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.

Overview

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

• 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, 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]
Engine/Motor
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%
Appliance
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)
others
Firearm	~30% (.300 Hawk ammunition)
Electrolysis of water	50%-70% (80%-94% theoretical maximum)

See also

• Energy efficiency (disambiguation)
• EROEI
• Exergy efficiency
• Figure of merit
• Fuel efficiency
• International Electrotechnical Commission
• Sensitivity (electronics)
• Thermal efficiency
• Relative cost of electricity generated by different sources
• Electrical efficiency
• Mechanical efficiency
• Figure of merit
• Heat of combustion
• Lower heating value
• Higher heating value
• Perpetual motion
• Power generating equipment efficiency and carbon footprint

References

Cited

1. ^ IEC/OECD 2008 Energy Balance for World, accessdate 2011-06-08
2. ^ "Motivations for Promoting Clean Diesels" (PDF). US Department Of Energy. 2006. http://www.epa.gov/midwestcleandiesel/publications/presentations/il-finance-09-06/eberhardt.pdf. ^{[dead link]}
3. ^ Miyamoto K. "Chapter 1 - Biological energy production". Renewable biological systems for alternative sustainable energy production (FAO Agricultural Services Bulletin - 128). Food and Agriculture Organization of the United Nations. http://www.fao.org/docrep/w7241e/w7241e05.htm#1.2.1%20photosynthetic%20efficiency. Retrieved 2009-01-04. 
4. ^ ^{a b c d e} Luminous efficacy#Lighting efficiency
5. ^ "Cree's blue LED has quantum efficiency of 35%". compoundsemiconductor.net. 2003. http://compoundsemiconductor.net/cws/article/news/18263. 
6. ^ ^{a b c} Light Pollution Handbook. Springer. 2004. http://books.google.com/books?id=61B_RV3EdIcC&pg=PA463&lpg=PA463&dq=meyer+nienhuis+efficiency&source=web&ots=urgXwn6sPi&sig=zaIKOAxRMmzKjVjaaFd8GsvXNLY&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA463,M1. 

General

External links

• The Intelligent Energy - Europe programme: the EU's tool for funding action towards a more energy intelligent Europe
• Does it make sense to switch to LED ?