What do you mean by alternative sources of energy? Give some examples.

Those energy sources, which can be used to preserve the non renewable sources of energy for future are called alternative sources of energy. For example hydro power energy, tidal energy, geothermal energy, biomass energy, solar energy, nuclear fission energy. 

Alternative energy is any energy source that does not use fossil fuels (coal, gasoline and natural gas). Renewable energy comes from natural sources that don't run out. ... The alternative energies that are already being used are solar, wind, geothermal, hydroelectric, tidal, biomass and hydrogen.Benefits of Renewable Energy: Environmental and economic benefits of using renewable energy include: Generating energy that produces no greenhouse gas emissions from fossil fuels and reduces some types of air pollution. Diversifying energy supply and reducing dependence on imported fuels.