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In physics and engineering, energy conversion is any process of converting energy from one form to another. Energy found in fossil fuels, solar radiation, or nuclear fuels needs to be converted into other energy forms such as electrical, propulsive, or heating to be useful. Often, machines are used to convert energy from one form to another. The efficiency of a machine characterizes how well (how efficiently) it can perform such a conversion.

Energy may be converted so that it may be used by other machines, or else to provide some service to society (such as heat, light, or motion). For example, an internal combustion engine converts the potential chemical energy in gasoline and oxygen into the propulsive energy that moves a vehicle. A solar cell converts solar radiation into electrical energy that can then be used to light a bulb or power a computer. The generic name for a device which converts energy from one form to another is transducer

The steam engine illustrates the energy conversion process. In a steam engine, burning coal heats water (the working fluid), which expands as a gas (steam) and moves a piston. The steam is then cooled in a radiator and once again brought near the hot coals in a cycle that keeps the steam engine moving. The steam engine uses the temperature difference between the hot coal and the radiator to move the piston. The operation of the steam engine is described from an energy perspective by the machine cycles of thermodynamics, from the fluid flow perpective by fluid mechanics, and from the heat flow perspective by the science of heat transfer.

In a steam turbine, steam is used to turn the blades of the turbine, which may then be used to turn a generator to produce electricity. This process also requires that the steam be cooled at some point in the process, because this cooling causes the pressure drop in one part of the system which causes the steam to move from one place to another, which causes the blades of the turbine to turn.

In a nuclear power plant, atoms of fissionable material such as uranium are split into fast-moving ions, and these products produce intense heat after they strike and are stopped by the materials of the fuel rods and reactor cooling fluid. This heat is then used to generate steam from water, which in turn is used very conventionally in a steam turbine to produce electricity.

Conversion of one form of energy to another may be done with very high or even perfect efficiency, unless the energy begins in the form of heat. The conversion of heat into other forms of energy is never perfect, and (according to the second law of thermodynamics) must always be accompanied by an increase in entropy, which is usually (though not always) accomplished simply by the further dissipation of a fraction of the heat into a colder bodies, still remaining as heat.

There are a few processes in which heat is converted entirely into work or other forms of energy, and in these, the device actually grows colder as it operates. Examples are a container of gas expanding, or a concentration cell chemical cell which works by allowing a chemical to diffuse and decrease in concentration. Such processes, however, are never reversible, as they require that the gas remain in expansion (expanded into space) and the chemical remain diffused into space. In this way, entropy considerations of the second law of thermodynamics are fulfilled without having to retain a part of the heat to accomplish it. Such processes can never therefore be full engines because they cannot operate in a cycle to return to their former state. Fully reversible engines which return concentrations of matter to their initial states in the conversion of heat to other types of energy, must end by dumping a fraction of the heat into a cold reservoir. Some of the heat energy that powers a steam engine or turbine, for example, must therefore always be "lost" as unconverted heat, which must be dissipated at a lower temperature to fulfil entropy concerns, without being turned into any useful work.

Specific examples:

• A human muscle's chemical energy converts to mechanical energy when moving. This can appear as kinetic energy when it appears as motion, or as potential energy when a weight is lifed against gravity or other force.
• The sun's electromagnetic radiation energy transforms one set of chemicals to another, during a plants' photosynthesis.

There are many different machines that convert one energy form into another:

• Fuel cells
• Geothermal power
• Heat engines, such as the internal combustion engine used in cars or the steam engine
• Hydroelectric dams
• Jet engine
• Nuclear reactor
• Nuclear fusion
• Ocean thermal power
• Solar cells
• Solar thermal
• Steam engine
• Wave power
• Windmills
• Piezoelectrics
• Dielectric materials

• Energy economics,
• Energy storage,
• Renewable energy
• Exergy
• Energy quality

Energy Conversion   Edit
Solar power:	Active solar | Barra system | Central solar heating plant | Energy tower | Photovoltaics | Solar cell | Solar combisystem | Solar panel | Solar pond | Solar power satellite | Solar power tower | Solar thermal energy | Solar tracker | Solar updraft tower | Passive solar | Trombe wall | Ocean thermal energy conversion
Wind power:	Wind farm | Wind turbine
Hydroelectricity:	Marine current power | Tidal power | Water turbine | Wave power | Run-of-the-river hydroelectricity
Biological:	Mechanical biological treatment | Anaerobic digestion | Biomass
Chemical:	Blue energy | Fuel cell | Hydrogen production
Geothermal power:	Earth cooling tubes | Deep lake water cooling
Electricity generation:	Distributed generation | Microgeneration | Sustainable community energy system
Storage:	Thermal energy storage | Seasonal thermal store

Sustainability and Development of Energy   Edit Conversion | Development and Use | Sustainable Energy | Conservation | Transportation