Research network on electrochemical energy storage,
Research center on batteries and supercapacitors

«Energy» as a resource

«Energy» as a resource

A few definitions and basic notions is an original concept for which it is difficult to give a definition other than a mathematical one.  The definition found in the dictionary is “what a system possesses if it is capable of producing an activity”; but it is not much easier to define “activity” and “system”

 In fact, there is an indirect but easy way to demonstrate what Energy is and this is to list the various forms in which it appears:

  • Electric = displacement of electric charges,
  • Nuclear = atomic nucleus cohesion,
  • Radiative = displacement of an electromagnetic field,
  • Chemical = bonds between atoms in the molecular structures,
  • Mechanical = Kinetics, mass displacement,
  • Calorific = disorderly movement of atoms/molecules.

 Thus, one can very easily observe that anything which is moving, hot (T>absolute zero), or emit radiations… contains or conveys Energy. Visibly, Energy is omnipresent around us. 

Energie possesses a few important properties:

  • It can change from one form into another but it can neither be created nor destroyed (= First principle).  Thus to speak of energy production is excessive.  Energy can only be changed, stored or conveyed.
  • Some of these changes have a single way of evolution and therefore are not equal (= Second principle).  Thus, during transformation it is better to avoid the passage via calorific energy owing to the Carnot principle. This is exemplified by the yield  of a thermal engine or a nuclear plant which solely ranges from 30-40%.  Energy is not destroyed but simply deteriorated.
  • Some forms of Energy can more easily be stored than others.  It is very easy to store mechanical energy and the best examples to do so  are  hydro-electric dams. In contrast storing photons which are particles associated to an electromagnetic wave and conveyed by light is not easy …. Whatever, it remains that the 3 forms of energy which are the most easily storable in a controlled manner are those related to electric, mechanical and chemical forms.  

 In the International System of Units, Joule is the amount of energy (J).  Our growing dependence on energy is connected to the discoveries made in the 18th century and to the ensuing technologies.  Initially, the human being’s only resort was muscles and biomass as sources of energy with no viable impact on the environment as the plants re-absorbed the emitted CO2 via photosynthesis.   This situation lasted until the end of the 18th century when James Watt (1736-1819) discovered the first steam engine able to efficiently change thermal energy into mechanical energy, which is at the origin of the first industrial revolution. Since then all technological breakthroughs in transportation as well as in industrial, agricultural and domestic fields have increased the rate of our energy consumption, which has become gigantic.  Presently, it is well over 14TW (14 1012 W), which equals the power of about 14000 nuclear reactors, or the hourly combustion of 5 super tankers (166 tons/hour).  Energy supplying has become one of the major preoccupations of our modern society.  Energy dependence and the ensuing geopolitical tensions, energy cost, climate change, are today central to the international debates.