Thermodynamics

Chapter 8EXERGY: A MEASURE OF WORK POTENTIALThe increased awareness that the world’s energyresources are limited has caused many countries toreexamine their energy policies and take drastic measuresin eliminating waste. It has also sparked interest in thescientific community to take a closer look at the energy conversiondevices and to develop new techniques to better utilizethe existing limited resources. The first law of thermodynamicsdeals with the quantity of energy and asserts that energy cannotbe created or destroyed. This law merely serves as a necessarytool for the bookkeeping of energy during a processand offers no challenges to the engineer. The second law,however, deals with the quality of energy. More specifically, itis concerned with the degradation of energy during a process,the entropy generation, and the lost opportunities to do work;and it offers plenty of room for improvement.The second law of thermodynamics has proved to be avery powerful tool in the optimization of complex thermodynamicsystems. In this chapter, we examine the performanceof engineering devices in light of the second law of thermodynamics.We start our discussions with the introduction ofexergy (also called availability), which is the maximum usefulwork that could be obtained from the system at a given statein a specified environment, and we continue with the reversiblework, which is the maximum useful work that can beobtained as a system undergoes a process between twospecified states. Next we discuss the irreversibility (also calledthe exergy destruction or lost work), which is the wasted workpotential during a process as a result of irreversibilities, andwe define a second-law efficiency. We then develop the exergybalance relation and apply it to closed systems and controlvolumes.ObjectivesThe objectives of Chapter 8 are to:• Examine the performance of engineering devices in light ofthe second law of thermodynamics.• Define exergy, which is the maximum useful work thatcould be obtained from the system at a given state in aspecified environment.• Define reversible work, which is the maximum useful workthat can be obtained as a system undergoes a processbetween two specified states.• Define the exergy destruction, which is the wasted workpotential during a process as a result of irreversibilities.• Define the second-law efficiency.• Develop the exergy balance relation.• Apply exergy balance to closed systems and controlvolumes.| 423