce the system output

by a factor of almost 0.9 [19].

3.5 Mismatch losses

During the operation of PV system the voltage level is changing and causes a

mismatch. This means the radiation and temperature change all the time and the

voltage level changes and eventually the operation cant be always in its

maximum power point. This drift in voltage is called mismatch and the factor for

mismatch loss can be taken 0.9 [19]. By using MPP tracker control in the inverter

it is possible to reduce the losses caused by this factor.

The PV generator is designed based on the requirement of daily estimated energy

per day as kwh/day. The average required energy for the summer and winter can

be used for the design.

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2 3 4 Where 1or it can be written as 0.94 0.9 can be used to find the net ener stomer for usage.gy available to the cu

or

2 Table 10: Design Parameters for Stand-Alone PV Systems

Summary of design overview

Characteristics Symbol Unit

Average daily energy

consumption

W Kwh/day

Real energy yield of PV

generator

Ereal Kwh/day

Ideal energy yield of PV

generator

Eideal Kwh/day

Nominal power of PV

generator

Ppv kWp

Factor of normalized

irradiation

Z2 h/day

Factor for temperature and

irradiation deviation

Z3 .

Factor for line and

conversion mismatching

Z4 .

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Summer excess: SE, kwh/day

Winter excess: WE, kwh/day

3.6 Determination of Summer Excess and Winter Reserve

In stand-alone PV system the determination of summer excess and winter reserve

is very importa o snt f r izing the battery and PV arrays.

Where W is daily average energy-consumption as indicated in Table 10.

A small house example can summarize the procedure for designing of stand-alone

PV system. A PV generator in a small house in London from the Table 5 has

0.5kw nominal power the incident angle is south-face 45 . In summer season

presented by august the net energy can be0.5kw*4h/day*1*0.88*0.76=1.34kwh/day. In winter season which illustrated by

December, the normalization factor for effective irradiation Z2 is 0.48h/day, the

energy can be calculated as 0.5kw*0.48h/day*1.55*0.99*0.76=0.28kwh/day.

Thus we can conclude that the winter requirement for small house would be

0.28kwh/day. If it is seen in Table 6 where the energy requirement of small house

is illustrated for winter and summer, it can be understood that the energy

production by 0.5kw PV generator can meet the energy requirement in winter

which is 0.28kwh/day.

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In Table 4 it can be seen that the summer requirement is 720wh/day and the

energy yield is calculated above is 1340wh/day. In summer there will be a big

amount of surplus in energy yield almost 160%.

The data required for energy balance of the system

1. Daily energy consumption should be listed in a table.

2. Factor Z2 normalization of the radiation at the locations should be known.

3. Factor Z3 correction of the orientation of PV generators.

4. Factor Z4 cell temperature and irradiation correction factor.

5. Overall factor V for lines, conversion and mismatch is required. V=0.76 is

cal tcula ed before based on assumptions.

3.7 A PV House Design Example

A small house of 3kW system is designed in Phoenix, US.

Assumptions:

The maximum energy consumption is 3kW.

The tilt-angle on the framework of the house is south-45 .

The ASE-300 module type is used having 0.77 performance ratio.

The design will be based on June.

The temperature factor Z 4 is assumed to be 1 because of the lack of daily

temperature data for the location.

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Result:

Z2 factor for Phoenix, US is shown in Table 7 which is 8.17.

Z3 is also shown in Table 8 for south-face 45 tilt angle 0.90.

V is the performance ratio 0.77 and Z 4 is 1.

2 3 430008.17 0.9 1 0529.8

3.7 Cable cross-section sizing

Table 11: Electrical Parameters and their Units

Electrical parameter Symbol Unit

Line length L M

Power transmitted in the

line

P W

mm 2 Line cross-section A

m/(*mm2)Electrical conductivity K

Percentage line loss (3%) . ..

System voltage V V

The cable cross-section can be calculated be

3%

For the design the cable loss is assumed to be 3% previously to allow the cables to

be able to handle additional power when it is required to add, should be

considered.

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3.8 Sizing the battery

In stand-alone PV systems the battery plays an important role in matching the

load requirement for the system. In the locations where there is a big fluctuationin irradiation we will need at least 2-3 days reserve for the summer months and 3-

5 days reserve for the winter months [12].

If the battery capacitys unit is Ah and total energy is wh then Ah=wh/V

where, V is the system voltage.

The battery capacity is Cn, the following equation can be used to calculate the

capacity [12].

