Alternative Energy Sources Extensive use is already made of hydropower sources where available but the potential for further expansion is limited, particularly in the developed countries. Tidal power, geothermal sources and ocean thermal gradients have limited and highly local potential. Wave power, solar energy and wind energy are more widely distributed, but only land based aerogenerators have approached economic viability except in very favourable circumstances. If the costs of solar power can be brought down, it could have potential in sunny regions but, like wind and biomass culture, it requires large tracts of land.

The variability and unpredictability of supplies from renewable sources in the absence of inexpensive large scale energy storage technologies, will limit their ability to satisfy global energy requirements, so that predictable, non-varying sources will continue to be needed for the bulk of supplies. This means reliance on fusion, fossil fuels, and fission with fast reactors.