Concept:
- The comparison becomes sharper if it is judged from the point of view of the future rather than the past, by asking which source can still be relied on fifty years from now.
- That single question separates the two groups on supply, on cost and on damage to the environment.
Step 1: Judge them on the supply that is left.
Coal, petroleum and natural gas took millions of years to form and are being used far faster than they can ever be replaced, so the stock keeps shrinking. Sunlight, wind, tides and heat from inside the earth arrive continuously and cannot be used up.
Step 2: Judge them on harm to the environment.
Burning conventional fuels releases smoke, carbon dioxide and other harmful gases, and it is the main cause of air pollution and of the growing concern over climate. The non-conventional group produces almost no such emissions while generating power.
Step 3: Judge them on cost.
Conventional sources are cheaper to start with, because the technology is old and widely available, but the fuel must be bought again and again. Non-conventional sources need a heavy investment in equipment at the beginning, after which the energy itself is free.
Step 4: Name the members of each group.
The conventional group holds firewood, cattle dung cake, coal, petroleum, natural gas and electricity. The non-conventional group holds solar, wind, tidal, geothermal, biogas and atomic energy.
Final Answer: Conventional sources are old, exhaustible, cheap to begin with and polluting. Non-conventional sources are newer, renewable, costly to install and clean, which is why the country is steadily shifting towards them.