# Thermal Efficiency and Isentropic Efficiency

In Chapters 4 and 5 we defined the thermal efficiency of a cycle as:
Isentropic
Turbine
• We want to define a new efficiency that can be used to evaluate the performance of a single process such as a pump, compressor, turbine, or even a nozzle.
• The thermal efficiency is based on the comparison of the performance of a real cycle to the performance of a hypothetical cycle which could convert heat completely into work.
• This hypothetical cycle violates the 2nd Law.
In Chapter 6, we learned that a Carnot Cycle has the maximum thermal efficiency and we showed that for any Carnot Cycle:
• In order to develop an efficiency for individual processes, we will compare the performance of real, irreversible processes to the performance of isentropic processes (adiabatic and internally reversible ).
• We will call this new type of efficiency the

## isentropic efficiency

and it will be represented by ηs
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### Ch 8, Lesson C, Page 1 - Thermal Efficiency and Isentropic Efficiency

• Thermal efficiency and coefficient of performance are great for comparing the performance of thermodynamic cycles, but they don’t work well for individual processes.
• For starters, the goal of every individual process may not be to do work or transfer heat.
• Even the performance of the individual processes that make up thermodynamic cycles cannot be compared on the basis of thermal efficiency.
• How could we calculate the thermal efficiency of an adiabatic pump, turbine or compressor.  We cannot !  They don’t exchange heat with any reservoir !
• The new efficiency that we will use to compare the performance of individual processes is called the isentropic efficiency.
• This efficiency is based on the comparison of a real process to an isentropic process.   That’s how it got its name !