Renewable energy sources are those which after being used can be regenerated naturally or artificially. Some of these renewable sources are subject to cycles which are kept more or less constant in nature. 

Renewable energy sources include the following:

• Tidal energy (tides)
• Hydraulic energy (reservoirs)
• Aeolic energy (wind)
• Solar energy (sun)
• Biomass energy (vegetation)
  
  The following pictures are graphic examples: