Photovoltaics
description
Transcript of Photovoltaics
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Photovoltaics
Technology Components and Systems
Applications
Clemson Summer School
4.6. – 6.6.2007
Dr. Karl Molter
FH Trier
www.fh-trier.de/~molter
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4.6.07 - 6.6.07 Clemson Summer School 2007Dr. Karl Molter / FH Trier / [email protected]
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Content
1. Solar Cell Physics
2. Solar Cell Technologies
3. PV Systems and Components
4. PV Integration into buildings
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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.
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1. Solar Cell Physics
• Solar Cell and Photoelectric Effect
• The p/n-Junction
• Solar Cell Characteristics
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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 %)
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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
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energy-states in solids:Band-Pattern
Atom Molecule/Solid
ener
gy-s
tate
s
• • • • • • • •
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energy-states in solids:Insulator
electron-energyconduction-band
valence-band
Fermi-level EF
bandgap EG
(> 5 eV)
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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
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energy-states in solids :metal / conductor
electron-energy
conduction-band
Fermi-level EF
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energy-states in solids:semiconductor
electron-energy
conduction-band
valence-band
Fermi-level EF
bandgap EG
( 0,5 – 2 eV)
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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:
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energy-states in solids:energy absorption and emission
electron-energy
conduction-band
valence-band
EF
+
-
h
Generation
+
-
h
Recombination
x
x
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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)
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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
Si14
B 5
P15
„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:
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N - Doping
Si Si
Si
Si
Si
Si
Si
Si
Si
P+
-
n-conducting Silicon
-
crystal view
conduction-band
valence-band
EF
- - - - -P+ P+ P+ P+ P+
majority carriers
donator level
energy-band view
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P - Doping
Si Si
Si
Si
Si
Si
Si
Si
Si
p-conducting Silicon
B- +
+
crystal
conduction band
valence-band
EF B- B- B- B- B-
majority carriersacceptor level
+ + + + +
energy-band view
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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
+ -Ed
Ud
depletion-zone
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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
+ -Ed
Ud
depletion-zoneE = h
-
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p/n–junction without irradiation(semiconductor diode)
crystal view
n-silicon
- - - - - - - - - - - -
- - - - - - - - - - - -
- - - - - - - - - - - -
- - - - - - - - - - - -
p-silicon
+ + + + + + + + + + + +
+ + + + + + + + + + + +
+ + + + + + + + + + + +
+ + + + + + + + + + + +
+
-diffusion
-
+
electrical fieldE- - - - - - - - - - - -+ + + + + + + + + + + ++ + + + + + + + + + + +
+ + + + + + + + + + + +
- - - - - - - - - - - -
- - - - - - - - - - - -
-
+
depletion zone
+
-
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p/n–junction with irradiationcrystal view
n-silicon
- - - - - - - - - - - -
- - - - - - - - - - - -
- - - - - - - - - - - -
- - - - - - - - - - - -
p-silicon
+ + + + + + + + + + + +
+ + + + + + + + + + + +
+ + + + + + + + + + + +
+ + + + + + + + + + + +
+
-diffusion
-
+
electrical fieldE- - - - - - - - - - - -+ + + + + + + + + + + +
+-
h
-
-
-
+
depletion zone
-
+
drift
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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
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Antireflection-coating
The real Silicon Solar-cell
~0,2µm
~300µm
Front-contact
Backside contact
n-region
p-region
-
+
h
depletion zone
- - - - - - - - - -+ + + + + + + + + +
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Equivalent circuit of a solar cell
RP
USG
RSISG
RL
UL
ILID
UD
currentsource
IPH
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
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Solar-Cell characteristics
ID ISG
RLUD=USG
ID
ISG / PSG
USG
solar-cellcharacteristics
ISG = I0 = IK
RL=0 RL=
Power
UD
diode-characteristic
ID
U0
Load resistance
UMPP
MPP
IMPP
MPP = Maximum Power Point
simplified circuit
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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
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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).
