Download all`inizio della lezione per verificare ciò che gli alunni

Lesson 2 (2 ore)
all’inizio della lezione per verificare ciò che gli alunni sanno già o che dovrebbero sapere prima di
trattare il nuovo argomento vengono svolte diverse attività di brainstorming come:
o warm-up questions
what is an I-V curve?
What are the differences between p-doped and n-doped semiconductors?
What is a diode?
Poi vengono scritte alla lavagna alcune parole chiave che fanno parte dei prerequisiti che gli
allievi devono assolutamente sapere.
Gli allievi vengono invitati a dare la spiegazione del significato
Junction
Power
Anode
Reverse biased
Working point
E rispondere alle domande:
Who can give me a definition of valence electrons?
Look at the I-V curve in the following text (solar cells): can you explain what the curve means?
Viene fornita una lettura tipo fill in the gaps di ripasso del diodo:
A semiconductor diode, the most common type today, is a crystalline piece of semiconductor
material with a p-n junction connected to two electrical terminals.
The most common function of a diode is to allow an electric current to pass in one direction (called
the diode's forward direction), while blocking current in the opposite direction (the
reverse direction). This unidirectional behaviour is called rectification, and is used to convert
alternating current to direct current , including extraction of modulation from radio signals in radio
receivers.
Most diodes today are silicon p–n junction diodes. Impurities are
added to semiconductor to create a region on one side that
contains negative charge carriers (electrons), called n-type
semiconductor , and a region on the other side that contains
positive charge carriers (holes), called p-type semiconductor .
When two materials i.e. n-type and p-type are attached together, a
momentary flow of electrons occur from n to p side resulting in a
third region where no charge carriers are present. It is called
Depletion region due to the absence of charge carriers (electrons and holes in this case). The crystal
allows electrons to flow from the N-type side (called the cathode) to the P-type side (called
the anode), but not in the opposite direction.
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Viene proiettato un video introduttivo (Solar Energy_Solar photovoltaic_Photovoltaic effect(3D
animation)) sul brano che verrà in seguito analizzato in classe
After the video answer these questions
1) How is the sun described?
2) How much energy does the sun deliver?
3) What are the other clean energies converted into electricity?
4) What does a photovoltaic cell do?
5) What are the features of a semi-conducting material?
6) What is the position of silicon un the periodic table?
7) What happens when silicon is combined with boron?
8) What does a photovoltaic cell consist of?
9) Why is a front and back contact installed to the layers?
10) Why is glass installed to the top of a photovoltaic cell?
11) What happens in the area between both layers?
12) What happens when a photon hits the silicon layer?
Mappa concettuale
Draw a mind map starting with the key word solar cell and then work ing pairs to compare it.
Lettura della del paragrafo solar cells del testo
Solar cells
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The solar cell is a diode. In a diode the relationship between the current and the voltage is non
linear , it depends on the behaviour of the p-n junction (fig.3): if the diode is forward biased (i.e
the + of the battery is on anode of the diode) above the voltage threshold
D1
(0.6V) the current flows in the circuit and the working point in the I-V
curve is in the first quadrant ; the equation is vo  v d  iR , i.e the diode
voltage is v d  iR  vo . (straight line 1 in the fig.5).
R
2
Vo
i
.
Fig.1
1
2
if the diode is reverse biased (i.e the + of the battery is on cathode of the diode) below the
breakdown voltage, the current is roughly zero (there are the minority
D1
i
carriers due to thermal agitation ) circuit and the working point in the I-V
.
curve is in the third quadrant ;the equation is vo  v d  iR , i.e the diode
R voltage is v d  iR  v o .Straight line 2 in the fig.5
Vo
Fig.2
2
I-V curve of the
diode.
Fig.3
If there is no battery in the circuit , the current doesn’t flow.
But if the p-n junction is enlightened some valence electrons can jump to the conduction band and
will flow freely in the material, like the minority carriers caused by thermal
i
agitation. Now the diode is a photovoltaic cell.
