Page 10 - Sources of Energy

Ocean Thermal Energy
The water at the surface of the sea or ocean is heated by the Sun while
the water in deeper sections is relatively cold. This difference in
temperature is exploited to obtain energy in ocean-thermal-energy
conversion plants. These plants can operate if the temperature difference
between the water at the surface and water at depths up to 2 km is
20 K (20°C) or more. The warm surface-water is used to boil a volatile
liquid like ammonia. The vapours of the liquid are then used to run the
turbine of generator. The cold water from the depth of the ocean is
pumped up and condense vapour again to liquid.
The energy potential from the sea (tidal energy, wave energy and
ocean thermal energy) is quite large, but efficient commercial exploitation
is difficult.

14.3.3 Geothermal Energy

Due to geological changes, molten rocks formed in the deeper hot regions
of earth’s crust are pushed upward and trapped in certain regions called
‘hot spots’. When underground water comes in contact with the hot spot,
steam is generated. Sometimes hot water from that region finds outlets
at the surface. Such outlets are known as hot springs. The steam trapped
in rocks is routed through a pipe to a turbine and used to generate
electricity. The cost of production would not be much, but there are
very few commercially viable sites where such energy can be exploited.
There are number of power plants based on geothermal energy
operational in New Zealand and United States of America.

14.3.4 Nuclear Energy

How is nuclear energy generated? In a process called nuclear fission,
the nucleus of a heavy atom (such as uranium, plutonium or thorium),
when bombarded with low-energy neutrons, can be split apart into lighter
nuclei. When this is done, a tremendous amount of energy is released if
the mass of the original nucleus is just a little more than the sum of the
masses of the individual products. The fission of an atom of uranium,
for example, produces 10 million times the energy produced by the
combustion of an atom of carbon from coal. In a nuclear reactor designed
for electric power generation, such nuclear ‘fuel’ can be part of a self-
sustaining fission chain reaction that releases energy at a controlled
rate. The released energy can be used to produce steam and further
generate electricity.

Do You Know? In a nuclear fission, the difference in mass, Δm, between the original nucleus and the

product nuclei gets converted to energy E at a rate governed by the famous equation,
2
E = Δm c ,
first derived by Albert Einstein in 1905, where c is the speed of light in vacuum. In
nuclear science, energy is often expressed in units of electron volts (eV): 1 eV = 1.602
–19
× 10 joules. It is easy to check from the above equation that 1 atomic mass unit (u)
is equivalent to about 931 mega electron volts (MeV) of energy.

Sources of Energy 251