Introduction

An engineer is interested in knowing the maximum useful work that can be obtained from a system when it is in a given state. Obviously, the work obtained will be the maximum only when the final state of the system is in equilibrium with the surroundings. In other words, till it reaches a state when there is no possibility of obtaining further work from the system due to interaction with the surroundings.

The availability of a given system is defined as the maximum useful work that can be obtained in a process in which the system comes to equilibrium with the surroundings or attains a dead state. Clearly, the availability of a system depends on the condition of the system as well as those of the surroundings.