2 / where;

W is the average daily consumption

F is the factor for the reserve days, how many days wanted to be reserved for

Vn is the system voltage

3.9 Charge Controllers

The most important feature of charge controller is to measure the battery voltage

and protects the battery against the overcharging. This can be achieved by the

following ways [12]

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Switching off the PV array when the charge cut-out voltage is exceeded, short-

circuiting the PV array with a shunt controller and adjusting the voltage with an

MPP charge controller.

The reserve diode which prevents the battery to be discharged via the array duringlow irradiation level is integrated to the charge controller.

Operation of batteries over long time of operation requires a charge controller to

be flexible [19]. The charge cut-off and discharge cut-off voltages are dependent

on the state of charge of the battery.

The main jobs of the charge controller are;

Allow the optimum charge for the battery.

Protect the battery from the overcharge.

Prevent the battery from unwanted discharge and from deep discharge.

Get information of state of charge of batteries.

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3.9.1 Series controllers

Figure 19: Series Charge Controller

When the voltage level reaches to the charge cut-off level the power from the PV

generator is blocked by the switch in S1 in Figure 19. After the voltage drops

again below the charging cut-off voltage level S1 switches back on.

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3.9.2 Shunt controllers

Figure 20: Shunt Charge Controller

When the charge cut-off voltage is reached charge controller continuously reduces

the power of the module. Since it reduces the power continuously the unwanted

power is short-circuited via the array, this creates heat in the system. This method

is usually used for battery when charging is safe and swift [19].

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3.9.3 MPPT charge controllers

Figure 21: MPPT Controller

During the operation of the PV array the temperature and irradiation are changing

continuously, this result in changing the I-V curve of the PV array consecutively

the maximum power point of the curve should be tracked to exploit the energy

from the PV more efficiently. MPP tracker is used with DC/DC converter by

regulation the voltage every a few minutes and passes through the characteristics

of PV array to determine the maximum point. DC/DC converter gets the power

from the specified point on the curve.

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3.10 Stand-alone PV operation principles

A stand-alone PV system is shown in Figure 22 with all necessary components

such as PV panels, DC-AC inverter, DC-DC converters, MPPT controller and battery.

Figure 22: Stand-Alone PV Operation [1]

where,

Ga is irradiation and Ta is the ambient temperature.

The PV panels are connected in series and parallel combinations to achieve the

desired power capacity. DC-AC inverter is used to convert DC input from the PVgenerator to AC output at certain voltage level and frequency to be used for

consumers. By the use of the battery, the excess power generated by the PV

system is stored to be used when required. The DC-DC bidirectional converter is

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boosting the voltage level from the battery side to the higher level of inverter

input side or it lowers the voltage from the inverter side to battery input side in

order to be able to charge the battery in an appropriate voltage level. The MPPT

controller is used to get the maximum power from the PV generator during the

operation.

3.10.1 Sizing Example of Stand-Alone System

As shown in the figure above the PV generator output is connected to a boost-

converter which has a fixed output of 400Vdc. The battery will be stored from

this point via step-down converter. The grid will be supplied via the inverter to

specified voltage and frequency level.

Assumptions:

The system capacity will be 15kW.

The PV generator has an output range of 120V-380V and connected to boost-

converter.

Grid system frequency is 60Hz and voltage is 120Vrms.

The selected battery voltage is 12V for each.

Results:

For the boost-converter;

Vo/Vin=1/(1-D) in this case to have a fixed voltage in the output the duty ratio

operating range D max=(400-120)/400=0.7, D min=(400-380)/400=0.05

For the inverter;

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Vac=(Vdc/2)xMa for this particular example ma=169.7/400=0.424

For the buck-boost converter;

Vo/Vin=D/(1-D), D=Vo/(Vo+Vin)=240/(240+400)=0.375 duty ratio is set.

For the battery;

The input voltage via buck-boost converter is 0.6x400=240Vdc

Which means 240/12=20 batteries are needed in series.

The resulting design is shown in Figure 23.

Figure 23: Stand-Alone PV System Design Example

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Chapter 4: Design and Installation of Large Scale PV Systems

4.1 Introduction

Photovoltaic technology which converts sunlight into electricity is growing very

fast among other energy sectors. This sector is being improved because of

ignitions of increasing oil prices and large amount of carbon emission from

different energy utility systems. In this research, it is planned to give the readers

essentials of PV systems in all aspects and consideration of the real design and

installation for 2MW large scale system.

