By David Hall
Exergy is a term commonly used to describe the work potential of energy. This is the amount of energy that can be extracted and used as useful work. It can also be known as ‘availability’ or ‘available energy’. The work potential of energy is a maximum value for the extractable useful work at a specified state. No process in 100% efficient, and therefore this is not a practical value, but rather a target for engineers to work towards.
For work output to be maximised, the process should end in dead state. Dead state is the term used to describe a system that is in thermodynamic equilibrium with the environment. This means it is at the same temperature and pressure as the environment, it has no kinetic or potential energy relative to the environment, and is unreactive with the environment. Additionally, there are no unbalanced forces (electrical, magnetic or surface tension effects) if relevant. Unless otherwise specified, dead state conditions are said to be T0 5 258C (778F) and P0 5 1 atm (101.325 kPa or 14.7 psia). A system has zero exergy at the dead state as no work can be produced.
Any system has immediate surroundings, surroundings and an environment. Although these terms all seem similar, they refer to different things. The term surroundings refers to anything outside the boundaries of the system, whilst the immediate surroundings refers to the portion of the surroundings that is directly affected by the process. The environment, is the region outside of the immediate surroundings that is not affect by the process at any point.
A system delivers the maximum possible work as it undergoes a reversible process from the specified initial state to the state of its environment, that is, the dead state. This represents the useful work potential of the system at the specified state and is called exergy. The value of exergy of a system does not however represent the actual work delivered by a work-producing device, but rather the upper limit on the amount it could produce assuming complete efficiency without violating any thermodynamic rules.
The exergy of the kinetic energy of a system is equal to the kinetic energy itself, as it is a form of mechanical energy which can be converted entirely into work.
Exergy of Kinetic Energy = xke = ke = V2 / 2 (kJ / kg)
With V being the velocity of the system relative to the environment. The same applies for the potential energy of a system.
Exergy of Potential Energy = xpe = pe = gz (kJ / kg)
With g being the gravitational acceleration and z the elevation of the system relative to a reference level in the environment. This does not apply for the internal energy, u, of a system or the enthalpy, h.