A fuel is defined as a material that stores potential energy and, when released, can be used as heat energy. A fuel can be stored as a form of chemical energy which is a source of heat energy, and at times, chemical energy that is released through oxidation without combustion.
Chemical fuels can be categorised into common solid fuels, liquid fuels and gaseous fuels, along with biofuels and fossil fuels. These fuels can also be divided into the basis of their occurrence. Primary fuels are natural as in the case of coal, petroleum and natural gas. Secondary fuels are artificial and include, charcoal, ethanol and propane.
The calorific value (CV) of a fuel is the heat available from that fuel when it is completely burned, expressed as heat units per unit of fuel weight or volume. The gross, or higher, value is determined in the laboratory using a calorimeter. It can be defined as the total heat released by the complete combustion of the fuel.
This is measured by the heat removed when cooling the products of combustion to a standard reference temperature, and it includes molecules contained within the fuel, and the vaporisation of any moisture present.
The net, or lower, value is determined by calculation and equals the gross calorific value minus the latent heat of the water vapour formed from the combustion of hydrogen and from any moisture present in the fuel.
Batteries have much lower energy per unit mass than common fuels like gasoline. This is diminished by the fact that batteries deliver their energy as electricity, which can be converted efficiently to mechanical work, and using fuels in engines results in a low efficiency of conversion to work.
Batteries convert chemical energy directly to electrical energy. A battery consists of a certain number of voltaic cells. Each cell consists of two half-cells connected in series by a conductive electrolyte.
The voltage developed across a cell’s terminals depends on the energy release of the chemical reactions of its electrodes and electrolyte.