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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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Lesson 9E: page 10 of 16

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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.