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Dispositivi e impianti per la conversionefotovoltaica
Claudio FiegnaMauro Zanuccoli
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Argomenti
Effetto fotovoltaico
Celle fotovoltaiche
Principio di funzionamento
Tecnologie e fabbricazione
Caratterizzazione sperimentale
Moduli fotovoltaici
Impianti fotovoltaici
Tipologie e classificazione
Esempi
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Argomenti (2)
Caratterizzazione sperimentale dei dispositiviPV (caratteristiche I-V)
Esempio di progetto (con sw dedicato)
Integrazione con l'architettura
Energy pay-back time
Il conto energia GSE
Mercato e tendenze del fotovoltaico
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Ivano Pola, SUPSI
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Energia solare e conversione
fotovoltaica
La potenza che colpisce latmosfera terrestre dicirca 170 milioni di miliardi di watt (170 PW).
In meno di unora il sole invia sulla Terra una
quantit di energia pari allintero consumocomplessivo mondiale annuale.
Il flusso di energia solare molto diluito edintermittente
La conversione fotovoltaica sfrutta il meccanismo digenerazione di carica elettrica prodotto dallaradiazione luminosa in un materiale semiconduttore
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Ivano Pola, SUPSI
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Development of PV technology
The photovoltaic (PV) effect was discovered in 1839
by Edmond Becquerel
After the introduction of silicon as the prime
semiconductor material in the late 1950s, silicon PVdiodes became available; main applications: TLC
equipments in remote locations and satelites
The oil crisis of 1973 led to public investments for
technology development
Since the beginning of the 1990s, ecological
considerations acted as a main driving force in
promoting PV solar energy
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Global Status of Solar Photovoltaics
By the end of 2007, the cumulative installed
capacity of solar photovoltaic (PV) systems
around the world had reached more than 9,200
MW. (1,200 MW at the end of 2000). Installations of PV cells and modules around
the world have been growing at an
solar electricity industry that it is now worthmore than an annual 13 billion
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Semiconductors - Silicon
Atomic Number: 144th Group4 Valence Electrons
Cubic Crystal
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Semiconduttori: bande di energia
Nei solidi cristallini gli elettroni possono assumere valori di energia totale compresiall'interno di bande di estensione finita, separate da intervalli di valori proibiti (gap).
Concetto di banda di valenza e banda di conduzione.
Transizioni inter-banda promosse termicamente o da radiazione elettromagnetica.
Lacuna: pseudoparticella a carica positiva corrispondente alla mancanza di un elettronein banda di valenza.
La conducibilit elettrica dipende in modo critico dalla concentrazione di elettroni (lacuneall'interno della banda di conduzione (valenza).
Tali concentrazioni sono controllabili mediante drogaggio
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Semiconductor Doping
In solar cell production the silicon has dopant atoms introduced to create a p-type and an n-typeregion and thereby producing a p-n junction. This doping can be done by high temperaturediffusion, where the wafers are placed in a furnace with the dopant introduced as a vapour. Thereare many other methods of doping silicon. In the manufacture of some thin film devices the
introduction of dopants can occur during the deposition of the films or layers.
A silicon atom has 4 relatively weakly bound (valence) electrons, which bond to adjacent atoms.Replacing a silicon atom with an atom that has either 3 or 5 valence electrons will thereforeproduce either a space with no electron (a hole) or one spare electron that can move more freelythan the others, this is the basis of doping. P-type doping, the creation of excess holes, is achievedby the incorporation into the silicon of atoms with 3 valence electrons, most often boron and n-type
doping, the creation of extra electrons is achieved by incorporating an atom with 5 valenceelectrons, most often phosphorus (see figure)
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Interaction of light with
semiconductors When light strikes the surface of a
semiconductor it is partially transmitted and
partially reflected;
The transmitted light is absorbed by thesemiconductor;
The energy associated to absorbed light
promotes the transition of electrons fromoccupied states (e.g. valence band) to the
higher-energy unoccupied states (conductions
band.
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Solar spectrum
=c
f
=hc
E
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Absorption of light in
semiconductors
Absorption of light in a direct-bandgap semiconductor (right) andabsorption coefficient as a function of photon energy in GaAs.
M.A. Green, Solar Cells, Univ. South Wales.
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Absorption of light in
semiconductors
Absorption of light in an indirect-bandgap semiconductor (right)and absorption coefficient as a function of photon wave-length inSilicon.
=
c
f=
hc
E
M.A. Green, Solar Cells, Univ. South Wales.
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Other absorption mechanisms
Phonon-assisted absorption in direct-gap
semiconductors;
free-carrier absorption (no electro-hole
generation)
two-steps absorption through an energy level
within the bandgap
electric-field assisted sub-bandgap absorption
effects of bandgap narrowing at large doping
levels
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Solar cells
Basic requirements for solar-cell operation:
optical generation of electron-hole pairs under sun
illumination: the band-gap must correspond to
wavelength included in the spectrum of solar light. Built-in electric field for separation of carriers.
low recombination rate low defect density.
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P-N Junction
Two region with opposite doping typeA strong Electric Field is present in the region close to the
junction plane
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Cella fotovoltaica convenzionale al silicio
Efficienza di conversione: 16% - 22%Massimo teorico: 31%Tecnologie alternative:Film sottile silicio amorfo (7% - 9%)Celle multi-giunzione (fino a 40%)Celle in materiale organico (basso costo, bassa efficienza 5%)
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Ivano Pola, SUPSI
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PN junction solar cell
M.A. Green, Solar Cells, Univ. South Wales.
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Conversion Efficiency
Efficiency requires:
large open-circuit voltage VOC
Low saturation current IO (dark I-V charact.)
Large short-circuit current ISC
Low IO --> low recombination rates
Large ISC --> small band-gap (downside: energywasted into heat generation).
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Ivano Pola, SUPSI
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Ivano Pola, SUPSI
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Tellururo di Cadmio CdTe
Ivano Pola, SUPSI
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Ivano Pola, SUPSI
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Ivano Pola, SUPSI