.
The equation 0  v d  iR , i.e. the voltage diode v d  iR and it is a battery.
The I-V curve goes down and the working point is in the fourth quadrant.
D1
R The working point is the intersection between the straight line 3( the circuit
equation) and the I-V curve.
Fig.4
Fig.5
3
If the light energy increases, the curve goes
further , the solar cell voltage and current
increase too..
Fig.6
The working point is chosen to get the maximum power Pmax  I * V , as you can see in the figure 7
( the curve is the same I have shown you before, but it is upside down ), therefore the load in the
circuit, R ,must suit the maximum power..In the figure 7. I sc stands for short circuit current and
Voc stands for open circuit voltage.
Fig.7
Solar
cells are photodiodes made to receive solar radiation
with great efficiency .The cell curve varies with the temperature and the light intensity as you can
see in the figures 8 , 9. The temperature decreases the highest voltage of the open circuit while the
light intensity increases it
Fig.8
Fig.9
Pu
% , Pu is the output power and
Pi
Pi is the incident radiation power Pi  G  S ,G stands for irradiance and S is the cell surface..
The most important parameter for the cell is its efficiency  
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The efficiency depends on the crystalline structure, the type of junction, the semiconductor material
used, and how the photovoltaic cells are made .
Subito dopo vengono eseguiti degli esercizi di verifica di comprensione in itinere
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True or false.
1. IF the working point in the I-V curve is in the third quadrant, the diode is forward biased F
2. When the diode is reverse biased below the breakdown voltage, the current flows in the
circuit. F
3. In the figure the diode is reverse biased. F
D1
R
2
Vo
i
.
4. Above the voltage threshold (0.6V) the current flows in the
circuit. V
5. If there is no battery in the circuit , the current flows in the circuit. F
6. When the p-n junction is enlightened, the current flows in the circuit, the working point is
in the third quadrant .F
7. If the light strikes the p-n junction , some electrons can jump from the valence band to the
conduction band. V
8. The diode in the figure is forward biased and the current flows from
i
.
cathode to anode. F
D1
R
9. When the diode is a photovoltaic cell ,the I-V curve goes down and the
working point is in the fourth quadrant. V
10. If the light energy increases, the solar cell voltage and current increase V
11. And the I-V curve goes further down.V
Read the paragraph solar cells and complete the following sentences:
1. The I V curve is …
2. The photovoltaic cell is a diode..
3. The working point is….
4. The maximum power Pmax  I * V in the photovoltaic cell is reached when …
5. I sc is…
6. When the temperature increases,……
7. The highest voltage of the open circuit increases when the light intensity….
8. The cell efficiency is….
9. Pi is the incident radiation power Pi  G  S ; G stands ….. and S…
10. The efficiency depends on……
Make sentences from the the words given
1) Efficency solar radiation,_ light energy_ maximum power_ working point_ forwad biased
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ESERCIZI ORALI
What is the difference between a diode and a solar cells?
What will happen to the Voltage open circuit , when the temperature increases?
And when the light intensity increases?
What is the differences between a diode reverse biased and a solar cell?
What are the main points we have made?
Per la valutazione abbiamo pensato ad esercizi simili a quelli forniti, aggiungendo
domande in inglese a cui rispondere in inglese o in italiano sulle due prime lezioni:
1. Make a list of renewable energy resources.
2. How many energy types can you obtain from the sun?
3. Give me definition of reverse bias.
4. What is the difference between a diode and a solar cell?
5. What will happen if the light striking on the solar cell increases?
6. Look at the I-V curve : where the diode is forward biased and where is revere biased?
7. Draw a circuit with a diode reverse biased.
8. In what way can you have a maximum power from a solar cells?
9. Give me definition of solar cell efficiency .
10. What is the incident radiation power ?
11. From what does the solar cell efficiency depend on?
e un riassunto guidato in italiano e/o in inglese scritto .
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