photovoltaic system summary

Czech Technical University in Prague

Faculty of Electrical EngineeringDepartment of Electroenergetics

PV Summary

Author:

Minh-Quan Dang

January 12, 2016Prague

CONTENTS

Contents

1 General considerations 11.1 What are the energy needs of mankind, trends and problems of fossil

fuels? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 What is renewable source and what are perspectives of different types? 41.3 What are the spectral properties and components of the sunlight? . . 51.4 What are the time variation properties and limits of the sun as a source? 6

2 Solar cell physical aspects 72.1 What is the main limit (hint: of the single-junction) solar cell efficiency? 72.2 What determines the generated voltage and the generated current in

solar cell? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3 Equivalent circuit of solar cell and diode equation. . . . . . . . . . . . 8

3 Solar cell Construction and Technology 93.1 Fabrication of silicon wafers . . . . . . . . . . . . . . . . . . . . . . . . 93.2 Fabrication of standard crystalline silicon solar cell . . . . . . . . . . . 103.3 Optical losses, improvements of standard crystalline cell technology . . 143.4 External Quantum Efficiency, its integral, types of losses . . . . . . . . 15

4 Thin film 174.1 Material requirements and application advantages of thin-film technolo-

gies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.2 Examples of mainstream and emerging thin film technologies, perspec-

tives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5 Construction and technology of PV modules 195.1 What determines the total power parameters of the module? . . . . . . 195.2 Effect of shading and its mitigation . . . . . . . . . . . . . . . . . . . . 195.3 Module fabrication components and steps, thin-film solar module inter-

connection scheme. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.4 Effect of temperature and its partial mitigation, what is NOCT? . . . 23

6 Autonomous PV systems 246.1 System with battery with / without regulator . . . . . . . . . . . . . . 246.2 Standalone household system and its dimensioning with / without backup

(hybrid). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7 On-grid PV system 277.1 Types of on-grid systems, sizing of PV inverter and string, requirements

for cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277.2 Types and topologies of inverters, detailed wiring of small and medium

system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287.3 Effect of PV on the grid (quality, voltage, stability) . . . . . . . . . . . 29

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8 Diagnostics 308.1 Diagnostics: type of failures, their consequences and methods to reveal 308.2 Maintenance, lifetime, monitoring, diagnostics using inverters, Potential

Induced Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318.3 I-V measurements: indoor/outdoor, requirements for sun simulator . . 338.4 Photo-/electro-luminescence: principle and type of failures . . . . . . . 338.5 Thermography: principle and type of failures . . . . . . . . . . . . . . 34

9 Economic consideration 359.1 Levelized Cost of Energy, effect of efficiency on the cost of electricity . 359.2 Structure of the end user electricity price, time evolution in yearly and

daily scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369.3 Billing scenarios, type of tariffs . . . . . . . . . . . . . . . . . . . . . . 379.4 Energy payback time, carbon footprint, recycle . . . . . . . . . . . . . 38

References 39

1 GENERAL CONSIDERATIONS

1 General considerations

1.1 What are the energy needs of mankind, trends and prob-lems of fossil fuels?

Energy is typically stored in the form of energy carriers (coal, gas, wood, etc.). Thisform of energy is typically called primary energy.

The pressure of the steam is again used to drive a generator that makes electricalenergy available at the exit of the power station. This energy is called secondaryenergy.

End Energy = Primary Energy - Secondary Energy - Losses

Figure 1: Energy flow

Figure 2: Human energy consumptions

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Primary sources by percentages [Oil - Coal - Gas - nuclear energy - biomass, wastes- Hydro - Renewable]

Problems:

1. Growing Energy Requirements

2. Tightening of Resources

3. Climate Change

4. Hazards and Disposal

Figure 3: Co2 problem

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Figure 4: Greenhouse effect

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1.2 What is renewable source and what are perspectives ofdifferent types?

Figure 5: Renewable sources

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1.3 What are the spectral properties and components of thesunlight?

Figure 6: Solar radiation

Figure 7: Types of irradiation

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1.4 What are the time variation properties and limits of thesun as a source?

While the solar radiation incident on the Earths atmosphere is relatively constant,the radiation at the Earths surface varies widely due to:

-atmospheric effects, including absorption and scattering;-local variations in the atmosphere, such as water vapour, clouds, and pollution;-latitude of the location; and-the season of the year and the time of day.The sun has different heights depending on the time of day and year. Air Mass

is the ratio between actual traveling distance of the light in comparison to the verticalpath though the atmosphere.

