Photovoltaics Technology Components and Systems Applications Clemson Summer School 4.6. – 6.6.2007...

Photovoltaics

Technology Components and Systems

Applications

Clemson Summer School

4.6. – 6.6.2007

Dr. Karl Molter

FH Trier

www.fh-trier.de/~molter

molter@fh-trier.de

4.6.07 - 6.6.07 Clemson Summer School 2007Dr. Karl Molter / FH Trier / molter@fh-trier.de

Content

1. Solar Cell Physics

2. Solar Cell Technologies

3. PV Systems and Components

4. PV Integration into buildings

IntroductionPhotovoltaics, or PV for short, is a solar power technology that

uses solar cells or solar photovoltaic arrays to convert light from the sun into electricity.

Photovoltaics is also the field of study relating to this technology and there are many research institutes devoted to work on photovoltaics. The manufacture of photovoltaic cells has expanded in recent years, and major photovoltaic companies include BP Solar, Mitsubishi Electric, Sanyo, SolarWorld, Sharp Solar, and Suntech. Total nominal 'peak power' of installed solar PV arrays was around 5,300 MW as of the end of 2005 and most of this consisted of grid-connected applications. Such installations may be ground-mounted (and sometimes integrated with farming and grazing) or building integrated.

Financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in many countries including Germany, Japan, and the United States.

1. Solar Cell Physics

• Solar Cell and Photoelectric Effect

• The p/n-Junction

• Solar Cell Characteristics

History

• 1839: Discovery of the photoelectric effect by Bequerel

• 1873: Discovery of the photoelectric effect of Selen (change of electrical resistance)

• 1954: First Silicon Solar Cell as a result of the upcoming semiconductor technology ( = 5 %)

Solar Cell and Photoelectric Effect

1. Light absorptionh

+2. Generation of „free“

charges

3. effective separation of the charges

Result: wearless generation of electrical Power by light absorption

energy-states in solids:Band-Pattern

Atom Molecule/Solid

• • • • • • • •

energy-states in solids:Insulator

electron-energyconduction-band

valence-band

Fermi-level EF

bandgap EG

(> 5 eV)

Terms:

Fermilevel EF: limit between occupied and non occupied energy-states at T = 0 K (absolute zero)

valence-band: completely occupied energy-band just be-

low the Ferminiveau at T = 0 K, theelectrons are „fixed“ (tightly bound)

inside the atomic structure

conduction-band:energy-band just above the valence-band, the electrons can move „freely“

bandgap EG: distance between valance-band andconduction band

energy-states in solids :metal / conductor

electron-energy

conduction-band

Fermi-level EF

energy-states in solids:semiconductor

electron-energy

conduction-band

valence-band

Fermi-level EF

bandgap EG

( 0,5 – 2 eV)

Electron-EnergyAt T=0 (absolute zero of temperature) the electrons occupy the

lowest possible energy-states. They can now gain energy in two ways:

• Thermal Energy: kT (k = Boltzmanns Constant, 1.381x10-23 J/K, T = absolute temperature in Kelvin)

• Light quantum absorption: h (h = Plancks Constant, h = 6.626x10-34 Js, = frequency of the light quantum in s-1).

If the energy absorbed by the electron exceeds that of the bandgap, they can leave the valence-band and enter the conduction-band:

energy-states in solids:energy absorption and emission

electron-energy

conduction-band

valence-band

Generation

Recombination

energy-states in semiconductorsphysical properties:

thermal viewpoint: The larger the bandgap the lower is the conductivity. Increasing temperature reduces the electrical resistance (NTC, negative temperature coefficient resistor)

optical viewpoint: the larger the bandgap the lower is the absorption of light quantums. Increasing light irradiation decreases the electrical resistance (Photoresistor)

doping of semiconductorsIn order to avoid recombination of photo-induced charges and to „extract“ their energy to an electric-device we need a kind of internal barrier. This can be achieved by doping of semiconductors:

IIIB IVB VB

„Doping“ means in this case the replacement of original atoms of the semiconductor-material (e.g. Si) by different ones (with slightly different electron configuration). Semiconductors like Silicon have four covalent electrons, doping is done e.g. with Boron or Phosphorus:

