The ancient Rigveda statement "Apsu Surye Mahatdhanam" (water and sun are the greatest riches) has great connotations for an energy-rich country like India. Let us consider this statement in the present context. We know that almost all the sources of energy available for human use emanate from the sun. Solar energy reaching the earths surface (assuming the earths diameter as disc diameter) is approximately 1.8 x 1014 kW, which is twenty thousand times more powerful than what we human beings utilise in organised activities such as transport, lighting, factories, etc. Though a part of it is reflected and a part of it absorbed by the ionosphere and atmosphere, it still comes to about 1kW per square metre (a few megawatts per square kilometre) in favourable locations and even at sea level. On an average, it is about 250 W/m2 at sea level and at greater heights where absorption by air is less, say places like Ladakh (in India), it would be six to seven times more - in the form of radiation. The optimum utilisation of a countrys resources depends on the mental attitude of the people in that particular time and space frame. The availability of easily usable forms of energy plays an important role in this context.
Solar energy in the form of radiation, interacts with the environmental material of earth in the following ways :
(i) Interaction with sea-water leads to the formation of water vapour leading to cloud formation, which ultimately pours down as rain. Thus sea water is raised to a higher potential energy and depending on the height at which water is collected, a potential source of hydroelectric power is born.
(ii) Radiation falling on the leaves of plants brings about photosynthesis thereby initiating the food chain. The product of photosynthesis thus becomes a source of power to be used as wood or charcoal for domestic and industrial purposes.
(iii) Fossil fuels such as petroleum and coal produced by the action of intense pressure and the resulting putrefaction of organic matter has undoubtedly become one of the major sources of fuel in use today.
(iv) Light rays falling on the earth heat up the surface, creating air pressure imbalances leading to the origin of winds. This raw force can be tapped to become a great source of power.
(v) In addition to the above, tidal energy is also being used as a potential source of power.
The author has discussed various sources of energy in the May 1999 issue of SEARCH.
Classification of energy sources
The sources of energy may be classified into two categories:
(a) Depleting sources - Coal, oil, petroleum etc used for running cars, buses, aeroplanes etc or generating electricity in thermal power plants are so called because the consumption rates far exceed production rates. Such sources of energy are the major causes of environmental pollution. USA is 85% dependent on fossil fuels such as petrol, coal etc and in India 72% of electrical power accrues from thermal power stations. These require the burning of coal in order to produce heat, giving out large amounts of CO2, SO2 and NO in the process. And these gases contribute to global warming. Global warming is supposed to be the cause of extreme precipitation events such as blizzards and heavy rainstorms. Not only are these inducing natural calamities but also rapid depletion of non-renewable resources are becoming a major cause for concern the world-over. Unfortunately, even the nuclear sources of energy belong to the depleting class.
b) Non-depleting sources of energy -
Direct radiation from the sun forms the basis of an incessant, non-depletable energy source, as long as the earth exists. Thus, electrical energy from solar cells, wind power, hydroelectric stations and sea-waves are all of the non-depleting type. The depleting types are polluting, while the non-depleting types are pollution-free.
The following cases enumerate the processes involved in converting mechanical energy to electrical energy.
In the case of hydroelectric power generators, height potential is exploited to give motion to the coil system.
In the case of wind power generators, wind force rotates the coil or magnet.
In the case of thermal power plants, coal is burnt creating steam pressure which in turn rotates the coil or magnets.
In case of nuclear power plants, heat is generated by nuclear reaction (mass defect is converted into heat). The remaining process then becomes akin to power generation in thermal power plants in that steam pressure rotates the coil or magnets.
a) Generator system with blades
The principle of conversion of mechanical power to electrical power in the above mentioned cases, elucidates the movement of coils or magnets of a dynamo. Due to motion of either of them, the magnetic lines of force are cut by the coil and electrons of the conducting coil are forced to move. This movement of electrons is called electricity. A simple dynamo using blades in the form of turbine to rotate the coil is shown here. The amount of power generation depends on the amount of magnetic flux (the number of lines of force) cut per unit time. The higher the speed of cutting lines of force due to the movement of the dynamo parts, the larger is the amount of power generated. Faradays law (taking into consideration Lenzs law as well) can be written as:
E=- (dØ / dt).........(1)
if flux Ø is in Webers and time t in seconds then E is in volts
b) Blade-less wind-power generators
Wind driven charged particles could be the main source of power. Dr Minardi, J E, at Dayton University has suggested that the cost of bladeless wind power generation could be as low as one fourth that of conventional generators with blades. These are apparently more applicable for use at or near the sea (as moisture is required). In this system, the attractor electrodes are at a higher potential relative to the colloid producing system. Wind forces the charged colloid particles past attractor electrodes to the collector electrodes. The collector voltage is dependant on load and current. This voltage is sensed by the feedback system which promptly sets about adjusting the attracter voltage.
Thus kinetic energy of air in motion is converted into electricity as induced electromotive force at the collector. Minardi showed that at wind speeds of below approximately 25 km/hr, this type of generator matches the performance of conventional wind power generator with blades. At wind speeds higher than this, the power output increases linearly.
This type (i.e. E F D) of generators could drastically reduce the disastrous effects brought about by the onslaught of cyclones, tornadoes etc by acting as effective speed breakers (as mentioned earlier by the author in SEARCH, Jan 2000 issue).
(c) In solar cells, a p-n junction acts as the semiconductor device When a photon strikes this surface it starts conducting leading to the phenomenon called photo-voltaic effect.
(d) Conversion of environ-mental heat into electricity (Ref: Energy Conversion
(e) Chemical conduction process This process is discussed under power storage below.
When large power generating systems, whether hydroelectric or thermal, supply power during full load hours, the power is fully utilised. But, during less load hours too much power wastage occurs, which cannot be controlled very easily. To avoid power wastage, large capacity energy storage systems are needed which calls for some major research work. Ordinarily, a cell is used as a power storage system. A cell is a device in which chemical energy is converted into electrical energy (i.e. electric current) and is released from a battery terminal through an electric circuit to where the current is required. There are two kinds of cells; one is a primary cell in which the chemical used cannot be reconstituted and the other is a rechargeable secondary cell. In the latter, the chemical can be reconstituted in its original form by passing current in the reverse direction to that of normal operation. The most commonly used secondary cells are lead-acid batteries.A lead acid battery has dilute H2SO4 as the electrolyte and constitutes basically two electrodes; one rich in lead and the other rich in lead dioxide. Such secondary batteries act as power storage systems. The US has recently added considerable improvements to the storage capacities of lead-acid batteries using nano-technology for making the plates of the battery.
High capacity rechargeable storage batteries are what the Indian industries require most today and therefore special attention should be paid to this field of research and development.