Figure 8: Air mass

where =Sun height angleThe limit of sun light source during nice day AM 1 EG = 1000W/m

2

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2 SOLAR CELL PHYSICAL ASPECTS

2 Solar cell physical aspects

2.1 What is the main limit (hint: of the single-junction) solarcell efficiency?

1. Silicon thickness

2. Carrier recombinations (radiative, Auger, via local centers)

3. Free carrier absorption

Figure 9: Carrier recombination

Figure 10: Factors limit the solar cell efficiency

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2 SOLAR CELL PHYSICAL ASPECTS

2.2 What determines the generated voltage and the generatedcurrent in solar cell?

Generated current depends on:

1. The area of solar cell

2. the number of photons come to cells surface

3. the spectrum of incident light

4. Optical properties(absorption and reflection)

5. the collection probability

Generated voltage depends on:

1. Recombination (measured by dark saturation current)

2. Thickness of bandgap (direct proportional)

2.3 Equivalent circuit of solar cell and diode equation.

Figure 11: Equivalent circuit of a solar cell

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3 SOLAR CELL CONSTRUCTION AND TECHNOLOGY

3 Solar cell Construction and Technology

3.1 Fabrication of silicon wafers

Czochralski process used to produce noncrystalline silicon.

1. Melting pieces of polysilicon in crucible at 1450oC

2. Seed crystal is dipped into the melt from above

3. Withdraw the the seed with light rotation

Figure 12: Czochralski method

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Figure 13: Wafer fabrication

3.2 Fabrication of standard crystalline silicon solar cell

Figure 14: Standard mass production (c-Si cells)

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Figure 15: etching of damaged layer

Figure 16: texturing

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Figure 17: Phosphorous diffusion

Figure 18: SiO 2 /SiN X deposition

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Figure 19: Al print screen

Figure 20: firing of contact pastes

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Figure 21: edge grinding

3.3 Optical losses, improvements of standard crystalline celltechnology

Optical losses

1. Reflection

(a) anti-reflective coating

(b) surface texturing

(c) use different layers with different thickness to minimum refection for specificwavelength.

2. Shading by means of contact finger

(a) Reduce finger distance

3. Parasitic absorption

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3.4 External Quantum Efficiency, its integral, types of losses

Figure 22: External Quantum Efficiency

Figure 23: Losses on solar cell

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Figure 24: Effect of losses

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4 THIN FILM

4 Thin film

4.1 Material requirements and application advantages of thin-film technologies

Material used for production of thin film cell must possesses a direct bandgap.Advantages

1. potentially cheap

2. flexible, lightweight

3. low material consumption

4. better temperature factor

4.2 Examples of mainstream and emerging thin film technolo-gies, perspectives

Thin film production technology

1. PVD = physical vapour deposition

(a) Magnetron sputtering (PVD)

2. CVD = chemical vapour deposition

(a) CVD (PECVD)Plasma enhanced

(b) metalorganic CVD

(c) chemical vapor transport

Major thin-film technologies

1. Thin-film Si solar cell

(a) absorber deposition: PECVD from SiH 4 with strong Hydrogen dilution defect passionating

(b) advantages non-toxic materials

(c) disadvantages Staebler-Wronski effect - low efficiency (10%) disorder -doping is problem - low voltage

2. CIGS or CIS

(a) advantages high efficiency (15 20%)

(b) disadvantages homogeneity issues of ternary system, limited supply of In,toxicity of CdS

3. CdTe

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4 THIN FILM

(a) absorber deposition many methods (vapour transport dep.) + post-deposition treatment by Cl - grain boundary passivation

(b) advantages low fabrication cost relatively high efficiency (15-20%)

(c) disadvantages toxicity of Cd limited supply of Te

4. AIIIBV multijunction cells

(a) absorber deposition Molecular Beam Epitaxy vacuum< 1010 mbar !

(b) advantages highest efficiency (46%)

(c) disadvantages active cooling necessary, solar tracker necessary, extremelyexpensive equipment,

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5 CONSTRUCTION AND TECHNOLOGY OF PV MODULES

5 Construction and technology of PV modules

5.1 What determines the total power parameters of the mod-ule?

The number of cells and type of interconnection determines the total power of themodule. Almost always cells are connected in series to increase the power and voltage.The current then depends on side of cell and its efficiency.

