2.1.3. Exergy as the Unifying Performance Metric

The identified need for an energy-based indicator needs to be put in the correct context. The process systems are evaluated based on certain requirements, which are intended to minimise or eliminate any adverse environmental impacts of the system.

Referring to Figures 3 and 4, the material outputs of each process system cannot simply be released to the ambient environment. Before release, they have to be brought to a certain desired state at the point of release to the environment characterised by composition (or an equivalent specification) and temperature. Naturally, suitable pressure also has to be selected and specified.

Such a state is usually defined by the environmental regulations concerning the corresponding natural storages. For instance, for wastewater discharge to environmental basins in the European Union, it is required that they contain a maximum of 25 mg/L BOD5 (Biological Oxygen Demand) at 20 ◦C [72], which can be used to estimate the content of the main contaminants.

Similarly, there are regulatory limits on effluent discharge temperature. For instance, King County, Seattle, US [73], allows a maximum of 40 ◦C at the entry of wastewater treatment plants. The Environmental Protection Agency of Taiwan [74] imposes limits from 35 to 42 ◦C for the points of discharge at sea, with the addition of a requirement that the water stream does not deviate from the surrounding surface water by more than 4 ◦C. The significance of this stipulation is that it relates the target stream temperature to that of the ambient conditions.

From the above reasoning, it becomes clear that all energy flows and storage contents that relate to the considered process systems are limited only to the energy that can be extracted as a difference from the conditions of the surrounding environment. This is equivalent to the definition of exergy, also known as availability [69].

In this case, the referenced environmental conditions are not necessarily the currently existing conditions but those mandated in the environmental regulations and standards. This provides a reference point for estimating the exergy balance (deficit or excess) to achieve zero deviation from the desired environmental conditions and minimise the potential environmental impacts.

The observations below aid in establishing the basis of the evaluation model:


The next section defines the necessary elements for using exergy as the metric to determine the quality of a process stream by defining exergy components associated with the stream, divided into assets and liabilities. The follow-up sections build on this by formulating the overall framework for exergy accounting and computing the exergy profit or loss associated with a process system.
