Key Concepts
The process of changing energy from one form to another. There are many conversion processes that appear as routine phenomena in nature, such as the evaporation of water by solar energy or the storage of solar energy in fossil fuels. In the world of technology, the term "energy conversion" is more generally applied to operations in which the energy is made more usable. Traditional examples include the burning of coal or natural gas in power plants to convert chemical energy into electricity and the burning of gasoline in automobile engines to convert chemical energy into propulsive energy for a moving vehicle. The conversion of energy in sunlight via solar cells into electricity, as well as the conversion of kinetic energy in wind into electricity, are further examples (Fig. 1). See also: Automobile; Coal; Electricity; Energy; Energy sources; Evaporation; Fossil fuel; Gasoline; Natural gas; Power plant; Propellant; Propulsion; Solar cell; Solar energy; Thermoelectric power generator; Thermoelectricity; Water
There are well-established principles in science which define the conditions and limits under which energy conversions can be effected. Examples include the law of the conservation of energy, the second law of thermodynamics, the Bernoulli principle, and the Gibbs free-energy relation. Recognizable forms of energy which allow varying degrees of conversion include chemical, atomic, electrical, mechanical, light, potential, pressure, kinetic, and heat energy. In some conversion operations, the transformation of energy from one form to another, more desirable form may approach 100% efficiency, whereas with others even a “perfect” device or system may have a theoretical limiting efficiency far below 100%. See also: Atom; Bernoulli's theorem; Chemical energy; Conservation of energy; Heat; Light; Nuclear fission; Nuclear fusion; Nuclear power; Pressure; Radioactivity; Thermodynamic principles
To take an example, a conventional electric generator, where solid metallic conductors are rotated in a magnetic field, actually converts 95–99% of the mechanical energy input to the rotor shaft into electric energy at the generator terminals. On the other hand, an automobile internal combustion engine operates at around only 20% efficiency, and even if it could be made perfect, might not exceed 60% for the ideal thermal cycle. Wherever there is a cycle which involves heat phases, the limitation of the Carnot criterion precludes 100% conversion efficiency, and for customary temperature conditions the ideal thermal efficiency frequently cannot exceed 50 or 60%. See also: Carnot cycle; Internal combustion engine
As a second example, in the prevalent method of producing electric energy in steam power plants, using fossil fuels or nuclear reactors (Fig. 2) for instance, there are many energy-conversion steps between the raw energy of fuel and the electricity delivered from the plant. Some of the steps for the fossils fuel power plant scenario include converting chemical energy from the fuel to the heat energy of combustion; heat energy released to heat energy of steam; heat energy of steam to kinetic energy of steam jets; jet energy to kinetic energy of rotor; and mechanical energy of rotor to electric energy at generator terminals. This is a typically elaborate and burdensome series of conversion processes. See also: Combustion; Electric power generation
Many efforts therefore continue to be made in eliminating steps in energy conversion processes to meet objectives such as improved efficiency, reduced weight, less bulk, lower maintenance, greater reliability, longer life, and lower costs. Other motivations historically for this elimination have been the needs of astronautics and satellite and missile technologies, as well as the need for new and superseding devices for conventional stationary and transportation services. Space and missile systems require compact, efficient, self-contained power systems. See also: Astronautics; Guided missile; Satellite (spacecraft)