Energy, Sources Of Energy sources are of two basic types, renewable and nonrenewable. Most of the industrial world is presently powered by nonrenewable fossil fuels - Coal, Petroleum, and Natural Gas - that, once used, cannot be replaced. Fission Nuclear Reactors are fueled by Uranium or Plutonium, themselves finite energy sources. Spent uranium can be converted to fissile plutonium in a breeder reactor, however, a process that makes Nuclear Energy almost infinitely renewable. Nuclear technology, however, has not yet developed either failproof reactors or a safe method for disposing of nuclear wastes. The development of nuclear fusion (whose end products are harmless) has so far been hindered by the difficulties of containing the fuels (plentiful light elements such as Hydrogen) at the extremely high Temperatures necessary to initiate and sustain fusion. Renewable energy sources include the energy from Water and Wind (see Wind Turbine, water wheel, Windmill); geothermal energy, the earth's internal Heat that is released naturally in geysers and volcanoes; tidal energy, the power released by the ebb and flow of the ocean's tides; biomass, the use of certain crops (including wood) or crop wastes either directly as fuel or as a fermentable source of fuels such as alcohol or Methane, for example, gasohol; and Solar Energy, which can be stored and used directly as heat, or transformed into Electricity through the use of Photovoltaic Cells. All these renewable energy sources are presently being tapped in some form, but none can replace fossil fuels without huge advances in the technologies needed to exploit them.