Thermodynamic efficiency

Thermodynamic efficiency is the ratio of useful work or energy output to energy input in a process or device. Its maximum is constrained by the energy-conversion process and, for heat engines, by the second law of thermodynamics.

First law of thermodynamics: The principle that energy is conserved, changing form or moving between a system and its surroundings. Energy balances identify the input, useful output, and losses used to calculate efficiency.
Thermodynamic system: A defined quantity of matter or region in space chosen for thermodynamic analysis. Efficiency depends on which device and boundary define the inputs and outputs.
Heat engine: A device that converts heat drawn from a hot source into work while rejecting some heat to a colder sink. Its efficiency is the work produced divided by heat absorbed.
Thermal efficiency: The ratio of useful work output to heat supplied in a thermal cycle or device. It is a common, narrower efficiency measure when the input is specified as heat.
Waste heat: Heat rejected or released without serving the intended useful output of a process. Unconverted input energy often leaves a device as rejected heat.
Second law of thermodynamics: The principle that the entropy of an isolated system cannot decrease in a spontaneous process. It rules out converting all heat into work in a cyclic heat engine.
Work (thermodynamics): Energy transferred across a system boundary by an organized force acting through a distance or equivalent generalized displacement. Many thermodynamic efficiencies count useful work as the desired output.
Steam turbine: A rotary machine that extracts work from expanding steam. Its performance can be assessed by comparing actual work with an ideal expansion.
Mechanical efficiency: The ratio of useful mechanical output to mechanical input in a machine or transmission. It isolates mechanical losses that may be only one part of a device’s total thermodynamic losses.
Thermal pollution: Environmental harm caused by human activities that raise the temperature of natural waters or other environments. Heat rejected by energy-conversion systems can contribute to local environmental warming.

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