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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
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Solar-cell characteristics (cSi)P = 0,88W, (0,18) P = 1,05W, (0,26)
P = 0,98W, (0,29)
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Solar-cell characteristics
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2. Solar-cell Technologies
• Materials
• Technologies
• Market shares and development
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MaterialsDefinition of semiconductor: This is a matter of electron configuration
Extract of periodic table:
Si14
Silicon (Si)
Ge32
Germanium (Ge)
Ga31
As33
Gallium-Arsenide (GaAs)
Cd48
Te52
Cadmium-Telluride (CdTe) P
15
In49
Indium-Phosphorus (InP)
Al13
Sb
51
Aluminium-Antimon (AlSb)
Copper, Indium, Gallium, Selenide (CIS)
Cu29
Se34
In49
Ga31
IIB IIIB IVB VB VIBIB
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Efficiency of different solar cells(Theory / Laboratory)
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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
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+ 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:
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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
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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
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+ 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
Cd)
Evaluation of Copper, Indium, Diselenide (CIS)
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+ 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
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Production process1. Silicon Wafer-technology (mono- or multi-crystalline)
Tile-production
Plate-production
cleaning
Quality-control
Wafer
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
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Production- Processmono- or multi-
crystalline Siliconcrystal growth process
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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
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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
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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
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Tandem-cell
Pattern of a multi-spectral cell on the basis of the
Chalkopyrite Cu(In,Ga)(S,Se)2
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Thin Si-Wafer
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energy payback time (EPBT)
BOS: Balance of System = inverter, cable, transport, assembly …
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Market Shares
Thin-FilmSi-Band-growth
multi-crystalline Si
mono-crystalline Si
of the main solar cell technologies
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Solar-Cell Manufacturer
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Worldwide installed PV-Power
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In Germany installed PV-Power
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PV-Module priceexperience curve: price per Wp against cumulative production
with Research & Development
without Research& Development
end of 2004
cumulative production in MWp
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3. PV Systems and Components
• PV System-Technology
• Solar Irradiation
• Energy yield and savings
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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.
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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!
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PV Modulein order to avoid this kind of failure, cells or cell strings are bypassed by diodes which shortcut the defective orshaded cell(s) :
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Grid-connected PV-System
Solar-Generator
inverter(virtualload)
DC
AC
protection-Diode
load utility-grid
Grid
The grid is involved as a temporary energy storage
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inverter concepts
Grid
=~
=~
…
module-integrated
=~
… … …
central
=~
=~
…
…
… …
string-inverter
…
…… …
==
==
=~
multistring-inverter
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PV – Solar Home System (SHS)with AC-Load
Solar-Generator
charge-regulator
DC
DC
Protection-Diode
Fuse
inverter
DC
AC
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
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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
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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:
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Solar irradiation characteristics(northern hemisphere, ~ 50° latitude)
tilt
south-eastsouth-west
west east
energy production with respect to optimal orientation
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Total solar irradiation
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Solar Irradiation in Germany
Data from 2002
Irradiation on horizontal surface between 900 (North)and 1300 (South) kWh/m² per year
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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
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Solar Irradiation worlwide(kWh/m² a) on horizontal surface
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Solar Irradiance worlwideAverage 1991-1993: (W/m²) on horizontal surface
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Example: practical energy gain
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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.
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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
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4. Building Integrated PV
• PV as a multifunctional part of buildings
• Examples
• further informationen
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4.1 Weather Protection
• Rain and wind tightness
• storm resistant
• climate-change resistant
• durable
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Example: Utility Tower in Duisburg
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Example: roof
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4.2 Thermal insulation
• In combination with usual heat-insulating materials
• In combination with heat insulating glass
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Example: special roof
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example: Swimming pool
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4.3 Heating / Air conditioning
• Combination of PV and thermal Energy-conversion (Air / Water)
• Optimization of PV Efficiency
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4.4 Shading
• Regulation by „Cell density“
• use of semitransparent cells
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Example: Shading
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4.5 Sound absorption
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4.6 Electromagnetic Absorption
• Faraday's cage principle
• Reduction of Electro smog inside of buildings
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4.7 Production of electrical energy
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Example: PV-Roof and Front,
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4.10 Design /Aesthetics
• PV facade and roof-elements are highly valuable building materials which may be adapted to many different Design-criteria
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Alwitra Solar-foil
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Solar-roof shingle
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Example: Sports-Center Tübingen
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Example: Fire-brigade
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Example: BP Showcase
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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)
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This Powerpoint Presentation can be downloadedfrom:
www.fh-trier.de/~molter
www.fh-trier.de/~molter