ifts fotovoltaico cfiegna 1

7/28/2019 IFTS Fotovoltaico CFiegna 1

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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



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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



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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

ifts fotovoltaico cfiegna 1

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