Energy Efficient Systems
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, it 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 includes effectiveness and efficiency. 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. In the UK appliances are measured by an alphabetic grading system (A) being the most energy efficient and (G) being the least energy efficient.
Related, more specific terms include:
- Electrical efficiency, useful power output per electrical power consumed.
- Mechanical efficiency, where one form of mechanical energy is converted to mechanical energy.
- Thermal efficiency or Fuel efficiency, useful heat and/or work output per input energy such as the fuel consumed.
- 'Total efficiency', e.g., for co-generation, useful electric power and heat output per fuel energy consumed. Same as the thermal efficiency.
Fuel heating values and efficiency
In Europe the usable energy content of fuel is typically calculated using the lower heating value of that fuel, i.e. the heat obtained by fuel combustion 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 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.