All energy ultimately comes from either the sun or the Earth
Fuel is any material that is burned or altered in order to obtain energy
Food is clearly a fuel
Wood, dung and animal fat were early fuels, burned by our prehistoric ancestors to create heat and possibly light
Fossil fuels are those formed over geological time from the partly decayed remains of plants or animals.
They include coal, oil and natural gas.
They can be used or combusted only once and are therefore finite
They become exhausted because they can only be replaced over considerable periods of geological time
They are classified as non-renewable sources of energy
During combustion, fossil fuels produce carbon dioxide, which has an impact on climate because it contributes to the greenhouse effect
Exploiting fossil fuels by quarrying, mining and pumping has other environmental
impacts, such as deforestation and the disruption of water tables and groundwater
supplies
Nuclear energy falls into the category of non-renewable
energy since supplies of the raw material, uranium, are finite
Renewable Sources
By contrast, renewable sources of energy are those
capable of natural regeneration on a human timescale
They provide almost continuous flows of energy
These sources include inland water, wind, the tides, ocean
waves, geothermal heat and the sun's rays
Sustainable sources of energy include wood, which can be
regrown, and the pumped storage water of a hydroelectric
power plant, which can be used time and time again
Some would also put nuclear energy in this category, because the
nuclear power industry is able to reprocess some spent fuel so that it can
be reused.
Primary Energy Sources
A distinction is made between primary and secondary energy
Primary energy is the energy
found in natural resources
Examples include coal, crude oil, sunlight, wind, rivers, vegetation and uranium
Secondary energy is primary energy that has been converted into a more convenient form, usually electricity