Q.6 What is Efficiency?

                                                        “What is Efficiency?”

Energy is available in various forms from different natural sources such as pressure head of water, chemical energy of fuels, nuclear energy of radioactive substances etc.  All these forms of energy can be converted into electrical energy by the use of suitable arrangement.  In this process of conversion, some energy is lost in the sense that it is converted to a form different from electrical energy. Therefore, the output energy is less than the input energy.  The output energy divided by the input energy is called energy efficiency or simply efficiency of the system.

             

                                             Efficiency,  η = Output energy / Input energy

As power is the rate of energy flow, therefore, efficiency may be expressed equally well as output power divided by input power i.e.,

                                             Efficiency,  η = Output power / Input power

Q.5 What is the Units of Energy?

                                                                  “ What is the Units of Energy?”

The capacity of an agent to do work is known as its energy.  The most important forms of energy are mechanical energy, electrical energy and thermal energy.  Different units have been assigned to various forms of energy.  However, it must be realized that since mechanical, electrical and thermal energies are interchangeable, it is possible to assign the same unit to them. 

1.                 Mechanical energy.  The unit of mechanical energy is newton- metre or joule on the M.K.S. or SI system. The work done on a body is one newton-metre (or joule) if a force of one newton moves it through a distance of one metre i.e., Mechanical energy in joules = Force in newton × distance in metres

2.                 Electrical energy.  The unit of electrical energy is watt-sec or joule and is defined as follows: One watt-second (or joule) energy is transferred between two points if a p.d. of 1 volt exists between them and 1 ampere current passes between them for 1 second i.e.,

Electrical energy in watt-sec (or joules)

                                             = voltage in volts × current in amperes × time in seconds

Joule or watt-sec is a very small unit of electrical energy for practical purposes.  In practice, for the measurement of electrical energy, bigger units viz., watt-hour and kilowatt hour are used.

                                                             1 watt-hour = 1 watt × 1 hr

                                                                     = 1 watt × 3600 sec

                                                                     = 3600 watt-sec

                        1 kilowatt hour (kWh) = 1 kW × 1 hr = 1000 watt × 3600 sec = 36 x 105 watt-sec.

3.                 Heat.  Heat is a form of energy which produces the sensation of warmth.  The unit* of heat is calorie, British thermal unit (B.Th.U.) and centigrade heat units (C.H.U.) on the various systems.

Calorie.  It is the amount of heat required to raise the temperature of 1 gm of water through 1ºC

                                                             i.e., 1 calorie = 1 gm of water × 1ºC

Sometimes a bigger unit namely kilocalorie is used. A kilocalorie is the amount of heat required to raise the temperature of 1 kg of water through 1ºC i.e.,

                                        1 kilocalorie = 1 kg × 1ºC = 1000 gm × 1ºC = 1000 calories

B.Th.U.  It is the amount of heat required to raise the temperature of 1 lb of water through 1ºF i.e.,

                                                                 1 B.Th.U. = 1 lb × 1ºF

C.H.U.  It is the amount of heat required to raise the temperature of 1 lb of water through 1ºC i.e.,

                                                                      1 C.H.U. = 1 lb × 1ºC

Q.4 What is source of Energy?

Answer

                                                   “Sources of Energy”

Since electrical energy is produced from energy available in various forms in nature, it is desirable to look into the various sources of energy.

These sources of energy are: (A) The Sun (B) The Wind (C) Water (D) Fuels (E)  Nuclear energy.

 

Out of these sources, the energy due to Sun and wind has not been utilised on large scale due to a number of limitations.  At present, the other three sources viz., water, fuels and nuclear energy are primarily used for the generation of electrical energy.

(A)   The Sun.  The Sun is the primary source of energy.  The heat energy radiated by the Sun can be focussed over a small area by means of reflectors.  This heat can be used to raise steam and electrical energy can be produced with the help of turbine-alternator combination.  However, this method has limited application because : (1) it requires a large area for the generation of even a small amount of electric power (2) it cannot be used in cloudy days or at night (3) it is an uneconomical method. Nevertheless, there are some locations in the world where strong solar radiation is received very regularly and the sources of mineral fuel are scanty or lacking.  Such locations offer more interest to the solar plant builders.

(B)   The Wind.  This method can be used where wind flows for a considerable length of time. The wind energy is used to run the wind mill which drives a small generator.  In order to obtain the electrical energy from a wind mill continuously, the generator is arranged to charge the batteries. These batteries supply the energy when the wind stops.  This method has the advantages that maintenance and generation costs are negligible. 

However, the drawbacks of this method are (a) variable output, (b) unreliable because of uncertainty about wind pressure and (c) power generated is quite small.

(C)   Water.  When water is stored at a suitable place, it possesses potential energy because of the head created.  This water energy can be converted into mechanical energy with the help of water turbines.  The water turbine drives the alternator which converts mechanical energy into electrical energy.  This method of generation of electrical energy has become very popular because it has low production and maintenance costs.

(D)  Fuels.  The main sources of energy are fuels viz., solid fuel as coal, liquid fuel as oil and gas fuel as natural gas.  The heat energy of these fuels is converted into mechanical energy by suitable prime movers such as steam engines, steam turbines, internal combustion engines etc.  The prime mover drives the alternator which converts mechanical energy into electrical energy.  Although fuels continue to enjoy the place of chief source for the generation of electrical energy, yet their reserves are diminishing day by day.  Therefore, the present trend is to harness water power which is more or less a permanent source of power.

(E)     Nuclear energy.  Towards the end of Second World War, it was discovered that large amount of heat energy is liberated by the fission of uranium and other fissionable materials.  It is estimated that heat produced by 1 kg of nuclear fuel is equal to that produced by 4500 tones of coal.  The heat produced due to nuclear fission can be utilised to raise steam with suitable arrangements.  The steam can run the steam turbine which in turn can drive the alternator to produce electrical energy.  However, there are some difficulties in the use of nuclear energy.  The principal ones are (a) high cost of nuclear plant (b) problem of disposal of radioactive waste and dearth of trained personnel to handle the plant.

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Q.3 Generation process of electrical energy.

                        “Generation Process of Electrical Energy”

The conversion of energy available in different forms in nature into electrical energy is known as generation of electrical energy.

Electrical energy is a manufactured commodity like clothing, furniture or tools. Just as the manufacture of a commodity involves the conversion of raw materials available in nature into the desired form, similarly electrical energy is produced from the forms of energy available in nature. However, electrical energy differs in one important respect. Whereas other commodities may be produced at will and consumed as needed, the electrical energy must be produced and transmitted to the point of use at the instant it is needed. The entire process takes only a fraction of a second. This instantaneous production of electrical energy introduces technical and economical considerations unique to the electrical power industry.

Energy is available in various forms from different natural sources such as pressure head of water, chemical energy of fuels, nuclear energy of radioactive substances etc. All these forms of energy can be converted into electrical energy by the use of suitable arrangements. The arrangement essentially employs an alternator coupled to a prime mover. The prime mover is driven by the energy obtained from various sources such as burning of fuel, pressure of water, force of wind etc. For example, chemical energy of a fuel (e.g., coal) can be used to produce steam at high temperature and pressure. The steam is fed to a prime mover which may be a steam engine or a steam turbine. The turbine converts heat energy of steam into mechanical energy which is further converted into electrical energy by the alternator. Similarly, other forms of energy can be converted into electrical energy by employing suitable machinery and equipment.