Electric Energy:
Electric energy is defined as the amount of work done in sending a current through a conductor.
We know, V= W/Q
W= VQ
W= VIT
Here, I is the current flowing through a conductor for a time T. When V is the potential difference across the two ends of a conductor. This is the expression for electric energy.
Units of electric energy:
1 joule= 1 volt×1 ampere×1 sec
There fore, 1 joule= 1 watt × 1 sec (1 volt × 1 ampere = 1 watt)
Bigger unit of electric energy= 1 watt hour
1 watt hour = 1 watt × 1 hour
= 1 watt × 3600 second
= 3600 watt second = 3600 joule
= 3600 × 10⁷ erɡ.
In practice, a still bigger unit, kilowatt hour, is used.
1 kilowatt hour = 1000 watt hour.
This is the unit which is used for calculations of electricity for domestic consumption and is called B.O.T. (Board of Trade) unit.
1 B.O.T. = 36 × 10⁵ J = 36 × 10¹² erg.
Sources of Electric Energy:
Electricity is a form of energy which is not available in material form like kinetic energy and potential energy. We have to obtain it by converting some other form of energy into electricity. For this, we shall require a source of energy. There are two types of sources of energy.
1. Non-renewable sources of electricity
2. Renewable sources of electricity
(a) Electricity from chemical energy. In some chemical reaction, ions are created. Movement of these ions across a cross-section results in flow of electric current. In this process, chemical energy is said to have been converted into electric energy. We can also arrange to store electrical energy, thus, constituting a battery. This technique can be used to develop sources of electricity to be used on a small scale. These sources can be used to perform experiment in science lab, to power a T.V. remote or to work an electric toy, etc.
(b) Electricity from fossil fuels. Heat produced by burning coal is given to water. Steam, this, produced is fed to an engine which operates a generator to produce electricity. Similarly, petrol and diesel is used as a fuel for engines to produce electricity. We do not have an unlimited stock of fossil fuel. The rate at which we are using these reserves tells us that the stock may get exhausted in about 50 years. This is the right time to plan for future sources of energy once fossil fuel is not available.
(c) Electricity from nuclear energy. Fission reaction of uranium is highly exothermic. This energy can also be utilised for production of electricity. Fission reaction is allowed to proceed in a controlled manner in a nuclear reactor. The heat produced is used to produce steam which works a turbine to get electricity. Uranium is extracted from the ground and we do not have unlimited stock of uranium also. Estimate tell us that stock of uranium may last about 60 - 70 years. Harnessing of nuclear energy is very expensive and is also not hazard free. It accompanied with lot of radioactive radiations which, in the event of a leakage, may prove to be disastrous.
2. Renewable sources of electricity. Sources which are available at all times and are capable of sustaining themselves are called Renewable sources e.g. water, air, sun, etc. We shall have to develop some techniques to harness these sources for long time production of electricity. Some of them have already been put to use while techniques are being developed for others.
(a) Hydropower plant. Water flowing through the rivers is being used for production of electricity. Water us allowed to fall from a height. It's potential energy is converted into kinetic energy. This falling water is made to spin a turbine which in turn operates the armature of a dynamo, this, producing electricity. For the construction of a hydropower plant we need to build a big reservior of water (in the form of a lake) at a certain height. For this, we shall have get a number of villages vacated resulting in large scale displacement of population. For this reason some governments do not favour building up big hydropower plants in spite of the fact that this a self sustaining source of electricity.
Hydropower generation plays a major role in countries which have abundant supply of water resources. Norway gets 89%, Canada 58%, Switzerland 55% of its electricity for this sources. 16% of the total global consumption is obtained from hydropower plants.
(b) Wind mill. Blowing wind has kinetic energy associated with itself. This energy can be converted into electric energy using a wind mill.
A wind mill [in Fig.] consists of a rotor having three blades capable of rotation about a horizontal axis. The rotor is mounted on a high steel tower whose base consists of a strong reinforced concrete. As wind strikes against the blades, which gets slowed down by imparting some of its kinetic energy to the blades which starts rotating. By means of a shaft rotational kinetic energy is conveyed to the armature of an electric generator. As the armature rotates in between the two poles pieces of a strong electromagnet, an e.m.f. is induced in it.
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| Wind mill. |
The power harnessed by the wind mill depends upon following factors:
(i) density 'ρ' of wind
(ii) area 'A' swept by the turbine blades
(iii) wind velocity v.
