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Energy system can refer to: 1  Electric power system, for supply, transmission and consumption of electric power. 2  Thermodynamic system, a physics concept for analysis of thermal energy exchange. 3  Bioenergetic systems, metabolic processes for converting energy in living organisms.
Energy can be considered in two categories - primary and secondary. 1  Primary energy is energy in the form of natural resources, such as wood, coal, oil, natural gas, natural uranium, wind, hydro power, and sunlight. 2  Secondary energy is the more useable forms to which primary energy may be converted, such as electricity and petrol.