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. 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. 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.
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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, i.e. the heat obtained by fuel combustion (oxidation), measured so that the water vapor produced remains gaseous, and is not condensed to liquid water. Using the LHV, a condensing boiler can achieve a "heating efficiency" in excess of 100% which violates the first law of thermodynamics. 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.
Efficiency in society
The energy efficiency of a consumer item is generally defined as the relative amount of power used (usually in the form of electricity) by an item in satisfying its purpose. For example, a washing machine is designed to wash clothes. The more energy efficient the washing machine, the less electricity that is consumed in performing this task. To fully evaluate the energy efficiency of a consumer item however, running costs and expected life must be included in the calculation. Energy consumption can be measured per kilogram, for example, when comparing the efficiency of a full washing machine load to a smaller one. Energy Star and the European Union energy label are energy labels system that allow buyers to easily make comparisons between the power consumption statistics of similar electrical appliances. Energy intensity is the macroeconomic measure of energy consumption. Energy demand management aims to reduce energy consumption on the demand side during specific times, while energy conservation is the broader practise of taking actions to increase energy efficiency.
Organisations promoting energy efficiency
International
Canada
United States
- Energy_Star, from EPA
