Renewable Energy Sources

3. Fully developed.
5. The basic physical and organizational structures needed for the operation of a society or enterprise.
10. The arrival of a notable person or thing.
13. Relating to the production of electric current at the junction of two substances exposed to light.
15. Limited in size or extent.
17. The quality of being inexhaustible; unlimited supply or resources.
18. Relating to or resulting from motion.
20. A problem or difficulty that must be overcome.

1. Relating to or produced by the internal heat of the earth.
2. Able to be replenished within a human lifetime.
4. Change something radically or fundamentally.
6. The state of being free from danger or threat.
7. The impact on the environment of human activity in terms of pollution and natural resource depletion.
8. Occurring in cycles; recurrent.
9. The act of controlling and making use of natural forces or resources.
11. Use up the supply or resources of.
12. Relating to or affected by tides.
14. Sending out light; shining or glowing brightly.
16. Make a hole in something with a revolving tool.
19. The quality of being intermittent; not continuous or steady.