PV systems can be classified into two categories; stand-alone systems and grid-

connected systems. In stand-alone system the produced energy from solar system

is matched with the total energy demand. Since the energy yield from PV often

does not match with the load, extra storage systems are used to store the energy

when it is not used completely. If the system is by the other type of energy such as

wind or fuel cell, it is called a photovoltaic hybrid system.

In grid-connected systems the public electricity grid functions as an energy store.

It is estimated that by the German solar energy society after the countries set up

the feed-in tariff for PV generation to encourage the investments, most of the

power generation will base on PV system in the following years. The figure

shows different application of PV systems.

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Figure 24: Application of PV systems [12]

4.2 Grid-Tied PV System

In a typical grid-tied solar system, the DC electricity produced by the PV array is

usually fed by cables into a PV array combiner box where they are connected

together. A cable form this junction box feeds the DC electricity to the grid-tied

inverter. The inverter converts DC to AC which is either consumed by the

building loads and appliances or fed onto the grid. The inverter is connected tomain AC circuit breaker panel / fuse box or directly to the incoming cables from

the grid.

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Figure 25: General Structure

4.2.1 Grid-Tied Inverter

Grid-tied inverters convert DC into single phase or three phase AC electricity at a

voltage and frequency suitable to be fed onto the grid. They are designated in

different sites depending on the size of PV array they are connected to.

4.2.1.1 Functions of Inverters

1. Convert DC to AC at a desired voltage and frequency.

2.

To maximize the output of the array under varying conditions of solar insulation in which it will operate by tracking the maximum power point

of I-V curve of the array in which it operates.

3. To ensure nonhazardous operation complying the electrical codes.

4.2.1.2 Technical Requirements

1. Generation of a pure sine wave, synchronous with the sine wave of the

grid.

2. Accurate tracking of the MMP of the array I-V curve.

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3. High frequency operation at full and lower loads.

Tracking the maximum power point is an important aspect of the inverters which

are used in PV systems.

As shown in the figure in each temperature and irradiation values the maximum

power point in I-V curve. Different techniques are used. The most general is to

sweep the array current, measure the array voltage and current and deduce the

maximum power point.

Figure 26: Maximum Power Point under Different Irradiation [27]

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4.2.2 Grid-Tied Inverters and PV Module Configuration

General selection of inverter for large scale grid-tied system is central inverter.

This is because of the compatibility and capability of carrying high power startingfrom 5 kW and more up to 500 kW.

When modules are connected in series the output current will be the same

as one module and the output voltage will be the sum of all the module

voltages.

When, modules are connected in parallel the total current will be the sum

of all the modules, and the total voltage will be the same as one module.

In grid-tied systems modules are usually connected in series strings. The

maximum voltage of a string of modules must be lower than the maximum

voltage of a string of modules must be lower than the maximum input voltage

rating of inverter.

Example:

Assume the specification for the PV module is given below.

Pnominal =200 W

VOC=72 V (@-10 o C)

Assume an inverter has a maximum Dc input of 600 V dc.

How could 3.2 kW array be assembled?

Solution:

3.2 kW/200W = 16 units of modules

872 = 576 maximum output voltage of one string is less then and appropriate for the given inverter type.

So we should use two 8-module strings in parallel to meet the voltage requirement

with the inverter.

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Figure 27: Basic Configuration for the given Example Showing the Modules,Strings and an Array

4.2.3 Selecting Inverter

4.2.3.1 Central Inverters

In large scale PV system all modules in the array are connected to single inverter.

Array will consist of several strings which are then connected to PV combiner

box before being connected to inverter. For large scale system the best way is to

use high capacity inverter as the central inverter.

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The advantage of using central inverter:

In large scale PV system

High array outputs up to now can be handled with a few inverters only.

Termination of the DC cables in the PV combiner box is relatively straightforward.

The Principal features of central inverter:

Centralized installation

Combined series and parallel connection of modules

Suitable for modules which have the same electrical characteristics

Suitable for PV array where it is subject to a uniform regime of solar insulation.

Figure 28: Central Inverter with Large Scale Array Configuration

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Inverter- PV Array Compatibility, Inverter Size and Location

Both the inverter and PV array need to be compatible with each other.

The size of the inverter should not be less than 90% of the peak wattage of

the array. For instance, if the array peak wattage is 100 kW the inverter should be in the range of (90 kW or 95 kW).

The inverters MPP range must match the operating voltage of the array.

The inverter must be compatible of withstanding the maximum array

voltage and current.

When the inverter is overloaded or overheated, it derates itself.

