There are various forms of energy and various classification methods. In daily production practice, the most commonly used energy classification methods are as follows:

(1) Classified according to the change level of physical form. According to whether the physical form changes, energy can be divided into primary energy and secondary energy. Under normal circumstances, people call the natural energy directly obtained from nature and directly used without any processing as primary energy, such as crude oil, raw coal, natural gas, nuclear fuel, solar energy, tidal energy, etc.; on the basis of primary energy, the energy that undergoes appropriate processing and changes its physical form is called secondary energy, such as coke, diesel, coal gas, kerosene, and hydrogen energy.

(2) Classified according to combustion performance. According to whether it can be used as a fuel, energy can be divided into fuel-based energy and non-fuel-based energy. Among them, fuel-based energy includes coal, oil, natural gas, peat, wood, etc.; non-fuel-based energy includes hydropower, wind energy, geothermal energy, ocean energy, etc.

(3) Classified according to energy source. According to different energy sources, energy can be roughly divided into three categories. The first category is the energy contained in the earth itself, such as nuclear energy, geothermal energy, etc.; The second category is energy from celestial bodies outside the earth, such as solar energy and wind energy, hydropower, ocean wave energy, biomass energy and fossil energy (such as coal, oil, natural gas, etc.) converted from solar energy;

The third category is from the gravitational force of the moon and the sun on the earth, and is dominated by the moon’s gravitational force, such as the tidal energy of the ocean.

(4) Classified according to commercialization level. According to whether it enters the commodity circulation link, energy can be divided into commodity energy and non-commodity energy. The former refers to the energy that is used as a commodity circulation link and is consumed in large quantities. Currently, it mainly refers to conventional energy sources such as coal, oil, natural gas, and electricity; the latter refers to traditional conventional energy sources that are self-produced and used without going through the commodity circulation link, such as firewood, straw, etc. .

(5) Classified according to current development and utilization status. According to the current development and utilization situation, energy can be divided into conventional energy and new energy. Conventional energy refers to energy that has been developed and utilized by people on a large scale, such as coal, oil, natural gas, hydropower, etc. These energy sources have been developed and used by humans for a long time, and related technologies are also very mature, and they all belong to conventional energy. New energy refers to energy that has not been developed and utilized on a large scale by humans, such as wind energy, solar energy, biomass energy, geothermal energy, ocean energy, combustible ice, etc. Some of these energy sources have only been discovered by mankind in recent years, and some have been recognized very early, but due to the limited technological level, they have not yet been applied on a large scale and commercialized. New energy is the mainstream energy in the future of mankind. Although its development and utilization technology is still in the large-scale development stage, it has been widely valued by all countries and regions in the world.

(6) Classified according to whether it is renewable. According to whether it is renewable, energy can be divided into renewable energy and non-renewable energy. In nature, energy that can be continuously regenerated and can be continuously developed and utilized by humans is called sustainable energy. Sustainable energy is a renewable energy source, such as wind energy, solar energy, biomass energy, etc.; in nature, there are also some energy sources that need to evolve for hundreds of millions of years to form, and once they are developed and utilized, they cannot be regenerated in a short period of time. Such energy sources are non-renewable, such as traditional energy sources such as coal, oil, and natural gas.

(7) Classified according to the degree of environmental pollution. According to the degree of pollution to the natural environment, energy can be divided into clean energy and non-clean energy. The so-called non-clean energy refers specifically to energy that will cause greater pollution to the natural environment in the process of development and utilization. Usually the development and utilization of fossil energy such as coal, oil, natural gas will cause great pollution to the natural environment, so they are all non-clean energy. The so-called clean energy refers specifically to energy that will only cause minor or even no pollution to the natural environment during the development and utilization process. For example, solar energy, wind energy, etc., these energy sources can be almost pollution-free and zero emissions in the process of development and utilization, so they are all clean energy sources. Of course, the pollution-free mentioned here presupposes the relatively clean use of energy.