SOURCES OF ENERGY
ALTERNATIVE
OR NON-CONVENTIONAL SOURCES OF ENERGY
The Sun has been radiating
enormous amount of energy for 5 billion years. It will continue for next 5
billion years.
Only a small part of
solar energy reaches the outer layer of the earth’s atmosphere. Nearly half of
it is absorbed in atmosphere and the rest reaches the earth’s surface.
- It is estimated that
India receives solar energy equivalent to more than 5,000 trillion kWh/year. Under
clear sky conditions, the daily average varies from 4 to 7 kWh/m2.
- Solar constant: It is the solar energy
received per unit of time per unit of area on the outer edge of the earth’s
atmosphere perpendicular to the Sun’s rays and at earth’s average distance from
the Sun. It is about 1.4 kJ per second /m2 or 1.4 kW/m2.
A black surface absorbs
more heat as compared to a white or a reflecting surface under identical
conditions. It can be demonstrated by the following activity:
o Take two conical flasks and paint one white and the other black.
Fill both with water.
o Place them in direct sunlight for ½ - 1 hour.
o Touch the conical flasks. Black is hotter than white.
Solar cookers are covered
with a glass plate to trap sunlight easily and prevent the heat to get escaped
(green-house effect).
Advantages of solar cooker:
o Pollution-free and economical.
o It is easy to handle and no chance of an accident.
o The nutrients in the food do not get destroyed.
Limitations of solar cooker:
o Cannot use during night and cloudy weather.
o More time to cook food.
o The direction of the solar cooker is to be changed
continuously towards the direction of the Sun.
These limitations are
overcome by using solar cells that convert solar energy into electricity.
A typical solar cell develops
a voltage of 0.5–1 V and produce 0.7 W electricity. Many solar cells are
combined to deliver enough electricity for practical use. It is called solar
cell panel.
Principal advantages of solar cells:
o They have no moving parts.
o Need only little maintenance.
o Work efficiently without using any focussing device.
o They can be set up in remote hamlets or rarely inhabited areas where
laying of power transmission line is expensive.
Silicon is used to make solar cells. It is abundant in nature but
special grade silicon is limited. Also, silver is used to interconnect cells
in the panel. So, manufacture of solar cells is very expensive.
Scientific & technological applications of Solar cells:
o In Artificial satellites & space probes
like Mars orbiters.
o In Radio or wireless transmission systems or TV relay stations
in remote locations.
o In traffic signals, calculators and toys.
Domestic use of solar
cells is limited due to its high cost.
Energy from the Sea
Tidal Energy
Due to gravitational
pull of the moon on the spinning earth, the water level in the sea
rises and falls. This phenomenon is called high & low tides.
The difference in
sea-levels gives tidal energy.
Tidal energy is harnessed
by constructing a dam across a narrow opening to the sea. A turbine
fixed at the opening of the dam converts tidal energy to electricity.
The locations where such
dams can be built are limited.
Wave Energy
-
Waves are generated by
strong winds across the sea.
-
Using devices, kinetic
energy of huge waves near the seashore is
trapped. It rotates turbine producing electricity.
Ocean Thermal Energy
The water at the sea surface
is heated by the Sun. But the deep water is cold. This temperature difference
is used to obtain energy in ocean-thermal-energy plants.
They can operate if the
temperature difference between surface water and deep water up to 2 km is 20°C
or more.
The warm surface-water is
used to boil a volatile liquid like ammonia. The vapours are used to run
the turbine of generator. The cold deep water is pumped up and condense vapour
to liquid.
The energy potential from
the sea (tidal energy, wave energy & ocean thermal energy) is quite large,
but efficient commercial exploitation is difficult.
Geothermal Energy
Due to geological
changes, molten rocks formed in the deeper regions of earth’s crust are pushed
upward and trapped in certain regions called ‘hot spots’.
When underground water meets
the hot spot, steam is generated. Sometimes hot water from that region comes to
the surface. They are called hot springs.
Steam in rocks → pipe → turbine → generate electricity.
The production cost is
low but commercially viable sites are very few.
New
Zealand & USA have many geothermal power plants.
Nuclear Energy
When the nucleus of a
heavy atom (e.g., uranium, plutonium or thorium), bombarded with low-energy
neutrons, it splits into lighter nuclei. It is called nuclear fission. As
a result, a tremendous energy is released.
It is due to the sum masses
of the product nuclei are less than that of the original nucleus. The difference
in mass (∆m) gets converted to energy (E). [Based on E
= ∆mc2 derived by Albert Einstein. c =
speed of light in vacuum].
The atoms that can
release nuclear energy are called nuclear fuel. E.g. Fission of a uranium
atom produces 10 million times the energy produced by the combustion of
an atom of carbon from coal.
Nuclear reactor: The devices to generate energy from nuclear fuels. Here, self-sustaining
fission chain reaction occurs at a controlled rate. The released energy is
used to produce steam and further generate electricity.
- In nuclear science, energy is expressed in units of electron
volts (eV). 1 eV = 1.602× 10–19 joules. So, 1 atomic mass unit
(u) is equivalent to 931 mega electron volts (MeV) of energy.
- Nuclear power reactors at Tarapur (Maharashtra), Rana Pratap
Sagar (Rajasthan), Kalpakkam (Tamil Nadu), Narora (UP), Kakrapar (Gujarat) and
Kaiga (Karnataka) have the capacity of less than 3% of the total electricity
generation capacity of our country. It is above 30% for many industrialised
countries.
Major hazards of nuclear power generation:
o Improper nuclear-waste storage and disposal causes environmental
contamination. Uranium continues to decay into harmful subatomic particles (radiations).
o Accidental leakage of
nuclear radiation.
High cost of installation,
high risk of environmental contamination
and limited availability of uranium prohibits large-scale use of nuclear
energy.
Nuclear energy was
first used for destructive purposes. The fundamental physics of the fission chain
reaction in a nuclear weapon (atom bomb) is similar to controlled nuclear
reactor, but both are engineered differently.
Nuclear
fusion
It is the joining of lighter nuclei into a heavier nucleus. E.g.
hydrogen or hydrogen isotopes create helium.
2H + 2H
→ 3He (+ n)
- It releases a tremendous energy, according to the Einstein
equation (Here, mass of the product is less than the sum of the masses of
original nuclei).
- It is the source of energy in Sun and other stars. It needs
considerable energy, extreme temperature & pressure.
- The hydrogen bomb is based on thermonuclear fusion
reaction. A nuclear bomb based on the fission of uranium or plutonium is placed
at the core of the hydrogen bomb. This nuclear bomb is embedded in a substance
which contains deuterium and lithium.
- When the nuclear bomb is detonated, the temperature is raised to
107 K in a few microseconds. It generates energy for the nuclear fusion
and a devastating amount of energy is released.
· The ultimate source of energy for biomass, wind
and ocean thermal energy is Sun.
· In geothermal energy & nuclear energy, ultimate
source is nuclear material.
· Ultimate source of energy of hydroelectricity
is the potential energy of water. For wave energy, it
is Sun because waves are formed due to wind and wind is formed due to
unequal heating of air by Sun.
