engine, the exergy can be simply defined as the energy input times the Carnot efficiency. Since many systems can be modeled as a heat engine, this definition can be useful for many applications. Terminology The term exergy is also used, by analogy with its physical definition, in information theory related to reversible computing. Exergy is also synonymous with: available energy, exergic energy, essergy (considered archaic), utilizable energy, available useful work, maximum (or minimum) work, maximum (or minimum) work content, reversible work, ideal work, availability or available work. Implications The exergy destruction of a cycle is the sum of the exergy destruction of the processes that compose that cycle. The exergy destruction of a cycle can also be determined without tracing the individual processes by considering the entire cycle as a single process and using one of the exergy destruction equations. Mathematical description An application of the second law of thermodynamics Exergy uses system boundaries in a way that is unfamiliar to many. We imagine the presence of a Carnot engine between the system and its reference environment even though this engine does not exist in the real world. Its only purpose is to measure the results of a "what-if" scenario to represent the most efficient work interaction possible between the system and its surroundings. If a real-world reference environment is chosen that behaves like an unlimited reservoir that remains unaltered by the system, then Carnot's speculation about the consequences of a system heading towards equilibrium with time is addressed by two equivalent mathematical statements. Let B, the exergy or available work, decrease with time, and Stotal, the entropy of the system and its reference environment enclosed together in a larger isolated system, increase with time: For macroscopic systems (above the thermodynamic limit), these statements are both expressions of the second law of thermodynamics if the following expression is used for exergy: where the extensive quantities for the system are: U = Internal energy, V = Volume, and Ni = Moles of component i The intensive quantities for the surroundings are: PR = Pressure, TR = temperature, μi, R = Chemical potential of component i Individual terms also often have names attached to them: is called "available PV work", is called "entropic loss" or "heat loss" and the final term is called "available chemical energy." Other thermodynamic potentials may be used to replace internal energy so long as proper care is