Electricity generation is the process of generating electric power from other sources of primary energy. Principal types of energy generation are thermal or combustion (gas, diesel, coal), nuclear and renewable. Electrical generation facilities include support facilities and operations, and may have associated facilities, such as a mode of transportation for fuel supply. Sustainable energy, specifically renewable energy, is an energy source that can be replenished, and it is essentially inexhaustible. Sustainable sources of energy include solar, wind, water (hydropower), biomass and geothermal. Non-renewable energy sources include coal, oil and natural gas. The sustainable production of electricity plays an increasingly greater role in reducing greenhouse gas emissions.