Among all energy sources we have, we distinguish:

• EXHAUSTIBLE (fossil fuels such as coal, oil, natural gas, shale oil and oil sands);
• PRACTICALLY INEXHAUSTIBLE (nuclear, medium/high temperature geothermal)
• RENEWABLE (hydro, wind, solar, tidal, ocean thermal gradients, biomass)

Renewable energy sources are almost all attributable to the sun as "engine" of the energy source and therefore they are considered inexhaustible and "clean". In fact, they are based on the idea of intercepting the energy that the sun in any case releases.

Thus the different technologies allow us to take advantage of the variety of ways in which the sun makes available its own energy. Wind turbines for wind, biomass plants for wood or livestock waste, ground heat exchangers for geothermal, etc.

In recent years many countries have become aware of the scenarios they would head if the development of the different countries would continue to be based on exhaustible fossil energy sources and with a high environmental impact. The different international agreements signed in favor of the environment since the early 90s, are leading to slow but significant improvements, such as the rules concerning the abolition of CFC refrigerant fluids in favor of the reduction of the ozone hole.

In this context, the role of renewable energy is becoming increasingly important for a really sustainable development support.