Units Of Energy And Energy Efficiency

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.

(i) 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

(ii) 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 10e5 watt-sec.

(iii) 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  kilo-calorie is used.A kilo-calorie is the amount of heat required to raise the temperature of 1 kg of water through 1ºC i.e .,

                                1 kilo-calorie = 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

Relationship Among Energy Units:

The energy whether possessed by an electrical system or mechanical system or thermal system has the same thing in common  i.e ., it can do some work.  Therefore, mechanical, electrical and thermal energies must have the same unit.This is amply established by the fact that there exists a definite relationship among the units assigned to these energies.  It will be seen that these units are related to each other by some constant.

(i) Electrical and Mechanical

                      1 kWh = 1 kW  ×  1 hr = 1000 watts  ×  3600 seconds = 36  ×  10e5 watt-sec. or Joules

                                            ∴ 1 kWh = 36  ×  10e5Joules

                    It is clear that electrical energy can be expressed in Joules instead of kWh.

(ii) Heat and Mechanical
 (a) 1 calorie = 4·18 Joules (By experiment)

 (b) 1 C.H.U. = 1 lb ×  1ºC = 453·6 gm  ×  1ºC = 453·6 calories = 453·6 × 4·18 Joules = 1896 Joules

                                               ∴ 1C.H.U. = 1896 Joules

(c) 1 B.Th.U. = 1 lb  ×  1ºF = 453·6 gm  ×  5/9 ºC= 252 calories = 252  ×  4·18 Joules = 1053 Joules

                                              ∴ 1 B.Th.U. = 1053 Joules

It may be seen that heat energy can be expressed in Joules instead of thermal units viz. calorie, B.Th.U. and C.H.U.
(iii) Electrical and Heat
(a) 1 kWh = 1000 watts ×  3600 seconds = 36  ×  105 Joules
                                                                   =36×10e5/4.18⋅ calories = 860 ×  10e3calories
                                           ∴ 1 kWh = 860 ×  10e3 calories or 860 kcal
(b) 1 kWh = 36  ×  10e5Joules  = 36  ×  10e5/1896 C.H.U. = 1898 C.H.U.
                                            [  1 C.H.U. = 1896 Joules]
                                          ∴ 1 kWh = 1898 C.H.U.
(c) 1 kWh = 36  ×  10e5 Joules = 36×10e5/1053 B.Th.U. = 3418 B.Th.U.
                                          [  1 B.Th.U. = 1053 Joules]
                                          ∴ 1 kWh = 3418 B.Th.U.
The reader may note that units of electrical energy can be converted into heat and vice-versa. This is expected since electrical and thermal energies are interchangeable.

Energy 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
Note that efficiency is always less than 1 ( or 100 % ).  In other words, every system is less than 100 % efficient.