# Thermal Efficiency & BWR of the A-S Brayton Cycle

Recall that the performance of a power cycle can be measured by its

, η.

## Back Work Ratio

:
The fraction of the work produced by the turbine that is consumed by the compressor.

40% - 80%

1% - 2%

Back Work Ratio for gas power cycles:

Back Work Ratio for vapor power cycles:

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### Ch9, Lesson E, Page 10 - Thermal Efficiency & BWR of the A-S Brayton Cycle

• There is nothing new about how thermal efficiency is calculated for the Air-Standard Brayton Cycle.
• Just remember that it is NOT reversible.  So, you cannot calculate the thermal efficiency of the cycle based on the temperatures of the two reservoirs with which the cycle interacts.
• Earlier in the chapter, we introduced the “Back Work Ratio.”
• The back work ratio is the fraction of the work produced by the turbine that is consumed by the compressor.
• The bad news is that the back work ratio for gas power cycles is very high, 40 to 80%
• The back work ratio for vapor power cycles is MUCH lower, as low as 1 or 2 %.
• So why does anyone build gas power cycles ?
• The equipment to run a gas power cycle is smaller and lighter than the equipment needed to run a vapor power cycle.  So, gas power cycles are especially well-suited to transportation applications.
• In addition, new developments in materials science and also process improvements can significantly improve the efficiency of the basic gas power cycle.
• We will learn more about improvements to the Brayton Power Cycle in the next lesson.
• Now, let’s see how we can use the 1st Law and the properties of isentropic processes to determine the thermal efficiency of an Air-Standard Brayton Cycle.
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