N - Doping

n-conducting Silicon

crystal view

conduction-band

valence-band

- - - - -P+ P+ P+ P+ P+

majority carriers

donator level

energy-band view

P - Doping

p-conducting Silicon

crystal

conduction band

valence-band

EF B- B- B- B- B-

majority carriersacceptor level

+ + + + +

energy-band view

p – type region

EFB- B- B- B- B-

+ + + +

n – type region

- - - -P+ P+ P+ P+ P+

p/n-junction without lightBand pattern view

--Diffusion

Diffusion

internal electrical field

depletion-zone

p–type region

EFB- B- B- B- B-

+ + + +

n–type region

- - - -P+ P+ P+ P+ P+

irradiated p/n-junctionband pattern view (absorption p-zone)

photocurrent

Internal electrical field

depletion-zoneE = h

p/n–junction without irradiation(semiconductor diode)

crystal view

n-silicon

- - - - - - - - - - - -

p-silicon

+ + + + + + + + + + + +

-diffusion

electrical fieldE- - - - - - - - - - - -+ + + + + + + + + + + ++ + + + + + + + + + + +

+ + + + + + + + + + + +

- - - - - - - - - - - -

depletion zone

p/n–junction with irradiationcrystal view

n-silicon

- - - - - - - - - - - -

p-silicon

+ + + + + + + + + + + +

-diffusion

electrical fieldE- - - - - - - - - - - -+ + + + + + + + + + + +

depletion zone

Charge carrier separation within p/n–junction

diffusion:from zones of high carrier concentration to zones of low carrier concentration (following a gradient of electrochemical potential)

drift:driven by an electrostatic field established across the device

Antireflection-coating

The real Silicon Solar-cell

~0,2µm

~300µm

Front-contact

Backside contact

n-region

p-region

depletion zone

- - - - - - - - - -+ + + + + + + + + +

Equivalent circuit of a solar cell

currentsource

IPH: photocurrent of the solar-cell

ID /UD: current and voltage of the internal p-n diode

RP: shunt resistor due to inhomogeneity of the surface and loss-current at the solar-cell edges

RS: serial resistor due to resistance of the silicon-bulk and contact materialISG/USG: Solar-cell current and voltage

RL/IL/UL: Load-Resistance, current and voltage

ISG = IL, USG = UL

Solar-Cell characteristics

ID ISG

RLUD=USG

ISG / PSG

solar-cellcharacteristics

ISG = I0 = IK

RL=0 RL=

diode-characteristic

Load resistance

MPP = Maximum Power Point

simplified circuit

Solar-cell characteristics

• Short-current ISC, I0 or IK:• mostly proportional to irradiation• Increases by 0,07% per Kelvin

• Open-voltage U0, UOC or VOC:• This is the voltage along the internal diode• Increases rapidly with initial irradiation• Typical for Silicon: 0,5...0,9V• decreases by 0,4% per Kelvin

Solar cell characteristics

• Power (MPP, Maximum Power Point)• UMPP (0,75 ... 0,9) UOC

• IMPP (0,85 ... 0,95) ISC

• Power decreases by 0,4% per Kelvin

• The nominal power of a cell is measured at international defined test conditions(G0 = 1000 W/m2, Tcell = 25°C, AM 1,5) in WP (Watt peak).

Solar cell characteristics

• The fillfactor (FF) of a solar-cell is the relation of electrical power generated (PMPP) and the product of short current IK and open-circuit voltage U0

FF = PMPP / U0 IK

• The solar-cell efficiency is the relation of the electrical power generated (PMPP) and the light irradiance (AGG,g) impinging on the solar-cell :

= PMPP / AGG,g

Solar-cell characteristics (cSi)P = 0,88W, (0,18) P = 1,05W, (0,26)

P = 0,98W, (0,29)

Solar-cell characteristics

2. Solar-cell Technologies

• Materials

• Technologies

• Market shares and development

MaterialsDefinition of semiconductor: This is a matter of electron configuration

Extract of periodic table:

Silicon (Si)

Germanium (Ge)

Gallium-Arsenide (GaAs)

Cadmium-Telluride (CdTe) P

Indium-Phosphorus (InP)

Aluminium-Antimon (AlSb)

Copper, Indium, Gallium, Selenide (CIS)

IIB IIIB IVB VB VIBIB

Efficiency of different solar cells(Theory / Laboratory)

Arguments for different technologies

• Potentially high efficiency

• Availability of material

• Low material price

• Potentially low manufacturing costs

• Stability of characteristics for many years

• Environment friendly and non toxic Materials and manufacturing process

+ Mass production efficiency between 15 - 18% (>23% in laboratory)

– A lot of raw material needed– Raw silicon costs are strongly varying in time+ Well known production process, but consumes much

energy, optimization by EFG and band-Technology+ Very good long term stability+ material almost pollution free+ Second place in market shares

