Materials For Solar Energy Applications Question Paper

Materials For Solar Energy Applications

Course:Bachelor Of Science

Institution: Kenyatta University question papers

Exam Year:2009

KENYATTA UNIVERSITY
UNIVERSITY EXAMINATIONS 2009/2010
FIRST SEMESTER EXAMINATION FOR THE DEGREE OF
BACHELOR SCIENCE AND BACHELOR OF EDUCATION

SPH 470: MATERIALS FOR SOLAR ENERGY APPLICATIONS

DATE: Wednesday 30th December 2009
TIME:
2.00pm – 4.00pm

INSTRUCTIONS
Answer question ONE and an other TWO
Q1.
a)
Define the term semiconductors?

[2marks]

b)
Name two materials commonly used as semiconductor.

Materials in the fabrication of integrated circuits.

[2marks]

c)
Name the three types of solids and state how they differ from one another.

[6marks]

d)
Distinguish between the valence band and the conduction bund.

[4marks]

e)
What are the Intrinsic and extrinsic semiconductors?
[3marks]

f)
Define the following terms:
i)
Solar
cell/photovoltaic
cell
ii)
Photovoltaic
effect
[3marks]

g)
What is meant by quantum efficiency (QE) of a solar cell? [3marks]

h)
Consider a single solar cell whose terminal voltage is 0.6 volts. Show

how you can connect 20 of this cells to provide a total of 12 volts.

[4marks]

i)
Solar cells behave like diodes. Explain this statement.

[3marks]

2.
a)
What is the advantage of a single crystal over other materials as a
semiconductor? [3marks]
Page 1 of 3

b)
What mostly affects the properties of semiconductor?
[3marks]

c)
What does Pauli’s exclusion principle
state?
[2marks]

d)
Clearly differentiate between the metals, semiconductors and insulators in
terms
of
their
electrical
behaviour.
[6marks]

e)
Define clearly and in detail the following terms;

i)
P-type and n-type Semiconductors
ii)
A
p-n
junction
[6marks]

3.
a)
Provide diagrams showing:
i)
Unbiased
p-n
junction

ii)
A forward based p-n junction
iii)
A
reverse
based
p-n
junction
[6marks]
b)
A p-n junction is forwarded biased through an external voltage V.

Using the symbols derive the rectifier equation for the diode.
[6marks]
c)
Usually, a single solar cell does not provide sufficient current and voltage for any

practical use. As such, cells without module are integrated with bypasses and

blocking diodes. Draw a diagram showing such kind of an arrangement. What is

the purpose of the diode in this
case?
[6marks]
d)
When light is absorbed by matter, electrons are usually exited higher energy states

within the material but the excited electrons quickly relax back toothier ground

state. What prevent this relaxation is a photovoltaic device?
[2marks]

4.
a)
Give a three major differences between a photovoltaic cell and a battery.

[6marks]

b)
A photovoltaic cell is illuminated with light of sufficient energy such that

a photocurrent is generated. Write the equation of the resulting short
circuit
current
density in terms of the quantum efficiency (QE), the

incident photon flux density bs (E) and the electronic charge q.

[3marks]

c)
Name three factors that affects the quantum efficiency of a photovoltaic

cell.

[3marks
Page 2 of 3

d)
Define the term dark current and provide an equation for the

corresponding dark current density Jdark. Give the meaning of the symbols

used.

[4marks]

e)
Provide a sketch showing the variation of bias voltage and the current

density J.

[4marks]

5.
a)
A solar collector is designed such that it has a duct through which air can

be pumped from one end to exit on the other end. Provide a simple
diagram
that
depicts
this
system.
[4marks]

b)
State two assumptions made when designing such a solar collector in (a)
above.
[4marks]

c)
Name three energy fluxes with respect to the absorber.
[3marks]

d)
The rate of absorption of energy is commonly given as a fraction of the

insulation. If the irradiance of the absorber is G, the surface area A, and

the transmission-absorption factor of the absorber ista , write the
equation
governing
this
rate absorption energy.

[3marks]

e)
According to the second law of entropy and energy production, the rate of
entropy
production
?s is given by,

- (
4 Ta )G ?
T
R
P
?
l

air
air
w,loss
? =
+ C
? In(
) -
ln(
)?l -
[6marks]
s
p
m
T
3
?
T
M
P
?
?
T
sun
air,in
o
air,in ?
a

Clearly state what each of the three terms in the equation represents.

f)
If an entropy current Is flows from a point at low temperature T1 to a

point at a lower temperature T2, then the energy is released at a rate Pth

such that:

Pth = (T1 – T2) Is

What name is given to Pth?

[1mark]

Page 3 of 3

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