5.2 Effect of shading and its mitigation

Figure 25: Cell in parallel

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Figure 26: Cell in parallel shaded

Figure 27: Cells in series

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Figure 28: Cells in series shaded

Solution using bypass diodes. Bypass diode is connected anti parallel to solar cell.If no shady then cell has positive voltage, then no current flows through diode. If shadyis there, then cell has negative voltage. The current from unshaded cells go throughdiode. When the current is small enough then the voltage at shady will be positiveagain and contribute to the total current.

Figure 29: Partial shading problem

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5.3 Module fabrication components and steps, thin-film solarmodule interconnection scheme.

Module components

Figure 30: Module components

Module production steps

1. Taking the cells

2. Transporting the cells

3. Soldering the cell strings

4. Positioning the cell strings

5. Feeding into the laminator

6. Installation of the module frames

Serial interconnection of thin film module by laser or mechanical scribing

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Figure 31: Thin film module

5.4 Effect of temperature and its partial mitigation, what isNOCT?

Figure 32: Effect of temperture

Operating temperature of cells in the module depends on the ambient tempera-ture and the module construction (Thermal resistance rth) then to reduce impacts oftemperature:

1. Use material with high heat transfer coefficient

2. Low heat conductivity with larger thickness

NOCT: Nominal Operating Cell Temperature is cell temperature at:

1. Ambient temperature 20oC

2. Irradiance 800 W/m2

3. Wind speed 1m/s

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6 AUTONOMOUS PV SYSTEMS

6 Autonomous PV systems

6.1 System with battery with / without regulator

Figure 33: Stand alone system without regulator

Figure 34: Stand alone system with regulator

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Regulator or charge controller tasks:

1. Overload protection

2. Deep-discharge protection (10.8V for Pb-battery)

3. Prevention of unwanted discharging

4. State-of-charge monitoring

5. Adjusting to battery technology (electrolyte/gel)

6. Voltage conversion (possibly)

7. MPP tracking (possibly) ensure PV system draws max. power from solar array.

6.2 Standalone household system and its dimensioning with/ without backup (hybrid).

Figure 35: off-grid PV applications

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Process of dimensioning the stand alone PV system:

1. Determination of the energy demand and optimization of consumption

2. Develop the concepts:

(a) What voltage levels?

(b) What type of PV system (DC, AC, combined AC/DC, with or withoutback-up generator)

3. Sizing the PV generator and storage battery

1

cycleCS +

GdGd0

CA 1

(a) Generator capacity

CA =GAGGd0

L

Where A and are area and conversion efficiency of PV; G is mean valueof daily irr; L is mean value of daily load.

(b) Battery capacity

CS =CuL

4. Dimensioning the charge controller

5. Dimensioning the cables

For PV with back-up system then smaller PV and battery system required. Systemwithout consumer limitations

For PV without back-up system then consumer limitation or smart solutions mustbe applied to limit the size of the system.

Cable requirements:

1. Diameter 4-6mm2

2. Water proof connectors

3. Must be separated positive and negative in different tubes

4. Loss < 1%

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7 ON-GRID PV SYSTEM

7 On-grid PV system

7.1 Types of on-grid systems, sizing of PV inverter and string,requirements for cables

Figure 36: Grid-on PV system

Figure 37: Hybird sytem

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7 ON-GRID PV SYSTEM

7.2 Types and topologies of inverters, detailed wiring of smalland medium system

Figure 38: PV installation

Figure 39: Converter topologies for PV inverters

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7 ON-GRID PV SYSTEM

7.3 Effect of PV on the grid (quality, voltage, stability)

Quality

1. Voltage: Maximum allowed voltage mismatch betweens PV and grid is 10%

2. Frequency: maximum allowed frequency mismatch between PV and grid is0.2Hz or 0.5 Hz

When V of f out of this range then PV system will be disconnected from the gridautomatically by its protective devices

Figure 40: Voltage profile with present of PV

PV system can supply active power but decrease power factor for the system.

