When forms of energy are classified in terms of resource, they can be divided into solid, liquid, and gas (usually natural gas) fuels, hydraulic, nuclear, electric, solar, bio, wind, ocean, and geothermal energy. Fossil fuels, specifically coal, petroleum, and natural gas, are created by a geochemical change of ancient creatures.
Unlike non-renewable energy that disappears once used, such as fossil fuel or nuclear fuel, types of energy that are renewed as a naturally, such as hydraulic, solar, bio, wind, ocean, and geothermal energy, are called renewable energy. In addition, forms of energy can be classified in terms of the flow from their resources to final consumption, and can be divided into primary, secondary, and final energy sources.
Primary energy is a type of energy that has not gone through any types of conversion. What can be used as direct energy is the resource itself and what can be used as energy after passing through a certain production process is a product with the process completed. It includes fossil fuels such as coal, petroleum, and natural gas (including LNG), hydraulic, nuclear, solar, bio, wind, ocean, and geothermal energy. Secondary energy is a type of energy produced from primary energy conversion (such as electric energy and all types of petroleum products), and final energy is a type of energy supplied to consumers for efficient uses (such as heat, light, and power).