What kind of energy is "clean energy"?

Clean energy is the symbol and synonym of environmental protection and good ecosystem. It can be divided into narrow sense and broad sense.

Clean energy in a narrow sense refers to renewable energy, such as hydropower, biomass energy, solar energy, wind energy, geothermal energy, ocean energy and so on. These energy sources can be replenished after consumption, with little pollution.

Clean energy in a broad sense includes choosing energy sources that have little or no pollution to the ecological environment in the process of energy production and consumption, such as natural gas, clean coal and nuclear energy.