I have begun to realize that almost all energy is solar
energy. People and animals do work;
their fuel is food. Food grows because
of sunlight. Plants take the solar
energy and convert it into food. Meat
just requires an extra step; the animals eat plants and convert them into
meat. So, animals and men run on solar
energy. The plants themselves can produce energy, for example by burning wood.
Hydrocarbons are also solar energy. Coal, oil and gas are simply old plants that
have been under pressure underground for millions of years, but the energy they
contain also came from the sun. Other
sources of energy from chemical reactions, e.g., rocket fuel made from hydrogen
and oxygen, usually require energy input from traditional (solar) sources to create the
chemicals that create energy when they combine. Hydrogen and oxygen have to be separated
using energy before they can be combined later.
Nuclear energy is about the only type of energy that does
not come from the sun. There are some relatively
unimportant sources of energy that are not based on solar, for example
geothermal energy that comes from volcanic heat underground. Ocean tides rising a falling similarly could
produce a small amount of energy.
However, wind and even wave energy is basically solar, caused by uneven
heating of the earth by sunlight.
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