The word you are searching for to fill in the blank is "alternative." Alternative energy sources are renewable energy sources that do not have fossil fuels as their origination point. As the world's supply of fossil fuels dwindles, it is necessary and imperative that we develop alternative energy sources that will continue to supply the energy we need. The current list of alternative energy sources are hydroelectric, wind, solar, nuclear, geothermal, and biomass. Each of these have their independence of fossil fuels to declare as a strength. A drawback would be they are dependent on an abundance of the raw material they require to manufacture the energy. For example, hydroelectric power requires a steady supply of flowing water. Wind power requires a constant supply of wind to work. Solar power requires a bright sunny day every day. Nuclear power produces dangerous radioactive waste. Biomass doesn't deliver as much thermal energy per unit as does current fossil fuel. While making progress in relying on alternative energy sources, we still have much to do to produce an effective replacement for fossil fuels in energy production.