Energy efficiency is the ratio between the useful energy output and the energy input of a thermodynamic system. The energy efficiency metric originates from the early development of thermodynamic cycles, such as the steam engine and internal combustion engine. However, the metric can be applied to any thermodynamic system. It serves as a performance criterion of the corresponding system. The figure illustrates a generic energy flow diagram of a thermodynamic system, with the system boundary shown by the dashed line.
General energy flow in a thermodynamic system
Efficiency in general is a measure of the economy of a process and quantifies the ratio of benefit to effort. In order to evaluate the utilization of the input energy of a system or process, the measure of energy efficiency has been established.
Energy efficiency is a dimensionless metric with values between 0 and 1 (0–100%) and defined as shown in this formula:
η = E(out) / E (in)
Besides this thermodynamic definition of energy efficiency, other relevant definitions include the “physical thermodynamic” definition or the “energy consumption intensity”. In this normalized measure, the energy efficiency is expressed as shown below Equilibrium or its inverse. In Eq., the unit of the numerator is application specific, for example, dried mass (kg), number of parts, heated volume, etc., while the denominator is the energy input.
η = physical useful output / E (in)
In production engineering, the system of interest is usually the production machine and/or the production system. Unlike energy conversion devices, the main objective of production machines is not to transform energy but to
manufacture products. Thus, the useful input energy, such as fuel and electricity, is lost as low-grade heat in exchange for final services of manufacturing products. These systems are also considered to be passive systems.
For systems like a production machine, the minimum work required for the corresponding process can be employed to identify the efficiency of the process. Comparison between this theoretical energy with the real required energy serves as energy efficiency metric of the process.
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