Updated: Jul 7, 2022
What is Efficiency?
Efficiency is nothing but the ratio of output from the system to the input provided to the system. Thermal efficiency is calculated for work-producing devices like engines, turbines.
Usually, the output is the thermodynamic work produced by the system and the input is heat input into the system. The higher the efficiency the better. Many engineers and companies try to improve the efficiency of the machines because if the efficiency is higher it can be said that, for the same work output the more efficient machine needs less heat input. The heat added into the system comes from natural resources and it cost us money, as well as these resources like coal, oil, natural gas, are depleting day by day.
What is COP?
Conceptually COP is similar to efficiency just that COP is calculated for work consuming devices like refrigerators and heat pumps. It is the ratio of heating or cooling effect to the work that is provided to the system from outside of the system.
What is the difference between COP and Efficiency?
Efficiency gives what fraction of total heat is getting converted into thermodynamic work whereas, COP tells us for desired cooling or heating effect what amount of work must be provided into the system. Usually (not always) COP is greater than 1.
Efficiency that is calculated as a fraction is multiplied by 100 and always informed in percentage.
When to use what?
For work producing devices, efficiency is calculated, and for work consuming devices, COP is calculated.