This means that the inverter is no longer able to process a portion of the array

power in the summer when the power at a peak.

Important: if the inverter or located at a place where temperature cannot be

reduces and stays at more than 70 o C, the inverter will regulate the output of the

array down to protect itself. Generally inverters have coolers to protect them from

high temperature.

4.2.3.2 Inverter Technical Specifications and Efficiency

The following specifications are important to match the inverter with the array

1. On the PV array (DC output) , or input side of the inverter

- DC nominal power and DC peak power.

- DC nominal current and DC peak current.

- DC nominal voltage and DC peak voltage.

- The MPP voltage range (maximum @-10 o C and minimum @ 70 o C )

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2. The grid side (output side of the inverter)

- AC nominal power output and AC peak power output.

- AC nominal current output and AC peak current output.

- Inverter efficiency over a range of loads 5%, 10 % ,

20%,.,100%,110%

Inverters operate at different efficiencies, depending on the load. This is

expressed in the inverters efficiency curve.

Figure 29: Efficiency of the Inverter under Different Irradiation and Durations [4]

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4.3 Design Considerations and 2 MW System Installations

4.3.1 Module Selection

PV module selection criteria:

The performance warranty in case of any problems,

Module replacement ease,

Compliance with natural electrical and building codes.

Manuals should be available to see the quality and characteristics of

module.

In this particular design, the most important issue has to be considered in PV

module selection is area occupancy. When Silicon and Thin-film are compared

Thin-film module occupies almost 1.35 times more area than a Silicon PV

module.

For instance consider Schott ASE-300-D6F/50 (Silicon Module) and First Solar

FS-272 module (thin film). Schott has a power of 300W with 2.43 m 2 surface

areas and most solar Thin-film has a power of 69.3 W with an area of 0.72 m 2.

When these data are considered it is seen that the land required by Schott modulewill be almost 35 % less.

Electric Data for ASE-300-D6F/50

The electrical data applies to standard test considerations (STC):

Irradiance at the module level of 1000 W/m 2, spectrum AM 1.5 and a cell

temperature of 25 o C:

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Table 12: Characteristics of AS-300 Module

Power (max) 300 W

Voltage @ Max. Power Point 50.6 V

Current @ MPP 5.9 Amp

Voc (Open Circuit Voltage) 63.2 V

Isc (Short Circuit Current) 6.5 Amp

Cell Temperature Coefficient

Power T k (P p) -0.47 %/ o C

Open-Circuit Tk (Voc) -0.38 %/ o C

Short-Circuit Tk (Isc) 0.1 %/ o C

Limits

Max. System Voltage 600 V DC

-40 o C to 90 o COperating module Temp.

Equivalent wind resistance Wind speed : 120 mph

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4.3.2 Inverter Selection

As discussed before, for high power applications large capacity inverter is

appropriate to use in this system. PV-powered inverter is one of the best invertershaving integrated MPPT central unit and synchronous with the grid. 260 MW size

in selected to use minimum number of inverters in this system.

4.3.2.1 Electrical Specifications

Table 13: Characteristics of PV-Powered Inverter

Continuous output power 260 kW

Weighted CEC efficiency 96.5 % (test)

Maximum DC input voltage (V DC) 600 V

DC peak power tracking range (V) 295-500 V

DC Nominal current (A) 918 Amp

AC Nominal voltage (V) 480

AC operating range (V) 422-528

AC frequency range (Hz) 59.3-60.5

Harmonic Distortion < 3 %

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As seen in the data sheet the input DC voltage range of the inverter is 295Vdc to

500Vdc. The output voltage is 480 volts. When the formulas in equations below.

2 sin 2

2 sin 2 2 sin 2

For V in,1=295 volts and output voltage is 480 volts ma=2*Vout/Vin,1

If we assume that the voltage is boosted to the level by duty ratio of 0.3 then the

actual inverter input voltage will be;

11 The Vdc for the inverter will be 295/ 1 0.7 98minimum and 500/0.7 1666.6volts. I s t e m i e calculated by,n this ca e h odulation ndex can b

2 480/9832 480/166 The modulation index value w n hill be i t e range of

0.9763 0.57

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4.3.3 First Proposal of PV system

Problem description:

In this proposal we will design a large scale PV system which has a size of

2MW. The data sheet for the module type ASE-300 Scott-solar is given on theappendix. The arrays will be sized and the inverters simultaneously. The

maximum modules will be used in the array for large scale inverter size and the

restrictions of the inverter input voltage and current will be considered.

