Energy resources are of two types. Those resources that are formed over millions of years and cannot be replenished are known as non-renewable energy resources, such as coal, natural gas, and crude oil. Those resources that can be replenished are known as alternative energy resources, such as, hydropower and wind power, biogas, and solar energy. Although nuclear energy too is inexhaustible, it is not considered an alternative energy resource because of the risks associated with it.
