19-10-2012, 05:12 PM
Non-Conventional and Conventional Sources of Energy
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NON-CONVENTIONAL
The contemporary non-conventional sources of energy like wind, tidal, solar etc. were the
conventional sources until James Watt invented the steam engine in the eighteenth century.
In fact, the New World was explored by man using wind-powered ships only. The nonconventional
sources are available free of cost, are pollution-free and inexhaustible. Man has
used these sources for many centuries in propelling ships, driving windmills for grinding corn
and pumping water, etc. Because of the poor technologies then existing, the cost of harnessing
energy from these sources was quite high. Also because of uncertainty of period of availability
and the difficulty of transporting this form of energy, to the place of its use are some of the
factors which came in the way of its adoption or development. The use of fossil fuels and
nuclear energy replaced totally the non-conventional methods because of inherent advantages
of transportation and certainty of availability; however these have polluted the atmosphere to
a great extent. In fact, it is feared that nuclear energy may prove to be quite hazardous in case
it is not properly controlled.
In 1973 the Arab nations placed an embargo on petroleum. People began to realise that
the fossil fuels are not going to last longer and that remaining reserves should be conserved for
the petro-chemical industry. But unfortunately, both nuclear and coal energy pose serious
environmental problems. The combustion of coal may upset the planet’s heat balance. The
production of carbondioxide and sulphurdioxide may adversely affect the ability of the planet
to produce food for its people. Coal is also a valuable petro-chemical and from long term point
of view it is undesirable to burn coal for generation of electricity. The major difficulty with
nuclear energy is waste disposal and accidental leakage (e.g. leakage at Chernobyl nuclear
power plant).
Introduction
Tidal or lunar energy as it is sometimes called, has been known to mankind since time
immemorial. Various devices, particularly the mills were operated using tidal power. In the
past water supply of London was pumped to a water tower by a mill operated by the tidal
power (which consisted of a large paddle wheel, mounted on a raft and fastened between two of
the piers of old London Bridge). The tidal power has been used to irrigate fields in Germany
and to saw firewood in Canada.
Tides are caused by the combined gravitational forces of Sun and Moon on the waters of
the revolving Earth. When the gravitational forces due to the Sun and the Moon add together,
tides of maximum range, called spring tides, are obtained. On the other hand, when the two
forces oppose each other, tides of minimum range, called neap tides, are obtained. In one year
there are approximately 705 full tidal cycles.
Basic Schemes
It has been suggested, that for harnessing tidal power effectively the most practicable
method is the basin system. Here a portion of the sea is enclosed behind a dam or dams and
water is allowed to run through turbines, as the tide subsides.
The power available from a given head of water varies as the square of the head and
since the head varies with the tidal range, the power available at different sites from tidal
energy shows very wide variation. Various tidal basin systems have, therefore, been evolved,
in order to overcome this wide variation in availability of tidal power.
Single Basin System
The simplest scheme for developing tidal power is the single basin arrangement, in
which a single basin of constant area is provided with sluices (gates), large enough to admit
the tide, so that the loss of head is small. The level of water in the basin is the same as that of
the tide outside. When the tides are high, water is stored in the basin and sluice gates are
closed. When the tides are falling, sluices are opened to allow water to go through the turbine
to generate power. A head of water is obviously required for the turbine to generate water.
This continues to generate power till the level of the falling tides coincides with the level of the
next rising tide.
The major disadvantage of this single basin scheme is that it gives intermittent supply
of power, varying considerably over the period of operation. It is for this reason that the tidal
power has not been developed on a large scale. Also with this scheme, only about 50 per cent of
tidal energy is available.
NON-CONVENTIONAL AND CONVENTIONAL SOURCES OF ENERGY 5
Two Basin System
An improvement over the single basin system is the two-basin system. In this system, a
constant and continuous output is maintained by suitable adjustment of the turbine valves to
suit the head under which these turbines are operating.
A two-basin system regulates power output of an individual tide but it cannot take care
of the great difference in outputs between spring and neap tides. This system, therefore, provides
a partial solution to the problem, of getting a steady output of power from a tidal scheme.
This disadvantage can be overcome by the joint operation of tidal power and pumped
storage plant. During the period when the tidal power plant is producing more energy than
required, the pumped storage plant utilizes the surplus power for pumping water to the upper
reservoir. When the output of the tidal power plant is low, the pumped storage plant generates
electric power and feeds it to the system. This arrangement, even though technically feasible,
is much more expensive, as it calls for higher installed capacity for meeting a particular load.
This basic principle of joint operation of tidal power with steam plant, is also possible
when it is connected to a grid. In this case, whenever tidal power is available, the output of the
steam plant will be reduced by that extent which leads to saving in fuel and reduced wear and
tear of steam plant. This operation requires the capacity of steam power plant to be equal to
that of tidal power plant and makes the overall cost of power obtained from such a combined
scheme very high. In the system shown in Fig. 1.1, the two basins close to each other, operate
alternatively. One basin generates power when the tide is rising (basin getting filled up) and
the other basin generates power while the tide is falling (basin getting emptied). The two
basins may have a common power house or may have separate power house for each basin. In
both the cases, the power can be generated continuously. The system could be thought of as a
combination of two single basin systems, in which one is generating power during tiding cycle,
and the other is generating power during emptying.