Carnot Efficiency

Carnot efficiency gives the upper limit on how much heat can be converted into useful work by any engine operating between two thermal reservoirs. It depends only on the absolute temperatures of the hot and cold reservoirs, because a reversible Carnot engine sets the best possible case allowed by the second law of thermodynamics: some energy must always be rejected to the cold reservoir. As the cold reservoir approaches the hot reservoir, the efficiency falls toward zero; as the cold reservoir gets much colder relative to the hot reservoir, the maximum possible efficiency rises.

Use this formula when you want the theoretical maximum efficiency of a heat engine, not the efficiency of a real machine. It is especially useful in thermodynamics problems involving power plants, engines, refrigerators viewed in reverse, and comparisons between operating temperatures.

• Using degrees Celsius instead of kelvin for T_H and T_C
• Forgetting that this is a maximum theoretical efficiency, not the actual efficiency of a real engine
• Reversing the temperatures or using T_H / T_C, which gives an impossible result