Energy is the capacity for doing work, generating heat and emitting light. It is measured as the total amount of work that the body can do. Energy is measured in units of caloric and joule. A kilo-calorie is the amount of energy or heat required to raise the temperature of 1 kg of water from 14.5°C to 15.5°C. The joule is defined as the amount of energy exerted when a force of one newton is applied over a displacement of one meter.



Energy is a basic concept in all science and engineering discipline. A very important principle is that energy is a conserved quantity, I.e., the total amount of energy in the universe is constant. As per the law of conservation of energy “Energy cannot be newly created. Energy cannot be destroyed. In a closed system, the total mass and energy remains unchanged. In a closed system, the energy is conserved. Energy is neither created not destroyed but converted or redistributed from one form to another such as from the wind energy into electrical energy or from chemical energy into heat etc.

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(a) Based on nature of availability of energy:

On the basis of nature of availability of energy source, the energy can be classified as follows:

(I) Primary resources

Primary energy sources can be defined as sources which are either found or stored in nature. These energy sources provide a net supply of energy. Examples: Coal, natural gas, oil, biomass, solar, tidal, hydro and nuclear energy.

(ii) Secondary resources

Secondary sources of energy are derived from the primary energy sources. Producing electrical energy from coal and hydrogen from hydrolysis of water are examples of this type of energy.

(b) Based on utilisation of energy: .

On the basis of utilisation of energy, the energy can be classified as follows:

(I) Direct source of energy

The direct sources of energy are obtained directly from the resources such as human labour, bullocks, and stationary and mobile mechanical or electric power units such as diesel engines, electric motor, power tiller and tractors.

(ii) Indirect sources of energy

The indirect sources of energy do not release energy directly but release it by conversion process. Some energy is invested in producing indirect sources of energy, Seeds, manures (farm yard and poultry), chemicals, fertilizers and machinery can be classified under indirect sources of energy. Again, on the basis of their replenishment, it can be further classified into renewable and non-renewable indirect source of energy.

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(iii) Supplementary sources of energy

Supplementary sources are defined as the energy sources whose net energy yield is zero. Those energy sources requiring highest investment in terms of energy insulation (thermal) is an example for this source.

(c) Based on traditional use:

On the basis of traditional use of energy source, the energy can be classified as follows:

(I) Conventional energy

Conventional energy source can be defined as a source which are wed traditionally and provides a net supply of energy. Examples: Thermal energy and hydro power energy,

(ii) New or Non-conventional energy

New or non-conventional energy sources are developed in recent past and produce no net energy. Though it may be necessary for the economy, they may not yield net energy. Examples of non-conventional energy sources are: solar energy, wind energy, tidal energy and biomass energy.

(d) Based on long term availability:

On the basis of long term availability of energy source, the energy can be classified as follows:

(I) Non-renewable energy sources

These are the energy sources that are derived from finite and static stocks of energy. Coal, oil, fossil fuels and nuclear fuels are example of conventional sources of energy. It cannot be produced, grown, generated or used on a scale which can sustain its consumption rate. These resources often exist in a fixed amount and are consumed much faster than nature can create them. The supply of these fuels is limited. It becomes very essential to use these fuels sparingly.

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(ii) Renewable energy sources

In this category, the energy sources which are direct in nature but can be subsequently replenished are grouped. The energies which may fall in this group are solar energy, wind energy, tidal energy, biomass energy, etc.

(e) Based on origin:

On the basis of origin of energy source, the energy can be classified as follows:

(I) Fossil fuels energy –

Energy obtained from fossil fuels such as coal, oil, natural gas etc.



(ii) Nuclear energy –

Energy obtained from nuclear fuels such as Uranium, Plutonium, and Thorium etc.



(iii) Hydro energy –

Energy obtained from water.



(iv) Solar energy –

Energy obtained from solar radiation.



(v) Wind energy –

Energy obtained from natural wind force.



(vi) Tidal energy –

Energy obtained from tides and waves.



(vii) Biomass energy –

Energy obtained from biomass fuels such as cow dung, vegetable waste etc.



(viii) Geothermal energy –

Energy obtained from natural temperature variation of present in the various depth of the earth.



(ix) Ocean thermal energy –

Energy obtained from natural temperature variation present in the various depth of the ocean.



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