"There are three different primary energy-supply system classes which may be used to implement the hydrogen economy, namely, fossil fuels (coal, petroleum, natural gas, and as yet largely unused supplies such as shale oil, oil from tar sands, natural gas from geo-pressured locations, etc.), nuclear reactors including fission reactors and breeders or fusion nuclear reactors over the very long term, and renewable energy sources (including hydroelectric power systems, wind-energy systems, ocean thermal energy conversion systems, geothermal resources, and a host of direct solar energy-conversion systems including biomass production, photovoltaic energy conversion, solar thermal systems, etc.). Examination of present costs of hydrogen production by any of these means shows that the hydrogen economy favored by people searching for a non-polluting gaseous or liquid energy carrier will not be developed without new discoveries or innovations. Hydrogen may become an important market entry in a world with most of the electricity generated in nuclear fission or breeder reactors when high-temperature waste heat is used to dissociate water in chemical cycles or new inventions and innovations lead to low-cost hydrogen production by applying as yet uneconomical renewable solar techniques that are suitable for large-scale production such as direct water photolysis with suitably tailored band gaps on semiconductors or low-cost electricity supplies generated on ocean-based platforms using temperature differences in the tropical seas."