Definition and classification of renewable energies

Classification of renewable energies by C. Martz, engineer ENSAIS

The aim of this multi-page dossier is to provide an overview of so-called renewable energy technologies with, for each, their advantages, disadvantages and limits.

We will also attempt a delicate critique of current energy policies. But let's start with a little definition.

What is renewable energy?

We consider as renewable, any source of energy which is renewed quickly enough to be considered as inexhaustible (hence its name) on the scale of man but also in certain cases of humanity (solar for example)!

Renewable energies come from regular or constant natural phenomena caused mainly by the Sun (solar energy but also hydraulic, wind and biomass energy, etc.), the Moon (tidal energy, certain currents: tidal energy, etc.) and the Earth (deep geothermal energy …).

Today, we often equate renewable energies with clean energies. This is not entirely true even if these energies are much less “dirty” than fossil fuels.

Indeed; renewable energies can also be opposed to fossil fuels, which, as we now know, are not inexhaustible on a human scale. We will see in this dossier that many links still unite fossil and renewable energies, at least from an economic point of view ...

Classification of renewable energies

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Renewable energies can be classified into 3 main categories according to their primary source of energy:

  • A) direct solar: processes using directly solar radiation or light.
  • B) indirect solar: processes that indirectly use the sun to provide another source of energy.
  • C) non-solar: not using solar radiation (but being able to use the forces of gravity of the Sun).

In any case, the Sun is the base of our energy source because without the sun, the Earth would not exist just like group C). But without Earth no greenhouse effect either… so let's not quibble too much…

A) Details of direct solar energies:

B) Details of indirect solar energies:

  • Land biofuels : Plants use solar energy to grow and develop. There are several types of biofuels: those requiring “refining” and others. Exclusively from agriculture. Find out more.
  • Marine biofuels : same remarks for terrestrial biofuels with the difference that it is algae and not plants. They have a very strong development potential.Find out more.
  • Solid biomass : mainly wood for heating but also some other fast growing plants (thistle in Spain for example).
  • Liquid biomass : can be assimilated to biofuels in the final product but the production process is radically different. This is a liquefaction of a solid fraction of biomass in particular by the Fisher-Tropsh process. Find out more.
  • The gaseous biomass : Biomass gasification: 2 known methods. Methanization of waste ou wood gasificationThe 1er process is more efficient than the second one.
  • Wind energy : without Sun, the wind does not exist. Undoubtedly the most “fashionable” renewable energy, but one of the least econologically efficient. This class includes the exploitation of wave energy.Find out more.
  • Hydraulic energy : whatever its application (mechanical or electrical) hydraulic energy would not exist without the cycle of water from the sun. It is the most exploited renewable energy in the world.
  • Geothermal or aerothermal energy : ie heat pumps. They capture their energy either in the ground or in the air. In both cases, the sun is the primary source.
  • Muscle or animal energy : that is to say the traction of the muscles. It is obviously an indirect solar energy since the energy comes from the food itself coming from the sun.

C) Details of non-solar energies:

  • The use of tides : tidal power plant of the Rance.
  • Deep geothermal energy : example of Soultz-sub-forests
  • The use of certain marine currents from the tides (created by the action of the Sun but also of the Moon and the rotation of the Earth) by zip lines. Find out more.

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