Figure 41: Incresing power factor

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8 DIAGNOSTICS

8 Diagnostics

8.1 Diagnostics: type of failures, their consequences and meth-ods to reveal

Figure 42: Type of failures and method to reveal

Figure 43: Diagnostics introduction

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8 DIAGNOSTICS

8.2 Maintenance, lifetime, monitoring, diagnostics using in-verters, Potential Induced Degradation

Figure 44: Causes of failures

Figure 45: Life time

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8 DIAGNOSTICS

Figure 46: Diagnostics using inverter

Figure 47: Potential Induced Degradation

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8 DIAGNOSTICS

8.3 I-V measurements: indoor/outdoor, requirements for sunsimulator

Indoor measurement

1. Continual sun simulators

(a) Small area (cell)

(b) Lab measurement (thin film)

2. Flash test

(a) large area (module)

(b) Flash duration 1-10 ms

(c) Error 2%

Outdoor measurement

1. Solar module analyzer

2. Temperature measurement

3. Irradiance measurement

- Correction to STC

-Error > 7%

Sun simulator requirement

1. Spectral mismatch

2. non-uniformity (max-min)/(max+min)

3. Short Term Instability (max-min)/(max+min)

4. Long Term Instability (max-min)/(max+min)

8.4 Photo-/electro-luminescence: principle and type of fail-ures

The ideal is to light up a solar module as LED. Silicon emit spectrum about 1150nm then can not be seen by human eyes, the special camera must be used. Dark regionare critical for cell performance.

1. Micro-crack (with/without effect on performance)

2. Screen printing error

3. interrupted metalization

4. low-collection region

5. low-lifetime region

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8 DIAGNOSTICS

8.5 Thermography: principle and type of failures

Defective cell will create hot spot (Joule heating) on PV module which can bedetected by thermometer camera.

The knowledge of the characteristic thermo-pictures of the various operating con-ditions makes thermography a very effective means of quality control of photovoltaicplants. It is very easy to recognize incorrectly connected and defective modules.

Figure 48: Solar module with 36 cells without bypass diodes: Shady acts as a load thatis massively heated by the remaining 35 cells

Figure 49: View of various degrees of shading: The transferred heat power reaches amaximum for degrees of shading between a quarter and half of the cell

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9 ECONOMIC CONSIDERATION

9 Economic consideration

9.1 Levelized Cost of Energy, effect of efficiency on the costof electricity

Figure 50: Levelised Cost of Energy LCOE

Figure 51: Simple COE

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Figure 52: Effect on efficiency on COE

Figure 53: LCOE

9.2 Structure of the end user electricity price, time evolutionin yearly and daily scale

Figure 54: Structure of electricity price

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Figure 55: Price variation

9.3 Billing scenarios, type of tariffs

Figure 56: Billing scenario

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Figure 57: Feed in tariff

9.4 Energy payback time, carbon footprint, recycle

Figure 58: Energy payback time

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REFERENCES

References

[1] Konrad Mertens. Photovoltaics: Fundamentals, Technology and Practice. JohnWiley & Sons, 2013.

[2] Ph.D. Mgr. Jakub Holovsky. Ae1m13ezf - electrochemical sources and photo-voltaics.

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General considerations What are the energy needs of mankind, trends and problems of fossil fuels? What is renewable source and what are perspectives of different types?What are the spectral properties and components of the sunlight? What are the time variation properties and limits of the sun as a source?

Solar cell physical aspectsWhat is the main limit (hint: of the single-junction) solar cell efficiency? What determines the generated voltage and the generated current in solar cell? Equivalent circuit of solar cell and diode equation.

Solar cell Construction and Technology Fabrication of silicon wafers Fabrication of standard crystalline silicon solar cell Optical losses, improvements of standard crystalline cell technology External Quantum Efficiency, its integral, types of losses

Thin filmMaterial requirements and application advantages of thin-film technologies Examples of mainstream and emerging thin film technologies, perspectives

Construction and technology of PV modulesWhat determines the total power parameters of the module?Effect of shading and its mitigation Module fabrication components and steps, thin-film solar module interconnection scheme. Effect of temperature and its partial mitigation, what is NOCT?

Autonomous PV systemsSystem with battery with / without regulator Standalone household system and its dimensioning with / without backup (hybrid).

On-grid PV systemTypes of on-grid systems, sizing of PV inverter and string, requirements for cables Types and topologies of inverters, detailed wiring of small and medium system Effect of PV on the grid (quality, voltage, stability)

DiagnosticsDiagnostics: type of failures, their consequences and methods to reveal Maintenance, lifetime, monitoring, diagnostics using inverters, Potential Induced Degradation I-V measurements: indoor/outdoor, requirements for sun simulator Photo-/electro-luminescence: principle and type of failures Thermography: principle and type of failures

Economic considerationLevelized Cost of Energy, effect of efficiency on the cost of electricity Structure of the end user electricity price, time evolution in yearly and daily scale Billing scenarios, type of tariffs Energy payback time, carbon footprint, recycle

References

photovoltaic system summary

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