Evaluation of mono-crystalline Silicon:

Evaluation of multi-crystalline Silicon:

+ Mass production efficiency between 12 - 14%– A lot of raw material needed– Raw silicon costs are strongly varying in time+ Well known production process, consumes less

energy than mono-Si+ very good long term stability+ material almost pollution free+ First place in market shares

Evaluation of amorphous Silicon (a-Si):

– Mass production efficiency only 6 – 8%+ Thin-Film Technology (<1µm), only few

raw material needed+ Well known production process, consumes

far less energy than crystalline Silicon+ large area modules can be manufactured in one step– long term stability only for efficiency between 4 – 6%+ material almost pollution free

+ Mass production efficiency 11 – 14%+ Thin-Film Technology (<1µm), only few raw material

needed+ large area modules can be manufactured in one step+ good long term stability – raw material not pollution free (Se, small quantity of

Evaluation of Copper, Indium, Diselenide (CIS)

+ Mass production efficiency up to 18%– some raw materials are rather rare– raw material very expensive– some production processes not suited for mass

production– long term stability not well known– raw material not pollution free (esp. As, Cd)

Evaluation of GaAs, CdTe and others

Production process1. Silicon Wafer-technology (mono- or multi-crystalline)

Tile-production

Plate-production

cleaning

Quality-control

Most purely silicon

99.999999999%

Occurence:

Siliconoxide (SiO2)

= sand

melting /

crystallization

SiO2 + 2C = Si + 2CO

Mechanical cutting:

Thickness about 300µm

Minimum Thickness:

about 100µm

typical Wafer-size:

10 x 10 cm2

Link to

Producers of Silicon Wafers

Production- Processmono- or multi-

crystalline Siliconcrystal growth process

Production - Process

EFG: Edge-defined Film-fed Growth

Less energy-consumptively than crystal-growth process

Thickness: about 100µm

Only few Silicon waste, since no cutting necessary

Silicon Band-Growth Process

Production Process

semiconductor materials are evaporated on large areas

Thickness: about 1µm

Flexible devices possible

less energy-consumptive than c-Silicon-process

only few raw material needed

Typical production sizes:1 x 1 m2

Thin-Film-Process (CIS, CdTe, a:Si, ... )

CIS Module

Technology -Trends

• Thin-Film Technology– few raw material needed– demand of flexible devices– production of large area cells / modules in one step

• enhancement of cell efficiency– Tandem-cell for better utilization of the solar spektrum– Light Trapping, enhancement of the light absorption– Transparent contacts– bifacial cells

• Solar-concentrating photovoltaics

Tandem-cell

Pattern of a multi-spectral cell on the basis of the

Chalkopyrite Cu(In,Ga)(S,Se)2

Thin Si-Wafer

energy payback time (EPBT)

BOS: Balance of System = inverter, cable, transport, assembly …

Market Shares

Thin-FilmSi-Band-growth

multi-crystalline Si

mono-crystalline Si

of the main solar cell technologies

Solar-Cell Manufacturer

Worldwide installed PV-Power

In Germany installed PV-Power

PV-Module priceexperience curve: price per Wp against cumulative production

with Research & Development

without Research& Development

end of 2004

cumulative production in MWp

3. PV Systems and Components

• PV System-Technology

• Solar Irradiation

• Energy yield and savings

PV-Systems

The basic photovoltaic or solar cell typically produces only a small amount of power. To produce more power, cells can be interconnected to form modules, which can in turn be connected into arrays to produce yet more power. Because of this modularity, PV systems can be designed to meet any electrical requirement, no matter how large or how small.

PV ModuleA PV-Module usually is assembled by a certain amount of series-connected solar-cells

typical open-.circuit Voltage using 36 cells: 36 * 0,7V = 25V

Problem: due to series connection, the failure of one cell (defective or shadow) reduces the current through all cells!

PV Modulein order to avoid this kind of failure, cells or cell strings are bypassed by diodes which shortcut the defective orshaded cell(s) :

Grid-connected PV-System

Solar-Generator

inverter(virtualload)

protection-Diode

load utility-grid

The grid is involved as a temporary energy storage

inverter concepts

module-integrated

… … …

central

… …

string-inverter

…… …

multistring-inverter

PV – Solar Home System (SHS)with AC-Load

Solar-Generator

charge-regulator

Protection-Diode

inverter

loadAccumulator

(storage)

Main difference to a grid connected System:- a local DC energy storage and DC/DC regulator is necessary- an additional DC/AC converter is necessary-> increase of Balance of System (BOS) costs

Solar-generator: Dimensioning I

• The solar-generator voltage and power has to be adopted to the load and storage (in case of a SHS) or the inverter (in case of a grid connected system)

• This is achieved by suitable series and parallel connection of PV-Modules

• SHS without inverter are mostly 12V or 24V and sometimes 48V DC-Systems.

