Download Sources of energy: the sun and the wind

Sources of energy: the sun and the wind
Another type of renewable energy comes from the sun. The use of solar energy is usually divided into
two main areas: solar thermal energy and solar electricity. The first uses the sun as a direct source of heat energy,
and is most commonly used for supplying hot water to houses and swimming pools. Solar electricity seeks to
convert light from the sun directly into electricity. This process is known as photovoltaic. For this purpose a solar
cell is used which is simply a semiconductor. The basic principle is that light falling on the cell provides energy
to the electrons within the structure, allowing them to break free of their bonds. These free electrons flow round a
circuit generating a current. Silicon is the most common semiconductor material used in solar cells because of its
low electrical conductivity as all the electrons in its structure are immobilized in bonds. The solar cell is made up
of two doped types of silicon (p-type and n-type) and the interface between them. The p-type silicon is made by
doping silicon with metals from group 13 (such as boron) which increase the conductivity by introducing
positive charge carriers known as “holes” because they are actually gaps within the crystallographic structure.
The n-type silicon, in which the main charge carriers are electrons, is produced by adding phosphorus to the
silicon. A typical solar cell has the n-type silicon on top, facing the sun with the p-type silicon beneath. At the pn function, the charge carriers between the two types of silicone combine, cancelling each other out. When the
light falls on the cell, energy is passed to some of the trapped electrons, which break free and become charge
carriers. Solar power is of such great interest because it is environmentally benign. It does not produce any
harmful emissions. Moreover, it is silent and does not need any fuel (other than sunlight).
Wind is also a source of renewable energy recognized worldwide. For this purpose wind turbines are
used which are of the horizontal-axis and vertical-axis types. In a horizontal-axis wind turbine, the whole rotor
needs to be rotated periodically about a vertical axis as the wind changes direction. Vertical-axis wind turbines
do not need to be rotated into the wind. Turbines increased in size rapidly from 71 kW in 1985 to 524kW in
1996. Outputs have also increased from around 475 kWh/m2/year with 30 kW machines to over 1000
kWh/m2/year with 500 kW machines. Higher output is achieved where winds are higher.
Small wind turbines are also used mainly for rural energy applications in developing countries. At these
smaller scales there are many different design types, adapted to specific needs. One advantage which comes with
a small scale is higher rotational speed. This enables many small motors to be directly connected to alternating
current (ac) or dc generators.
The question arises if wind is economic. The answer depends on the local wind regime, the price interest
rates and bank loan periods. However, as wind technology becomes cheaper and more reliable, it is increasingly
being used to provide power in the developing world. Rural electrification for homes, villages, farms and small
industries can be achieved more cheaply and quickly with wind energy. It is also used to meet specific needs for
electricity, such as cattle fences and traffic signs in remote regions. The interest in wind energy results from the
urgent need for electricity in the Third World and the need to find non-polluting energy sources. It is rapidly
becoming cost competitive with the conventional fossil sources of electricity generation.
Answer the questions:
1.
What is the difference between solar thermal and solar electricity areas?
2.
How does a solar cell work?
3.
What are the advantages of solar power?
4.
What are two types of wind turbines?
5.
Where do small wind turbines find application?
Match the words with the definitions.
b) axis
c) turbine
d) remote
e) charge
f) generation
g) carrier
h) solar
i) circuit
j) semiconductor
k) output
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
closed system wires through which electricity can flow
from the sun or using energy from the sun to produce electric power
line divides a symmetric shape into two equal halves
thing that carries something
amount of something produced by a person, machine, etc.
material which allow electricity to move through it
far away in distance
type of machine to produce power
causing to exist or production
amount of electricity that an electrical device stores or carries