What does alternative energy include?

The concept of alternative energy has arisen in order to enable wider utilization of renewable energy sources. Alternative energy in the narrow sense is simply any energy source that can replace oil; while alternative energy in the broad sense refers to energy sources that can replace fossil fuels currently in use (fossil fuels include oil, natural gas, and coal), and most new energy sources are alternative energy sources, including solar energy, nuclear energy, wind energy, and ocean energy.