Something that does not seem to have been properly exploited is energy derived from the orbit of the moon, IE => tidal energy in the form of water movement. Splits into 2 forms, wave energy and tidal movements. There are areas around the world that have higher than 10 metres tidal difference and it the right context, there is massive power available that would convert directly into electricity.
Solar has its uses if there will ever be a decent technique for power storage and wind energy has a similar advantage if you can put it in the right place but both are intermittent where anything that works off the rotation of the planet and the orbit of the moon is far closer to being consistent without the time and atmospheric drop outs.