ABSTRACT
Objectives ◾ Physical meaning of availability ◾ Optimum work, external (or useful) work, and work by medium in a closed system. ◾ Generalized availability analysis for any system, optimum work, irreversibility (loss in work),
and relation to entropy generation, absolute availability (ψ), and relative availability (ψ') or exergy
◾ Illustration of availability concepts for open and closed systems and band diagrams ◾ Œermomechanical equilibrium (TM), thermo-mechanical-chemical equilibrium (TMC)
and chemical availability for open and closed systems
4.1 Introduction Œe Second Law illustrates that the energy contained in a system in the form of thermal or internal energy relative to the ambient cannot be entirely converted into work in a cyclic process even though the system may exist at a higher temperature than its ambient. On the other hand, if an equivalent amount of energy is contained in the same system in the form of potential energy that energy can be entirely converted into work. Œerefore, 1000 kJ of thermal energy contained in a system at a temperature of 1000 K that interacts with an ambient at 300 K can at most potentially provide only 700 kJ of electrical work (through a Carnot heat engine) while 1000 kJ of potential energy in the same system can produce possibly 1000 kJ of electrical work.