The grid voltage is 20kV at 60 Hz frequency. The power will be carried from the

PV side through the inverters having output voltage of 460Vac.

The cabling, transformer and inverter sizing also will be discussed in detail.

Solution:

1. Number of modules :

Peak power of Schott AS-300 module; P max=300 W

Estimated modules to use in 2 MW system = = 6,666 modules.

2. Voltage values (high, low, nominal outputs) of modules must bedetermined.

Max. Voltage is determined for (-10 o C), V oc, and the lowest is in summer

(high temperature) time.

Voc, at low temperature (-10 o C) has to be calculated.

VMPP, IMPP, Voc for (25 o C) is given in data sheet, which are

[VMPP =50.6 V, I MPP =5.9 A, V oc =63.2 V @25 o C]

Voltage temperature coefficient T c,Voc = -0.38 %/ o C

Current temperature coefficient T c,Isc = 0.1 %/ o C

Power temperature coefficient T c,PP = -0.47 %/ o C

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And Maximum System Voltage for this system is 600 V.

251035 , 70 25 45

@10 63.6. 63.6 35 0 3@ 1050.6 0.3850.6 5 0

@7050.6 0.38 50.6 45 03. Since the system is very large (2 MW) the inverter should be selected to

be as large as possible.

260-kW inverter is available in the market; however PV-powered is

selected.

From the data sheet of PV-Powered inverter:

Maximum DC input voltage V DC= 600 V,

V pv,MPP lower = 265 V

V pv,MPP upper = 500 V

DC nominal current, I DC nominal = 918 Amps.

4. How many modules in one string? How on ay?many strings in e arr

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. , 106007.8 76.9

. 918

5.9 .155 Maximum number of modules in one array is; 1558=1250 modules

Total power of one array = 1240 300 W = 372 kW which is very large for a

single inverter not appropriate!

We have to consider estimated power for one array such as 270 kW:

2700.95=260 kW, therefore inverter is appropriate.

If we calculate the estimated power for the array to the 8 modules, then string

power will be

270 kW/ (8300) =112.5 @ 112 strings in parallel can be used for this design.

Figure 30: Configuration of the Array (112 strings with 8 modules) with theCentral Inverter

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Total number of modules in this case is; 1128=896 modules, then

896300=268.8 kW which is suitable for 260 kW inverter.

If we use 110 strings and 8 modules in one string, then total power will be

8112300=268.8 kW for one array , 268.8 0.965=260.7 kW which is

good for 260 kW inverter.

If we use 8 inverters, that is, 8 arrays;

Total power will be 8268.8=2.15 MW which is the system power for

maximum efficiency usage.

Figure 31: General Structure of the Arrays with the Inverters

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The reason for 2.15 MW proposals is that since our system with these inverters

can handle up to 2.15 MW better to use maximum number of modules to reduce

the cost as overall and to utilize the system more efficiently.

Figure 32: Network Model of 2.15MW System, Single Line Diagram

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Multiple Interconnected Inverters: The highest efficiency in grid-tied inverters is

at their full load condition. If the load reduces the efficiency will decrease. If each

array shown in the single line diagram is connected to one inverter and there is no

interconnection between the inverters, when the solar insulation reduces the array

output will decrease. However, in multiple interconnected inverter method, this

situation is overcome by coordinating communication between the inverters and

when the arrays output is less they will feed only one inverter as much as the

output power increases they will switch on one more inverter to meet the power

requirement from the arrays. This continues until it comes to the point of the

maximum power during the time and all the inverters are switched on running at

full load condition. The same sequence is also for when the output of the arrays

are decreasing they start to switch off the inverters one by one to match the output power of the array with the inverters. This communication will give us two

different advantages; first inverter lifetime will increase, second the inverters will

run in their highest efficiency.

4.3.4 Description of AC Transmission Side and AC Line Model

From 2.1 MW PV capacity, (295-500) V-DC input from the arrays to the

inverters, connect all the inverters to the same bus at 460 V-AC , then step up the

voltage to 20 kV with 5 local bus network . Then transfer the power through an

overhead line for 30 km to the nearest transmission line at 63 kV. At 63 kV-AC

bus we have 10 bus local networks on 20 kV bus. For 13 km transmission line

different models were tried and Magpie is selected.

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Table 14: 13.2132 kV Class One phase Neutral Return Line Model

Conductor DC

Resistance

Inductance

( /km)

L

Susceptance

(S/km)

C

Current

ratings

proiect_taymur_eyup_ohio (1)

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