Energy conversion efficiency

Energy conversion efficiency
Output energy is always lower than input energy
Efficiency of Power Plant, World total 2008

Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.



Energy conversion efficiency is not defined uniquely, but instead 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.

\eta = \frac{P_\mathrm{out}}{P_\mathrm{in}}

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 (aka Gross Heating Value etc.) or LCV (aka 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

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, which definition 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

Conversion process Energy efficiency
Electricity generation
Gas turbine up to 40%
Gas turbine plus steam turbine (combined cycle) up to 60%
Water turbine up to 90% (practically achieved)
Wind turbine up to 59% (theoretical limit)
Solar cell 6%-40% (technology dependent, 15% most often, 85%-90% theoretical limit)
Fuel cell up to 85%
World Electricity generation 2008 Gross output 39%, Net output 33%.[1]
Combustion engine 10-50%[2]
Electric motors 30-60% (small ones < 10W); 50-90 (between 10-200W); 70-99.99% (above 200W)
Natural process
Photosynthesis up to 6% [3]
Muscle 14% - 27%
Household refrigerators low end systems ~ 20%; high end systems ~ 40-50%
Incandescent light bulb 0.7-5.1%,[4] 5-10%[citation needed]
Light-emitting diode 4.2-14.9%,[4] up to 35% [5][dead link]
Fluorescent lamps 8.0-15.6%,[4] 28% [6]
Low-pressure sodium lamps 15.0-29.0%,[4] 40.5% [6]
Metal halide lamps 9.5-17.0%,[4] 24% [6]
Switched-mode power supply currently up to 95% practically
Electric shower 90-95% ( Overall it would be more efficient to use a heat pump, requiring less electric energy[citation needed])
Electric heaters 100% (all energy is always converted into heat)
Firearm ~30% (.300 Hawk ammunition)
Electrolysis of water 50%-70% (80%-94% theoretical maximum)

See also



External links

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