Nuclear fusion promises unlimited clean energy forever. Fusion, unlike fission, does not produce long lasting radioactive waste. People have been trying to build fusion reactors for about 70 years, however no one has come close, due to the difficulty of holding the atoms in place at the super high temperatures required for fusion.
However, a MIT spinoff, Commonwealth Fusion Systems (CFS), is planning a working prototype fusion reactor by 2025
If the CFS technology is viable (both technically and commercially) it will have a dramatic impact on the future of energy supply, rendering obsolete fossil fuel solutions for electricity generation according to Hugh Bradlow at Tech Omens.