The International Energy Agency's Renewable Energy Working Party defines renewable energy as follows.
"Renewable energy is derived from natural processes that are replenished constantly. In its various forms, it derives directly from the sun, or from heat generated deep within the earth. Included in the definition is electricity and heat generated from solar, wind, ocean, hydropower, biomass, geothermal resources, and biofuels and hydrogen derived from renewable resources."
To further detail this definition it is possible to divide renewable energy sources into three categories:
- Direct solar energy
- Indirect solar energy
- Non-solar renewables
The majority of renewable energy sources derive that energy from solar radiation. Direct solar energy refers to solar thermal energy conversion and solar photovoltaics. Indirect solar energy includes wind power, wave power and biofuels.
Non-solar renewables are those that do not depend on solar radiation. There are two sources of non-solar renewable energy - tidal and geothermal.
It is very important to highlight the difference between a primary energy source and an energy storage medium.
Importance of renewable energy
As fossil fuel resources have depleted and environmental concern has increased, renewable energy has become a very important engineering sector. It is undoubtable that in the future a large proportion of energy used by the human race will be derived from a diverse range of renewable sources.
Renewable energy and sustainability
Renewable energy can be regarded as one of the fundamental premises for building a sustainable global society.
A truely sustainable energy source should accordingly not only be renewable, but also apply the principles of sustainability.
That is, to look at the energy production and use in light of endured maintainance of our biosphere, the biodiversity and the social stability of the human culture.