**Energy conversion**, the transformation of energy from forms provided by nature to forms that can be used by humans.

Over the centuries a wide array of devices and systems has been developed for this purpose. Some of these energy converters are quite simple. The early windmills, for example, transformed the kinetic energy of wind into mechanical energy for pumping water and grinding grain. Other energy-conversion systems are decidedly more complex, particularly those that take raw energy from fossil fuels and nuclear fuels to generate electrical power. Systems of this kind require multiple steps or processes in which energy undergoes a whole series of transformations through various intermediate forms.

Many of the energy converters widely used today involve the transformation of thermal energy into electrical energy. The efficiency of such systems is, however, subject to fundamental limitations, as dictated by the laws of thermodynamics and other scientific principles. In recent years, considerable attention has been devoted to certain direct energy-conversion devices, notably solar cells and fuel cells, that bypass the intermediate step of conversion to heat energy in electrical power generation.

This article traces the development of energy-conversion technology, highlighting not only conventional systems but also alternative and experimental converters with considerable potential. It delineates their distinctive features, basic principles of operation, major types, and key applications. For a discussion of the laws of thermodynamics and their impact on system design and performance .

**General Considerations**

Energy is usually and most simply defined as the equivalent of or capacity for doing work. The word itself is derived from the Greek *energeia: en*, “in”; *ergon*, “work.” Energy can either be associated with a material body, as in a coiled spring or a moving object, or it can be independent of matter, as light and other electromagnetic radiation traversing a vacuum. The energy in a system may be only partly available for use. The dimensions of energy are those of work, which, in classical mechanics, is defined formally as the product of mass (*m*) and the square of the ratio of length (*l*) to time (*t*): *ml** ^{2}*/

*t*

*. This means that the greater the mass or the distance through which it is moved or the less the time taken to move the mass, the greater will be the work done, or the greater the energy expended.*

^{2}**Development of the concept of energy**

The term energy was not applied as a measure of the ability to do work until rather late in the development of the science of mechanics. Indeed, the development of classical mechanics may be carried out without recourse to the concept of energy. The idea of energy, however, goes back at least to Galileo in the 17th century. He recognized that, when a weight is lifted with a pulley system, the force applied multiplied by the distance through which that force must be applied (a product called, by definition, the work) remains constant even though either factor may vary. The concept of vis viva, or living force, a quantity directly proportional to the product of the mass and the square of the velocity, was introduced in the 17th century. In the 19th century the term energy was applied to the concept of the vis viva.

Isaac Newton’s first law of motion recognizes force as being associated with the acceleration of a mass. It is almost inevitable that the integrated effect of the force acting on the mass would then be of interest. Of course, there are two kinds of integral of the effect of the force acting on the mass that can be defined. One is the integral of the force acting along the line of action of the force, or the spatial integral of the force; the other is the integral of the force over the time of its action on the mass, or the temporal integral.

Evaluation of the spatial integral leads to a quantity that is now taken to represent the change in kinetic energy of the mass resulting from the action of the force and is just one-half the vis viva. On the other hand, the temporal integration leads to the evaluation of the change in momentum of the mass resulting from the action of the force. For some time there was debate as to which integration led to the proper measure of force, the German philosopher-scientist Gottfried Wilhelm Leibniz arguing for the spatial integral as the only true measure, while earlier the French philosopher and mathematician René Descartes had defended the temporal integral. Eventually, in the 18th century, the physicist Jean d’Alembert of France showed the legitimacy of both approaches to measuring the effect of a force acting on a mass and that the controversy was one of nomenclature only.

To recapitulate, force is associated with the acceleration of a mass; kinetic energy, or energy resulting from motion, is the result of the spatial integration of a force acting on a mass; momentum is the result of the temporal integration of the force acting on a mass; and energy is a measure of the capacity to do work. It might be added that power is defined as the time rate at which energy is transferred (to a mass as a force acts on it, or through transmission lines from the electrical generator to the consumer).

Conservation of energy (see below) was independently recognized by many scientists in the first half of the 19th century. The conservation of energy as kinetic, potential, and elastic energy in a closed system under the assumption of no friction has proved to be a valid and useful tool. Further, upon closer inspection, the friction, which serves as the limitation on classical mechanics, is found to express itself in the generation of heat, whether at the contact surfaces of a block sliding on a plane or in the bulk of a fluid in which a paddle is turning or any of the other expressions of “friction.” Heat was identified as a form of energy by Hermann von Helmholtz of Germany and James Prescott Joule of England during the 1840s. Joule also proved experimentally the relationship between mechanical and heat energy at this time. As more detailed descriptions of the various processes in nature became necessary, the approach was to seek rational theories or models for the processes that allow a quantitative measure of the energy change in the process and then to include it and its attendant energy balance within the system of interest, subject to the overall need for the conservation of energy. This approach has worked for the chemical energy in the molecules of fuel and oxidizer liberated by their burning in an engine to produce heat energy that subsequently is converted to mechanical energy to run a machine; it has also worked for the conversion of nuclear mass into energy in the nuclear fusion and nuclear fission processes.