Classical thermodynamics is concerned primarily with the macrostructure of matter. Engineering thermodynamics has undergone a revolution, both in terms of the presentation of fundamentals and in the manner that it is applied. Energy is a fundamental concept of thermodynamics and one of the most significant aspects of engineering analysis. Energy can enter and exit a control volume by work and heat transfer. Vapor and gas power systems develop electrical or mechanical power from energy sources of chemical, solar, or nuclear origin. The Clausius inequality provides the basis for introducing two ideas instrumental for quantitative evaluations of processes of systems from a second law perspective: entropy and entropy generation. The Clausius inequality can be demonstrated using the Kelvin–Planck statement of the second law, and the significance of the inequality is the same: The equality applies when there are no internal irreversibilities as the system executes the cycle, and the inequality applies when internal irreversibilities are present.