In the actual process of energy utilization, a large amount of energy is first converted into heat energy, which is then converted into mechanical energy, electrical energy, chemical energy and light energy through heat engine, thermoelectricity, thermal reactor, blackbody radiation, etc. for reuse
Generally speaking, the energy used through the conversion into heat energy accounts for more than 90% of the total energy used. It is very important for the sustainable development of contemporary society to deeply analyze the forms and characteristics of energy conversion and make rational use of heat energy. In the process of energy conversion, the total energy is conserved, but the quality of energy is different.
The difference of energy in quality is essentially the difference of the order degree of the system. Expressed by entropy, free energy can be a measure of energy quality. According to the second law of thermodynamics, the quality of energy is high or low, and the transformation process is directional. The output energy in energy conversion can usually be divided into easy to use high-grade energy and difficult to use low-grade energy. In all the energy utilization processes, the generation and loss of low-grade energy are inevitable. Generally speaking, the energy available for output is mainly electrical energy, mechanical work or thermal energy, of which electrical energy and mechanical energy belong to high-quality energy and can be completely converted into mechanical work. However, the quality of thermal energy is low, and only part of it can be converted into mechanical work. For example, in real life, the heat released after coal combustion can be used to boil water, cook and warm; It can also be used to produce steam, which can be converted into mechanical energy by steam engine, or into electrical energy by steam turbine generator. Electric energy can be converted into mechanical energy, light energy or internal energy through electric motors, lamps or other electrical appliances.
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