Primary energy is the energy available in the environment and directly usable without transformation. Given the energy losses during each stage of transformation, storage and transport, the quantity of primary energy is always higher than the quantity of final energy available.
There are multiple sources of primary energy:
Primary energy therefore essentially concerns thermal energy and mechanical energy.
Thus the mechanical energy produced by a windmill is a form of primary energy. On the other hand, if this mechanical energy is converted into electricity, as in the case of wind turbines, the electricalenergyproduced is considered to be a type of secondary energy.
Diagram of energy flows in the United States (2002). On the left, the primary energy consumed by the country, on the right, the fraction of energy actually used (yellow). © Energy & Environment Directorate, Lawrence Livermore National Laboratory, public domain