• To compensate voltage loss at the charge-regulator / inverter and the cabling, the nominal voltage of the modules should always be slightly above the minimal required input voltage of the charge-regulator / inverter

Solar-generator: Dimensioning II

• Orientation of the module surface (Azimuth) : Northern Hemisphere to South, Southern Hemisphere to North (Deviations less than ± 30° reduce the energy gain less than 5%

• Guide: Inclination (tilt angle) ~ latitude of locationmore steeply: more energy gain during spring / autumnmore flat: more energy gain in summer

• Sun-Tracker is expensive and complicated (moving parts) and increases the energy gain by only 10 to 15%

The dimensioning of the solar generator depends also on the solar irradiation conditions of the location and the orientation of the module surface:

Solar irradiation characteristics(northern hemisphere, ~ 50° latitude)

south-eastsouth-west

west east

energy production with respect to optimal orientation

Total solar irradiation

Solar Irradiation in Germany

Data from 2002

Irradiation on horizontal surface between 900 (North)and 1300 (South) kWh/m² per year

Solar irradiation in the USA

Shown is the average radiation received on a horizontal surface across the continental United States in the month of June. Units are in kWh/m2

Solar Irradiation worlwide(kWh/m² a) on horizontal surface

Solar Irradiance worlwideAverage 1991-1993: (W/m²) on horizontal surface

Example: practical energy gain

Energy-Yieldis dependent on:

• location / Climate middle-Europe: 700 – 900 kWh per kWp installed PV-Power

• Orientation (Tilt, Azimuth)± 20° deviation ± 5% Energy-loss

• PV-Technologydetermines area needed and efficiency

• eventually additional use (aesthetics, weather proof,SHS)

• pollution free electricity generation,CO2 reduction etc.

Incentives for solar generated electricity (EEG, in Germany)

Grid connected system, electricity produced is totally feed into the grid

The table shows the amount paid per kWh solar electricity produced:

year 2004 2005 2006 2007 2008

Building integrated 57,4 ct 54,53 ct 51,80 ct 49,21 ct 46,75 ct

More than 30 kW 54,6 ct 51,87 ct 49,28 ct 46,82 ct 44,48 ct

More than 100 kW 54,0 ct 51,30 ct 48,74 ct 46,30 ct 43,99 ct

Facade- bonus 5,00 5,00 ct 5,00 ct 5,00 ct 5,00 ct

Open-land systems 45,7 ct 43,42 ct 40,60 ct 37,96 ct 35,49 ct

4. Building Integrated PV

• PV as a multifunctional part of buildings

• Examples

• further informationen

4.1 Weather Protection

• Rain and wind tightness

• storm resistant

• climate-change resistant

• durable

Example: Utility Tower in Duisburg

Example: roof

4.2 Thermal insulation

• In combination with usual heat-insulating materials

• In combination with heat insulating glass

Example: special roof

example: Swimming pool

4.3 Heating / Air conditioning

• Combination of PV and thermal Energy-conversion (Air / Water)

• Optimization of PV Efficiency

4.4 Shading

• Regulation by „Cell density“

• use of semitransparent cells

Example: Shading

4.5 Sound absorption

4.6 Electromagnetic Absorption

• Faraday's cage principle

• Reduction of Electro smog inside of buildings

4.7 Production of electrical energy

Example: PV-Roof and Front,

4.10 Design /Aesthetics

• PV facade and roof-elements are highly valuable building materials which may be adapted to many different Design-criteria

Alwitra Solar-foil

Solar-roof shingle

Example: Sports-Center Tübingen

Example: Fire-brigade

Example: BP Showcase

Information sources in the Internet (selected)

• U.S. Department of Energy (http://www1.eere.energy.gov/solar/technologies.html)and links within these pages

• Wikipedia(http://en.wikipedia.org/wiki/Solar_cells)and links within this page

• Software: Valentin Energy Software: PVSOL, Meteonorm(http://www.valentin.de/index_en)

This Powerpoint Presentation can be downloadedfrom:

www.fh-trier.de/~molter

Photovoltaics Technology Components and Systems Applications Clemson Summer School 4.6. – 6.6.2007...

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