Power P developed by the wind mill is given by, P = 1/2 ρ × A × v³
If r = radius of revolution of turbine blades Power = 1/2 ρ × πr² × v³
It can be seen that :
(i) Increasing the length of blades 2 times results in increase in power by 4 times.
(ii) Increasing the velocity of wind 2 times results in increase in power by 8 times.
The operation of such turbine requires wind speed to be in the range (4 - 25) ms-1. The maximum output is obtained at speed range (12 - 25) ms-1 .
It is never possible to convert 100% energy of wind into electricity due to following reasons.
(i) There is a limit known as 'Betz limit' according to which turbine blades cannot extract more than 59.26% of energy from the wind. Beyond this limit more and more air tends to go around the turbine rather than going through it with air pooling up in front.
(ii) Electric generator is also not 100% efficient to convert whole input (rotational kinetic energy) into output (electric energy).
(iii) Some energy is listed in overcoming friction between parts of the assembly.
Power conversion by a wind mill is measured in terms of coefficient of power denotes by 'Cp'. It has been shown that coefficient of power greater than 35% has not been attained as yet.
Large scale electricity generation through wind mill requires installation of huge wind turbines (about 50 m in large) in a region where we have large wind velocity. Naturally, the region cannot be in a density populated town. So, electricity generated has to be transmitted to the distant places. Now a days there has been a greater awareness of harnessing wind power. Some people have installed smaller wind turbines at their house tops for using electricity for domestic purpose.
(c) Solar power plant. God is showering immense amount of energy on our planet for the benefit of human being irrespective of cast and creed. It is for us to harness these energy for our practical use.
(i) Small scale use of solar energy. Sun rays, when incident on a photosensitive material, liberate electrons from it. The phenomenon is known as 'photoelectric effect'. Based upon this principle, photo cells have been developed which can be used as sources of electricity for small scale purpose i.e. for operating small electrical devices like calculator, camera, etc.
(ii) Large scale use of solar energy. To harness solar energy for industrial purpose we have to build power plants using energy from sun. To achieve this, we shall have to concentrate solar energy by using large parabolic reflectors. Efforts are being made in this direction in a number of ways.
(i) Concentrated heat from reflectors is made to heat an observer which is placed on the focus of the reflector. The observer is, generally, a synthetic oil which gets heated to about 700°C. This heated oil transfer energy to a secondary circuit which produces steam to drive a conventional turbine or generator to produce electricity. Solar one project in Nevada costing about $ 250 million projected to produce 124 million kWh of energy per year is spread over 160 hectares using 760 mirrored throughs to concentrate heat.
(ii) In another type of solar power plant an array of reflecting mirrors concentrate energy on tunes containing hydrogen. This energy pressurizes hydrogen to power the four cylinder reciprocating solar sterling engine which in turn drives a generator, thus, producing electricity.
(iii) In another type of plant, a huge chimney is built. Near the base of the chimney reflectors are used to heat air which rises through the chimney. Hot air, as it rises up, operates a turbine in conjunction with a generator to produce electricity.
Solar input may be, both diffuse and intermittent (due to night and clouds). Therefore, generation of electricity from solar power has a low capacity factor. It is, generally, less than 15%. Moreover the power cost is also two to three times that of conventional sources.
(d) Electricity from geothermal energy. Inside of the earth is very hot. This heat is, generally, due to radioactive decay taking place at a depth of a few kilometres below the surface of earth.
(i) There are places where hot underground steam can be trapped. This steam is used to operate a turbine to produce electricity.
(ii) There is another technique known as 'hot dry rock technique' to harness geothermal energy. There are regions on earth where fractures in rocks are observed. These may be natural or artificial one. Water pumped into these fractures, gets converted into the steam and is subsequently used for production of electricity.
(e) Electricity from biofuel. Wood or any other crop which can be burnt to produce heat is a biofuel. Production of ethanol and biodiesel from fermentation of sugarcane is also an example of biofuel. Ethanol and biodiesel can be directly used as fuel for transport vehicles. Using biofuel is another way of utilizing the solar energy stored them. This source of energy has its own hazard. Crops are, generally, situated large distances away from the situation of power plant. So, we shall have to use our water resources to grow these crops. Again burning of biofuel is accompanied with emission of gases which is harmful to the environment. Considering the cost of transport, use of water resources etc. We find that our inputs will be much more costly than the outputs.
(f) Some other renewable sources of energy. A part from those discussed above there are some sources of energy of which can also be utilised for production of electricity.
- Wave energy.
- Ocean thermal energy.
- Tidal energy.
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