Energy conversion efficiency
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Energy conversion efficiency (η) is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The input, as well as the useful output may be chemical, electric power, mechanical work, light (radiation), or heat.
Energy conversion efficiency depends on the usefulness of the output. All or part of the heat produced from burning a fuel may become rejected waste heat if, for example, work is the desired output from a thermodynamic cycle. Energy converter is an example of an energy transformation. For example a light bulb falls into the categories energy converter. Even though the definition includes the notion of usefulness, efficiency is considered a technical or physical term. Goal or mission oriented terms include effectiveness and efficacy.
Generally, energy conversion efficiency is a dimensionless number between 0 and 1.0, or 0% to 100%. Efficiencies may not exceed 100%, e.g., for a perpetual motion machine. However, other effectiveness measures that can exceed 1.0 are used for heat pumps and other devices that move heat rather than convert it.
When talking about the efficiency of heat engines and power stations the convention should be stated, i.e., HHV (a.k.a. Gross Heating Value, etc.) or LCV (a.k.a. Net Heating value), and whether gross output (at the generator terminals) or net output (at the power station fence) are being considered. The two are separate but both must be stated. Failure to do so causes endless confusion.
Related, more specific terms include
- Electrical efficiency, useful power output per electrical power consumed;
- Mechanical efficiency, where one form of mechanical energy (e.g. potential energy of water) is converted to mechanical energy (work);
- Thermal efficiency or Fuel efficiency, useful heat and/or work output per input energy such as the fuel consumed;
- 'Total efficiency', e.g., for cogeneration, useful electric power and heat output per fuel energy consumed. Same as the thermal efficiency.
- Luminous efficiency, that portion of the emitted electromagnetic radiation is usable for human vision.
Fuel heating values and efficiency
In Europe the usable energy content of fuel is typically calculated using the lower heating value (LHV) of that fuel, the definition of which assumes that the water vapor produced during fuel combustion (oxidation), remains gaseous, and is not condensed to liquid water so the latent heat of vaporization of that water is not usable. Using the LHV, a condensing boiler can achieve a "heating efficiency" in excess of 100% (this does not violate the first law of thermodynamics as long as the LHV convention is understood, but does cause confusion). This is because the apparatus recovers part of the heat of vaporization, which is not included in the definition of the lower heating value of fuel. In the U.S. and elsewhere, the higher heating value (HHV) is used, which includes the latent heat for condensing the water vapor, and thus the thermodynamic maximum of 100% efficiency cannot be exceeded with HHV's use.
Example of energy conversion efficiency
This article is missing information about clear definition of the energy conversion efficiency for light sources. The lighting efficiency is given by the luminous efficacy which does not allow to give a simple percentage without specifying what "100%" would be. If there is an ISO standard or another reliable source defining the energy conversion efficiency in lighting, please cite it. . (May 2012)
|Conversion process||Conversion type||Energy efficiency|
|Gas turbine||Chemical to electrical||up to 40%|
|Gas turbine plus steam turbine (combined cycle)||Chemical/thermal to electrical||up to 60%|
|Water turbine||Gravitational to electrical||up to 90% (practically achieved)|
|Wind turbine||Kinetic to electrical||up to 59% (theoretical limit)|
|Solar cell||Radiative to electrical||6–40% (technology-dependent, 15-20% most often, 85–90% theoretical limit)|
|Fuel cell||Chemical to electrical||up to 85%|
|World Electricity generation 2008||Gross output 39%||Net output 33%|
|Lithium-ion battery||Chemical to electrical/reversible||80–90% |
|Nickel-metal hydride battery||Chemical to electrical/reversible||66% |
|Lead-acid battery||Chemical to electrical/reversible||50–95% |
|Combustion engine||Chemical to kinetic||10–50%|
|Electric motor||Electrical to kinetic||70–99.99% (> 200 W); 50–90% (10–200 W); 30–60% (< 10 W)|
|Turbofan||Chemical to kinetic||20-40%|
|Photosynthesis||Radiative to chemical||up to 6%|
|Muscle||Chemical to kinetic||14–27%|
|Household refrigerator||Electrical to thermal||low-end systems ~ 20%; high-end systems ~ 40–50%|
|Incandescent light bulb||Electrical to radiative||0.7–5.1%, 5–10%|
|Light-emitting diode (LED)||Electrical to radiative||4.2–53% |
|Fluorescent lamp||Electrical to radiative||8.0–15.6%, 28%|
|Low-pressure sodium lamp||Electrical to radiative||15.0–29.0%, 40.5%|
|Metal-halide lamp||Electrical to radiative||9.5–17.0%, 24%|
|Switched-mode power supply||Electrical to electrical||currently up to 96% practically|
|Electric shower||Electrical to thermal||90–95% (multiply with the energy efficiency of electricity generation for comparison with other water-heating systems)|
|Electric heater||Electrical to thermal||~100% (essentially all energy is converted into heat, multiply with the energy efficiency of electricity generation for comparison with other heating systems)|
|Firearm||Chemical to kinetic||~30% (.300 Hawk ammunition)|
|Electrolysis of water||Electrical to chemical||50–70% (80–94% theoretical maximum)|
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