Irreversibility and Lost Work

Ch8, Lesson D, Page 1 - Irreversibility and Lost Work

• In Lesson 8C, we defined the isentropic efficiency.
• It is a better measure of the performance of a system than the thermal efficiency because it compares the actual performance to the performance of an ideal, isentropic process.
• Now, let’s consider a real compressor that requires 500 kW of power.
• An isentropic compressor might consume just 300 kW of power.
• That isn’t so bad. The isentropic efficiency is 60%.
• But, where did the extra 200 kW of power go?
• What if we built our real and isentropic compression systems so that the inlet states are the same in the 2 processes and the outlet states are also the same in the 2 processes?
• In that case, there would be only one way for the missing 200 kW to get out of the system: Heat transfer to the surroundings.
• So, the lost work is energy converted to thermal energy at the temperature of the surroundings.
• This is a key point.
• We usually assume that we have an infinite supply of thermal energy available at the temperature of the surroundings.
• And, this energy is free.
• Anything that is free and available in infinite quantities has no real value.
• That is why we call this lost work. It is energy converted to a zero value or “dead” state.
• This leads me to consider lost work in a heat exchanger processes.
• Heat transfer through a finite temperature difference is irreversible.
• Any irreversibility means there is lost work, even though there is no shaft work in the process.
• So, our goal in this lesson is to quantify lost work for compressors, turbines, and especially heat exchangers.