Chemically Stored Energy

In processes where the combustion of fuels is used for obtaining mechanical energy, the chemical energy stored in the fuel is converted into thermal energy that in turn is converted into mechanical energy. The mechanical energy thus obtained may be converted into electrical energy by means of an electric generator. Also, when a storage battery or a dry cell is discharged, chemical energy is converted into electrical energy.

Typical values of energy for the three most common fuels, in terms of Btu per pound, are: coal = 14000; gas = 20000; and oil = 18000.

These are large quantities of energy in relation to the amount of energy that can be stored in an inductance, capacitance, or flywheel. This is evident since 1 Btu = 1055 j. Hence the energy in a pound of coal is approximately 1.5 x 10⁷ j. It can be shown that a 5-kva, 60-cycle capacitor, the type that is used to correct power factor on distribution systems, stores a maximum of approximately 13.3 j under rated conditions. The kva requirements in terms of capacitance to store the amount of energy present in a pound of coal would then be

The largest a-c generators today have ratings less than 1.5 million kva; this corresponds to about 1.71 million hp if the rated power factor of the generators is 0.85, which is quite common. In terms of capacitors, the one pound of coal contains stored energy equivalent to that of 1.13 million capacitors, each rated at 5 kva.

The energy in a pound of coal corresponds roughly to the kinetic energy in a flywheel 9 ft in diameter, weighing a ton, and rotating at a speed of 3600 rpm.